Visualisation de l`écoulement dans un système vis/fourreau. Suivi en

Transcription

N° d’ordre 2008-ISAL-0124
Année 2008
Thèse
PhD dissertation
Visualisation de l’écoulement dans un système vis/fourreau.
Suivi en ligne du mélange polymère/nanocharges.
Visualization of the flow in a screw/barrel system.
Real-time monitoring of polymer/clay mixing.
Présentée devant
Presented to
Institut National des Sciences Appliquées (INSA) de Lyon
Pour obtenir le grade de
In partial fulfillment of the requirements
for the Degree of
Docteur
Doctor of Philosophy
selon l’arrêté du 30/03/1992
according to the French law
(decree of 30/03/1992)
Dans la spécialité
In the area of expertise
Matériaux Polymères et Composites
Polymer and Composites Materials
Au sein de
In the frame of
École Doctorale Matériaux de Lyon
Antonella ESPOSITO
Ingénieur des Matériaux
Materials Engineer
Soutenue le 5 décembre 2008
devant la commission d’examen:
Defended on December 5th 2008
in the presence of the examination jury:
Rapporteur
José Maria KENNY (Professor, University of Perugia, Italy)
Rapporteur
Serge BOURBIGOT (Professeur, ENSC Lille)
Examinateur
Jean-François GERARD (Professeur, INSA de Lyon)
Examinateur
Jean-Jacques FLAT (Ingénieur Docteur, ARKEMA CERDATO)
Directeur de Thèse
J.-Y. CHARMEAU (Professeur, INSA de Lyon)
Directrice de Thèse
J. DUCHET-RUMEAU (MdC, INSA de Lyon)
Ingénierie des Matériaux Polymères (IMP) – UMR CNRS #5223
Laboratoire des Matériaux Macromoléculaires (LMM)
Referee
Referee
Examiner
Examiner
PhD advisor
PhD advisor
a papà Antonio Senza il tuo incoraggiamento non sarei arrivata fino a qui
a mamma Ivana Il mio punto di partenza, sei tu...
a Feliciano Segui sempre la tua strada
Andata e Ritorno
sed ad meliora
semper
e poi
a tutte le
persone che amo
e che mi vogliono
bene, e che per questa
pura e semplice ragione
m’hanno sempre e da
sempre sostenuta,
incoraggiata, e
soprattutto
attesa
Un pensiero particolare va ai miei nonni,
che vedono il tempo fuggire via senza fare i bagagli,
per i quali il mio ritorno non è mai troppo presto,
e che spero non partiranno mai senza di me.
Jesus ALAJARIN MARTINEZ Pierre ALCOUFFE (et le Vietnam) Firas ALMOUSTAFA Jenny ALONGI (e il famoso baffo) Marco AMICI (e
Irena) Angelica ANTONIELLI D’OULX (Erasmus) Kamira AOUACHRIA (et la rose du désert) Rafael ARIOLI (et le collier de coquillages)
Murat ARLI Maëlenn AUFRAY Ludivine AUGRY (et son papa) France AZZOPARDI Céline BAGUENARD Jean BALCAEN (et la Visiovis)
Liubov BARDACH Najoua BARHOUMI (et le voile interdit) Amélie BARON Claire BARRES (et Arianne…) Julien BARRUET Ahmed Ali
BASFAR (et la rose rouge) Sylvain BAUDU (et Céline) Ludovic BEAL Pierre BEAUDET Redha BELLA (et Ninou) Ditza BEN-ZION Hynek
BENES (et toutes ses Ianas) Mallaouia BENGOUA (Mallou, et l’histoire sans fin du complément labo…) Gwladys BENISTANT (et Alain)
Bruno BERAL (SAMPE) Yves BEREAUX (et les randonnées et les crêpes et la musique) Vincent BERLIER Julien BERNARD Sandra
BINAULD (et Morgan) Françoise BISCARA (et le squatteur toxicomane) Anne BLOND (et la cabine peinture) Gisèle BOITEUX (mais n’en
parlons pas…) Liliane BOKOBZA (JNC15) Dimitris BOLLAS (et Natasa et Nikos et Ppappas) Mélanie BORDES Luigi BOTTA Ghalia
BOUKELLAL (JEPO35) Serge BOURBIGOT (soutenance de thèse) M’hamed BOUTAOUS (et la sacoche volée) Séverine BOYER
(JEPO35) Nadia BRAHMIA (et Mikaël) Raphaël BRUNEL (et chaque jeudi soir en sortant du labo) Annarita CASCONE (et Pascal)
Philippe CASSAGNAU (et tous ses chapeaux) Nelly CHAGNEUX (JEPO35) Mikaël CHAILLY (et Nadia) Aurélia CHARLOT Jean-Yves
CHARMEAU (et Odile et Juliette et Mathilde) Thomas CHAUSSEE (JEPO35) Jiang-Feng CHEN (et les cours de chinois et la soupe au
chocolat) Sambor CHHAY Antonio CHIECHI (SAMPE Europe) Ecaterina CHILAT (JEPO35) Jae-Won CHO (et la tortue dorée) Dieu
Huong CHU Nicolas CINAUSERO (SAMPE France) Sandra CLEMENSON (JEPO35) Floraine COLLETTE (JEPO35) Gaël COLOMINES
Yves-Marie CORRE Magali COULAUD (et les pauses café et les "petites" mésaventures…) Cécile COUSTAL (et Séb et tout le reste…)
Romain CREAC'HCADEC (JNC15) Agnes CREPET Caroline CREUZET Lizong DAI Céline DAMEZ Emmanuelle DANET (et la
marseillaise!) Emilie DEGOUE Guillaume DELAITTRE (JEPO35) Maxime DERANCY (et le bon voisinage) Sandra DIRE Eric
DROCKENMULLER (et Francesca) Jannick DUCHET-RUMEAU (et Pascal) Florence DUPASQUIER (et la Thaïlande) Jérôme DUPUY
Lama ELIAS Françoise FENOUILLOT Christiane FERRET (et le 1er cour de français) Paola FINOTELLO (Erasmus) Jean-Jacques FLAT
(soutenance de thèse) Etienne FLEURY Nadège FOLLAIN Nicolas FORTIN Elsa FRANCHINI (et ses chignons et Abdou) Olivier GAIN (et
la moto) Jean-Paul GALLETIER (SAMPE) Arnaud GALLON (et Claire et Mathis) Jocelyne GALY Franck GAUDIN Estelle GAUTIER (et
Loïc) Jean-François GERARD (le chef !) Frédéric GILLE (et les scouts et les concerts et la politique et la patate) Nelly GIMENEZ (et
Jérôme et Isabelle et le nouveau né) Henri GIRARDY (SAMPE) Emilie GONCALVES Guilene GOOUREY Fabrice GOUANVE Joackym
GRANAL (SAMPE) Raffaella GUBBIOTTI (la Raf nazionale) Nadia GUERROUANI (et son pourrimère) Abdoulallye GUEYE Johann
GUILLEMINOT (SAMPE France) Chady HAKME Lofti HAMITOUCHE (SAMPE France) Sanna HAVERI (SAMPE Europe) Lina HENAO
(et la Colombie et sa bonne humeur… et Michelin) Marcelo HERNANDEZ-AVILA (et le Mexique) Christian HOCHARD (JNC15) Amélie
HOUEL (et son pH…) Hikmet HOUICHI (et les randonnées et la boue et la chanson des schtroumpfs et la femme qui parlait au basilic) Ye
HUA Marie-Pierre INNOCENTE (et son/mon appartement dans les pentes) Emilie JACQUELOT (et son bébé) Bénédicte JACQUETFRANCILLON (et un bocal de cornichons) Boris JAFFRENNOU Stéphane JEOL James JOHNSON (SAMPE) Christophe KAUFFMANN
(SAMPE) José Maria KENNY (le
grand chef!) Greet KERCKHHOFS
(SAMPE Europe) Magorzata
KNAPIK (Nanofun-Poly) Constandinos
the first KONSTANTIS (SAMPE)
Adrie KWAKERNAAK (SAMPE) Jean
LACHAUD (SAMPE France)
Sébastien LADET (et la Cécile nationale)
Nadir LAHLALI Khalid le maître
LAMNAWAR Yoann LAMY Fabrice
LANDAIS (SAMPE) Brigitte
LATOUR (et le 2ème cour de français)
Massimo LAZZARI (NanofunPoly) Jihean LEE (SAMPE USA-Europe)
Michael LEMOINE (JEPO35)
Pierre LISSAC (JMC) Sébastien LIVI
Frédéric LORTIE (le grand… tout
court!) Luis LUNA PEREZ (un des cours
de français) Abdou MAAZOUZ
Sara MAIEZ-TRIBUT (et Laurent et le
gros ventre et puis Rémi) Francesca MANGONI (e la mia tesi di laurea) Gilbert MARTIGNAGO Françoise MARTIN (ou Sylvain) Matthieu
MARTINEAU (Erasmus) Valérie MASSARDIER Benoît MALLET Françoise MECHIN Andrea MEDICI (et Nanofire et Elsa) Flavien MELIS
Julien MESNAGER (et la visite chez Michelin) Marie-Laure MILAN (et le changement de bureau) Cheima MNEKBI Nizar MNIF Mahdi
MNIF (et le kidnapping et la Tunisie que j’aime) Maël MOGUEDET (et Véronique et ses deux anges) Saber MOHAMMADI (et sa petite
famille) Yannick MOLMERET (JEPO35) Fabrice MONTEZIN (JEPO35) Bruno MOUGIN (et la philo-photographie) Jean-Damien
MULLER (et piou-piou) Marc MULLER (SAMPE Europe) Amapola MUNUERA (SAMPE Europe) Loubna NAJEMI (JEPO35) Addisalem
NEGASH (cours de français) Van NGO THI THANH Viviane O’BRIEN (et Lyon International) Meigui OU (cours de français) Jean-Pierre
PASCAULT (le grand grand chef !) Michela PASQUALETTI (Miky ou Pas ?) Francesca PEDITTO (et Eric) Jean-Marc PELLETIER (et
l’Ecole Doctorale) Hervé PERIER-CAMBY (ou Gilbert) Edith PEUVREL-DISDIER (JEPO35) Guillaume PIBRE Emilie PICARD Lionel
PICARD (et Hélène) Pascal PICHON Maribel PINEDA (et le Mexique et Rafael et Ximena) Delphine PLATEL Pavel PODANY (SAMPE
Europe) Tomasz POKROPSKI (et la Pologne, and a year in the merde) Isabelle POLO (et Evan et son deuxième bout de chou Eléa) Isabelle
PONCELET (ma che ?! et sa maman Lucienne, son fils Sylvain, et puis Thiaïs et Poussy et tous les autres animaux…) Simina POPA-NITA
Julien PORTAL Daniel PORTINHA Patricia POULARD (SAMPE) Charlyse POUTEAU (PEP et RAID DAHU 2008) Arnaud PREBE (et
Céline) Olivier RACCURT (CEA) Corinne RATTON-BENOIT (et ses deux merveilleux enfants) Raphaël REGISSER Fanny RICHARD
(JEPO35) Rocio-Noemi RIVAS-ARAIZA Maurice ROCHET (NCT ABG) Steven RODGERS (SAMPE) Laurent ROUGEAU Alain ROUSSEAU
(et la responsabilité ATG) François ROUSSET Riccardo RUGGERONE (SAMPE Europe) Pascal RUMEAU (et Jannick) Daniela RUSU
(JEPO35) René SAINT-LOUP (et la facette noire de la noblesse française) Xavier SAMAIN (JEPO35) Mara SAPONARO (et le TER LyonGenève) Henry SAUTEREAU (et tous les salons du vin) Adil SBIAI Henrik SCHMIDT (SAMPE Europe) Domenico SCIABOLETTA (et le
noir de carbone) Elena SERRANO-TORREGROSA (et le tabagisme passif qui fait du bien) Romain SESCOUSSE (JEPO35) Gérard SEYTRE
Keyhan SHAHRIARI (cours de français) Layth SLIMAN (cours de français) Rodolphe SONNIER (et la République Tchèque) Manel SORBA
Pavla ŠVIGLEROVA Emilie TAILLON Valeria TAGLIAZUCCA Agnieszka TERCJAKS (et Elena et la boucle est bouclée) Julie TEUWEN
(SAMPE Europe) Marie-Claire THIVEND (et l’aide à l’insertion professionnelle) Morgan TIZZOTTI (et Sandra) Hervé TOLLENAERE
(molto così) Valentina TORNABENE (e zio Franco Giancane) Alain TRANQUARD (et Gwladys) Christophe TRAVELET (JEPO35) Laurent
TRIBUT (et Sara) Matthieu VALLE Pascale VALOT (et la Côte d’Azur) Perrine VAN NIEUWENHUYSE Philippe VAUTEY (SAMPE) Karel
VELECHOVSKY (et le Beaujolais) Ruben VERA (et les rayons X) Ronen VERKER (SAMPE Europe) Jérémy VIALE (et Tours) Guy
VILLEVIEILLE (et le bouquin italien) Demian VON OSTEN (Erasmus) Paul WILLIAMSON (SAMPE Europe) Aristide WOLFROM (et
Question pour un Champion) Nikos ZAFEIROPULOS (Nanofun-Poly) Nathalie ZYDOWICZ Evgeny ZELIKMANN (Nanofun-Poly)…
Ero bambina e già se ne parlava: la fuga dei cervelli. In tutta onestà, non credevo
potesse succedere anche a me… non in Francia, e soprattutto non così giovane. E invece
eccomi qua, alla fine di tre anni gradevolmente sofferti sulla tesi di dottorato, alla soglia
di un quarto anno da insegnante/ricercatrice assunta con contratto a tempo determinato,
ma espatriata a tempo indeterminato e… senza più cervello (dicono che sia in fuga).
C’est vraiment la fin des haricots.
Cette thèse a été financée par le Ministère National de l’Enseignement Supérieur
et de la Recherche (MNESR) français, mais s’inscrit dans le cadre du réseau européen
d’excellence pour les nanotechnologies NanoFun-Poly (Nanostructured and Functional
Polymer-based Materials and Nanocomposites). Ce réseau a été constitué en 2004 et est
actuellement en train d’évoluer vers un European Centre for Nanostructured Polymers
(ECNP). Le financement était destiné à un(e) étudiant(e) ressortissant(e) d’un des pays
européens hormis la France : c’est justement grâce à la communication promue au sein
du réseau que ce travail a pu voir le jour.
TABLE OF CONTENTS
INTRODUCTION
Chapter I
1
PROCESSING OF NANOCOMPOSITES
Melt compounding
3
I-1 --------- MIXING AND PROCESSING ------------------------------------------------------ 6
I-1.1
Melt processing in screw/barrel systems ................................................ 8
I-1.2
Melt conveying in the meter section .................................................... 15
I-1.3
Mixing in the molten state.................................................................... 17
I-1.3.1
Mixing steps ..................................................................................... 17
I-1.3.2
Laminar mixing ............................................................................... 18
I-1.3.3
Mixing of highly viscous fluids by helicoidal screws ...................... 21
I-1.3.4
Mixing of solid particles with a molten polymer ............................. 23
I-1.3.5
Distributive and dispersive mixing .................................................. 26
I-2 --------- MACRO-, MICRO-, NANOCOMPOSITES --------------------------------------- 27
I-2.1
Nanocomposite morphology
Agglomeration, aggregation, dispersion and distribution .................... 31
I-2.2
Techniques for morphological analysis................................................ 37
I-2.2.1
Morphological characterizations ex situ ......................................... 38
Transmission Electron Microscopy (TEM) 38
X-Ray Diffractometry (XRD) .................... 41
Other techniques......................................... 44
I-2.2.2
Morphological characterizations in situ ......................................... 48
Winch ......................................................... 50
Laser Doppler Velocimetry (LDV) ............ 50
Particle Imaging Velocimetry (PIV) .......... 52
Antonella ESPOSITO
PhD INSA de Lyon (2008)
Table of contents 1/5
Ultrasound Doppler Velocimetry (UDV) ... 53
Other techniques ......................................... 54
I-2.2.3
Local probes .................................................................................... 55
Laser Induced Fluorescence (LIF) .............. 55
Concentration field ..................................... 57
Mixing time ................................................ 57
I-2.2.4
Computer simulation........................................................................ 60
Identification of chaotic flow zones ........... 60
Quantification of laminar mixing ............... 61
I-3 ---------- PHOTO-FUNCTIONALIZATION OF NANOFILLERS ---------------------------- 62
I-3.1
Lamellar fillers...................................................................................... 63
I-3.1.1
Structure and chemistry ................................................................... 63
I-3.1.2
Photo-functionalization methods ..................................................... 65
I-4 ---------- A DEEPER INSIGHT INTO THE STATE OF THE ART --------------------------- 67
I-R --------- REFERENCES ------------------------------------------------------------------- 85
Chapter II
PHOTO-FUNCTIONALIZATION
Lamellar fillers
95
II-1 --------- MATERIALS -------------------------------------------------------------------- 96
II-2 --------- PHOTO-FUNCTIONALIZATION METHODS ------------------------------------ 98
II-2.1
Clay swelling (A) ..................................................................................98
II-2.2
Dry compounding (B) ...........................................................................99
II-2.3
Melt compounding (C) .........................................................................99
II-2.4
Cation exchange processing (D) .........................................................100
II-3 --------- CHARACTERIZATIONS -------------------------------------------------------- 101
II-3.1
X-Ray Diffractometry (XRD).............................................................102
II-3.2
ThermoGravimetric Analysis (TGA) .................................................102
II-3.3
Elemental Analysis (EA) ....................................................................103
II-3.4
Fourier Transform InfraRed (FTIR) spectroscopy .............................104
II-3.5
TGA coupled to FTIR spectroscopy (TGA-FTIR) .............................105
II-3.6
Spectrofluorimetry ..............................................................................106
II-4 --------- REFERENCE MEASUREMENTS ----------------------------------------------- 108
Antonella ESPOSITO
II-4.1
Reference XRD measurements .......................................................... 108
II-4.2
Reference TGA measurements........................................................... 109
II-4.3
Reference EA measurements ............................................................. 111
II-4.4
Reference FTIR spectra ...................................................................... 112
II-4.5
Reference fluorescence spectra .......................................................... 115
II-5--------- EVALUATION OF THE PERFORMED PHOTO-FUNCTIONALIZATION
METHODS AND CHOICE OF THE PROTOCOL
-------------------------------- 116
II-6--------- OPTIMIZATION OF THE PROTOCOL
FOR CATION EXHANGE PROCESSING (D) ----------------------------------- 122
II-6.1
Choice of the fluorescent molecule .................................................... 123
II-6.2
Influence of the fluorescent molecule concentration ......................... 129
II-6.3
Complementary characterizations (C30B 0.25MC RhP) ................... 133
II-6.4
Further general comments about the efficiency of the
photo-functionalization ...................................................................... 139
II-7--------- CONCLUSIONS ---------------------------------------------------------------- 144
II-R -------- REFERENCES ------------------------------------------------------------------ 146
Chapter III PHOTO-FUNCTIONAL COMPLEXES
Cation exchange processing
149
III-1-------- MATERIALS ------------------------------------------------------------------- 150
III-2-------- PHOTO-FUNCTIONALIZATION PROTOCOL ---------------------------------- 152
III-3-------- CHARACTERIZATIONS -------------------------------------------------------- 152
III-4-------- REFERENCE MEASUREMENTS ----------------------------------------------- 152
III-4.1
Reference XRD measurements .......................................................... 152
III-4.2
Reference TGA measurements........................................................... 154
III-4.3
Reference EA measurements ............................................................. 156
III-4.4
Reference FTIR spectra ...................................................................... 157
III-4.5
Reference fluorescence spectra .......................................................... 160
III-5-------- CHARACTERIZATION OF THE PHOTO-FUNCTIONAL
INORGANIC/ORGANIC COMPLEXES
Antonella ESPOSITO
----------------------------------------- 160
III-5.1
Photo-responsive CNa+ 0.25CEC RhP ...............................................160
III-5.2
Photo-responsive C30B 0.25MC RhP ................................................161
III-5.3
Photo-responsive C10A 0.25MC RhP ................................................162
III-5.4
Photo-responsive C15A 0.25MC RhP ................................................164
III-5.5
Comparison of the photo-responsive complexes ................................166
III-6 -------- CONCLUSIONS----------------------------------------------------------------- 172
III-R ------- REFERENCES ------------------------------------------------------------------ 174
Chapter IV PROCESSING
Real-time monitoring of mixing
175
IV-1-------- VISIOVIS ----------------------------------------------------------------------- 176
IV-1.1 Original configuration ........................................................................177
IV-1.1.1 Components and utilization ...........................................................179
IV-1.1.2 Advantages and limitations ............................................................182
IV-1.2 Evolutions of the configuration ..........................................................186
IV-1.2.1 From 3D lighting to 2D laser plan ................................................187
IV-1.2.2 Position of the CCD cameras ........................................................192
IV-1.2.3 Optical fiber and in-line spectrofluorimetry ..................................193
IV-1.2.4 Calibration of the detection systems .............................................195
IV-1.3 Actual configuration ...........................................................................202
IV-1.3.1 Objectives .......................................................................................204
IV-2-------- EXPERIMENTAL PROTOCOL -------------------------------------------------- 205
IV-2.1 Acquisition of data ..............................................................................210
IV-2.1.1 Images ............................................................................................211
IV-2.1.2 Videos.............................................................................................211
IV-2.1.3 In-line fluorescence spectra ...........................................................213
IV-3-------- PROCESSING OF THE ACQUIRED DATA-------------------------------------- 215
IV-3.1 Images .................................................................................................215
IV-3.1.1 Standard deviation of image luminosity ........................................217
IV-3.1.2 Discrete Fourier Transform (DFT) of textured images .................218
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IV-3.1.3 Validation of data processing ........................................................ 225
IV-3.2 Videos................................................................................................. 231
IV-3.3 Fluorescence spectra .......................................................................... 232
IV-4 ------- SOME EARLY RESULTS ------------------------------------------------------- 233
IV-4.1 Comparison of different photo-active lamellar fillers ........................ 233
IV-4.2 Comparison of different amounts of filler.......................................... 237
IV-4.3 Regulation of the back pressure ......................................................... 242
IV-5 ------- CONCLUSIONS ---------------------------------------------------------------- 246
IV-R ------- REFERENCES ------------------------------------------------------------------ 247
Chapter V
VISIOVIS TECHNICAL PROGRESSES
Problems and suggestions for further amelioration
248
V-1 -------- MATERIALS ------------------------------------------------------------------- 249
Model fluid .................................................... 249
Photo-functional fillers .................................. 254
V-2 -------- EQUIPMENT ------------------------------------------------------------------- 257
Configuration................................................. 257
Light source ................................................... 257
Barrel ............................................................. 258
Screw ............................................................. 258
Feeding .......................................................... 259
V-3 -------- PROCESSING OF THE ACQUIRED DATA ------------------------------------- 259
V-4 -------- INTERPRETATION AND VALIDATION OF THE RESULTS -------------------- 260
V-4.1
Real-time sampling ............................................................................ 261
XRD .............................................................. 262
Rheology ....................................................... 264
V-4.2
Computer simulation .......................................................................... 266
V-R -------- REFERENCES ------------------------------------------------------------------ 267
CONCLUSIONS
269
APPENDIX
Résumé détaillé en français
Antonella ESPOSITO
INTRODUCTION
We live in a time which cherishes the extremes – when people dream of having
the biggest benefits with the smallest effort. It’s happening to the materials engineers
and to the chemists, as well… the smallest the length scale on which they can control
the matter, the biggest their hopes of getting incredible performances. The question is:
should we get satisfied with a power we have only on a laboratory scale? Polymer/clay
nanocomposites are the new Eldorado for materials engineers: the number of scientific
papers published on this topic in unceasingly increasing. The progresses accomplished
in the research laboratories are pushing the global market of the advanced materials to
turn to the world of the nanocomposites, but the reality is that no market segments (or
only limited ones) will free up if the scientific progresses won’t be accompanied by a
proved economic efficiency. Nanocomposites will be considered economically efficient
when it will be possible to process them by conventional tools, following a protocol
whose outcomes can be guaranteed on an industrial scale, that is for medium to large
production rates. It is time to evaluate whether the existing processing tools can be used
Antonella ESPOSITO
1
INTRODUCTION
also to obtain this new class of materials, or they should be adapted to satisfy the actual
and future technological needs, and how. The answers come from a clear definition of
the objectives, the understanding of the mechanisms involved, a deep knowledge of the
available tools and the capability of envisioning which is the best solution to capitalize
on (or modify) them.
This work represents a starting point in this direction. Our objective was to set up
a new method to visualize how do lamellar fillers (e.g. clays) – which are susceptible of
generating nanocomposite morphologies – behave when mixed with molten polymers in
a screw/barrel system. Chapter I will dive deeper into topics such as melt compounding,
the mixing mechanisms, the nanocomposites and their processing, the issues about their
morphology and its characterization, the notions of aggregation, dispersion, distribution,
etc. An extensive review of the state of the art about process monitoring by fluorescence
will also be presented. Chapter II will answer to the question “How to visualize a multiscale lamellar filler which doesn’t have any native optical activity? Which procedure to
photo-functionalize clays?”. Chapter III will prove that clay photo-functionalization can
be performed by cation exchange processing, and the experimental protocol established
and optimized in Chapter II is suitable to photo-functionalize several commercial clays.
In Chapter IV we present Visiovis, the original tool which symbolizes our answer to the
question “How to visualize polymer/clay compounding in a screw/barrel system, whose
walls are generally opaque? How to detect multi-scale lamellar fillers, how to process
the acquired data, and how to interpret the results?”. In Chapter V we’ll make a point
about the progresses accomplished on Visiovis, trying to identify the difficulties we
have not yet overcome and suggesting some solutions for further ameliorations,
whenever a problem has been encountered. A brief conclusion precedes the Appendix,
in which we reported the Matlab codes written purposely for Visiovis data processing.
We preferred not to abuse of the Appendix to promote a regular rhythm of reading.
Each chapter can be read independently from the others, as we regularly reminded the
global context of the study. A multimedia CD-Rom accompanies the manuscript, for all
the contributions which cannot be written down or otherwise commented. The reader
disposes also of a bookmark, particularly useful because it contains any otherwiseforgotten-or-probably-lost chemical formula and sample nomenclature. Enjoy!
2
Chapter I
Melt compounding
Polymer properties directly depend on polymer chemistry. All the polymers that
are members of the same “family” are uniquely identified by their identical “chemical
origin” (the repeating unit) and approximately have the same physical, mechanical and
thermal properties – which differentiate them from the members of the other polymer
families. Sometimes, even though they belong to the same family, polymers can slightly
differ from each other because some of their properties are influenced by a specific
choice of processing route and parameters. Undeniably, significant modifications of the
properties of a given polymer can be achieved by its copolymerization in presence of
one or more monomers having different chemical structures – by the way, shouldn‟t we
consider any copolymer as the member of a brand new family of polymers, rather than
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3
Chapter I
as the modification of an existing one? All of this just to emphasize the fact that the
only way left to significantly modify and tailor polymer properties without changing its
intimate chemistry is compounding – which is nothing else than adding to the polymer
some other components, chemically and/or physically heterogeneous, and dispersing
them into the host matrix – typically by melt processing. Additives contribute to the
formation of a new material (a polymer composite) which keeps belonging to the same
family and nevertheless has different properties with respect to the neat polymer. The
properties of a polymer composite can diverge from those of the neat polymer matrix to
different extents, according to the chemical, physical and morphological nature of the
additives, as well as their physical and/or chemical interactions with the host polymer
matrix [1]. Any processing step involving compounding (e.g. the formulation of the
masterbatch to produce composite pellets by extrusion) could be crucial for the whole
processing of polymer composite materials. The factors which can originate most of the
problems encountered during compounding are [2]:
 the chemistry of the polymer matrix (which can engender a semicrystalline or
an amorphous structure and which determines the glass transition temperature, the melt
rheological behavior, etc.);
 the physical-chemical properties of the modifiers (inorganic or organic fillers,
pigments, flame retardants, UV radiation absorbers, curing agents, etc.);
 the percentage of modifier to be added to the mixture;
 the method chosen for compounding.
As a result, quality and performances of polymer composites don‟t exclusively
depend on the physical and chemical properties of the ingredients chosen for its receipt,
but also on the capability of the processing step to achieve the best dispersion of all the
components of the mixture in the entire volume of the processed polymer – to an extent
which should allow to assume that any portion of the polymer composite, whatever its
location, has the same target properties [1].
Compounding (to tell it differently: the production of a masterbatch, and thus the
formulation of composite pellets) typically involves three steps – all of them critical to
the achievement of good results [2]: feeding, mixing and pelletizing.
4
Melt compounding
The choice of a proper feeding system is crucial to a point that it often influences
the results of compounding in spite of the fact that the subsequent step of mixing may
be highly performing. Traditionally, dry fillers are added directly to the molten polymer
evolving into the extruder – but it frequently happens that shearing isn‟t sufficient to
break filler aggregates and agglomerates. Certainly, it isn‟t accidentally that Dupuy and
Bussi [3] patented a new dispersion process for submicron fillers in thermoplastics. The
aim was to optimize filler desagglomeration before feeding it into the extruder: the filler
is suspended in a liquid phase – which helps desagglomeration – then fed to the system
by pumping the suspension at the entrance of the extruder, where pellets are still solid.
In relation to mixing, it must guarantee some specific conditions but, primarily, it
should assure the achievement of an adequate degree of dispersion of the modifiers into
the polymer matrix and a good homogenization of the compound. Mixing is typically
performed by an extruder (single- or twin-screw, the latter being co-rotating or counterrotating) or by an internal mixer (continuous or discontinuous).
Last step involves the fabrication of pellets from the formulated compound in the
molten state and, analogously to the previous steps, may also influence the final results:
the equipments for injection molding, for instance, require pellets having all the same
dimensions, regularly shaped and homogeneous to assure good performances [2]. If it
has been proved that high-quality products obtained by injection molding require highquality1 pellets, nobody could exactly tell whether this condition is necessary and also
sufficient, or it is only necessary and thus requires something else to get good products.
Indeed, both extrusion and injection molding involve melt processing: the objective of
the former is to formulate composite pellets, whereas the latter makes the composite
pellets melt again in order to reshape the material in the form of the final product. In
addition, both extrusion and injection molding are performed in more or less complex
systems (screw/barrel systems). In conclusion, the question we should try to answer to
is the following: how do fillers exactly behave when mixed to a molten polymer evolving
into a screw/barrel system?
1
We stressed that “high-quality composite pellets” means “regularly-shaped, uniformly size-distributed
and homogeneous pellets”. Starting from now, “high-quality” will implicitly and more specifically means
“characterized by a good filler dispersion and distribution into the polymer matrix”.
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Chapter I
I-1
MIXING AND PROCESSING
Up to now, we‟ve just reminded something universally recognized and admitted:
mixing is a crucial issue for polymer composites processing.
Generally speaking, the combined activities of mixing and stirring are certainly
very ancient and maybe represent one of the very first organized activities of the human
beings: probably, mixing two or more ingredients to get a final product has always been
such an ordinary and almost instinctive doing that rarely captivated our attention. Little
by little (and much earlier than the first industrial revolution), these two activities have
become much more than a hobby for (processing) engineers. By the way, it is worthy to
emphasize that, as any other engineering activity, mixing and stirring have always been
subjected to requirements that, of course, engineers couldn‟t really decide to ignore (e.g.
some volume and/or dimension limitation, the reutilization of an existing container, the
presence of other interfering tool or, occasionally, a specific behavior of the ingredients
of the mixture). Anyhow, having one or more requirements to fulfill doesn‟t prevent
from making (reasonable) suggestions to ameliorate both the processing equipments and
parameters. Sometimes, the simple fact of knowing the advantages and (even more
interesting!) the disadvantages of a given processing system is worthy investigating. In
recent times, as the oil industry progressed, the increased need for mixing organic (and
frequently viscous) fluids introduced a further requirement: the necessity of shearing
(not only stirring!) mixtures. As the market expanded and more products derived from
the differentiation of the oil industry – that is, as the market of plastics developed – the
equipments specifically devoted to highly performing mixing became more and more
complex. Today, mixing and processing are more than ever strictly connected.
Nowadays, several diversified tools are available to stir a fluid, or to compound
it with one or more additives and then to mix everything together: rotating mechanical
stirrers and mixers, vibrating systems, pumping or jet systems, equipments operating an
external recirculation of the fluid, ultrasounds probes, etc. As a general rule, the stirrers
explicitly designed for homogenization are highly performing for the generation of fluid
movements, whilst the mixers designed to ameliorate the dispersion of a phase into
6
Melt compounding
another are primarily performing in terms of energy dissipation [4]. Mechanical systems
are by far the most common industrial systems for stirring and mixing – especially for
plastics. Any other procedure is considered beneficial only when specific requirements
appear to be for some reason incompatible with the traditional equipments. Ultrasounds
probes, for instance, are extremely efficient to generate intense local fluid movements,
to a point that they could be indispensable when mixing should be limited to small
portions of the whole volume of processed material (e.g. to facilitate chemical reactions
or develop specific morphologies [5][6]), but their use is limited by obvious scaling-up
difficulties: that‟s why ultrasound probes haven‟t yet found large application at the
industrial scale. Most of the industrial stirring and mixing processes are nowadays
performed by means of tools stably joint to a rotating axis, which is itself adjusted in a
cylindrical pan. The industry of plastics has developed its own specific equipments,
rather adjusting a screw into a cylindrical barrel. Definitely, most industrial mechanical
mixing processes involving plastics are performed in screw/barrel systems – which exist
in many different configurations having a more or less complex geometry.
In a still medium, mixing is controlled by diffusion and conforms to Fick law. To
accelerate transfer phenomena and, thus, to accelerate mixing, it is necessary to generate
a movement in the volume of the fluid (flow) so that the contact between the ingredients
is regularly renewed by the mechanical action exerted by the mixer. Stirring and mixing
elements are commonly classed on the basis of the direction(s) in which the fluid is
expulsed when exiting the volume swept during rotation. If several stirring and mixing
elements can be easily classed as axial, radial or mixed (whether they generate an axial,
a radial, or a combination of axial and radial flow), the equipments used by the industry
of plastics are often too complex to be unequivocally classed that easy – we previously
cited screw/barrel systems. Nonetheless, performing processing requires performing
mixing, which in turn greatly depends on the choice of the most suitable equipment, and
obviously starts with an optimum design of the mixing elements (or, with reference to
the screw/barrel systems, an optimum design of the screw profile). Once the system
geometrically optimized, it is necessary to find the optimum processing parameters –
rotational speed and residence time, for instance. The best way to make the best choice
is to clearly know which are the desired results and how it is possible to obtain them –
Antonella ESPOSITO
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Chapter I
in other words, the best choice is based on the comprehension of the phenomena
(mixing mechanisms, filler dispersion and distribution) but also on the knowledge of the
available processing tools (essentially the screw/barrel systems).
I-1.1
Melt processing in screw/barrel systems
We already called attention to the fact that compounding is typically performed
by melt processing – undeniably, both extrusion and injection molding involve melting.
Melting isn‟t a simple phase transition for polymers – on the contrary, it strictly depends
on polymer properties. If the polymer is semicrystalline, its fusion forms a low-viscosity
phase only in case the processing temperature is higher than the melting temperature of
its crystalline portion – temperature which is normally rather higher than glass transition
temperature. Conversely, for amorphous polymers, talking about a melting temperature
is somehow meaningless: in fact, the polymer starts flowing as soon as the value of the
processing temperature gets higher than its glass transition temperature. In any case, as
glass transition is a phenomenon which occurs in a range of temperatures rather than at
a specific temperature, as a general rule the processing temperature is set to a value
considerably higher than glass transition temperature, with the intention of facilitating
polymer flowing2.
A certain fluidity of the processed material – more or less accentuated, depending
on its molecular weight and on the mismatch between processing and glass transition
temperature – helps reducing the internal friction between the macromolecular chains
(intrinsically due to the fact that polymer is flowing, thus its macromolecular chains are
moving with respect to each other) and could avoid the inconveniences of excessively
high shear rates (e.g. chain breaking, thus reduction of the average molecular weight).
By the way, applying high shear rates to the processed polymer is sometimes necessary
to obtain specific (morphological) properties: melting first, then conveying and mixing
phenomena are typically accompanied by a shear rate which is not only responsible for
any eventual orientation of the macromolecular chains, but can also be involved in the
2
It is well known that polymer viscosity depends on temperature and typically decreases as temperature
increases.
8
Melt compounding
distribution, dispersion and orientation of reinforcing additives (fillers). These factors
can significantly influence the properties of the final products [7].
Figure I-F1 Schematic representation of the main processing steps for thermoplastic polymers:
extrusion (top) followed by injection molding (bottom).
Composite materials in which the matrix is a thermoplastic polymer are typically
processed by extrusion (the step properly devoted to compounding, i.e. during which the
neat polymer is melt-mixed with all the required additives to formulate the masterbatch
and then the composite pellets) followed by injection molding (the step which uses the
composite pellets previously formulated by extrusion to fabricate the final products), as
schematized in Figure I-F1. As a matter of fact, any thermoplastic polymer composite
processed by extrusions and injection molding undergoes double melting: a first time
for the formulation of the masterbatch, and a second time to be injected into the mould.
In the hypothesis that the masterbatch can be considered a high-quality compound and
that, hence, the composite pellets are also perfectly homogeneous (i.e. filler particles are
completely dispersed and uniformly distributed into the matrix), the critical step which
still has to be evaluated, in terms of performances and with particular attention to the
properties of the final products, is injection molding. Since the simple fact of filling up a
mould causes high gradients of velocity in the injecting channels, any process involving
injection risks to be concerned by some phenomena of filler segregation in some parts
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Chapter I
of the injected pieces. In the case of pigments, segregation (which essentially means bad
distribution3) produces only an effect of color heterogeneity; in contrast, when fillers are
meant to reinforce, any morphological phenomena directly affect mechanical properties,
thus the piece doesn‟t have the same designed mechanical properties in all its portions.
To avoid these problems, one solution could be to ameliorate the moulds by designing
them as capable of strategically diminishing local shear [8]; anyway, even with perfect
moulds, the main requirement remains a highest quality of the initial masterbatch, of the
formulated composite pellets and of the melt compound in the injection screw/barrel
system – to be assured by using chemical compatibilizing agents, if necessary [5].
As previously said, the screw/barrel systems for polymer and polymer composite
processing are quite complex and may have different geometrical (screw) profiles. The
main parameters to describe screw profiles are essentially three4:
 the nominal diameter (D);
 the total flighted length (L);
 the efficient flighted length, i.e. the portion of flighted length which effectively
contributes to processing5.
The flighted length of a plasticating screw typically consists of three sections –
easily identifiable as geometrically different (Figure I-F2):
 the feed section – having a constant deep channel;
 the transition section – having a channel of decreasing depth;
 the meter section – having a constant swallow channel.
The feed section transports the pellets received by the feeding system (which are
still solid); the transition section operates the plastication process by melting pellets;
the meter section (in which pellets are meant to be totally melted) conveys and keeps
3
We‟ll later get deeper into the notions of distribution, dispersion, agglomeration, aggregation…
More precisely, screws are normally defined by their geometrical parameters normalized with respect to
their diameter – in particular by the ratio L/D (length over nominal diameter).
5
This parameter is valid only for injection screws. Indeed, in the reciprocating injection screws, only a
portion of their flighted length continuously contributes to processing: the screw extremity is unavailable
when the unit is totally retracted.
4
10
Melt compounding
mixing the masterbatch6 to assure the best physical, chemical and thermal homogeneity
of the evolving compound.
Figure I-F2 Scheme illustrating the typical geometrical configuration of a plasticating screw. It
is interesting to observe that the flighted length can be divided in three main sections: the feed
section, the transition section and the meter section. Please note that the right portion of this
scheme corresponds to the entrance of the screw/barrel system, whereas the left portion
corresponds to its exit [9].
The ratio between the channel depths in the feed section and in the meter section
corresponds to the screw compression ratio – a further parameter characterizing screw
profiles. In addition to the abovementioned parameter, the absolute depth of the channel
can also be relevant: indeed, a screw having a quite deep channel is highly performing
in terms of transport, but requires high values of torque for rotation; conversely, a screw
having a quite swallow channel requires higher rotational speeds to convey the fluid at
the same transport rate, but also lower values of torque. Besides, the shear rate imposed
to the polymer by a screw having a deep channel and rotating at low speed is less than
the shear rate generated by a screw having a shallow channel but rotating at high speed.
Finally, the relative lengths of feed, transition and meter sections are also responsible of
6
When preparing for injection moulding, the composite pellets formulated by extrusion can be further
compounded with an additional amount of neat polymer in order to dilute the initial concentration of the
masterbatch and obtain the target concentration for the final products.
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Chapter I
screw performances: to improve the homogeneity of the molten compound, for instance,
it may be worthy to increase the length of the meter section. Most conventional screws
have a square pitch, viz. their pitch is similar to their diameter, and even if most screws
have a single flight, it is possible to find screws with a double flight [10].
V tot
Figure I-F3 On the left: the vector V represents the apparent total velocity of the barrel with
respect to the screw and results from the addition of two components (Vz and Vx), respectively
longitudinal and perpendicular to the screw flights. On the right: in the case of injection screws,
the vector V must be corrected by considering the translation of the screw inside the barrel [9].
The three steps of melt processing corresponding to each of the aforementioned
flighted sections of screws can be modeled by mathematical equations and, under given
hypothesis, collected in a unique global model. This work is exclusively aimed to the
evaluation of the last step – the one corresponding to the meter section – in which the
evolving material is supposed to be fully melted. During screw rotation (the movement
which, both in extrusion and injection molding units, is associated to the plastication
process), convection and transportation are supported by the drag forces exerted on the
matter longitudinally with respect to the flights, and the recirculation flow responsible
of mixing is due to the component of the velocity perpendicular to the flights (Figure IF3). The feature which essentially makes the difference between extrusion and injection
molding is the fact that, while rotating, reciprocating injection screws also translate
longitudinally inside the barrel, whereas extrusion screws execute a simple rotational
movement. Thus, in the case of injection molding, the velocity field globally imposed to
the material evolving into the screw channels isn‟t purely circumferential, but has also a
non-zero radial component (Figure I-F3, on the right). In general, the radial component
12
Melt compounding
of velocity is rather small in comparison to the circumferential components, and the
contribution due to the longitudinal translation of injection screws can be neglected: for
this reason, the behavior of reciprocating injection screws has been often assimilated –
just to simplify models – to the behavior of a single-screw extruder.
Figure I-F4 Illustration of the main steps of the plastication process in screw/barrel systems [9].
When passing through the flighted length of the screw, the polymer experiences
several effects, for each section generates different solicitations (Figure I-F4):
 in the feed section, the pellets are packed together and proceed as if they were
an elastic solid;
 in the transition section, the pellets are melted by the heating system around
the barrel and with the help of friction and viscous dissipation of energy;
 in the meter section, the molten compound is homogenized (if necessary) or
simply stirred, and conveyed towards the exit of the screw/barrel system.
As previously said, the role of the feed section is essentially to bring the pellets
into the system and push them towards the transition section. The phenomena occurring
in this first portion of the screw/barrel system are controlled by gravity, analogously to
what happens in an Archimedean screw – even if for big screws rotating at high speeds
the centripetal forces can become relevant, as well. Initially, the polymer pellets aren‟t
at all compacted and tend to roll on each other and to rearrange; then, they‟re rapidly
Antonella ESPOSITO
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Chapter I
packed together and continue proceeding as if they were a unique and elastic solid. As
solicitations increase, the mechanisms become more and more dependent on the friction
between the polymer and the barrel surface, as well as between the polymer and the
screw surface. The feed section, anyway, is not the section which influences the most
screw performances – although an inadequate feeding may have relevant effects on the
following steps of melting and pumping. Generally speaking, the sections which could
influence the most the performances of a screw/barrel system are the transition and the
meter sections.
The role of the transition section is to melt the pellets, previously compacted by
the feed section. Pure solid conveying ends when a thin film of molten polymer is
formed at the interface between the compact solid and the hot barrel: melting is initially
controlled by the formation and development of the film of molten polymer parallel to
the screw flights; then, as the film reaches a critical value of thickness 7, the action
exerted by the screw flights becomes rather scraping and a pool of molten polymer in
continuous recirculation is formed by the side of the pushing surface of screw flights.
As a consequence, the width of the residual solid packed pellets diminishes and the pool
of molten polymer grows till the melting process is complete.
With reference to the meter section (melt conveying), it is worthy to underline
some more differences between extrusion and injection. In single-screw extruders, it is
common to use screw profiles characterized by long meter sections in order to stabilize
the production rate of molten polymer and consequently assure a good homogeneity of
the masterbatch: this precaution is necessary because extrusion is a continuous process
and any variation directly influences the quality of the products. In the case of injection
molding, conversely, melting/mixing and injection aren‟t directly connected and can be
treated separately – which means that a slight variation of the processing parameters
doesn‟t necessarily influence the quality of the final products or, anyway, consequences
aren‟t as direct as in the case of extrusion. Moreover, extrusion requires high pressures
to convey the molten polymer to the exit of the screw/barrel system, whereas injection
molding exploits a total different mechanism to supply the pressure necessary to fill up
7
This critical thickness value normally corresponds to slightly more than the space between the barrel
and the crest of screw flights [9].
14
Melt compounding
the mould. For such reasons, the meter section is relatively less important for injection
molding than it is for extrusion – fundamentally because, as previously explained, the
main role of the meter section is to assure that melting is complete, that the compound is
homogeneous and that the pressure is sufficient to overcome the resistance due to the
smaller section at the exit of the system (back pressure). Briefly, in injection screws the
meter section can be rather short – surely shorter than a quarter of the overall flighted
length and even shorter if the processed polymer is easily melted but highly viscous in
the molten state.
I-1.2
Melt conveying in the meter section
A simple analysis of melt conveying shows that the flow of a Newtonian fluid in
a simple rectangular channel (in the hypothesis that the curvature of the surface and the
thickness of the screw flight are negligible) is determined by two contributions8 [9]:
Q
 2 D 2 hN sin  cos 
2
 Dh3 sin   p 

 
12 
 z 
(I-E1)
 the contribution of the drag flow, i.e. the flow generated by the velocity of the
screw relative to the barrel, in particular by its component parallel to the screw flights;
 the contribution of the pressure flow.
 p

 0  , the second contribution is nil
In the absence of a gradient of pressure 
 z

and the global flow rate Q depends only on the drag flow; conversely, in the presence of
 p

 p

 0  or increases 
 0
a gradient of pressure, the global flow rate Q diminishes 
 z

 z

as shown in Figure I-F5.
8
Q is the global flow rate, D is the nominal diameter of the screw, h is the flight depth, N is the number of
screw revolutions per minute,  is the helix angle, p is the pressure,  is the viscosity of the fluid.
Antonella ESPOSITO
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Chapter I
Figure I-F5 Velocity field in a screw channel having a simple rectangular geometry and in the
presence of a positive (a) and negative (b) pressure gradient, respectively [9].
If an increasing back pressure is applied to the system, the second contribution
influences more and more the global flow rate Q. If some obstacles are added to make
the flow more complex, Q diminishes and the residence time increases, thus mixing is
improved and compound homogeneity is ameliorated, as well. When designing screw
profiles, all these aspects must be taken into account to choose the optimum geometrical
parameters (Figure I-F2). Meter sections design always results from a compromise
between flow rate and mixing efficiency (linked to flow complexity and recirculation).
Changing screw rotational speed could radically modify the melting process, as well:
when speed increases, Q increases and the conversion of mechanical into thermal
energy by viscous dissipation is enhanced. However, the majority of polymers have a
pseudoplastic behavior – their viscosity in the molten state diminishes if the imposed
shear rate increases – which limits the effect of speed rising on viscous dissipation. In
reality, increasing screw rotational speed requires longer flighted sections to accomplish
complete polymer melting. In order to optimize the equipments for high values of speed
or flow rate, the latest trend is to use longer screws; in practice, the best performances
can be obtained by simply choosing the lowest rotational speed compatible with all the
16
Melt compounding
other processing conditions such as the time required for a complete production cycle.
When production cycles have to be short, processing time must be reduced and the
phenomenon which contributes the most to plastication is viscous dissipation; on the
contrary, when production cycles can be long, processing time is less constraining and
conductive heat transfer from the barrel surface becomes the most relevant factor.
I-1.3
Mixing in the molten state
Mixing is nothing else than using mechanical energy to develop a velocity field
and induce a fluid movement in order to homogenize the concentration field present in a
fluid and initially characterized by high gradients. In spite of its universality, mixing is
poorly understood and generally difficult to characterize: the first unified treatment of
the mixing of fluids from a kinematical point of view has been done by Ottino [11].
It is universally admitted that obtaining a mixture statistically perfect right down
to the molecular scale is quite hard and could require intolerably long processing times.
Defining the perfect mixing equipment implies the possibility of achieving the absolute
homogeneity of property fields such as concentration, temperature, etc. in the entire
volume of processed material. In practice, assuring such conditions is considered to be
almost impossible: even if the concentration field appears perfectly homogeneous at the
macroscopic scale, it could not be true at the microscopic or molecular scale.
We should say: it was considered to be almost impossible. Recent progresses in
materials science engaged processing engineers in increasingly tough challenges: new
classes of materials morphologically structured down to the nanoscale (polymer-clay
nanocomposites) are nowadays attracting the interest of both academic and industrial
researchers. Henceforth, the new objective will be: obtaining high-quality compounds
down to the molecular scale – possibly by means of optimized existing equipments.
I-1.3.1
Mixing steps
Beek and Miller (1959) described the mixing process of two fluids by four steps,
summarized in Table I-T1 [4] with the corresponding mechanism and length scale.
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Chapter I
Table I-T1 Mixing steps: mechanisms and relative length scales [4]
MIXING STEP
Macroscopic
Mesoscopic
Microscopic
Microscopic
MECHANISM
Dispersion of the fluids into each other thanks to
the velocity field
Size reduction of the interdispersed parcels of
fluid in consequence of turbulence (if present)
Formation of microcoils, stretching and enrolling
of spirals
Molecular interdiffusion of the fluids into each
other from the spirals whithin the microcoils
LENGTH SCALE
Macroscopic
 cm
Taylor
 mm
Kolmogoroff
10-100 m
Batchelor
0.5-5 m
If spatial scales are easily assignable, temporal scales obviously depend on the
processing parameters (especially the rotational speed of the mixing element), as well as
on the physical and chemical properties of the ingredients of the mixture.
Laminar mixing is based on the mechanisms corresponding to the last two steps,
which in this particular case we would rather define as macroscopic mixing (formation
and cyclic stretching, cutting, recombining of multilayered structures 9) and microscopic
mixing (molecular diffusion at the interfaces of the multilayered structures).
For high viscosities and in the absence of turbulence, microscopic mixing is the
slowest step, thus diffusion is the phenomenon limiting the whole mixing process. If we
decompose the concentration in an average and a fluctuating value, one can affirm that
macroscopic mixing corresponds to the ensemble of phenomena contributing to uniform
the local average value of the concentration in a given volume of the processed mixture,
whilst microscopic mixing corresponds to the phenomena contributing to the reduction
of the local fluctuations with respect to the average concentration value. In the presence
of fillers, these mixing steps are correlated only to distributive mixing.10
I-1.3.2
Laminar mixing
The phenomena controlling flow and mixing directly depend on the conditions in
which processing is carried out. In general, one distinguishes two flow regimes (laminar
9
These mechanisms will be better illustrated in the next paragraph (§ I-1.3.2).
The difference between distributive and dispersive mixing will be clarified in § I-1.3.5.
10
18
Melt compounding
and turbulent) separated by a transition regime. It was in 1883 that Reynolds discovered
the existence of different flows into a channel: its experiment consisted in visualizing a
colored tracer, isokinetically injected in the centre of a transparent tube, while mixing to
a fluid flowing at a known flow rate (Figure I-F6). This technique allowed to visualize
the different behaviors of the tracer. The parameters which determine the flow regime
during processing are grouped into the Reynolds number, representing the ratio between
inertial and viscous forces11:
Re 
D

(I-E2)
water
tracer
laminar
regime
water
tracer
water
tracer
transition
regime
water
water
tracer
turbulent
regime
water
Figure I-F6 Reynolds experiment [4].
The laminar flow regime occurs when flow velocity is very low or fluid dynamic
viscosity is very high and is facilitated by a small characteristic dimension of the system
D. In a laminar flow, the fluid can be imagined as a stack of extremely thin layers (fluid
lamellae) which proceed by slipping on each other without crossing. The movement is
transferred from a fluid lamella to the adjacent ones by friction. Adjacent lamellae don‟t
move at the same velocity: the difference produces balancing opposite drag forces that
generate a shear stress  [Pa]. The gradient of velocity between two adjacent fluid
lamellae associated to the shear stress is called shear rate  [s-1].
 is the dynamic viscosity,  is the density, D is the characteristic dimension of the geometrical system
and  is the velocity of the fluid.
11
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Chapter I
The ratio shear stress over shear rate is defined as the fluid viscosity  

.

Therefore, viscosity can be considered as a coefficient describing the ability of the fluid
to transfer the movement from a lamella to the adjacent ones during lamellar flow.
stretching
cutting
stretching
recombining
cutting
Figure I-F7 Mixing mechanisms in laminar flow: stretching, cutting, recombining [4].
The mixing mechanisms for highly viscous fluids are schematically illustrated in
Figure I-F7. Under the solicitations exerted by the rotating screw, the fluid stretches and
assumes the shape of lamellar elements. During stretching, the contact surface between
adjacent lamellae increases and the distance between them diminishes. The lamellae can
also fold, get cut and recombine. This cycle of morphological modifications repeatedly
occurs until the characteristic minimum lamellae thickness is achieved, viz. until fluid
lamellae are sufficiently thin to consider, at the macroscopic scale, that the compound is
homogeneous. Actually, as previously underlined, no mixture should be considered
perfect if molecular diffusion hasn‟t rendered it homogeneous also at the microscopic
(or even nanoscopic) scale; in practice, this condition has been traditionally neglected as
it would have probably required too much time to be accomplished but, with the advent
of polymer-clay nanocomposites, all these considerations have to be taken into account
and even extended to the molecular scale (one of the mechanisms of clay exfoliation is
20
Melt compounding
thought to be diffusion of polymer macromolecules into clay galleries 12). If stretching is
assured by the velocity gradients developed into the system, cutting and recombining of
several lamellae are caused by three-dimensional fluctuating phenomena which perturb
the flow. An example of such periodic phenomena is the rotation of the screw itself and,
more particularly, the regular movement of the flights in the processed volume of fluid.
Mixing is aimed to the achievement of lamellae as thin and homogeneous as possible.
I-1.3.3
Mixing of highly viscous fluids by helicoidal screws
Highly viscous fluids are frequent in industrial applications – in some industrial
fields (plastics, food, drug and cosmetic industries) they even represent the most used
fluids. The main difficulty associated to the mixing of highly viscous fluids is that the
only possible flow regime is the laminar flow, as previously enlightened. In the absence
of turbulence, there are no vortexes to facilitate mixing: the movement of the fluid is
only assured by the convection movement imposed by the rotating screw as well as, to a
negligible extent, by molecular diffusion (Figure I-F8).
Figure I-F8 Mixing mechanisms in laminar flow: convection and molecular diffusion [4].
Being aware of the limitations imposed by laminar flow, it isn‟t surprising that
the equipments used to process molten polymers are specific for each application and
12
Polymer-clay nanocomposites morphology (intercalated, exfoliated, etc.) will be described in § I-2.
Antonella ESPOSITO
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Chapter I
must be specifically designed for. A good design of screw profile is essential, for it is
the movement induced by the rotation of the screw that directly determines the quality
of mixing. It is desirable that any fluid element spends most of its residence time close
to screw flight tips, in order to exploit the action of convection to the maximum extent:
that‟s why the mixing elements specifically designed for highly viscous fluids typically
sweep a significant volume of fluid13.
Helicoidal screws are, essentially, Archimedean screws. They can continuously
generate an axial movement of the fluid in both the longitudinal directions (“forwards”
and “backwards”), depending on the direction of rotation. The fluid evolves in the space
comprised between two adjacent screw flights, the root surface of the screw and the
internal surface of the barrel. The average trajectory of any element of fluid can be
statistically identified, and its knowledge may greatly help ameliorating the quality of
the final products. Each elementary volume of fluid statistically follows the same
trajectory and undergoes the same shear history if fluid can be considered Newtonian.
Indeed, at a given temperature, the viscosity  can be considered an intrinsic property
of the fluid only in case it is Newtonian – for the viscosity of non-Newtonian fluids is a
local property, which depends on the local conditions of flow (namely, on the shear rate
imposed by the rotating screw into the screw/barrel system).
The homogenization of a compound when fluids are highly viscous is a process
which may be extremely long if compared to the same process performed by the same
equipment on a mixture of fluids having a low viscosity. The reason, as observed15, is
the absence of turbulence typical of laminar flow: the only mechanism responsible for
mixing is convection, and molecular diffusion can be neglected because too slow for the
industrial rhythms of production. Mixing time can be expressed in the form of an
adimensional number which, in reality, stands for the number of screw revolutions
necessary to achieve a mixture with a sufficiently good quality. This observation, along
with all the considerations about the mechanism which prevalently influences mixing,
13
Here is the reason why screws occupy as much space as possible inside the barrel, and their diameter is
often very close to the diameter of the barrel. This last feature is also due to the difficulty of stirring and
renewing the fluid in proximity of the barrel surface.
14
We introduced the notion of viscosity in § I-1.3.2.
15
Find some explanations in § I-1.3.2.
22
Melt compounding
spontaneously lead to the conclusion that mixing time depends on the geometry of the
screw/barrel system but also on the rheology of the fluid. Even though – as Reynolds
showed16 – mixing can be studied with the help of tracers, the introduction of additives
into non-Newtonian highly viscous fluids must be careful, for macromolecular materials
(and in particular their rheology) could be highly sensitive to the presence of smaller
heterogeneous molecules. In other words, the risk is to alter the observed phenomena…
Let‟s consider a screw/barrel system filled with a homogeneous fluid evolving in
steady-state conditions. If, at a given time t 0 , a small amount of a second heterogeneous
phase is fed to the system, the moieties of such added phase follow the flow established
into the system until complete homogenization of the mixture (concentration C 0 ). With
mixing going on, the initial concentration is gradually diluted: the function C (t ) tends to
zero. The added phase can be considered as a tracer and, by definition, should fulfill the
following requirements:
 it should have the same physical properties (i.e. the same density) of the fluid;
 it should be inert, i.e. neither react nor be dissolved during measurement.
The time necessary for the curve C (t ) to reach its asymptote corresponds to the
time spent by the tracer molecules into the screw/barrel system, i.e. the residence time.
By diving C (t ) by the total amount of tracer, the normalized concentration represents the
Residence Time Distribution (RTD). Since the tracer is supposed to reliably follow the
flow, its behavior reproduces the behavior of the fluid. Investigations like this one are
commonly used to evaluate the mixing efficiency of screw/barrel systems: what’s about
mixing solid particles with a molten polymer?
I-1.3.4
Mixing of solid particles with a molten polymer
Processing highly viscous fluids is sometimes aimed to the creation of particular
morphologies – whether the fluid is compounded or not with fillers or other additives.
Indeed, textures in polymers can be due to specific macromolecular arrangements or to
the presence of additional heterogeneous phases. One objective of polymer composite
16
See Figure I-F6 in § I-1.3.2.
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Chapter I
processing could be, for instance, the alignment of fibers (or any other filler) or the
development of a semicrystalline structure with preferentially-oriented crystallites.
A system containing both melt (fluid) and solid phases presents some additional
stirring problems due to the intrinsic nature of such a kind of mixture. These problems
can be summarized as follows:
 the solid phase has a different density from the melt phase (typically higher):
a phase separation by sedimentation is often spontaneous in the absence of movement –
thus, stirring is firstly aimed to generate the movement to contrast sedimentation;
 contrarily to fluids, solids can‟t be deformed and have a completely different
mechanical behavior – therefore, the presence of a solid phase generally modifies the
rheological behavior of the fluid;
 matter and energy transfers between the phases may occur and are typically
accelerated by stirring and mixing.
How solid particles can remain suspended in a flowing fluid? Intuitively, if the
particles are sufficiently small or have a density similar to the density of the fluid, they
will behave exactly as the fluid – except for the possibility of undergoing deformations.
But how to explain that particles having a size up to millimeters and a density lower or
higher than the fluid can remain suspended, whereas they would float or sediment if
plunged in the same fluid but in the absence of stirring? Actually, even if a solid particle
is isolated, the movement of the fluid will exert on it some forces and transport it. It is
evident that, for high concentrations, the shocks between particles and the resistance to
the movement due to their proximity will delay the process of separation of the particles
from each other – that is, dispersing and distributing particles into the fluid won‟t be the
same as for lower particle concentrations. The mechanical equilibrium of one spherical
particle plunged in a fluid is a topic well known by the experts of fluid dynamics and by
processing engineers [4] and the sedimentation and aggregation of relatively small solid
particles from liquid suspensions are phenomena frequently encountered in a variety of
manufacturing processes. More details about the mixture of solid particles with liquids
can be found in specialized books [12][13] and specifically dedicated works [14]. We‟d
like to remind that the flow in presence of suspended solid particles can be described by
24
Melt compounding
adimensional parameters such as Reynolds (Rep), Froude (Frp) and Archimedes (Ar)
numbers. Once again, we are not particularly interested in the adimensional description
of the flow in the presence of solid particles, thus we won‟t give more details; however
it is noteworthy to observe that on the basis of such adimensional numbers big particles
(1 mm) can follow turbulent flows, are sensitive to gravity effects and rapidly sediment,
whereas smaller particles (1 m or less) are insensitive to turbulence and gravity and do
not sediment: they follow the flow and behave as if the fluid was homogeneous [4]. It is
clear, thus, that particle size represents a key factor drastically influencing not only flow
behavior, but also the possibility of characterizing it.
Melt mixing with conventional fillers to obtain polymer micro-composites is
surely easier to characterize than melt mixing with smaller multiscale fillers (clays) to
obtain polymer nanocomposites17. Describing particle breaking and aggregation is
possible by choosing a property of interest (e.g. particle size dp) and expressing the
number of particles having a given value  of such property at the time t and in the point
of coordinates ( x, y, z ) as n( , x, y, z, t )d . The distribution n(d p , x, y, z , t ) is the solution
of an equation of partial derivatives called “equilibrium of particle populations” which
can be written as following (in its general form and for a finite volume V into which the
number n can be considered as a constant) [4]:
(Vn)
 Qs ns  V [ Dbr  Dag ]  Qe ne  V [ Bbr  Bag ]
t
(I-E3)
The distributions B and D respectively indicate the number of particles appearing
or disappearing in consequence of breaking (br) of particles with d > dp or aggregation
(ag) of particles with d < dp, per unit volume and time18. The possibility of taking into
account particle breaking and aggregation implies a potentially more complex analysis
of the flow – indeed, in the presence of a filler to be mixed up with a molten polymer,
both distributive and dispersive mixing are important to obtain high quality products. If
distributive mixing is moderately affected by particle morphology and size (except for
sedimentation, concerning mostly big and dense particles), dispersive mixing is much
17
The notions of micro- and nanocomposites will be introduced in § I-2.
The hypothesis is that no nucleation, no crystalline growth, no precipitation and no dissolution of the
particles occur.
18
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Chapter I
more related to the physical properties of the filler particles, as well as to their chemical
compatibility with the molten polymer in which they have to be dispersed.
Melt mixing with lamellar fillers to obtain polymer-clay nanocomposites is
complex, to the point that many researchers are still looking for a deeper understanding
of the mechanisms of clay exfoliation and dispersion during melt compounding [15-18]
– which would finally help finding the most appropriate parameters to optimize in order
to improve dispersive mixing.
I-1.3.5
Distributive and dispersive mixing
Aggregation and agglomeration are the names commonly given to the processes
during which two particles in suspension collide and remain jointed to each other. These
phenomena are prevalently binary, since ternary or high-order collisions are rare. On the
other hand, disaggregation and desagglomeration (or more generically breaking) occur
as a consequence of the rupture of aggregates and agglomerates. These notions are quite
generic but rather adequate for macro- and micro-composites, whose filler particles are
big enough and have a sufficiently simple morphology19. In contrast, nanocomposites
morphology (in particular polymer-clay nanocomposite morphology) makes everything
more complicated and the words aggregation/agglomeration, dispersion/distribution are
erroneously considered equivalent – their utilization is sometimes confusing. The same
uncomfortable feeling of inadequateness affects also nanocomposite processing.
Up to now, we described mixing and processing issues primarily as they suit to
conventional polymer composites: after all, the main requirements for a high quality
mixing appeared to be a good distributive mixing and an optimized residence time of the
compound in screw/barrel systems. They certainly remain crucial for processing control
but there are other factors, typically requiring a multiscale approach [19], that have to
be considered as well: dispersive mixing and a suitable chemical compatibilization of all
the ingredients of the compound are additional key parameters to control nanocomposite
processing. Nowadays, the literature provides ample evidences that nanocomposites can
19
Breaking by collision with the rotating screw or the barrel surface, for example, is almost impossible
when the particles have a size dp<100 m (unless it happens to aggregates or agglomerates) [4].
26
Melt compounding
be formed by melt processing, and several reasons make melt processing the preferred
method to obtain nanocomposites for commercial purposes. However, while it might be
anticipated that melt processing conditions would have an important influence on the
morphology of the processed nanocomposite, until recently the literature contained no
definitive conclusions about the optimum process conditions [20]. On the contrary,
many articles focus on the importance of the chemistry used to modify the surface of the
nanofillers without including the role of processing – as stressed by Dennis et al. [15] –
or eventually taking into account processing conditions but to a lesser extent, or with
less marked results [21]. This is the reason why we decided to focus on nanocomposites
processing: to understand which requirements should processing have to obtain high
quality nanocomposites. But before that, some morphology issues have to be clarified.
I-2
MACRO-, MICRO-, NANOCOMPOSITES
We‟ve already reminded that, in general, any intimate association of immiscible
phases, capable of conferring to the final compound some specific properties which raw
materials didn‟t singularly have, can be considered as a composite. In particular, any
composite having a polymer as the prevalent phase is labeled as polymer composite. But
a composite can also be defined as the dispersion, whether organized or not, of one or
more secondary phases in a primary phase20. Depending on the size of the moieties of
secondary phase dispersed in the matrix, composites can be classed in three categories:
 macro-composites;
 micro-composites;
 nano-composites.
Macro-composites correspond to traditional composites, reinforced by fibers that
can be long or short, continuous or discontinuous and disposed more or less regularly
into the matrix. The critical issue to assure a good performance of macrocomposites is,
chiefly, the optimization of the interface between the polymer matrix and the secondary
phase. This category of composites have their greatest advantage in the improvement of
20
Primary and secondary only refer to the relative amount of the phase of interest in the compound.
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Chapter I
the mechanical properties and, in relation with this aspect, it is worthy to remind that the
stresses produced by a mechanical solicitation are efficiently transferred from the matrix
to the reinforcing phase only if the phases are intimately and strongly connected 21 [22].
Contrarily to macrocomposites (which nowadays still represent the majority of
the composites produced at the industrial scale), micro-composites can be thought as a
transition between traditional composites and the emerging nanocomposites. In the past,
the major interest in using clays for polymer enhancement was to break clay aggregates
into clusters so that polymers were reinforced by microsized fillers. The results obtained
with clusters, however, weren‟t much better than those obtained for the corresponding
macrocomposites: indeed, it is easy to guess that the excellent properties owned by each
silicate layer composing clays can‟t be properly exploited with such a morphology [23].
Additionally, the weak interaction between clay platelets22 transform clay clusters in
damage initiation sites by locally reducing mechanical resistance, thus facilitating the
formation and propagation of cracks [24].
A nano-composite is a composite in which the secondary phase consists of
moieties having at least one of their three dimensions of the order of the nanometer – or
anyway less than a hundred nanometers [25]. The idea from which the development of
nanocomposites started is certainly related to some frustration due to the quite ordinary
results of microcomposites but also, someway, to chance. First, as the most interesting
properties of clays are largely due to the structure of the individual silicate layers, it was
quite spontaneous to consider exploiting each clay platelet rather than clay clusters; in
addition, as each clay cluster can contain hundreds (even thousands) platelets, another
advantage of disaggregating clays consists in reducing the amount of filler to be added
to the polymer matrix to get high-performance composites. As a matter of fact, if the
optimum level of filler loading is 60% for macrocomposites [22] and almost as high as
60% for microcomposites [3], for nanocomposites a level of filler loading typically less
than 5% wt can remarkably improve the initial properties of the neat polymer matrix
21
Of course, the properties of traditional macrocomposites depend also on other parameters such as the
diameter and the length of the fibres, their length distribution, the volume fraction of fibres and the way
the reinforcement is arranged into the matrix.
22
The structure of clay minerals is described in § I-3.1.
28
Melt compounding
[24]. Indeed, it is particularly in the domain of low filler loadings that nanocomposites
result much more high-performing than traditional composites. Finally, comparing to
traditional composites, nanocomposites have the advantage of improving not only the
mechanical properties, but also the barrier properties, electrical conductivity and fireresistance potentiality of the matrix without affecting its optical transparency [24][26].
Nevertheless, the effective development of polymer-clay nanocomposites started
mostly by chance, when a group of researchers working for Toyota got the very first
surprising results by in-situ polymerization of nylon-6: before that, even if some studies
had been conducted about polymer-clay nanocomposites, this new class of materials
didn‟t have the expected success as the first works didn‟t regularly result in dramatic
improvements of the properties [24]. The first patents about the fabrication of
nanocomposites from nylon-6 and clays have been registered in the latter part of the
„80s [27][28] but the first commercial application of these materials appeared on the
market only few years later [24] and the factor which made it possible was
unsurprisingly related to a better control of filler dispersion in the matrix. Even
nowadays, nanocomposites haven‟t yet find a veritable market segment – not as much
as traditional composites, and in spite of their numerous advantages – maybe because
several physical and chemical mechanisms involved in nanocomposites formation
haven‟t yet been understood and several aspects of nanocomposite processing still have
to be optimized [23]. Controlling nanocomposite processing means controlling
nanoparticle size distribution, as well as dispersion and distribution into the matrix, and
the fact that such a level of control is the critical condition to exploit the exceptional
properties of polymer nanocomposites is nothing else than an evidence, today.
We previously defined a nanocomposite as a composite containing a secondary
phase composed of moieties having at least one of their three dimensions of the order of
the nanometer. It is possible to further class nanocomposites according to the number of
dimensions which are of the order of the nanometer (< 100nm).
Nanocomposites can contain nanoparticles having [25]:
 one nano-dimension;
 two nano-dimensions;
 three nano-dimensions.
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Chapter I
When nanoparticles have only one nano-dimension (e.g. a thickness about 1 nm)
and a shape factor of at least 25, they have the appearance of thin layers (nanoplatelets).
The nanoparticles having such morphology and most commonly used to form polymer
nanocomposites are double-layer hydroxides (DLH) and swelling23 clays (smectites).
The most important smectite is montmorillonite (MMT), an aluminosilicate constituted
of an octahedral alumina sheet stacked between two tetrahedral silica sheets [23][25].
More details about lamellar fillers and MMT structure are available in § I-3.1.
Nanoparticles with two nano-dimensions (the third being much bigger) can appear
as empty tubular (nanotubes) or bulk fibrous (nanofibers and nanowhiskers) particles,
having a diameter less than 100 nm and a shape factor of at least 100 [23].
The group of nanoparticles with three nano-dimensions includes metallic nanoparticles, fullerenes, and other isometric nanoparticles derived from oxides and based
on sulphur, selenium, nitrogen, carbon, e.g. magnetite nanoparticles (Fe3O4), quantum
dots (CdS, CdSe). These particles are not supposed to act as a mechanical reinforcement
but rather contribute to specific properties (magnetic, optical, conductive, etc.).
We focused our attention on processing of nanocomposites containing particles
having one nano-dimension, i.e. lamellar fillers and, more particularly, swelling clays.
The addition of clays to polymers has been proven to greatly improve their mechanical
performances, fire retardancy and barrier resistance potentialities, as well as thermal and
electrical conductivity, whereas optical properties are preserved. Clays are frequently
used to prepare nanocomposites and, nowadays, many reviews are available about this
topic [24][26][29-43]. In spite of the fact that the inorganic platelets are generally
required to perfectly disaggregate into the organic matrix24, clays are often affected by
dispersion and/or distribution problems [44-46]: a method well adapted to monitor the
evolution of the morphology during processing would greatly help the development of
polymer-clay nanocomposites.
23
Swelling clays are also known as expandable clays.
For most applications it is generally believed that the greatest benefits are achieved when the platelets
are fully dispersed into the matrix [15].
24
30
I-2.1
Melt compounding
Nanocomposite morphology
Agglomeration, aggregation, dispersion and distribution
At the very beginning, fillers such as talc and calcium carbonate were added to
polymers primarily to reduce the cost of the final products and, additionally, to increase
their rigidity [3]. Today it is universally admitted that adding microsized fillers to
thermoplastic polymers has many other advantages: reduction of thermal deformations,
improvement of thermal stability and fire-resistance, tailoring of optical properties, etc.
[47]. However, the addition of microsized fillers to polymers frequently reduces their
impact strength, since mineral particles represent zones of stress concentration [23].
Any filler participates someway to the improvement of material properties and,
in any case, the presence of fillers always alter the rupture mechanisms of the polymer
matrix – according to filler shape, dimensions and size distribution, compatibility with
the polymer and, most of all, dispersion and distribution into the matrix. In theory, the
mechanical reinforcing efficiency of fillers should increase as the average particle size
decreases [48] – moreover, reducing filler size should improve matrix rigidity without
affecting its resiliency [3]. But the real revolution of nanofillers is the development of
an extraordinarily extended interface with the polymer matrix: the available surface
dramatically increases when filler diameter is lower than 100 nm (Figure I-F9) [23].
Figure I-F9 Available Surface Area per Unit Volume as a function of filler Particle Diameter in
the case of perfectly dispersed spherical filler particles [23].
Therefore, the smallest the particles, the biggest the available surface: here is the
reason of the strong interparticle interactions experienced by nanoparticles – to a point
Antonella ESPOSITO
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Chapter I
that the formation of aggregates is almost unavoidable. The presence of aggregates has
to be carefully avoided: when composites are subjected to mechanical solicitations, the
presence of aggregates can cause premature failures and modifies the micromechanical
behavior (thus the rupture mechanisms of the polymer) in an unpredictable manner
(Figure I-F10) [48]. Even the presence of a few aggregates can greatly (negatively)
affect the impact strength and reduce composite performances, annulling the benefic
effects of a filler with reduced size [3]. In conclusion, the addition of nanofillers to
thermoplastic polymers is certainly a good method to improve their properties, but if
the nano-particles are perfectly dispersed and homogeneously distributed into the
matrix.
Figure I-F10 Comparison of a polymer composite having a heterogeneous (on the left) and a
homogeneous (on the right) particle distribution. Influence of the particle distribution on the
micromechanical behavior and the fracture mechanisms [48].
The available filler surface directly determines the composite interface which, in
turn, controls the quality of the interactions between the filler particles and the polymer
matrix and consequently influences the properties of the final products. Somehow, the
biggest challenge of nanocomposites could seem similar to the traditional challenges of
macrocomposites: being able to perfectly control the quantity and quality of the fillermatrix interface. In reality, nanofiller reduced dimensions make it more complicated.
In traditional macrocomposites, the interface is defined as the region starting in
the point of the fiber where the properties of the composite are different from fiber bulk
properties, and ending in the point of the matrix where the properties of the composite
32
Melt compounding
are exactly the same as those of the neat polymer25. If we consider the interparticle
spacing as a function of the particle size and in the hypothesis that the filler particles are
perfectly dispersed into the matrix, we realize that at low filler loadings the interface
coincides exactly with the volume of the matrix (Figure I-F11) [23]. The interparticle
spacing for fillers having an average diameter of 15 nm and a volume concentration of
10%, for instance, is about 10 nm: even if the interface measured only few nanometers,
when moving from the filler surface one would observe that the whole matrix has a
behavior which is different from the one had in the absence of the filler [23].
Figure I-F11 Interparticle Distance as a function of the Volume Fraction of Nanoparticles for
different particle sizes, in the case of perfectly dispersed spherical particles [23].
According to the aforementioned definition, nanocomposites have an interface at
least one order of magnitude bigger than traditional macrocomposites: but which is the
exact definition of interface in the case of nanocomposites? We could probably affirm
that interface in nanocomposites is represented by any portion of the matrix in which
the macromolecular chains are immobilized because of the presence of the nanofiller –
meaning that one should be able to easily tell the interface from the unmodified neat
polymer matrix. However, it frequently happens that the properties of a nanocomposite
change continuously from the nanofiller surface towards the neat matrix. Moreover, the
relationship between nanocomposite properties and average particle size isn‟t absolutely
linear. As for macrocomposites, the interface doesn‟t exclusively depend on the particle
25
Such properties could be the chemical composition, the mobility of the macromolecular chains, the
polymerization degree, the polymer crystalline fraction, etc. [23].
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Chapter I
diameter, but also on the quality of the interactions between the filler particles and the
polymer matrix – the strongest such interactions, the thickest the portion of polymer
whose macromolecules have a reduced mobility: the presence of the nanofiller induces
a continuous (rather than discrete) variation of the nanocomposite properties [23]. The
relationship between particle size, filler surface and resulting interface – everything gets
more complicated if we consider that nanoparticles may have a different shape factor
according to the number of nano-dimensions26. The geometrical appearance of a particle
modifies the relationship existing between surface and volume (Figure I-F12) [40].
Figure I-F12 Surface area/volume relations for various particle geometries [40].
Nanofillers can have different size, shape factor, chemical composition and, thus,
nanocomposites morphology may be very different according to nanofiller properties.
But nanocomposite morphology depends also on how the secondary phase is arranged
into the primary phase. Filler spatial arrangement can be better defined in terms of [23]:
 aggregation (or agglomeration), which essentially depends on the physical
and/or chemical interactions between filler particles;
 dispersion, which denotes whether the filler particles are physically separated
from each other (i.e. isolated) or not;
 distribution, which quantifies nanocomposite global homogeneity, because it
measures the ability of the filler particles (whether isolated or not) to homogeneously
occupy the entire volume of the polymer matrix.
26
We have previously explained how it is possible to class nanocomposites according to their number of
nano-dimensions (§ I-2).
34
Melt compounding
Nanocomposites can have good distribution but poor dispersion (Figure I-F13
(a)), poor distribution and dispersion (Figure I-F13 (b)), poor distribution but good
dispersion (Figure I-F13 (c)) or good distribution and dispersion (Figure I-F13 (d)) [23].
(a)
(c)
(b)
(d)
Figure I-F13 Schematic illustration of the possible aggregation and/or distribution outcomes in
composites: (a) good distribution but poor dispersion, (b) poor distribution and poor dispersion,
(c) poor distribution but good dispersion, (d) good distribution and good dispersion [23].
It is obvious that, to obtain nanocomposites with good filler dispersion, the first
obstacle to surmount is represented by the spontaneous tendency that nanofillers have to
aggregate: a choice between avoiding aggregation, assuring disaggregation during nanocomposite processing or – why not? – both of them. The difficulty of this task is due to
the fact that, no matter their morphology, all nanoparticles form aggregates stabilized by
forces (ionic and Van der Waals interactions, hydrogen bonding, etc.) which sometimes
are stronger than the interactions between filler surface and polymer matrix [25].
The interactions between clay platelets, for instance, are stabilized by the anionic
attraction of mobile cations localized in the interlayer spacing27: these cations, highly
hydrated, render clay galleries highly hydrophilic and hinder clay intercalation by most
of the organic macromolecules, prevalently hydrophobic. Thus, sometimes the hydrated
27
Some more details are available in § I-3.1.
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Chapter I
(hydrophilic) inorganic cations have to be exchanged for organic (hydrophobic) cations.
As organic cationic molecules are bigger than inorganic ones, the exchange induces clay
swelling (increase of clay interlayer spacing). More information about MMT structure,
cation exchange process and organically-modified clays (organoclays) are available in §
I-3. Organoclays can be then compounded with polymers, and several methods exist to
produce polymer-clay nanocomposites [26][38]: compounding, independently from the
chosen procedure, is supposed to produce intercalation or complete exfoliation (also
called delamination) of clay platelets by polymer chains, according to the mechanisms
previously proposed [15-17]. But polymer-clay nanocomposites can develop such a
complex morphology that it appears impossible to establish a simplified, unambiguous
description – as demonstrated by the variety of morphological descriptions present in
literature [49]. Biswas and Sinha Ray [29] individuated three categories of polymer-clay
nanocomposites: conventional composites (in which clay tactoids exist in their original
aggregated state, with no intercalation of the polymer into clay lamellae); intercalated
composites (in which the insertion of polymer into the clay structure occurs in a
crystallographic regular fashion, regardless of the clay to polymer ratio, and normally
only a few molecular layers of polymers are intercalated into clay galleries); exfoliated
composites (in which the individual clay layers are separated in a continuous polymer
matrix by average distances which depend on loading, typically with a lower content of
clays than in intercalated composites). Dennis et al. [15], in contrast, differentiated four
possible morphological outcomes: tactoid (clay layers remain unseparated), intercalated
(a small amount of polymer matrix diffuses into clay galleries but causes less than 2030 Å of separation between clay layers), intercalated disordered and delaminated (or
exfoliated) (polymer further separates clay layers, e.g. 80-100 Å or more) Figure I-F14,
on the left. Still different, Schadler [23] distinguished conventional composites with
tactoids from intercalated, ordered and disordered exfoliated nanocomposites (Figure IF14, on the right). Eckel et al. [50] stated that polymer-clay nanocomposite morphology
can be “simply” classified as exfoliated (or delaminated) (clay platelets are dispersed as
discrete sheets with the single platelets having “no observable” association with other
silicate platelets), tactoid28 (clay platelets are stacked face to face in clay particulates)
28
Tactoids tend to be less than 100 silicate sheets thick and can range down to a few layers thick [50].
36
Melt compounding
and agglomerated tactoid29 (bigger clay particulates are made of multiple tactoids).
Homminga et al. [51] gave one more version. Finally Liu et al. [44] recently reviewed
the apparently delicate issue of the morphology of polymer nanocomposites reinforced
by clays: these examples are just meant to point out how complex could be to describe
polymer-clay nanocomposite morphology. Well, the techniques to characterize it aren‟t
expected to be any more anodyne.
Figure I-F14 Schematic illustrations of the terminology used by Dennis et al. [15] (on the left)
and Schadler [23] (on the right) to describe polymer-clay composite morphology.
I-2.2
Techniques for morphological analysis
The difficulties encountered in describing the morphology of polymer-clay nanocomposite affect also the techniques for their morphological analysis. Analogously to
several other properties, morphology can be investigated by techniques ex situ (off-line)
or in situ (on-line). The latter can be performed just by complex equipments (generally
available only in research laboratories) in particular conditions and, sometimes, with the
help of model materials. Processing engineers have been looking for new techniques to
perform on-line process monitoring since a long time – certainly long time before the
advent of polymer nanocomposites. However materials evolve, their applications don‟t
29
The agglomerated tactoids can be several microns large and are comprised of loosely bound tactoids.
The agglomerated tactoids can be distinguished from non-agglomerated tactoids by their relatively close
proximity to one another relative to the overall dispersion of the clay [50].
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37
Chapter I
stop to differentiate and progress unavoidably demands the development of more and
more innovating techniques of characterization. The methods to assess dispersion [52]
and distribution of fillers have to follow, as well.
I-2.2.1
Morphological characterizations ex situ
The most commonly used techniques for morphological characterization ex situ
of polymer-clay nanocomposites are X-Ray Diffractometry (XRD) and Transmission
Electron Microscopy (TEM), although some researchers tried also to employ Scanning
Electron Microscopy (SEM) and Differential Scanning Calorimetry (DSC). Biswas and
Sinha Ray [29] lately proposed a review of some relevant results obtained on polymerclay nanocomposites by XRD, TEM, SEM and DSC. In the next paragraph we rapidly
review the main principles of TEM and XRD and we point out their limitations.
Transmission Electron Microscopy (TEM). TEM is the sole technique able to
visualize morphology and qualitatively estimate nanofiller dispersion by providing an
image of the sample in which one can appreciate some specific structural features of the
nanocomposite such as the shape, size and local repartition of nanofiller particles, both
on the surface and in the volume of the sample (observations can be made on a cross
section of the sample, usually obtained by microtome slicing). Contrarily to the photons
used for light microscopy, electrons have short wavelengths: typical energy values used
to perform electron microscopy are 100-300 keV (corresponding to theoretical optimum
resolutions of 0.2 nm), which is comparable to the interatomic distances of the matter.
Such a high spatial resolution makes TEM a technique well adapted to the investigation
of nanocomposite morphology. Nevertheless, TEM analysis is only possible on samples
which are transparent to electron beams, viz. very thin films (thickness < 120 nm) [25]:
therefore, it is necessary to cautiously prepare the samples, following a procedure to be
chosen according to the nature of the sample itself. Independently on the method used
for sample preparation, one should assure that the chosen cross section is representative
of the entire nanocomposite. TEM images show dark features representing the fillers on
a clear background representing the polymer matrix – since the atoms of most of the
38
Melt compounding
fillers are heavier and stop the transmission of the electrons more than the hydrocarbon
macromolecular matrix [25]. As previously stressed, polymer-clay nanocomposites can
have several different morphologies typically quite difficult to describe. To simplify the
task, let‟s say that during clay intercalation (i.e. while polymer chains diffuse into clay
galleries) two morphological outcomes are possible: a structure made of regular stacks
of clay platelets alternated to polymer layers (intercalated) or a quite homogeneous but
rather irregular morphology (exfoliated or delaminated). Please note that these represent
only two morphological models: in reality, polymer-clay nanocomposite morphology is
partially intercalated and partially exfoliated, both regular and disordered, and may even
contain tactoids in specific portions of the sample. TEM is the best characterization tool
to identify all the possible morphological outcomes with all their variety, but can only
visualize a reduced portion of the sample: how to be sure that the sample has the same
morphology in any of its portions? In other words, how to be sure that the observed
sample is really representative of the whole final product?
Figure I-F15 Low-magnification (on the left) and high-magnification (on the right) TEM image
of the same PS-clay nanocomposite (5% wt) [49]. High-magnification shows an intercalated/
exfoliated morphology, whereas low-magnification reveals small (a) and large (b) tactoids.
An example of TEM image is shown in Figure I-F15 [49] but the literature offers
plenty of papers in which impressive TEM images of different nanocomposite systems
are reported [15-17][45][50][53][54], often substantiated by results obtained by other
techniques (XRD, rheology measurements, etc.). Some authors made considerable
Antonella ESPOSITO
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Chapter I
efforts to find some quantitative methods to exploit TEM images. Dennis et al. [15], for
instance, judged the dispersion level of polymer-clay nanocomposites by counting the
number of platelets or intercalates seen in twelve 6.25 cm2 cutouts from a sheet of paper
laid over a photomicrograph printed at 130,500  magnification, and averaging them to
get a number representing delamination and dispersion for each image: they considered
that the higher the number, the better the delamination and dispersion of the sample are.
Eckel and coworkers [50] observed that, except for few works, TEM is used only
quantitatively to characterize dispersion in polymer-clay nanocomposites and, therefore,
tried to assess a quantitative use of TEM images. First of all, they proved a significant
dependency of TEM results on sample preparation and, in any case, on the dimensions
and orientation of clay platelets: they showed that clay features can be observed easily
(and independently from their orientation) only if sufficiently thick (i.e. thicker than the
microtomed section); as their thickness approaches the width of a single silicate layer,
they are observable only edge on (i.e. only if they are perpendicular to the sample slice).
They also recognized that a qualitative description of clay structures is insufficient to
adequately describe clay dispersion, and may be misleading for several reasons: a small
area may not be representative of the overall structure; the morphology of polymer-clay
composites can be comprised of multiple clay structures; clay structures typically have a
range of different sizes. For these reasons, Eckel et al. decided to quantify dispersion
and considered using several methods: measuring particle size (but they found it tedious
and requiring too many samples, thus they excluded it), measuring particle density (they
proceeded as Dennis et al. [15]) and measuring the linear intercept distance between
particles. A higher particle density at a fixed (or normalized) content of clay indicated a
better dispersion. About linear intercept measurements: they placed an array of parallel
lines over the micrograph and then divided the total length of the lines by the number of
times the lines intersect a clay structure30 – a smaller linear intercept distance indicated
a better dispersion. In conclusion, the linear intercept method has several advantages:
 it is quicker, less tedious and more objective;
30
In samples with highly oriented clay particles, the authors placed the array of lines perpendicularly to
the dominant clay axis.
40
Melt compounding
 in the case of exfoliated nanocomposites, it does not depend on the length of
the silicate platelets;
 for an aligned exfoliated nanocomposite, it provides a theoretical mean linear
intercept distance by dividing the initial clay interlayer spacing by clay volume fraction.
It‟s noteworthy that the values obtained with this method tend to be slightly smaller
than the theoretical ones (i.e. dispersion is underestimated) because of a stereological
error31. A disadvantage of the linear intercept method with respect to the particle density
method is the dependence on orientation: clay particles showing a random orientation in
the sample plane will have a larger linear intercept distance than clay particles having a
preferred in-plane orientation.
Vermogen et al. [45] recently observed that more and more researchers are using
TEM image analysis to characterize the dispersion of polymer-clay nanocomposites and
proposed a novel method to exploit TEM micrographs by image processing and an
appropriate statistical analysis. According to some corrections they had to take into
account, they could compute thickness average  t  , length average  L  , aspect ratio
average  AR  , interparticle distance   II  and     (parallel and perpendicular to
the length, respectively) and average particle density per m2 for each classed tactoid.
Indeed, the more meticulous TEM analysis gets, the more evident the morphological
complexity of polymer-clay nanocomposite becomes: to develop such an exhaustive
and methodic procedure, Vermogen and coll. couldn‟t avoid assuming some hypothesis
and had to define six classes of tactoids to be able to compare different samples.
X-Ray Diffractometry (XRD). If qualitative TEM can visualize all the possible
morphological outcomes in polymer-clay nanocomposites (parenthetically, we reported
proofs that quantitative TEM is developing but is not yet sufficiently reliable as a standalone tool), another technique appears handier, quicker, cheaper than TEM. In general,
XRD is used firstly to establish whether the nanocomposite has or not an intercalated
31
Stereological errors in TEM arise from the image projection of microtomed sections and, thus, increase
with thicker slices and smaller particles. This error affects both the linear intercept and the particle density
measurements.
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Chapter I
morphology and then, if morphology is intercalated, an approximate quantification of
clay interlayer spacing32 can be done on the basis of the X-ray diffraction patterns.
Figure I-F16 WAXD patterns (a) and corresponding TEM images (b) of three different types of
polymer-clay nanocomposite [38].
In a well-ordered layered structure, the basal reflections conform to Bragg law
and are expected to generate harmonic series of diffraction peaks. Natural clay minerals
such as montmorillonite (MMT) are characterized by an initial interlayer spacing of
about 1.2 nm, but modifications by cation exchange with an organic cation bearing a
long alkyl chain swell clay galleries and increase the interlayer spacing to a minimum of
1.5 nm up to a maximum of 3.0 nm, according to the length of the alkyl chain and to its
configuration33 [25]. Any change in the X-ray diffraction pattern of the nanocomposite
sample (peaks appearing, disappearing, shifting, broadening or narrowing, heightening
or lowering) may provide valuable information about the interactions between the clay
32
33
This notion will be clarified in § I-3.1, where the structure of lamellar fillers has been rapidly described.
XRD measurements are currently performed on crystalline powders to characterize their structure.
42
Melt compounding
and the polymer matrix. Morphological assessments of polymer-clay nanocomposites
are thus performed by observing position, shape and intensity of any peak produced by
X-ray diffraction from clay platelets in the range 2 = 1-10°: in practice, the presence of
one or more peaks indicates that the nanocomposite has an intercalated morphology
(with the clay platelets stacked in a crystallographic order and separated by an interlayer
spacing which depends on the position of the peak(s)), and when the peaks broaden or
disappear, clays are completely exfoliated and isolated platelets are dispersed into the
polymer matrix at the molecular level [23][29][38].
An example of XRD patterns (correlated to TEM images) are shown in Figure IF16 [38] but, once again, the literature offers lots of papers in which authors use XRD
to characterize polymer-clay nanocomposite morphology [15-17][29][53-58]. It is
worthy to remind that intercalated and exfoliated morphologies are only some ideal
configurations to make comparisons easier, since the real morphology of polymer-clay
nanocomposites is typically a mixture of tactoids having different sizes (the biggest
visualizable only by light microscopy [45][54], the smallest also by TEM), intercalated
(quantified by XRD), exfoliated (observable only by TEM) clay structures. None of
these techniques is stand-alone, since none of them can unmistakably describe the
morphological outcomes of nanocomposite. Researchers are gradually getting aware of
the limits of the available techniques and do not hesitate to point out the difficulties
encountered and the uncertainties not yet explained. Indeed, several ambiguities in XRD
data risk to complicate nanocomposite characterizations. Eckel et al. [50], for instance,
properly reminded that the basal reflections of lamellar fillers and of the corresponding
nanocomposites do not always form harmonic series of diffraction peaks: it‟s quite the
opposite regularly happening. This observation is easily explained by mixed-layering:
the interlayer spacing between clay platelets is a mixture of two or more types 34. Mixedlayering generates significant uncertainty in the estimation of the interlayer spacing, the
classification of the structural ordering and the exact evaluation of morphology. By the
way, even in the absence of mixed-layering, the interactions between the lamellar fillers
and the polymer are difficult to discern from X-ray diffraction patterns and, in addition,
34
Clay minerals are considered mixed-layered if their peak position (compared to their nominal Bragg
position) exceeds 0.75% [50]. Such phenomenon is also known as the Hendricks-Teller effect [142].
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Chapter I
instrumental parameters35, particle size, defect density and strain effects can affect (to a
minor extent, surely) the observed peaks and, in particular, their width. Because of the
plate-like morphology of lamellar fillers36, preferred orientation can affect XRD results
(more intense peaks indicate a higher clay content or a significant preferred orientation),
as previously reported [50]. In conclusion, the absence or presence of basal reflections
could actually indicate whether exfoliation has occurred or not, but other factors such as
clay dilution, preferred orientation and artifactual peak broadening should be taken
into account: a dilution of the clay content can otherwise result in the false conclusion
that exfoliation has occurred and, conversely, a preferred orientation can result in the
false conclusion that exfoliation has not occurred. XRD is perhaps handy (though not
uncontroversial) to quantify dispersion, but surely cannot help evaluating distribution.
Other techniques. We presented TEM and XRD from a critical point of view:
these techniques are certainly limited and cannot be considered sufficient, if performed
separately, to characterize polymer-clay nanocomposites. Nevertheless, nowadays they
represent the most commonly used techniques and will surely keep being essential for
any morphological characterization ex situ. Some researchers keep comparing XRD and
TEM either to find out whether the respective results are comparable, or to understand
at once which technique is the most effective [49][50][56]. In opposition, many other
researchers have started looking for complementary techniques in order to provide some
alternative tool for polymer-clay nanocomposite morphological characterization. Clay
multiscale structure and the morphology of the resulting polymer-clay nanocomposites
rather require a complete panel of analytical techniques covering several length scales.
Rheology is increasingly used to evaluate nanocomposite morphology. The most
simplistic approach consists in considering that an increase of complex viscosity at low
shear rates (flow measurements) or low frequencies (dynamic measurements) indicates
a good dispersion of the clay platelets into the polymer matrix. Indeed, clay dispersion
in a melt is due to both macroscopic and local shear and results from the stress transfer
of the melt medium to the silicate layers. During melt processing, the macroscopic shear
35
36
Instrumental broadening is expected to contribute no more than 0.2 2.
Lamellar fillers are described in § I-3.1.
44
Melt compounding
imposed by the mixing tool makes clay agglomerates peel, then the local shear due to
the viscosity of the matrix lets polymer chains intercalate in clay galleries [15][45]. The
increase of viscosity at low shear rates or frequencies would be related to the hindrance
exerted on polymer macromolecules by the presence of the clay platelets, dispersed at
the molecular level [45]. In reality, rheology provides more information if the slope of
the viscoelastic moduli is also considered (network formation, percolation), and not only
at low frequencies and shear rates.
Wagener and Reisinger [59] developed a semi-quantitative37 method which, on
the basis of the shear thinning exponent, lets compare the extent of delamination of clay
platelets stacks. This method is one of the first rheological methods for morphological
analysis of thermoplastic polymer nanocomposites containing lamellar fillers. Before
Wagener and Reisinger, the literature contained several papers describing melt rheology
as a potential method to analyze polymer-clay nanocomposites [60], but no publications
disclosing some practical approach to quantify shear thinning with the perspective of
comparing clay exfoliation or nanoscale distribution – even though pronounced shear
thinning had been already found to be a characteristic feature of truly nano-dispersed
polymer-clay composites [61]. Under specific conditions, rheological pseudo solid-like
behaviors indicate edge-to-face interactions of the clay platelets with each other or with
clay tactoids, which would contribute to the construction and mechanical stabilization
of mesoscale card-house structures of silicate layers; at higher shear rates or under the
prolonged action of slow shear forces, such structure would get broken in consequence
of an increasing alignment of the platelets – which is responsible for the shear thinning
effect. Of course any filler – especially for high loadings and no matter its morphology,
(i.e. whether lamellar or not) – modifies the rheological behavior of polymer melts [62]:
thus, rheology appears as a universal tool for the analysis of filler dispersion in molten
polymers even for composites different from polymer-clay composites [63].
Lately, Vermant et al. [64] used again rheology to compare polymer-clay nanocomposites processed by melt mixing: they employed steady-state as well as transient
37
The term semi-quantitative means that there is no unequivocal relation between the shear thinning
exponent and the degree of clay exfoliation. It also means that the average number of clay platelets per
tactoid for a given nanocomposite cannot be calculated by the shear thinning exponent – better used for
direct comparisons of the quality of exfoliation.
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Chapter I
nonlinear measurements to better separate the contributions of flow-induced orientation
of clay tactoids, and of clay particles networking. They determined the conditions under
which the rheological properties are dominated by the presence of clays, then analyzed
the low frequency linear viscoelastic behavior by the scaling concepts of fractal theories
(in order to get the degree of network formation by clay exfoliation) and assessed the
quality of clay dispersion on the basis of the high frequency behavior of the viscoelastic
moduli. Contrarily to TEM, the macroscopic samples used for rheology measurements
offer an integral “vision” of the composite morphology with increased reliability – this
is an obvious advantage in comparison with methods using smaller samples, more prone
to microscale heterogeneity. However, melt rheology data are strongly influenced by the
flow and shear history of the sample: it is necessary to carefully distinguish thixotropy
(which could provide interesting information) from any other effect due to the enhanced
dispersion got by the rheometer – this is the reason why establishing a good protocol for
rheological measurements could be delicate and surely represents the most difficult part
of the whole characterization procedure. In addition, prolonged exposure to heat (during
long lasting measurements) may start the thermal degradation of particularly sensitive
samples. By the way, Vermant et al. showed that, if a suitable experimental protocol is
established, scaling laws for fractal networks can be used to assess clay dispersion in the
melt at low clay contents. The percolation threshold can be determined from the clay
concentration effects on the elastic moduli, i.e. from a deviation from viscoelastic-liquid
to elastic-solid behavior. This percolation value also provides an average particle aspect
ratio for the entire nano-composite: above the percolation threshold, the elastic moduli
and the onset strain to disrupt the network can be scaled with clay volume fraction to
yield a fractal dimension characterizing the network. Changes in the high frequency
moduli were also considered as an indication of dispersion. Since then, rheology has
been used more and more frequently to characterize the structure and melt behavior of
polymer-clay nanocomposites [17][65][66].
TEM isn‟t the only microscopy used for morphological analysis of polymer-clay
nanocomposites. A first proof has been given when discussing TEM: clay minerals and
their nanocomposites have multiscale structures which require a set of characterization
techniques sweeping several length scales. For such reason, even light microscopy could
46
Melt compounding
contribute to morphological analysis – some features, even if definitely undesirable, can
be visualized only on a microscopic scale and mustn‟t be neglected [45][51][54].
If TEM is known to provide useful information on nanocomposite morphology,
Scanning Electron Microscopy (SEM) has never been considered particularly effective,
as reported by Biswas and Sinha Ray [29]. However, some researchers use SEM [51]
and try to develop a method to extract quantitative information from the images (as for
TEM) to assess filler dispersion – although filler is not necessarily lamellar [67]. Atomic
Force Microscopy (AFM) has been recently used as well [68], but the usefulness of this
technique is, analogously to SEM, still questionable [50].
Intercalated morphologies in which the distance between regularly stacked clay
platelets is more than 70 Å can be observed but are quite rare. In this situation, XRD
performed in the typical conditions (2 = 1-10°) becomes useless, since it characterizes
only intercalated morphologies with an interlayer spacing comprised between 10 and 40
Å [38]. If Wide-Angle X-ray Scattering (WAXS) is insufficient, or when the polymer
separates clay platelets and their spatial distribution isn‟t anymore characterized by a
regular, periodic arrangement, researchers use Small-Angle X-ray Scattering (SAXS).
This technique exploits the difference of electron density of clay platelets with respect
to the polymer matrix – which lets detect the presence of the clay from the lowest-angle
fraction of the diffraction pattern. Cser and Bhattacharya [55] published an interesting
work about the methods based on X-ray radiation and able to detect clay platelets in
polymer-clay nanocomposites. They suggested to be careful about the interpretation of
XRD data and showed that the small-angle portion of the XRD scattering curves can be
deconvoluted according to particle and reciprocal lattice scattering in order to estimate
the ratio of exfoliated to intercalated clay structures. Sinha Rya and Okamoto [38] lately
reviewed some results obtained by coupling Wide-Angle X-ray Scattering (WAXS) and
Small-Angle X-ray Scattering (SAXS). These techniques are currently used coupled for
morphological characterizations [69].
Apparently, Differential Scanning Calorimetry (DSC) has also been successfully
used to obtain evidences of polymer intercalation: Biswas and Sinha Ray [29] reported
that Giannelis et al. observed a different thermal behavior of intercalated PS-organoclay
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Chapter I
in comparison with the bulk PS polymer. The fact is that both the neat polymer and the
physical mixture PS-clay clearly exhibited the characteristic glass transition temperature
at 96°C, whereas it seems that the intercalated hybrid did not show any transition in the
temperature range 50-150°C. They also reported that soon after Krishnamoorti et al.
demonstrated that the local and global dynamic behavior of confined polymer chains is
markedly different from the bulk and can be easily visualized by DCS, as shown in the
case of intercalated PS-clay or PEO-clay (absence of any thermal indication of glass or
melting transitions). DSC is rarely used to investigate nanocomposite morphology: this
technique is probably more interesting to analyze the behavior of systems in which the
presence of lamellar fillers not only induces a particular morphological configuration,
but has also a catalytic effect on the formation of the composite.
Finally, some researchers used Solid-State Nuclear Magnetic Resonance (NMR)
to characterize polymer-clay nanocomposites. VanderHart et al. [70-72] demonstrated
that solid-state NMR can be used to quantitatively characterize the dispersion of various
organoclays in PS. The direct influence of the paramagnetic Fe3+ (embedded in the
aluminosilicate layers of montmorillonite)38 on polymer protons within about 1 nm from
the surface of clay platelets creates relaxation sources which, via spin diffusion,
significantly shorten the overall proton longitudinal relaxation time. They used such
relaxation time as an indicator of clay dispersion in PS and showed that this approach
correlated reasonably well with XRD and TEM observations. Indeed, they presented a
double choice: a less complete NMR assay of dispersion that is significantly faster than
TEM, or a slower but more complete NMR analysis – with sampling times comparable
to TEM, information rivaling that of TEM, and a substantial advantage: the fact of
performing bulk characterizations of the polymer-clay nanocomposite of interest.
I-2.2.2
Morphological characterizations in situ
It‟s interesting to observe that, since the beginning, fluid dynamics appears to be
a relevant parameter to understand the relationships between mixing and processing: as
38
The structure of montmorillonite is rapidly described in § I-3.1.
48
Melt compounding
soon as technology progressed enough and the available technical resources permitted
it, numerous sensors have been conceived and designed to evaluate the performances of
mixing devices. In most cases, these new detection systems were preliminary assembled
and tested on a laboratory scale: it‟s what happened with the hot-wire anemometry, the
Laser Doppler Velocimetry (LVD) and the Particle Imaging Velocimetry (PIV), just to
cite some examples. Parenthetically, it is in the field of stirring, mixing and processing
that the first codes for computer-aided simulation of processing have been generated.
Besides, while progressing in the comprehension of mixing and processing and in the
evaluation of the performances of standard devices for quite standard applications, new
emerging fields impose further challenging requirements: that‟s what is truly happening
with polymer-clay nanocomposites.
The pans (or the barrels) are usually cylindrical and must necessarily be made of
a material compatible with the processed products: steel is often a good solution, even if
plastic39 and glass40 recipients have also been used [4]. In most cases, pans and barrels
are not transparent – making any direct visualization impossible – and processing is thus
reduced to a mysterious “black box”: one can completely know the pristine state of the
processed materials, as well as the properties of the final products, but there‟s no way to
know which is the correlation between all these factors (initial properties, process, final
properties) and how to control them. The study of the mechanisms involved in polymer
melting has been made primarily thanks to experimental investigations: Maddock [73]
firstly conceived and tested a technique which, subsequently, let study the flow in the
plastication section of extrusion and injection screw. The technique required waiting for
the processing system to reach a steady state condition, and then stopping the rotational
movement of the screw and rapidly quenching the evolving material by cooling down
the screw/barrel system with cold water recirculating around the barrel. The screw and
the solidified material were then extracted from the barrel and the block of polymer41
was unrolled and cut – normally one cut per revolution (or half revolution) of the screw.
Nowadays, this method is still used (as demonstrated by Moguedet [74]) but not exactly
39
Plastics are light, even if poorly resistant to the internal pressure and to mechanical solicitations.
Glass is inert, compatible with almost everything, but brittle.
41
This technique already uses optical methods to better identify the different portions of the sample –
typically by covering the pellets with some coloured contrast agent [9].
40
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Chapter I
practical. Moreover, a proper and complete flow characterization firstly requires the
measurement of the local velocity vectors of the fluid. Several techniques can be used to
determine the velocity field of a fluid, but most of them require complex equipments
and a certain level of know-how which are a strict prerogative of research laboratories.
Here we report some information about the existing techniques [4].
Winch. It‟s the easiest and most accessible tool to determine the velocity field of
a fluid. This technique is obviously intrusive – the winch must be immersed into the
flow – and implies the risk of perturbing the flow stream. Therefore, winch is reserved
to the flow characterization in big containers, i.e. when the size of the winch (thus, the
induced perturbation) is negligible in comparison to the size of the stirring elements. It
essentially consists of a small wheel: once introduced in the flow, the wheel turns and
its rotational speed is recorded. By repeating this operation in several points of the flow,
the correlation with the local values of velocity is made by means of a calibration curve.
Laser Doppler Velocimetry (LDV). Contrarily to the previous technique, LDV
is a non-intrusive optical system, whose principle is based on the measurement of the
velocity of some particles (naturally present or appositely introduced into the fluid) by
means of a laser source. Of course, the particles are meant to be small enough to follow
the flow streams without modifying them (negligible drag effects). Velocity is measured
as follows: when two coherent beams of the same monochromatic wavelength (laser
beams) cross, their intersection produces a network of interference fringes, i.e. a series
of alternated bright and dark bands. This network, typically shaped as an ellipsoid of
revolution, is the volume of interest for measurement (~ mm3). When tracing particles
pass through the volume of interest, they diffuse the laser light in the bright bands but
are not detectable in the dark bands. The frequency at which particles diffuse light is a
function of the component of their velocity in the plan of the laser beam, as well as of
the distance between the interference fringes. If 0 is the laser wavelength and is the
angle formed by the laser beams at their convergence, the instant velocity u of a tracing
particle traversing the volume of interest can be calculated directly from the Doppler
frequency fd of the recorded signal:
50
Melt compounding
fd 
u
y

2 u sin( )
2
(I-E4)
0
motor
optical unit
on
mobile support
laser beams
splitter
laser source
laser beams
photomultiplier tube
oscilloscope
cylindrical pan
introduced in a
rectangular pan
(transparent)
covariance
analyzer
computer
Figure I-F17 Laser Doppler Velocimetry (LDV) [4].
As the interference fringes are stationary, the Doppler frequency doesn‟t depend
on the direction of the flow. To take into account directional effects, a Bragg cell is
added to one of the laser sources so that a constant frequency shift is applied to the
interfering beams: as a result, the interfering plans aren‟t anymore immobile but rather
pass through the volume of interest with a constant speed. The measured value now
corresponds to the sum of the particle velocity and the passage speed. The light diffused
by the particles is collected by a photomultiplier that converts luminosity in an electrical
signal, which in turn will be processed to get the velocity of the fluid. An oscilloscope is
also integrated to the acquisition system in order to check the quality of the acquired
signals. The LDV technique provides, in each position, a time-averaged value of a given
velocity component and its fluctuating value – Reynolds decomposition of velocity is
particularly adapted to detect turbulence or any other instability of the flow. To get the
other components of the velocity vector, it is necessary to rotate the laser beams to align
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Chapter I
the volume of interest to the desired direction. The main limitation of LDV is the need
for a perfectly transparent system (fluid and equipment). Such constraint obliges to use
water (or another transparent model fluid) and suitable transparent equipments, whose
walls can be traversed by a laser beam without diminishing its power or compromise its
optical coherence. Besides, some particular shapes of the transparent pan in which the
model fluid evolves cause serious optical problems: cylindrical barrels should rather be
externally shaped as parallelepipeds to avoid any divergence of the laser beams due to
the curved surface of the cylinder. LDV technique is schematized in Figure I-F17.
Particle Imaging Velocimetry (PIV). PIV is a non-intrusive technique which
lets acquire, almost instantaneously, the velocity field of a flow in a given geometrical
plan. This technique can be defined by comparison with the LDV technique. If the LDV
technique provides the average value of a given component of the velocity vector in a
given position of measurement, the PIV technique rather provides the instant value of
two components of the velocity vector (the two components corresponding to the plan of
measurement). However, like LDV, PIV technique also requires a perfectly transparent
system (fluid and equipment). The principle of this technique consists in recording the
position of small particles formerly introduced into the fluid: by performing two records
shifted of short time lapses it is possible to follow the discrete movement of the particles
and deduce their speed. Recording is made on optical plans of measurement generated
by a laser sheet: a camera is placed perpendicular to the laser sheet and records images
in which the particles are visible as bright points. The plans of measurement have to be
discretized in multiple small areas: for each area, a vector of velocity is recorded and
averaged. This discretization step is important for it partially influences accuracy: if the
areas are too large, most gradients of velocity won‟t probably be detected; if they‟re too
small, there won‟t be statistically enough tracing particles to perform image processing.
The time lapse between two successive images is an important parameter, as well: it is a
function of the order of magnitude of the velocity vectors expected to be measured, as
well as of the size of the areas in which the plans of measurement are discretized. Once
the images acquired, processing is based on the use of a function of crossed correlation
to calculate the most probable trajectory of the visualized particles by two-dimensional
52
Melt compounding
Rapid Fourier Transform. This way, it is possible to obtain an instant velocity field on a
given plan of the flow; the average velocity field can be obtained by averaging several
(sometimes thousands) instant velocity fields. Figure I-F18 illustrates PIV technique.
fluid containing
a tracer
pulsed laser
laser sheet
function of crossed
correlation
CCD camera
image n
image n+1
discretized
area
velocity
vector
Figure I-F18 Particle Imaging Velocimetry (PIV) [4].
Ultrasound Doppler Velocimetry (UDV). UDV differs from LDV and PIV for
the fact that it doesn’t require a transparent system (neither the fluid nor the equipment)
but rather a system in which the sound propagation is good. Most of the liquids and any
equipment having metal walls are adapted to such a technique, which has the advantage
of being adaptable to real fluids and industrial (non specific) equipments. It essentially
consists in using ultrasound probes to emit intermittent sequences of pulsed waves and
collect, by the same transducer (i.e. the ultrasound probe switched in mode reception),
the echoes produced by small particles introduced into the fluid at a frequency which
gets shifted because of the Doppler effect. The frequency shift provides a measure of
particle velocity, supposed to be similar to fluid velocity. If the value of frequency shift
is completed by the estimation of the total time needed by the emitted waves to go and
the echoes to return, it is possible to estimate the distance of the particle with respect to
the ultrasound probe. The limitation of UDV technique is the way velocity is measured:
the value obtained corresponds to the projection of the particle velocity vector on the
Antonella ESPOSITO
53
Chapter I
axis determined by the ultrasound probe. Since flows are typically three-dimensional, it
could be difficult to extrapolate the real velocity vector from its projection: hence, UDV
is better suited to analyze a mono-dimensional rather than a complex three-dimensional
flow. UDV is schematized in Figure I-F19.
wall 2
flow
particle B
particle A
wall 1
transducer
emission
reception
Figure I-F19 Ultrasound Doppler Velocimetry (UDV) [4].
Other techniques. There exist some other techniques for the characterization in
situ of the flow, but they are less common. Lagrangian techniques [11][75] consist in
following the trajectory of a single particle plunged in the fluid rather than choosing a
position in the flow and wait for the particle to pass in the volume (or plan) of interest.
Such particle can be either real (tracer) or virtual (element of fluid assumed to have no
volume) depending if the experiment is performed on a pilot equipment or by computer
simulation. In the former case, the particle can be tracked by a video apparatus (if fluid
is transparent and the particle is big enough) or by any other system capable of detecting
some specific radiation (e.g. gamma radiation) eventually emitted by the particle (e.g.
Positron Emission Particle Tracking, PEPT). The trajectory is a function of the flow but
also of the physical properties of the tracing particle (density, dimensions, shape).
54
I-2.2.3
Melt compounding
Local probes
Local probes are used to obtain the local concentration value in a given position
of the flow. Probes typically require a preliminary calibration, for concentration cannot
be directly measured but is be rather deduced from the variations of specific physical
and/or chemical properties of the flowing materials.
Formerly, colorimetry was limited to the observation of samples recovered only
at the exit of the mixing system [76]. Since then, progresses have been done in terms of
local probes quality – mostly thanks to the advent of optical fibers, and especially on
account of the constructors of spectrophotometers. Nowadays, accurate results can be
obtained by using fluorescent tracers and, recently, a successful employ of infrared
spectroscopy has also been reported in the presence of particles absorbing specific
ranges of radiation wavenumber. Conductivity is another property commonly exploited
to measure mixing time and Residence Time Distribution (RTD). High temporal (ms)
and spatial (m) resolution probes can be used when visualization is otherwise
impossible (e.g. for equipments with opaque walls). There are also some techniques
based on pH measurements: pH probes cover bigger volumes (mm3) and have a slower
time-response (s) in comparison with conductivity probes; on the other hand, their
response is less affected by bubbles and undesired suspended particles. Finally, a
possibility is offered by thermal methods: the concentration field can be deduced from
temperature if, for example, the tracing agent is a fluid warmer (or colder) than the
observed one, and the probe consists of a platinum resistance. However, the temporal
resolution of thermal methods is mediocre (the probe is too big), the utilization of
viscous fluids is delicate (their viscosity depends on temperature) and sensitivity is
limited by the admissible temperature mismatch between the fluid and the tracer
(excessive mismatches could locally modify the flow behavior).
Laser Induced Fluorescence (LIF). LIF techniques are advantageous for their
resolution and sensitivity. Analogously to most techniques of characterization in situ,
LIF is restrained to laboratory applications. Fluorescent probes absorb the radiation at a
characteristic wavelength and then emit a fraction of the absorbed energy at a longer
Antonella ESPOSITO
55
Chapter I
wavelength42. If the fluorescent tracer is adequately diluted, the intensity of its emission
is proportional to the incident luminous power P0 as well as to the local concentration of
tracer C and to the concentration C averaged on the whole optical pathway throughout
the solution. Rhodamines are the most frequently used class of fluorescent tracers: they
are typically excited by a radiation having a wavelength of 470-520 nm and they emit at
a wavelength comprised between 550 and 600 nm. LIF is then performed by injecting a
given concentration of fluorescent tracer in the continuous flow where is focused a lowpowered laser: the emission intensity is then collected perpendicularly to the incident
laser by a photomultiplier equipped with a filter. The advantage of LIF is represented by
its excellent temporal (ms) and spatial resolution (10 m), along with the fact that it‟s
non-intrusive and that the admissible tracer dilution can be relatively low (10-5 or even
10-6 mol·dm-3). The disadvantages are the need for equipments with transparent walls
and the fact that it‟s only a local measurement. This last problem has been solved by the
introduction of a laser sheet (Plan Laser Induced Fluorescence, PLIF).
PLIF is a particularly powerful technique to investigate the concentration field of
a fluorescent tracer. This technique was firstly used by Van Cruyningen and coll. (1990)
and soon after by Mahouast (1993) before being further developed by Houcine and coll.
(1996), Marcant and coll. (1997) [4]. The intensity of fluorescence emission is usually
mapped by a CCD camera equipped with a filter, then converted to a concentration field
by means of a preliminary calibration of the detection system. CCD cameras permit a
direct evaluation of the mixing system in order to optimize its geometrical profile, the
flow rate, the injection procedure and the feeding position.
The preliminary calibration of the detection system typically proceeds as following:
 the system is filled up with water (or any model fluid) and residual luminosity
(background) is measured Io(x, y, t);
 the system is emptied and filled up again with a solution of fluorescent tracer
with the target mixture concentration C* and the luminosity corresponding to the target
mixture is measured I*;
42
Some more information about fluorescence and spectrofluorimetry will be given in § II-3.6.
56
Melt compounding
 the system is emptied and filled up again with the fluid of interest, the tracer is
injected and the luminosity field of the local emission intensity is measured I(x, y, t).
Houcine and coll. (1996) reported that the local instant concentration of tracer
can be expressed as:
C ( x, y, t ) 
I ( x, y, t )  I o ( x, y) *
C
I * ( x, y)  I o ( x, y)
(I-E11)
In some cases, this technique can provide an index to estimate the quality of the
mixing process [4]. Such index can be derived from the average and variance values of
the concentration field or from the mixing time (more precisely from the RTD).
Concentration field. The local average concentration C ( x, y, z ) is obtained by
averaging a large number n of local values of concentration C ( x, y, z, t ) measured in a
given position. A reduced concentration value
C ( x, y, z )
can be useful to indicate (if
C * ( x, y, z )
different than 1) the deviation of the mixture from the ideal case of a macroscopically
perfect mixture. The temporal variance of concentration is expressed as:
 2 ( x, y , z , t )  c 2 


1 n
2
C ( x, y , z , t )  C ( x, y , z )

n k 1
(I-E12)
and could help characterizing mixing dynamics (high variance values correspond
to significant fluctuations of the concentration). An advantage of the variance analysis is
that the contributions due to vortexes of different size are additives. Anyway, not all the
probes are enough sensitive to detect the spatial and temporal phenomena responsible of
the fluctuations of concentration [4].
Mixing time. Mixing time is defined as the time necessary the get a good quality
mixture. However, the notion of good quality mixture is relative, subjective and depends
on the requirements imposed to the final products and on the techniques used to verify
that such requirements have been fulfilled. For laminar flows, a local criterion to get the
degree of mixing is the thickness of fluid lamellae (also called striations)43; moreover, if
43
Laminar mixing has been introduced in § I-1.3.2 but a more detailed description is due to Ottino [11].
Antonella ESPOSITO
57
Chapter I
the flow is non-chaotic, the number of striations is proportional to the number of screw
revolutions and the thickness of the striations is inversely proportional to the number of
screw revolutions. Contrarily to turbulent mixing, laminar mixing can be affected by
“dead zones”, in which the flow is almost stationary: only molecular diffusion assures
some matter exchanges between looping fluid lamellae (the existence of multiple loops
can require an infinite mixing time, for it would mean that the cyclic stretching, cutting
and recombining mechanisms typical of laminar mixing are not possible). Therefore, the
presence of loops can be a problematic issue for polymer processing.
There are several methods (experimental and computer aided) to measure mixing
time. Experimental methods include global and local techniques. Global techniques are
based on the visualization of the mixing process directly into the equipment of interest.
Of course, these techniques can be used only for equipments having transparent walls
and with transparent fluids: their use is limited to specific laboratory equipments filled
with model fluids. Anyway, the possibility of visually detect the zones in which mixing
isn‟t efficient (maybe because of loops) represents a great interest. The difficulty is due
to the fact that the interpretation of the results obtained by visual technique may depend
on the observer and the results themselves (qualitative) can‟t help understanding mixing
dynamics. The only way to exploit qualitative information is to use it for comparison of
different mixing system and to search a solution for eventual mixing inefficiencies. One
possibility of visualization is given by rapid reactive systems whose kinetic depends on
the mixing conditions and the main result is a color change of the fluid44.
Local techniques require placing a concentration probe in one or more positions
of the flow and following the response upon injection of a tracer. The possible response
of the local probe can be modeled as shown in Figure I-F20 [4]:
44
In this case, it is important to check that the reaction generating the color change doesn‟t significantly
modify the rheological behavior of the fluid, and that the amount of tracer introduced is negligible with
respect to the volume of the mixing system. Moreover, the method used to inject the coloring solution
into the system must be clearly stated in the experimental protocol, for results can dependent on feeding
position and rapidity.
58
Melt compounding
measured
property
global trend
equilibrium
time
measured
property
equilibrium
time
measured
property
equilibrium
time
Figure I-F20 Possible responses of a local probe used for a local characterization technique of
the flow in order to measure the mixing time necessary to accomplish a good quality mixing [4].
Trend I – typical of a laminar flow – the probe periodically detects the passage of
the tracer and its response gets attenuated with time. After a given number of passages,
an equilibrium value (corresponding to the macroscopically perfect mixture) is reached;
Antonella ESPOSITO
59
Chapter I
if the asymptote of the curve is shifted with respect to the equilibrium value, the mixing
system is affected by some dead zones. Such a curve can always provide a mixing time.
Trend II – typical of a situation in which either the injection or the probe are
placed in a dead zone of a laminar flow – the tracer arrives to the probe exclusively by
diffusion at the interfaces of the recirculation loops, thus very slowly. Such a response
usually cannot inform about the mixing time.
Trend III – typical of a turbulent flow – as the mechanisms of turbulent diffusion
assure a rapid dispersion of the tracer in the fluid, it is common to observe just one peak
on the probe response (sometime no peaks are visible, depending on the position of the
probe relative to the position of injection). Turbulence produces signal fluctuations,
which are even better resoluted if the time-response of the probe is rapid.
These methods (both global and local) are well adapted to quantify mixing when
the knowledge of flow mechanisms is partially available. If the system generates several
recirculation loops, the result strongly depends on the position of the probe with relation
to the position in which the tracer is injected. Thus, it is always preferable to perform
first a global visualization (just to understand in which position it would be better to
place the probe and inject the tracer) followed by a local technique.
I-2.2.4
Computer simulation
Computer simulation represents a very useful (and probably the most common)
tool to visualize and understand the influence of fluid dynamics [11] on processing.
Parenthetically, we‟ve already reminded that it is exactly in the field of stirring, mixing
and processing that the first codes for computer simulation have been generated. Here,
we just want to give some examples of computer simulation of laminar flows.
Identification of chaotic flow zones. We‟ve previously introduced the notion of
laminar mixing (§ I-1.3.2) insisting on the fact that thermoplastic polymer processing is
never supported by turbulence: nonetheless, there‟s a form of chaotic mixing which may
be considered a particular type of laminar mixing and typically occurs when a periodic
perturbation is applied to the flow. The effect produced on two fluid elements initially
60
Melt compounding
very close is that the perturbation makes them move away from each other: if the
distance of the fluid elements is an exponential function of the number of perturbations
(i.e. of time) the flow is considered chaotic. Conversely, the laminar flow is non-chaotic
if the distance is a linear function of time. Chaotic flow obviously facilitates mixing, for
it separates two fluid elements much faster than non-chaotic flow. Mixing occurring in
conditions of chaotic flow is called chaotic mixing. Each fluid element evolving in a
zone of chaotic flow could statistically occupy all the available positions – at least over
long processing time. This property is used to visualize by computer simulation whether
a zone of flow is chaotic or not: a significant cross section is chosen and its intersections
with the trajectories of the fluid elements are visualized (Poincaré sections) to identify
non-chaotic zones – which are then represented by the absence of points (Figure I-F21).
Figure I-F21 Example of Poincaré section obtained by computer simulation and showing two
zones of non-chaotic laminar flow [4].
Quantification of laminar mixing. A fluid experiencing laminar flow appears
made of layers (fluid lamellae) slipping on each other, as previously described. Two
main variables can quantify laminar mixing and be deduced from the analysis of
striations45 [4][11]:
 the thickness of the striations – which provides the distance between adjacent
fluid elements and therefore determines which mechanism (between molecular diffusion
and convective mixing) is the most probable;
45
Information about striations can be found also in § I-1.3.2 and I-2.2.3.
Antonella ESPOSITO
61
Chapter I
 the surface of contact of adjacent striations – which greatly influences any
transfer coefficient and, thus, the kinetics of chemical reactions (if present) between the
components to be mixed.
Again, laminar flow is considered chaotic when the thickness of the striations
decreases exponentially with time, whereas the surface of contact, of course, increases
exponentially. As it is difficult to estimate the value of these variables – especially when
the test involves three-dimensional flows – computer simulation represents a valuable
tool to support experiments. An example of computer-aided quantification of laminar
mixing is reported in Figure I-F22.
Figure I-F22 Quantification of the thickness of the striations for a flow in a micro-channel: (a)
Poincaré section (vertical cross section of the micro-channel) and (b) estimation of the thickness
of the corresponding striations [4].
I-3
PHOTO-FUNCTIONALIZATION OF NANOFILLERS
We pointed out all the difficulties which can be encountered during polymer-clay
nanocomposite processing by melt compounding in geometrically complex screw/barrel
systems. The geometrical complexity of the equipments, the morphological issues
associated to the multiscale structure of such materials, the disadvantages of the existing
62
Melt compounding
tools for morphological characterizations ex situ – as well as the progress of the modern
on-line process monitoring techniques – pushed us to try conceiving a detection system
capable of performing real-time monitoring of melt compounding of a polymer with a
lamellar filler. Our proposal was preparing us a new challenge: how to characterize in
situ polymer-clay nanocomposite morphology, being aware of the difficulties we could
encounter? Our detection system should be sensitive to multiscale morphologies and
compatible with some specific property of the materials of interest, i.e. lamellar fillers.
Which properties do lamellar fillers have? Which possibilities of modification?
I-3.1
Lamellar fillers
Lamellar or plate-like fillers are so called because of their morphology – they are
made of platelets having different composition and spatial arrangement. The dimensions
of each platelet typically range in the following scales: few nanometers (thickness), tens
of nanometers (width) and from tens of nanometers to few micrometers (length). Having
a thickness of few nanometers is enough to class lamellar mineral fillers in the group of
nanofillers46. Their reduced dimensions are responsible for a high specific surface (from
100 up to 1000 m2g-1) and their particular morphology confers them a high aspect ratio
(from 100 up to 1000). The most widespread lamellar fillers belong to the family of 2:1
(double layer minerals) phyllosilicates (montmorillonite) but other families of lamellar
fillers are also used to prepare polymer composites, e.g. polysilicates (magadiite) and
Layered Double Hydroxide (hydrotalcites). Lamellar fillers can be natural or synthetic.
For our purposes, we selected natural lamellar fillers (cheaper than synthetic) belonging
to the family of phyllosilicates (more interesting for commercial application, as their use
is already widespread but not yet optimized).
I-3.1.1
Structure and chemistry
Montmorillonites (MMTs) are naturally-occurring multiscale lamellar mineral
fillers which consist of crystalline hydrated aluminosilicates containing two repeating
46
The notion of nanocomposite and nanofiller has been introduced in § I-2.
Antonella ESPOSITO
63
Chapter I
units: tetrahedral and octahedral sheets. In particular, each MMT platelet consists of
one octahedral Al(OH)3 layer stacked between two tetrahedral SiO4 layers – the former
containing edge-shared octahedra and the latter corner-linked tetrahedra. If each silicate
tetrahedron contains one Si atom in its centre, and all gibbsite octahedrons contain only
Al atoms, the overall structure is electrically neutral. However, during the geological
processes which lead to the formation of MMTs, some structural atoms are replaced by
other atoms having the same size (isomorphic substitution) but lower electrical charge:
Si4+ can be replaced by Fe3+ or Al3+, and Al3+ can be replaced by Fe2+ or Mg2+. These
substitutions generate a deficiency of positive charge and justify the fact that the stacks
of platelets are globally negative. The permanent negative charge associated to the
immobilized anions47 is equilibrated by the presence of singly- and doubly-charged
mobile cations (Na+, K+ and Ca2+, Mg2+) which act as counterions (i.e. compensating
cations). These counterions are principally grouped in the interlayer space (also known
as clay gallery) – which separates the platelets and whose dimension (basal spacing
d001) depends on the nature of the cations and the degree of clay hydration [23][25] –
but some of them are also located on the external surface of the platelets. The surface
counterions can be readily exchanged with other inorganic or organic ions in all clay
minerals, whilst the interlayer counterions are only accessible in expandable (swelling)
clays. MMTs are expandable clays: their interlayer inorganic cations can be replaced by
other cations having a higher affinity for the fixed ionic sites on the platelets – without
any major change of the crystalline structure but an increase of its basal spacing. The
d001 value of MMT in the form of powder may vary between 9 and 60 Å [77] depending
on the nature and the concentration of the molecules absorbed between the layers and,
in the presence of a polymer matrix, it could even attain a hundred of ångströms. The
general formula which describes MMT structure is Al2-xMgx[Si4O10](OH)2NaxnH2O.
Silicate platelets represent MMT elementary particles (thickness ~ 1 nm): MMT primary
particles include 5 to 10 platelets (size 8 to 10 nm) and MMT aggregates consist of
several MMT primary particles stacked together without any preferential orientation
(size 0.1 to 10 m), as shown in Figure I-F23 [78].
47
The maximum layer charge for most 2:1 clay minerals is about 1.00 per formula unit [79].
64
Melt compounding
Figure I-F23 Clay multiscale structure: elementary particles (each single platelet), primary
particles (5 to 10 platelets) and aggregates (several primary particles stacked together without
any preferential orientation) [78].
This multiscale structure includes voids at any organization level, which explains
the outstanding aptitude of MMTs to swell in water or aqueous solutions [80][81] by
reversibly gaining and losing water molecules. MMTs can absorb water by hydration of
the compensating cations and/or by capillarity penetration, within clay galleries as well
as within the voids associated to inter-particle and inter-aggregate porosity [78]. We
won‟t provide much more information about clay minerals and MMTs, for nowadays
these lamellar mineral fillers probably represent the nanofiller the most commonly used
by researchers – most of the scientific papers currently published about polymer-based
nanocomposites contain some results obtained with clays (or at least cite someone else‟s
results). Many reviews on polymer-clay nanocomposites are available in the literature.
For our purposes, we would just underline that natural MMTs don‟t have any specific
optical property, i.e. they have no spontaneous optical activities. As we were planning
to conceive a new detection system for real-time monitoring of the melt compounding
of polymer and lamellar fillers by fluorescence, we firstly needed to find a suitable
photo-functionalization method to render clays photo-active48.
I-3.1.2
Photo-functionalization methods
There are several ways to modify 2:1 phyllosilicates [82]. Clay functionalization
can be carried out by adsorption, whether involving chemical linkages (chemisorption
48
The experimental progression which led us to the establishment of a photo-functionalization protocol
suitable to our purposes is reported in Chapter II. Further applications of such protocol will be discussed
in Chapter III.
Antonella ESPOSITO
65
Chapter I
or grafting), electrostatic interactions (compensation of electrical charges) or only weak
intermolecular interactions (physisorption) between the adsorbent (clay platelet surface)
and the adsorbate (any chemical bearing one or more functional groups). The adsorbent
surface may be either the external surface of clay platelets, or the internal surface of
clay galleries, or even the border of clay platelets – depending on the electrical charges
on the surface of the layers, the nature of the interlayer compensating cations and the
sterical hindrance effect due to the difference between the adsorbate molecular size and
the pristine d001 spacing of the clay. When the sterical effect is moderate and a liquid
phase is present to promote molecular diffusion, the adsorbate may migrate in between
the clay platelets, getting trapped into the clay galleries and eventually forcing clay to
swell49 (i.e. increasing its basal spacing). The ordinary procedure to adsorb a chemical
containing a charge-bearing group (i.e. an ionized molecule) on the internal surface of
clay galleries is cation exchange process: the cations born by the adsorbate are then
substituted to compensating cations. If the adsorbate is an organic molecule containing a
charge-bearing group with a high affinity for clay immobilized anions (e.g. a quaternary
ammonium salt), the cation exchange process modifies clay nature (from hydrophilic to
hydrophobic) to an extent which depends on the nature of the organic molecule. This
procedure generates organically-modified clays (or organoclays), characterized by an
improved compatibility with most of the polymers. If the adsorbate bears more than one
function (e.g. cationic fluorescent dyes contain charge-bearing and fluorophore-bearing
groups), cation exchange process is still possible and suitable to functionalize clays, for
example by tailoring their optical properties. Whatever the nature of the adsorbate,
whatever the procedure used to functionalize clays and regardless of the driving forces
which make it possible, diffusion is critical because it influences molecular mobility.
If a suitable photo-functionalization method is found to render MMT photoactive, and if the fluorescence behavior of the photo-functionalized lamellar filler can be
controlled and correlated to any possible spatial organizations of the platelets (tactoids,
intercalated, exfoliated), performing real-time monitoring of the melt processing in a
screw/barrel system by fluorescence would present a double advantage:
49
This concerns only expandable clays.
66
Melt compounding
 the reduced dimensions of the tracing particles (even if agglomerated) would
consent to the photo-functional clays to follow the flow – which is a characterization of
distributive mixing;
 the environmentally-sensitive optical properties of the photo-functional clays
would detect (by spectrofluorimetry) any morphological change down to the molecular
scale50 – which would be a characterization of dispersive mixing.
I-4
A DEEPER INSIGHT INTO THE STATE OF THE ART
Cation exchange process is potentially adequate to confer specific (e.g. optical)
properties to any layered (natural or synthetic) mineral filler able to exchange cations by
dispersing without dissolving in solutions containing water and/or an organic solvent
[83-91]. Bujdak [92] has recently analyzed and critically reviewed most of the older
papers dealing with the photo-functionalization of clay minerals and their interactions
with cationic organic dyes. López Arbeloa and coworkers [93] focused their review on
the interactions between rhodamine dyes and clay layered films.
MMT is the most common lamellar mineral filler chosen by researchers to study
its interactions with rhodamines [86][88][89][91][93][94] and other organic dyes
[90][95-98], but other minerals have also been evaluated (fluor-taeniolite [84], laponite
B [94][97][99][100][101], saponite [85][98][102], sepiolite [103], fluorohectorite [97],
synthetic mica [98], zeolite [87], kaolinite [102]) as well as other cationic organic dyes
(1,1'-diethyl-2,2'-cyanine [85][98], oxazine-4 [90], azobenzene [95], methyl green
[103], methylene blue [96][97][101][102], acridine orange [104], thiamine [97]).
Fluorescent molecules are fickle but versatile tracers, sensitive to any difference
in the surrounding environment (chemical composition, atomic arrangement, molecular
configuration, physical confinement, temperature). Against any preconceived opinion,
the use of fluorescent molecules isn’t only a prerogative of biologists [105-109]:
fluorescent tracers and dyes have already been successfully used in the field of polymer
processing – to monitor shear effects on phase segregation in the mixture of two
50
The detection of intercalation and exfoliation by fluorescence is possible only if photo-functionalization
targets clay galleries.
Antonella ESPOSITO
67
Chapter I
miscible polymers by in situ fluorescence quenching51 [110], to evaluate the mixing
efficiency of an internal batch mixer [111], to estimate the RTD during extrusion [112],
to measure the temperature of molten polymers during extrusion and injection molding
[113-116], for real-time monitoring of biaxially stretched polypropylene films [117]
and, recently, to assess the size and distribution of fillers in a polymer matrix [118].
Considering the number of available on-line monitoring techniques based on optical
properties or detection systems52 and the multiplicity of diversified applications [119]
[120], fluorescence offers huge potentialities in more than a research field.
Rhodamine dyes (whether in the cationic or non-cationic form) have been used
since a long time [83] and are nowadays quite familiar to many research teams. Indeed,
they represent the ideal probes to study heterogeneous systems thanks to the strong
dependence of their absorption and fluorescence emission on the properties of the host
matrix: their optical properties are environmentally sensitive and according to the theory
of exciton splitting depend on the arrangement and the spatial configuration of the dye
molecules (monomers, dimers, J-aggregates, H-aggregates)53 [86]. The organization of
the photoactive species within clay inorganic microstructure depends on the host-guest
and guest-guest interactions: the adsorption of rhodamine onto clays sometimes leads to
metachromasy in the absorption spectrum (i.e. shift of the main absorption band) [86].
Nile Blue (whether in the cationic or non-cationic form) shows clear confinement
effects on the amplitude of the Stokes shift54 and the dynamics of solvation in ethanol,
when confined to sol-gel glasses with 50 Å and 75 Å average pore size. In particular, its
fluorescence emission undergoes a blue shift for confinements inferior to 7.5 nm [121].
If nano-confined in 5 nm pores of the same sol-gel glass but in a non-polar medium
such as dodecane, Nile Blue shows an even more pronounced blue shift, from 664 nm
(ethanol) to 648 nm (dodecane) [122]. Nile Blue A Perchlorate (NBAP) has already
been used for cation exchange processes on commercial clays [122-124].
51
For the notion of fluorescence quenching, please see § II-3.6.
See § I-2.2.2 and § I-2.2.3.
53
These notions will be found again in Chapter II (PHOTO-FUNCTIONALIZATION – Lamellar fillers).
54
The Stokes shift is the wavelength shift of the emission compared to the absorption peak of fluorescent
molecules. A simple explanation and some more details about fluorescence will be given in Chapter II.
52
68
Melt compounding
9-anthracenemethanol (hydroxylmethyl anthracene) has been previously used as
UV-fluorescent tracer by Cassagnau et al. to study mixing processes in a batch mixer
for miscible polymers blends, low-viscosity-ratio miscible blends and immiscible
blends [111], and to evaluate the residence time distribution in a twin-screw extruder
[112]. Zhang et al. [125] recently developed a similar (but ameliorated55) detection
system and tested it to assess the local RTD in co-rotating twin-screw extruders.
A group of researchers at the National Institute of Standards and Technology
(NIST) at Gaithersburg (Maryland, USA) have been working for a long time on process
monitoring by fluorescence, using fluorescent local probes for different applications.
Fluorescence-based measurements of temperature profiles during polymer processing
[113] was one of the first utilizations they made of fluorescent probes – with the
purpose of monitoring the packing and cooling phase of injection molding (the optical
sensor was positioned in the wall of the mold cavity and consisted of a sapphire window
at the end of a sleeved ejector pin into which an optical fiber had been inserted) [126]
[127]. Afterward, they adapted this method of temperature measurement to extrusion
(once again via standard instrumentation ports)56 [114] and to the process of biaxially
stretched polypropylene (PP) films (in this case the sensor with optical fibers, polarizing
elements and lenses was mounted above the film, as it was processed in a tenter frame
oven stretching machine) [117] – demonstrating the versatility of their technique.
Other research groups inspired by this new characterization technique developed
their own instrumentation to monitor polymer processing. Cassagnau et al. [111], for
instance, developed a UV-fluorescence monitoring device to evaluate in situ the mixing
efficiency of an internal batch mixer. They investigated mixing of both miscible and
immiscible polymers, as well as mixing of a molten polymer with an additive having
low viscosity (plasticizer). They used an optical probe similar to that conceived by the
researchers at the NIST, consisting of a system of optical fibers able to both transmit the
UV excitation radiation to the processed polymer and to collect the fluorescence
55
Two probes simultaneously measured RTD in any two different locations of the extruder, providing the
possibility of calculating the local RTD in between the two locations by deconvolution methods based on
a statistical theory for RTD.
56
For extruders, it‟s typically the standard ½ inch diameter sensor port normally used for temperature and
pressure transducers.
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Chapter I
emission of the tracer. One optical fiber ( 0.9 mm) carries the excitation light and
twelve other fibers ( 0.1 mm) transmit the fluorescence emission to a photomultiplier:
the total diameter (1.5 mm) probes a sample area which is large compared to the mixing
scale but small compared to the volume of the chamber. As their work was intended to
quantify the mixing of different materials in molten EVA, the UV tracer was dispersed
in a masterbatch of each material of interest (polymers or plasticizer) and an amount57
of masterbatch (1% wt anthracene) was added to the evolving mixture58. They collected
fluorescence intensity vs. time curves (Figure I-F24) at different rotational speeds (5, 30,
50, 120 rpm): all the curves showed several peaks59 whose intensity gradually decreased
with time and reached a mean value (asymptote). These curves show trends typical of
laminar mixing (trend I in Figure I-F20). Actually, the recorded fluorescence intensity
corresponds to the emission integrated on the volume of few millimeters in front of the
probe, thus characterizes only the macroscale segregation of the tracer.
Figure I-F24 UV-Fluorescence device implemented on a Haake Rheomixer 600 batch mixer by
Cassagnau et al.[111]. Example of the curves characterizing mixing, scheme of the chamber
equipped with the probe and details of the cross section of the optical fibers.
57
All experiments were carried out with 50g of material (total amount) evolving in the chamber of the
batch mixer. The total amount of anthracene used was about 0.7 mg, viz. 15 ppm anthracene in the blend.
58
As the fluorescence curves recorded by this method strongly depend on the experimental conditions of
injection of the tracer, they chose to simply drop the tracer on the molten flow stream [111].
59
The earliest peaks can be associated to a saturation of fluorescence emission because they correspond
to a high local concentration of anthracene right in front of the device window [111].
70
Melt compounding
The mixture of highly viscous miscible polymers was investigated by mixing the
masterbatch EVA-tracer in the neat EVA. The mixture of a low viscosity additive with a
highly viscous molten polymer can provide information complementary with respect to
the information got by torque variation (rather representative of the global behavior of
the mixture): Cassagnau and coll. found that torque variation and fluorescence emission
do not measure the same scale of mixing. The authors also highlighted that their method
(as any other method which uses a masterbatch to introduce the fluorescent tracer in the
material of interest) informs only about distributive mixing.
The UV-fluorescence monitoring device assembled by Cassagnau and coll. [111]
has been adapted by Poulesquen et al. [128], in collaboration with Cassagnau‟s equipe,
to twin-screw extruders in order to provide experimental validation of theoretical RTD
predictions previously made by Poulesquen and Vergnes [129]. Lately, Cassagnau et al.
[112] applied their method to study the mixing mechanisms of liquid/polymer in a twinscrew extruder by fluorescence-based measurement of the RTD. The fluorescence-based
methods using on-line instruments equipped with optical fibers to assess local RTD
during polymer processing are becoming more and more popular, as confirmed by the
increasing number of research groups working on this topic [125].
However, since Maddock [73] divulged the experimental procedure60 which, for
the first time, visualized (indirectly) polymer processing and illustrated the mechanisms
of plastication, the researchers have been looking for an experimental method to directly
visualize polymer melting and mixing into screw/barrel systems. Some researchers kept
using the procedure indicated by Maddock. Someone considered equipping the existing
screw/barrel systems for extrusion and injection molding with transparent windows only
in the zones thought to be critical for processing [130][131], so that images from inside
the system could be captured by in-line non-invasive techniques. Certainly, many visual
analyses of mixing in screw/barrel systems have been done – including those devoted to
the mixing in the meter section61 – frequently performed by numerical simulation [132].
60
This technique, proposed in the late 1950s, was known as “screw freezing” or “screw crash” technique.
Alemaskin et Manas-Zloczower [132] properly remind that, although nowadays in polymer processing
single-screw extruders are mostly used as pumping devices for processes such as injection or blow
molding, their mixing efficiency should not be underestimated.
61
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Chapter I
Indeed, we‟ve already mentioned62 that numerical simulations provide an opportunity to
study the mechanisms of flow and mixing in geometrically complex systems without
performing the actual experiments. Anyway, in spite of difficulties, some researchers
considered the possibility of building specific screw/barrel systems having a completely
transparent barrel; Moguedet, in the manuscript of its PhD thesis [74], quickly reviewed
the evolution of the main techniques for flow visualization in screw/barrel systems since
the pioneer “screw freezing” technique. By the way, it was in the framework of its PhD
that Moguedet (with the help of his collaborators) conceived, designed and assembled a
pilot screw/barrel system to visualize the 3D trajectories of a single fluorescent particle
plunged in a transparent fluid – the barrel being entirely transparent [74]. More detailed
information about the system assembled by Moguedet and coll. (called Visiovis) will be
given in Chapter IV – as Visiovis represented also the nucleus of the present work.
So far, it is obvious that we‟re particularly interested in: melt processing, in-line
process monitoring, flow visualization, fluorescent local probes… indeed, although online monitoring of polymer melt processing doesn‟t necessarily have to be performed by
a visual detection system (acquisition of images and/or videos by a CCD camera) or
exploiting the fluorescence properties of some local probes, such techniques remain the
most used in the literature when visualization supposed to help comprehension. On-line
monitoring can be visual without using fluorescence (Ing et al. [133] performed in-line
monitoring of particles in a polymer melt during extrusion using a scanning particle
monitor, Figure I-F25) and, as proved, UV-fluorescence methods don‟t need visualizing
to characterize processing (Lutzen et al. [134] performed in-line monitoring of polymer
additives during extrusion using a simple UV spectrometer – to cite one more example).
Figure I-F25 A schematic diagram of
the scanning particle monitor used by
Ing et al. [133] to visualize particles
in molten polymers during extrusion
for applications to quality control.
62
See § I-2.2.4.
72
Melt compounding
In reality, we are rather interested in real-time monitoring of polymer-clay
nanocomposite processing, which implies a real-time monitoring of melt compounding
and could be helped by a direct visualization of the flow in screw/barrel systems. When
processing polymer nanocomposites, conventional techniques of flow visualization can
only characterize distributive mixing – since acquiring images and videos of a mixture
containing nanofillers almost makes no sense. Nonetheless, visualization keeps being
valuable for nanofillers have specific optical properties (e.g. for fluorescent nanofillers).
In this case, a fluorescence emission sufficiently intense could be enough to visualize in
situ nanofiller distributive mixing by a CCD camera. Besides, the optical properties of
fluorescent nanofillers – if properly controlled – could inform on dispersive mixing
(something that macroscale techniques – such as CCD cameras – could never do).
Once again, fluorescence isn‟t the only property which can be used to monitor
polymer-clay nanocomposite processing: Kummer et al. [135], for instance, are actually
developing a method for the in-line characterization of polymer-clay nanocomposites by
NIR (and UV/VIS) spectroscopy, in combination with microscopic methods. Even the
research group at the NIST developed, in association with the fluorescence techniques,
a method based on dielectric spectroscopy to monitor polymer/filler compounding
[136]: what‟s more, they correlated the degree of exfoliation to the dielectric properties
and the light transmission properties of nylon11/clay nanocomposites probed by an online dielectric slit die they specifically designed for process monitoring [137]. However,
in our opinion, fluorescent probes still represent the most suitable tool to characterize
clay aggregation and, conversely, dispersion (i.e. dispersive mixing), thanks to the
incredible versatility of the techniques based on fluorescence measurements. We‟re not
alone in our convictions, since Yilmaz and Alemdar [138] proved that fluorescent
surfactants can be used for both controlling and measuring the size or organoclay
aggregates. In addition, the proficient research activity carried out since a long time at
the NIST confirms the huge potentialities of the fluorescence techniques for process
monitoring: we aforementioned that they recently extended the use of fluorescence
spectroscopy to real-time monitoring of the morphology developed in polymer-clay
nanocomposites during melt processing. We‟ll rapidly review the results divulged by
this group of researchers.
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The researchers at NIST chose fluorescence spectroscopy for process monitoring
because one advantage of fluorescence measurements is that, whilst traditional optical
methods require the probing light to be perfectly transmittable throughout the thickness
of the sample, fluorescence measurements can be carried out by exciting the sample and
collecting its response from the same side. This property made fluorescence attractive to
implement a new real-time characterization method on the existing equipment (typically
having opaque walls). Nanocomposites are frequently prepared by melt compounding of
a polymer with lamellar mineral fillers – usually montmorillonite. Cationic fluorescent
dyes are easily adsorbed on clay mineral surfaces by cation exchange process: previous
works have shown that the intensity of their fluorescence emission depends on both the
nature of the adsorbent clay and the concentration of the adsorbate dye – the latter being
correlated to the Cation Exchange Capacity (CEC) of the clay. Natural clay can contain
significant amounts of Fe in the octahedral layers: some authors assert that the presence
of iron can cause fluorescence quenching – but quenching can also occur at high levels
of adsorbed dye (concentration quenching) because of dye-dye interactions, as shown in
Figure I-F26 [122].
Figure I-F26 Fluorescence spectra of an organically modified clay containing different amounts
of Nile Blue (excitation wavelength 407 nm, integration time 4s) [122].
74
Melt compounding
The researchers at NIST considered exploiting this property of fluorescence (due
to the interactions of fluorescent cationic dyes and clay minerals) to monitor polymerclay nanocomposite processing by means of a new multipurpose instrument, conceived
to be mounted at the exit of an extruder to obtain real-time dielectric and fluorescence
spectroscopy during melt compounding. Bur et al. [136] described the system and
demonstrated its capabilities by presenting, at the Polymer Processing and Engineering
Conference in Bradford UK (2003), the results of real-time monitoring of nylon-6 and
EVA compounded with two commercial organoclays (Cloisite ® 15A and 30B) and
doped with benzoxazolyl stilbene (BOS) at concentrations less than 10-5 % wt. The
samples used for testing had been prepared by melt compounding the powdered clays
and the dye with the polymer (4% and 2% wt clay) in a twin-screw extruder at 30 rpm.
The tests revealed that significant differences in the dielectric dispersion parameters are
observed for polymer-clay nanocomposites according to their morphology (aggregated,
intercalated or exfoliated). A correlation between the degree of exfoliation of the clays,
the dielectric properties (i.e. the Maxwell-Wagner and the  relaxations) of the polymer
composite and its light transmission were soon after reported by probing a compound of
nylon-11 with three commercial organoclays (Cloisite ® 15A, 20A and 30B) [137]. It
seems that, at the beginning, the new multipurpose instrument was oriented to dielectric
spectroscopy, rather than fluorescence… they actually recorded fluorescence spectra too
(Figure I-F27) and tried to correlate their observations with WAXS and TEM results.
Figure I-F27 Fluorescence spectra of EVA doped with BOS and compounded with Cloisite ®
15A (left) and Cloisite ® 30B (right) clays [136].
WAXD and TEM showed that compounding EVA with Cloisite ® 15A at 194°C
generated an intercalated nanocomposite (clay galleries were expanded by 20%), while
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Chapter I
Cloisite ® 30B remained aggregated upon mixing with EVA in the same conditions.
They observed that the fluorescence spectra of pure BOS in EVA and of the compound
BOS/EVA/C30B were identical to the spectrum for pure BOS dissolved in chloroform:
they concluded that the fluorescence behavior of free BOS in EVA/30B composite is
not affected by the presence of aggregated clays (Figure I-F27 on the right). Conversely,
the intercalated EVA/15A nanocomposite causes a radical change of BOS fluorescence
spectra (Figure I-F27 on the left): they interpreted such shift in the spectrum as a result
of the local change of electric field experienced by dye molecules in their environment.
For an intercalated dye molecule, the difference between an organic environment (the
matrix) and an inorganic polar environment (clay galleries) can represent a factor of 1.5
concerning the internal electric field, which could explain a change of the magnitude of
its Stokes shift. Clay concentration in the polymer matrix is several orders of magnitude
greater than dye concentration, thus large clay surfaces absorb few dye molecules. They
had to assume that the dye is associated in the intercalated regions of the clay and that
this is the reason of the observed spectral shift [136]. We think that the simple addition
of BOS to the polymer-clay compound isn‟t as sensitive as expected to its
morphological changes. Fluorescent probes can be used in two ways to study the spatial
arrangement of organoclays in polymers [138]:
 the fluorescent dye can be simply added to the compound as a local probe of
the system (extrinsic fluoroprobe) – thus the probe measures physical properties of the
host medium such as polarity, viscosity and hydrophobicity;
 the fluorescent dye can be previously adsorbed on the clay particles and act as
an organoclay-bonded label (intrinsic fluoroprobe).
Apparently, the use of fluorescent dyes as intrinsic fluoroprobes has been proved
to be more sensitive and particularly adapted to monitor clay (des)aggregation [138].
Indeed, soon after they developed a new procedure to monitor intercalation and
exfoliation in melt-processed polymer-clay nanocomposites by LIF spectroscopy of a
dye used as an intrinsic fluoroprobe [122] and reported some early results obtained with
nanocomposites prepared compounding an organoclay with polystyrene and polyamide6 (Figure I-F28, respectively on the left and on the right). The organoclay had been
formerly exchanged with Nile Blue A Perchlorate (NBAP) as reported elsewhere [124].
76
Melt compounding
Figure I-F28 On the left: LIF spectra (excitation wavelength 407 nm, integration time 800 ms)
of PA-6 processed with 2% of a photo-functionalized organoclay (NBAP level 10% CEC) at
240°C and two different residence times, compared to a melt dispersion of NBAP in PA-6 as
reference (240°C, integration time 1s). On the right: LIF spectra of PS nanocomposites with 2%
of the photo-functionalized organoclay at different processing temperatures and residence times,
compared to a melt dispersion of NBAP in PS (180°C) (exc. wavelength 407 nm, integration
time 6s) [122].
They effectively confirmed that NBAP is a sensitive fluorescence probe if coexchanged into clay galleries with traditional quaternary ammonium salts by cation
exchange process: the early results indicated that different morphologies (intercalated
and exfoliated) can be distinguished on the basis of the appearance and/or change of the
relative intensity of fluorescence peaks as a consequence of the decreased fluorescence
quenching due the development of the nanocomposite structure. In other words, they
assumed that concentration quenching dominates until clay platelets get physically
separated by polymer intercalation – then fluorescence emission appears. After that, the
interlayer spacing increases and fluorescence emission grows more intense, until clay
platelets get totally exfoliated and the fluorescent dye can be considered as unconfined.
As expected on the basis of previous studies about Nile Blue [121], while the interlayer
spacing increases the wavelength of the main fluorescence emission peak undergoes a
shift – which confirms that fluorescence emission depend on the local nanostructure and
nano-confinements below 7.5 nm can cause significant blue shifts. The comparison with
other techniques correlated polymer intercalation with emissions centered around 565
nm and clay exfoliation with emissions at 605 nm and higher.
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At that moment the group of researchers used the analytical microscale technique
developed by Bourbigot et al. [72] and other conventional techniques (TEM and XRD)
to validate the results obtained with the novel procedure based on NBAP as an intrinsic
fluoroprobe [124]. They characterized the samples of PA-6 compounded with NBAPMMT by a mini-extruder at 240°C during two different processing times (1 and 7 min).
All the conventional tools for morphological analysis ex situ revealed that the sample
extruded for 1 min had a microcomposite intercalated morphology (and a purple color),
whereas the sample extruded for 7 min showed a far more exfoliated morphology, with
individual clay platelets uniformly distributed in PA-6 matrix (and a bright red color)
(Figure I-F29). One possible explanation for color change is that as the layers separate,
the effective NBAP concentration decreases, reducing dye-due interactions which cause
fluorescence concentration quenching (Figure I-F26). They hypothesized that the 1 min
sample is purple because of a mixture of aggregated clay (blue, as NBAP is a dark blue
powder) and disordered intercalated or exfoliated clay (red). LIF measurements showed
that the color change is also accompanied by a fluorescence emission at 605 nm (red)
which grows in intensity as the clay exfoliates [123].
Figure I-F29 PA-6 compounded with NBAP-MMT (2% wt) by a mini-extruder at 240°C for 1
min (A) and 7 min (B). Sample (A) resulted to be a mixture of aggregated and intercalated clay,
whereas sample (B) resulted to be a mixture of disordered intercalated and exfoliated clay
(according to TEM, XRD and NMR results, and justified by LIF measurements [123]) [124].
The early results obtained with NBAP [122] and the correlations established
between morphology and fluorescent emission of the intrinsic fluoroprobe [124] have
78
Melt compounding
been soon after confirmed again by the same group of researchers [123] with two of the
most commonly used organoclays: Cloisite ® 15A and ® 30B. They exchanged the
selected commercial organoclays with NBAP molecules following a slightly different
cation exchange process [83] to replace, in water-ethanol mixture, 1% of the surfactant
(in both the organoclays, the surfactant is an organic cation bearing a long alkyl chain).
After that, they compounded 4% wt of the photo-functionalized clays with nylon-11 by
a batch mixer instrumented with an optical fiber sensor to collect real-time fluorescence
spectra [114]. After mixing C15A-NBAP with the polymer at 195°C for 5 min, they got
a morphology significantly but not completely exfoliated, i.e. consisting of a
combination of exfoliated, intercalated and aggregated clay. Even worse, the compound
contained clay particles visible to the naked eye. The absence of fluorescence emission
was thus attributed to the high concentration of dye clusters in clay galleries (i.e. to
concentration quenching). By acquiring a fluorescence emission spectrum of the
compound right after processing (mixing at 195°C for 10 min), they showed that NBAP
doesn‟t experience anymore fluorescence quenching and fluorescence emission gets
intense (Figure I-F30): they supposed that, during compounding, dye molecules are
released from clay galleries when exfoliation occurs (TEM analyses supported their
hypothesis that exfoliation is necessary for NBAP molecules to express fluorescence).
Figure I-F30 Real-time fluorescence spectra obtained during mixing of nylon-11 at 195°C with
Cloisite ® 15A doped NBAP (on the left) and Cloisite ® 30B doped NBAP (on the right). Left:
thirteen spectra over a period of 10 min. Right: eight spectra over a period of 9 min [123].
The change in the spectra over a period of 10 min of compounding showed the
development of the fluorescence emission: in particular, two peaks (509 and 605 nm)
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Chapter I
changed intensity in opposite directions as a function of mixing time. These changes
follow the release of dye (from clay galleries) in the form of concentrated clusters that
are subsequently dispersed as mixing proceeds. The peak at 605 nm, initially suppressed
by concentration quenching, became more intense as the dispersion of NBAP molecules
through the matrix proceeded, i.e. as dye-dye interactions diminished. By plotting the
intensities ratio of the peaks at 605 nm and 509 nm (I605/I509) as a function of time, they
obtained the graph shown in Figure I-F31 (on the left). They observed that after 10 min
exfoliation was incomplete and suggested that the positive slope for long processing
times could be explained by two phenomena: a better dispersion of the dye molecules
(which minimizes dye-dye interactions) and the progress of exfoliation.
Figure I-F31 The ratio of fluorescence intensities vs. the mixing time is plotted for the Cloisite
® 15A doped NBAP mixed with nylon 11 (on the left, 605 over 509 nm) and for the Cloisite ®
30B doped NBAP mixed with the same polymer (on the right, 609 over 504 nm) [123].
The results obtained mixing C30B-NBAP with nylon-11 are different from the
results obtained with C15A-NBAP: the intensity of fluorescence emission at 605 nm
remained relatively small compared to that at 509 nm, although a shoulder in the curve
was observed in the vicinity of 605 nm. Plotting the ratio I605/I509 vs. time (Figure I-F31
on the right) showed that the reshape of fluorescence spectra was smaller compared to
the previous case: they concluded that the composite with C30B-NBAP had substantial
aggregate and intercalated microstructure that did not allow NBAP molecules to migrate
from clay galleries. They confirmed the limited extent of exfoliation by means of realtime dielectric and light transmission measurements [140].
80
Melt compounding
Recently, the same group of researchers published some more results [139]: in
practice, they just transposed the fluorescence-based method previously established to
monitor polymer-clay compounding and clay exfoliation in a batch mixer [123] to a
twin-screw extruder. They confirmed that twin-screw extruders are much more efficient
in producing clay exfoliation than batch mixers. However, even if the composite based
on C30B resulted better exfoliated by the extruder in comparison with the batch mixer,
the morphological differences between C15A and C30B persisted.
Before concluding, it‟s worthy to rapidly describe the optical probe used by the
researchers at the NIST to obtain the results here reviewed.
Figure I-F32 The standard ½ inch bolt modified by Bur et al. [136] to host an optical probe.
The system has been reproduced also by Cassagnau et al. [111].
The optical probe developed in NIST laboratories consists of a bundle of seven
200 m core optical fibers placed into a sleeved standard ½ inch sensor bolt with a
sapphire window at its extremity (Figure I-F32). It operates in two possible modes:
 to measure light transmission through the evolving fluid;
 to measure fluorescence emission of a dye eventually present in the fluid.
In the transmission mode, one of the fibers transmits light from the light source through
a focusing lens, then the sapphire window, then the fluid; transmitted light reflects off
the far stainless steel surface (please remind that this probe was designed to be mounted
on a slit at the exit of an extruder) and reverses its path through the fluid, the sapphire
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Chapter I
window and the focusing lens. The reflected light is collected by the other six fibers and
transmitted to a photomultiplier detector. For fluorescence measurements, the difference
is that the optical fibers which collect the signal are rather connected to a spectrometer.
Such a probe could probably be adapted to any existing processing equipment – the
only requirement being the availability of an access point to the processing volume. If
this is impossible, the dispersion mechanisms associated to melt processing and mixing
could be both visualized and characterized in situ by a system such as the transparent
Couette flow cell that Mighri and Huneault used to visualize the dispersion of model
fluids [141]. Indeed, the use of photo-functionalized clays with such a flow cell would
be interesting to definitely confirm the relationship between specific morphologies and
the expression of fluorescence by a cationic fluorescent dye confined into clay galleries.
Table I-T2 Summary of the most relevant works present in the literature (to be continued 1/3)
Ref. [126]
Tracer
Polymer matrix
Fillers
Processing
Analytical technique
Excitation light
Detection system
Ref. [114]
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [141]
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Bur et al. (1997)
DiMethylAmino DiPhenyl Hexatriene / Bis-Pyrene Propane (BPP)
PolyEthylene (PE) / PolyStyrene (PS)
Injection molding
Fluorescence intensity measurements
Xenon arc lamp
Bifurcated optical fibers probe adjusted in mold ejector pin channels
Bur et al. (2001)
Bis-Pyrene Propane (BPP), BenzOxazolyl Stilbene (BOS), perylene
Poly (MethylMethAcrylate) (PMMA), Poly Carbonate (PC)
Extruder
Fluorescence-based temperature measurements
Laser or Xenon arc lamp
Bifurcated optical fibers probe through existing instrumentation ports
Polymer pellets (5% of the total amount) coated by solution dye doping
Mighri et al. (2001)
Drop of model fluids colored with red pigments
Poly DyMethylSiloxane (PDMS) (5, 10 and 30 Pa·s)
Two counter-rotating concentric cylinders (outer quartz, inner steel)
Visual Couette rheology
High-resolution digital camcorder + macro lens + digital chronometer
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Melt compounding
Table I-T2 Summary of the most relevant works present in the literature (to be continued 2/3)
Ref. [111]
Cassagnau et al. (2003)
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [112]
Hydroxymethyl anthracene (9-anthracenemethanol)
Ethyl Vinyl Acetate (EVA) + additives (miscible/immiscible/low viscosity)
Haake Rheomix 600 batch mixer
Mixing efficiency evaluated by intensity of fluorescence emission
Mercury lamp (filter 380 nm)
Optical fibers probe
1% wt tracer in a masterbatch of the material of interest = 15 ppm total
Cassagnau et al. (2005)
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [136]
Hydroxymethyl anthracene (9-anthracenemethanol)
Ethyl Vinyl Acetate (EVA) + additives (miscible/immiscible)
Intermeshing self-wiping co-rotating twin-screw extruder
RTD measured by the intensity of fluorescence emission
Mercury lamp (filter 380 nm)
Optical fibers probe as in [111]
1% wt tracer in a masterbatch of the material of interest = 15 ppm total
Bur et al. (2003)
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [137]
BenzOxazolyl Stilbene (BOS)
Nylon-6, Poly Ethyl Vinyl Acetate (EVA copolymer)
Cloisite ® 15A and Closite ® 30B
-5
Compound (4% and 2% wt clay, 10 % wt BOS) twin-screw extruder
Spectrofluorimetry and dielectric spectroscopy (online)
n.a.
New multipurpose instrument for on-line process monitoring (slit die)
Presentation of the slit die, BOS as extrinsic fluoroprobe!
Lee et al. (2004)
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [122]
Nylon-11
+
Cloisite ® 15A, Cloisite ® 20A and Cloisite ® 30B, Cloisite ® Na
Compound (4% wt clay) twin-screw extruder (198°C for 4 min)
Dielectric spectroscopy and conventional light transmission (on-line)
n.a.
Cell equipped with a dielectric sensor and an optical sensor (slit die)
Correlation morphology vs. dielectric properties vs. light transmission
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Nile Blue A Perchlorate (NBAP), Methylene Blue (MB)
PolyStyrene (PS), PolyAmide-6 (PA-6)
Two laboratory-modified organoclays
Compounding (2% wt clay) mini-extruder, different temperature and time
Spectrofluorimetry (off-line)
Several (results obtained at 407 nm)
Bifurcated optical fibers probe
Spectra at room temperature AFTER extrusion: perspective of using the
method for on-line monitoring of nanocomposites DURING extrusion
Notes
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Chapter I
Table I-T2 Summary of the most relevant works present in the literature (3/3)
Ref. [124]
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Ref. [123]
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [74]
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [125]
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Ref. [139]
Tracer
Polymer matrix
Fillers
Processing
Excitation light
Detection system
Notes
Gilman et al. (2004)
Nile Blue A Perchlorate (NBAP)
PolyAmide-6 (PA-6)
Organoclay (high temperature stable trialkyl imidazolium-based cation)
Melt compounding (2% wt clay) mini-extruder, different mixing times
Laser Induced Fluorescence (LIF) (on-line), XRD, TEM, NMR (off-line)
n.a.
n.a.
Bur et al. (2004)
Nylon-11
Cloisite ® 15A and Closite ® 30B
Compound (4% wt clay) batch mixer
Spectrofluorimetry
Diode laser Power Technologies (407 nm, 30 mW)
Optical fibers probe
Further cation exchange process (replaced 1% clay surfactant)
Moguedet (2005)
Sample of fluorescent nylon ( 0.4 mm)
Poly DyMethylSiloxane (PDMS) (100 Pa·s)
Transparent screw/barrel system modeling meter section (Visiovis)
Particle Tracking Velocimetry (PTV) (detection of the brightest point)
UV diodes (4) (400 ± 5 nm)
CCD cameras (4)
More details and the origins of Visiovis in Chapter IV
Zhang et al. (2006)
Masterbatch pellets (PS + anthracene, concentration 1, 3, 5 or 10% wt)
Polystyrene (PS)
Co-rotating twin-screw extruder, different screw configurations
New in-line fluorescence-based RTD measuring system
UV high-pressure mercury lamp (125 W)
Bifurcated optical fibers probe + photomultiplier
Reference to Hu G.H., Kadri I., Picot C. Polym. Eng. Sci. 39, 930 (1999)
Bur et al. (2007)
Nylon-11
Cloisite ® 15A and Cloisite ® 30B, laboratory-modified organoclay
Compound (4% wt clay) batch mixer and co-rotating twin-screw extruder
Spectrofluorimetry
Violet laser Power Technology (407 nm, 30 mW)
Optical fiber probe in standard ½ inch instrument port - slit die (extruder)
Clay photo-functionalization by a second cation exchange process
(replaced surfactant 5% mol C15A and C30B, 1% mol other organoclay)
84
I-R
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[135] Kummer S., Müller J., Fischer D. In- and off-line characterization of nanocomposites by
NIR spectroscopy, UV/VIS-spectroscopy in combination with microscopic methods. 4th
international symposium on NAnostructured and FUNctional POLYmer-based
materials and Nanocomposites (NANOFUN-POLY), 16-18 April 2008, Rome.
[136] Bur A.J., Roth S.C., Lee Y.-H. et al. In-line monitoring of dielectric and fluorescence
spectroscopy during polymer/filler compounding. Polymer Processing and Engineering
Conference. July 2003. Bradford (UK).
[137] Lee Y.-H., Bur A.J., Roth S.C. Correlation between degree of exfoliation, dielectric
properties, and light transmission of nylon11/clay nanocomposites probed by an online
dielectric slit die. Proceedings of the Society of Plastics Engineers: Annual Technical
Conference. ANTEC 2004, Vol. 1 (Processing), 1279-1283.
[138] Yilmaz Y., Alemdar A. Fluoro-surfactant as a tool for both controlling and measuring
the size of the organoclay aggregates. Appl. Clay Sci. 2005, 30, 154-164.
[139] Bur A.J., Roth S.C., Start P.R. et al. Monitoring clay exfoliation during polymer/clay
compounding using fluorescence spectroscopy. Trans. Inst. Meas. Control 2007, 29 (5),
403-416.
[140] McBrearty M., Bur A.J., Roth S.C. Variation of electrical properties with exfoliation
condition in nanocomposites. Proceedings of the Society of Plastics Engineers: Annual
Technical Conference. ANTEC 2002, Vol. 3 (Special Areas), 882-886.
[141] Mighri F., Huneault M.A. Dispersion visualization of model fluids in a transparent
Couette flow cell. J. Rheol. 2001, 45 (3), 783-797.
[142] Hendricks S., Teller E. X-ray interference in partially ordered layer lattices. J. Chem.
Phys. 1942, 10, 147-167.
[143] Lee S.M., Park J.C., Lee S.M. et al. In-line measurement of residence time distribution
in twin-screw extruder using non-destructive ultrasound. Korea-Australia Rheology J.
2005, 17 (2), 85-97.
94
Chapter II
Lamellar fillers
A great portion of this chapter corresponds to the content of a paper
recently submitted for publication.
As previously highlighted (§ I-2), the addition of clays (hydrophilic by nature) to
polymers (mostly hydrophobic) may greatly improve their properties, but in case the
particles are perfectly disaggregated, dispersed and distributed into the matrix1. Cation
exchange processes are commonly used to improve clay compatibility with polymers,
thus facilitating a more intimate mixture of the fillers with the matrix. On-line
monitoring of polymer nanocomposite processing is difficult – mainly because of filler
reduced dimensions – and in the case of polymer/clay nanocomposites it is even more
difficult because of clays multiscale structure2. A suitable, additional modification of
clays may confer them peculiar fluorescence properties in order to monitor their
exfoliation (hence dispersion) and distribution during processing. Moreover, fluorescent
clays could be employed to trace pollution in soils and conceive optical devices, for the
interactions of clays with cationic organic dyes may lead to the formation of controlled
supramolecular assemblies.
1
2
Agglomeration, aggregation, dispersion and distribution have been introduced in § I-2.1.
More details about clay multiscale structure are available in § I-3.1.1.
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The main goal of this first part of the work was to find a suitable experimental
protocol well adapted to confer peculiar emission properties to commercially available
organically-modified clays, in the perspective of using such inorganic-organic opticallyactive complexes for real-time monitoring of polymer-clay nanocomposite morphology
during processing. We firstly selected an organically-modified clay (Cloisite ® 30B)
and we focused our attention on the photo-functionalization process: in the following
chapter, all the steps we went through to find the most suitable protocol (which turned
out to be cation exchange processing) and the optimization of the parameters to be used
for such protocol (essentially the choice of the fluorescent molecule and its optimum
concentration in the exchanging medium) are presented. Clay modification has been
performed by several methods with three different fluorescent molecules, viz. by clay
swelling, dry and melt compounding (9-anthracenemethanol) and by cation exchange
processing (Rhodamine 6G Perchlorate, Nile Blue A Perchlorate). Modified clays have
been washed and characterized (both before and after washing, when possible) by XRay Diffractometry (XRD), ThermoGravimetric Analysis (TGA), Elemental Analysis
(EA), Fourier Transform InfraRed (FTIR) spectroscopy, TGA coupled to FTIR (TGAFTIR) spectroscopy and spectrofluorimetry. Accurate investigations of the molecular
arrangement of the organic guests (fluorescent molecules) interacting with the inorganic
host structure (clay platelets and galleries) are necessary to understand the mechanisms
of the photo-functionalization process (whether the adsorption involves monomers,
dimers or aggregates, and which kind of aggregates), to estimate the quality of the
photo-functional inorganic-organic complexes (whether they’re photo-active or not and
which kind of information can be deduced from fluorescence emission) and to
profitably use them for nanocomposite process monitoring.
II-1
MATERIALS
MMT-MT2EtOH
(Cloisite®
30B,
Southern
Clay Products,
USA),
a
montmorillonite organically modified with a quaternary ammonium salt (MT2EtOH =
methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium chloride), was purchased
and used as received. The initial Modifier Concentration (MC) given by the supplier is
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Lamellar fillers
90 meq/100g of clay and tallow fatty chains have the following composition: 65% C18,
30% C16 and 5% C14. Its photo-functionalization has been performed in the presence
of the following Fluorescent Molecules (FM): 9-anthracenemethanol, Nile Blue A
Perchlorate, Rhodamine 6G Perchlorate (Sigma-Aldrich, France). Raw materials and
the structure of the corresponding relevant chemicals, as well as their molecular weight
Mw and their maximum dimension Dmax estimated by molecular modeling (Materials
Studio – Accelrys Software Inc.), are listed in Table II-T1. Permuted water has been
ion-exchanged right before using it, assuring a quality of 18 M -cm. Any other solvent
involved in clay photo-functionalization and washing (benzyl alcohol, tetrahydrofurane
and ethanol) was purchased from Sigma-Aldrich and used as received.
Table II-T1 Raw materials and the corresponding relevant chemicals
MW
ACRONYM
TRADE or
IUPAC NAME
SUPPLIER
C30B
Cloisite ® 30B
MMT-MT2EtOH
Southern Clay
Products
(USA)
anth
9-anthracene
methanol
Sigma-Aldrich
(France)
208.26
10 Å
NBAP
Nile Blue A
Perchlorate
Sigma-Aldrich
(France)
417.84
15 Å
RhP
Rhodamine 6G
Perchlorate
Sigma-Aldrich
(France)
543.01
14 Å
CHEMICAL
[g/mol]
360.80
Dmax a
29 Å
average b stretched c
a
The maximum dimension of each chemical (D max) has been estimated by molecular modeling
(Materials Studio v.4.1.0.0, Accelrys Software).
b
MT2EtOH includes tallow fatty chains (T) having the following composition:
65% C18, 30% C16, 5% C14.
c
A molecule bearing one or more long chains may assume different molecular conformations.
To estimate MT2EtOH maximum dimension, the hypothesis of linearity of the longest tallow fatty chain
has been done.
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II-2
PHOTO-FUNCTIONALIZATION METHODS
The aim of the photo-functionalization is to graft/adsorb a fluorescent molecule
onto the clay silicate layers or at their edges, or to introduce it into the clay galleries.
Several methods have been tested in order to photo-functionalize the selected clay: (A)
clay swelling in a solution containing the fluorescent molecule; (B) compounding of the
clay with the dry fluorescent molecule in a batch mixer; (C) compounding of the clay
with the molten fluorescent molecule; (D) cation exchange processing in the presence of
the fluorescent molecule.
II-2.1
Clay swelling (A)
C30B has been introduced in a benzyl alcohol solution containing an amount of
9-anthracenemethanol corresponding (in terms of moles) to twice the initial modifier
concentration of the organically-modified clay (2MC). 9-anthracenemethanol is a nonionic molecule and talking about Cation Exchange Capacity (CEC) is actually inexact.
Anyway, we dealt with it as if it was a monovalent cationic molecule just to provide a
kind of criterion to estimate the amount of fluorescent molecule required for the photofunctionalization. The solution has been sonicated for 10 minutes by an ultrasounds
probe and let swell at room temperature for at least 12 hours. As a reference, the same
procedure has been done without the fluorescent molecule.
This method is schematized in Figure II-F1.
Figure II-F1 Schematic of the photo-functionalization method by clay swelling.
This method has been performed exclusively with 9-anthracenemethanol.
98
II-2.2
Lamellar fillers
Dry compounding (B)
C30B and an amount of 9-anthracenemethanol corresponding (in terms of moles)
to twice the modifier concentration of the organically-modified clay (2MC) have been
introduced in a batch mixer (Haake Rheomix, PolyLab System) and let mix at 40 rpm
for 5 to 15 min at 160°C to 200°C – the high temperature being supposed to locally melt
the fluorescent molecule and thus help mixing. Once again, we decided to consider 9anthracenemethanol as if it was a monovalent cationic molecule just to provide a kind
of criterion to estimate the amount of FM required for the photo-functionalization.
This method is schematized in Figure II-F2.
Figure II-F2 Schematic of the photo-functionalization method by dry compounding.
This method has been performed exclusively with 9-anthracenemethanol.
II-2.3
Melt compounding (C)
A considerable excess of 9-anthracenemethanol has been melted (m.p.160°C) in
an oil bath at 180°C and, while stirring, C30B has been added to the liquid phase in a
weight ratio anth/C30B between 1 and 3. Samples have been repeatedly washed by THF
and recovered by centrifugation (10 min at 3500 rpm) till a colorless supernatant was
obtained (meaning that no more fluorescent molecule could be extracted by washing),
then let dry under exhaust hood for several days at room temperature. This method is
schematized in Figure II-F3.
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Chapter II
Figure II-F3 Schematic of the photo-functionalization method by melt compounding, followed
by washing. This method has been performed exclusively with 9-anthracenemethanol.
II-2.4
Cation exchange processing (D)
C30B has been cation-exchanged with two cationic organic dyes (Nile Blue A
Perchlorate and Rhodamine 6G Perchlorate), introduced in two different concentrations
(corresponding to 1 and 0.25 times the initial modifier concentration of the organicallymodified clay, denoted 1MC and 0.25MC respectively) in a solution 90/10 permuted
water/ethanol. The use of permuted water is always worthy to be preferred in order to
get rid of any undesired cation. The correct amount of ethanol to be introduced in the
exchanging medium has been determined as the smallest amount of co-solvent
necessary to completely dissolve the fluorescent molecule, i.e. in order to get a saturated
solution. First, the permuted water has been warmed up to 80°C. Then, the organicallymodified clay has been added to the stirring hot water and let disperse for at least 15
minutes. Stirring speed has been adjusted to the value at which a vortex just started to
form, which seems to be the optimal condition to get a homogeneous mixing [1].
Separately, the fluorescent molecule has been dissolved in the required amount of
ethanol and the obtained solution has been added to the clay suspension. Once started,
the cation exchange process has been performed under mechanical stirring for 24 hours.
After additional 24 hours in the still exchanging medium, the photo-functionalized clay
has been separated from the liquid by centrifugation (20 min at 4000 rpm), washed with
ethanol and recovered anew by centrifugation. The described washing procedure has
been repeated until a sufficiently colorless supernatant was obtained (meaning that no
more fluorescent molecule could be extracted by washing), then let dry under exhaust
hood for several days at room temperature. The method is schematized in Figure II-F4.
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Lamellar fillers
Figure II-F4 Schematic of the photo-functionalization method by cation exchange processing,
followed by washing. This method has been performed with both NBA (top) and RhP (bottom).
II-3
CHARACTERIZATIONS
Characterizations were mainly aimed to verify whether the fluorescent molecules
adsorbed or not on clay surfaces, and where they are located, as well as to check the
efficacy of the washing procedure – supposed to be selective, i.e. capable of washing
out the excess of dye without removing the chemicals cation-exchanged into clay
galleries. In addition, characterizations to test the photo-activity of the samples have
also been performed (spectrofluorimetry).
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Chapter II
II-3.1
X-Ray Diffractometry (XRD)
X-Ray Diffractometry (XRD) is widely used to quantify the interlayer spacing of
crystalline structures, thus it is well appropriate to assess whether the fluorescent
molecules have made clay swell or not by diffusing into their galleries. In order to
measure the d001 spacing of the pristine and photo-functionalized clays, the value of 2
angle must vary between 1 and 10°: the d001 distance is then referred as a peak on the
intensity vs. 2 plot. A shift of the peak to lower values of 2 indicates an increase of
clay interlayer spacing ( d001>0) corresponding to the intercalation of the organic
moieties into clay galleries.
Wide-Angle X-ray Diffractometry (WAXD) has been performed at room
temperature in the range 2 = 0.1-10° and with a scan rate of 0.5° per minute on a Bruker
D8 Advance diffractometer with a goniometer having a Bragg-Brentano geometry in –
configuration, with a 500 mm focalisation diameter and a cupper anode to generate X
rays (33 kV, 45 mA,
= 1.54 Å).
Ruben Vera (Centre de Diffractométrie Henri Longchambon, Université Claude
Bernard, Lyon-1) is kindly acknowledged for having performed XRD measurements.
II-3.2
ThermoGravimetric Analysis (TGA)
In spite of its high sensitivity to the dimension of clay galleries (and, thus, to the
presence of chemical species confined in between the silicate layers), XRD isn’t able to
detect any molecule which is adsorbed/grafted onto the external surface of the silicate
layers or at their edges. ThermoGravimetric Analysis (TGA) measurements may help
completing clay characterizations by investigating the effects of functionalization and
washing on the thermal stability and the degradation mechanisms of the samples
compared to their references, i.e. the pristine commercial clay and the pure fluorescent
molecule. It has been observed [2][3] that peaks corresponding to weight losses between
150°C and 250°C (up to 300°C) typically indicate the presence of organics which could
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Lamellar fillers
be simply physisorbed (or eventually interacting by hydrogen bonding) either on the
external surfaces of the silicate layers or at their edges. Such organic fraction either
didn’t undergo the cation exchange process, or stayed unconfined, or maybe entered the
galleries but remained in a peripheral position. On the other hand, weight losses in
between 300°C and 550°C rather correspond to organics intercalated into clay galleries
[3-6]. Indeed, organics inside clay galleries are better insulated and thus protected from
thermal degradation; in addition, the eventual products of an initial thermal
decomposition are slowed down from diffusing to the gas phase by the presence of the
silicate layers (barrier effect) [5]. A suited washing procedure is expected to reduce the
peaks corresponding to the excess of fluorescent molecule and to any chemical
physisorbed at the edges or on the external surfaces of the silicate layers, still preserving
the peaks which denote the chemicals intercalated into clay galleries [3].
TGA has been accomplished by performing continuous ramp experiments from
room temperature up to 580°C with a heating rate of 10°C/min, in aluminium pans, under
nitrogen atmosphere and with a gas flux of 90 mL/min (Q500 Thermogravimetric
Analyzer, TA Instruments).
II-3.3
Elemental Analysis (EA)
Along with TGA, Elemental Analysis (EA) completes (but doesn’t exhaust) the
set of characterizations which can be made on a functionalized clay. Indeed, this
technique allows estimating the relative amount of any specific element (but the
oxygen) present in the sample. By performing EA of each sample before and after
washing, and by comparing these results to the ones obtained for the raw materials
(essentially the pristine C30B clay), it is possible to evaluate the effects of the photofunctionalization as well as those of the washing step. The limiting factor is that such a
technique gives only relative elemental compositions, meaning that the amount of any
detected element has to be “normalized” to be interpreted. By burning some milligrams
of the sample at 1050°C in the presence of He and O2, the main elements detected by
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Chapter II
the CHN analyzer are converted to CO2, H2O and NxOy respectively. Subsequently, the
nitrogen oxide is reduced to N2 on cupper at 450°C and all the products are separated by
column chromatography.
EA of the raw materials and of the samples has been performed giving to the
elements to be analyzed the following priority: Na, C, N, H and Si. We chose Si as the
reference, since it is the only main constituent of clay platelets (the other element, the
oxygen, is not detectable by a CHN analyzer) which is not supposed to vary in
consequence of a cation exchange process and/or of a washing procedure.
EA measurements have been carried out on a CHN analyzer having the following
khatarometer detection limits: 0.10 % N, 0.30 % C and 0.30 % H (uncertainty ± 0.30 %).
Normalization has been done in relation to the Si % content detected in pristine C30B.
We acknowledge the technical staff at the Service Central d’Analyse CNRS in
Solaize for EA characterizations.
II-3.4
Fourier Transform InfraRed (FTIR) spectroscopy
Fourier Transform InfraRed (FTIR) spectroscopy provides information about the
chemical bonds or the molecular structure of a material, whether organic or inorganic.
By submitting the sample to an infrared beam in a given range of wave number values,
bonding and molecular vibrations are excited at their characteristic frequencies and their
spectrum of absorbed or emitted vibrational energies is collected [7][8]. Such a
spectrum is sensitive to the nature of the atoms involved in the bond, the nature of the
bond itself (single or double covalent bond, hydrogen bond…), the molecular
conformation (C=C cis or trans…). By comparing the FTIR fingerprint of each sample
to the one obtained for the pristine C30B clay, it is possible to detect the presence of
any additional chemical and, eventually, its surroundings (i.e. whether it is adsorbed
onto silicate layers or is present as a free excess).
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Lamellar fillers
FTIR spectra have been obtained at room temperature on a Nicolet Magna-IR 550
spectrometer on pellets obtained by mixing the sample with dried KBr (2% wt ca) and
pressing the mixture by a hydraulic press (10 tonnes ca). Right after, pellets have been
rapidly mounted in a stainless steel disk holder for mid-infrared measurements and
spectra have been collected in the wavelength range 400-4000 cm-1 at a resolution of 4
cm-1 using 50 signal-averaged scans.
II-3.5
TGA coupled to FTIR spectroscopy (TGA-FTIR)
TGA coupled to FTIR spectroscopy (TGA-FTIR) is particularly useful to probe
and understand the thermal degradation pathways of organic samples [9], which usually
involve the release of several volatile products. This technique greatly helps identifying
the combustion products emitted while performing conventional TGA characterizations,
leading to a better assignment of the degradation peaks to a specific chemical present
into the sample. There are several ways in which the TGA-FTIR data may be displayed:
(1) a tabular display of the mass losses and the gases which are observed in each region;
(2) the display of individual spectra at various temperatures; (3) the display of spectra at
several temperatures in a stacked plot; (4) the display of the infrared absorbance for
some peak(s) overlaid on the TGA curve [9]. Some authors [10] may prefer to present
the TGA-FTIR results in the form of a Gram-Schmidt plot, which shows information
related to the total IR absorbance of the evolved gases in the whole spectral range.
TGA-FTIR has already been used to study the thermal decomposition products evolved
during the degradation of several commercially available organoclays by Cervantes-Uc
et al. [10].
TGA-FTIR characterizations have been performed on a TG 209 F1 Iris coupled to
a Tensor 27 BRUKER, with an heating rate of 10°C/min, under a flux of synthetic air (20
M% O2, 80 M% N2, H2O < 3ppm) of 80 mL/min. Spectra have been collected in the
wavelength range 600-4400 cm-1 at a resolution of 4 cm-1 using 5 signal-averaged scans.
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Chapter II
Pascal Rumeau and the technical staff at the Institut Français du Textile et de
l’Habillement IFTH (Ecully) are kindly acknowledged for TGA-FTIR measurements.
II-3.6
Spectrofluorimetry
Fluorescence is a three-stage process that generally occurs in polyaromatic
hydrocarbons or heterocyclic molecules called fluorophores or fluorescent dyes. In the
first stage, a photon of a given energy is supplied by an external source (e.g. an
incandescent lamp or a laser) and absorbed by the fluorescent molecule, creating an
excited electronic singlet state. This excited state exists for a finite time, during which
the molecule undergoes conformational changes and is subject to several interactions
with its molecular environment. Such processes partially dissipate the energy of the
excited state, yielding a relaxed singlet excited state and thus producing a fluorescence
emission. Finally, a photon is emitted returning the molecule to its ground state. Due to
the energy dissipations in the excited-state lifetime, the emitted photon has a lower
energy (longer wavelength) than the absorbed one: the difference in energy (or
wavelength) of the two photons is called the Stokes shift. The sensitivity of any
fluorescence technique strictly depends on this fundamental parameter, which allows
emission photons to be detected against a low background, clearly isolated from
excitation photons. In theory, fluorescence is a cyclical process: unless the fluorophore
is irreversibly destroyed in the excited state (phenomenon known as photobleaching), it
can be repeatedly excited and detected.
When performing spectrofluorimetry characterizations, it is first necessary to
collect the fluorescence excitation spectrum of the sample and to identify its maximum:
although the fluorescence emission spectrum doesn’t depend on the excitation
wavelength, the best sensitivity is obtained when the excitation source is at its highest
efficiency with respect to a given sample. Under the same conditions, the fluorescence
emission spectrum of an isolated fluorophore (monomer) in solution is supposed to be
identical in shape to its fluorescence absorption spectrum – the only difference being the
Stoke shift. Nevertheless, it may happen that a chemical and/or physical change in the
fluorophore environment modifies the shape of its fluorescent emission spectrum.
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Lamellar fillers
Emission spectra are generally more sensitive to any change in the molecular
environment than absorption spectra, but the latter ones may be helpful to characterize
the configuration and the supramolecular arrangement of the fluorescent dyes.
Indeed, fluorophores can assemble in dimers and/or higher-order aggregates:
any possible configuration produces a well-defined absorption spectrum and doesn’t
assure the same fluorescence emission. J-aggregates (head-to-tail molecular
arrangements) typically exhibit a narrow absorption peak (the so-called J-band) which
is red-shifted with respect to the monomer band (bathochromic shift), whereas Haggregates (tail-to-tail molecular arrangements) show a broader absorption peak (Hband) which is rather blue-shifted with respect to the monomer band (hypsochromic
shift). On the basis of the exciton theory, J-aggregates are fluorescent whereas Haggregates are affected by self-quenching. Interactions, either between two fluorophores
or between a fluorophore and the other species in the surrounding environment, can
produce environment-sensitive fluorescence emission.
Fluorescence quenching is a bimolecular process which reduces (and sometime
zeroes) the fluorescence signal intensity without reshaping the emission spectrum: it can
result from transient excited-state interactions (collisional quenching) or from the
formation of non-fluorescent ground-state species (i.e. H-aggregates). Quenching tends
to occur when high fluorophore concentrations are used. Clearly, spectrofluorimetry is
an essential tool to check the result of a photo-functionalization process.
We proceeded as following: (1) we measured the fluorescence excitation and
fluorescence emission spectra to identify the optimum excitation wavelength and
estimate the Stoke shift; (2) we recorded several fluorescence emission spectra as a
function of different concentrations of fluorophore or photo-functionalized clay in order
to determine the value of concentration at which fluorescence quenching is observed;
(3) we finally established a calibration curve (maximum fluorescence emission vs.
concentration) in non-quenching conditions for each sample.
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Chapter II
Spectrofluorimetry has been performed at room temperature on a steady-state
FS920 spectrofluorimeter (Edinburgh Instruments, UK) with a high spectral resolution
(signal to noise ratio > 6000:1), using ethanol as solvent (for the fluorescent molecules)
or suspension medium (for the pristine C30B and the photo-functionalized clays). The
spectrofluorimeter covers the wavelength range from 200 to 1670 nm using two
detectors: a photomultiplier R928 for UV-Vis scans (up to 870 nm) and a solid InGas TE
G8605-23 detector for IR scans. The excitation source is a continuous Xenon Arc lamp
(450W) coupled to two Czerny-Turner DMX300X 1800tr/mn monochromators, one for
UV excitation (focal length 300 nm) and one for the visible wavelength range (focal
length 500 nm).
Olivier Raccurt and the head of the Laboratoire des Technologies des Traceurs
L2T (Département des Technologies des Nanomatériaux, Commissariat à l’Energie
Atomique CEA, Grenoble) are particularly acknowledged for the material and technical
support, which allowed us to get spectrofluorimetry characterizations of the samples.
II-4
REFERENCE MEASUREMENTS
First of all, the fluorescent molecules of interest as well as the pristine C30B clay
have been characterized by XRD, TGA, EA, FTIR spectroscopy and spectrofluorimetry
in order to get reference measurements.
II-4.1
Reference XRD measurements
XRD shows that 9-anthracenemethanol has a crystalline peak corresponding to
14.5 Å, which considerably diminishes after melting followed by spontaneous cooling
at room temperature, as shown in Figure II-F5a.
NBAP has a small peak at 10.2 Å (Figure II-F5b), RhP has two marked peaks at
9.8 Å and 12 Å (Figure II-F5c). None of these peaks could affect the interpretation of
XRD measurements on the photo-functionalized clays.
C30B has an initial interlayer space d001=17.5 Å (Figure II-F5d).
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Lamellar fillers
Figure II-F5 Reference XRD measurements: (a) 9-anthracenemethanol as received and
after melting at 180°C followed by cooling down to room temperature, (b) Nile Blue A
Perchlorate, (c) Rhodamine 6G Perchlorate, (d) pristine Cloisite ® 30B.
Considering that the maximum molecular dimension Dmax estimated under the
hypothesis of linearity of the longest tallow fatty chain (stretched molecule) is 29 Å
(Table II-T1), this d001 value is consistent with a paraffinic configuration of MT2EtOH
molecules into clay galleries with an angle around 37°, which is actually close to the
values previously proposed in the literature [11].
II-4.2
Reference TGA measurements
9-anthracenemethanol undergoes thermal degradation in two steps (240°C and
317°C) and is completely degraded starting from 350°C (Figure II-F6a). NBAP loses
more than 30% of its initial weight in the temperature range 200-400°C, following a
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Chapter II
complex three-steps mechanisms (higher peak at 261°C followed by lower peaks at
303°C and 341°C) (Figure II-F6b). RhP halves its weight in the temperature range 200500°C: this weight loss mostly corresponds to a single sharp peak at 338°C, as shown in
Figure II-F6c.
Figure II-F6 Reference TGA measurements: (a) 9-anthracenemethanol as received and
after melting at 180°C followed by cooling down to room temperature, (b) Nile Blue A
Perchlorate, (c) Rhodamine 6G Perchlorate and (d) Cloisite® 30B in its pristine state and
after washing with ethanol (recover by centrifugation 20 min at 4000 rpm).
C30B (Figure II-F6d) undergoes two weight losses between 150°C and 500°C.
The first weight loss (21% ca at 253°C) corresponds to a partial physisorption of the
MT2EtOH molecules at the edges or on the external surface of the platelets, since the
peak regularly lowers after washing. The fact that it doesn’t completely disappear
means that a portion of MT2EtOH is well intercalated but in a peripheral position with
respect to the clay gallery, as reported by Davis et al. [6]: such portion of surfactant
cannot be washed away (since it underwent cation exchange) but it is not thermally
110
Lamellar fillers
stabilized by the presence of the inorganic silicate platelets, thus it degrades at the same
temperature as the physisorbed surfactant. The second weight loss (30% at 360°C)
corresponds to the intercalation of the MT2EtOH molecules into clay galleries (this
perfectly corresponds to the % weight loss on ignition given by the supplier). The
organic fraction for each step of weight loss has been evaluated by identifying, isolating
and integrating the main peaks of the TGA derivative curves, calculating their area and
deducing the weight percentage as the percentage of the corresponding area, referred to
the total amount of weight loss up to 550°C. We haven’t characterized the thermal
behaviour of the clay samples for T>580°C because we were solely interested in the
interactions of the cationic organic dyes with the clay and no organic can resist to
temperatures higher than 600°C. In addition, it has been already shown (and we directly
checked) that the next step of the thermal degradation of natural and organicallymodified clays would have been the dehydroxilation of the aluminosilicate framework
[2][4][5] starting after 500°C and corresponding to a peak centred around 645°C.
The experimental modifier concentration for pristine C30B clay (83 meq/100g)
has been calculated on the basis of the organic fraction corresponding to the loss of
intercalated chemicals, since this is the only fraction of surfactant which has surely
replaced the inorganic cations in clay galleries. The theoretical CEC for each cationic
fluorescent molecule (239 meq/100g of NBAP and 184 meq/100g of RhP) has been
calculated with relation to 100g, since each single molecule (and thus the whole amount
of organic cationic dye) is supposed to be able to replace clay inorganic cations. On the
contrary, 9-anthracenemethanol has a zero CEC since it’s a neutral molecule.
II-4.3
Reference EA measurements
Reference EA values are resumed in Table II-T2. The Si % content detected in
the pristine C30B has been chosen as the reference value to get the Normalization
Factor (NF) for all the other measurements performed on the photo-functionalized clay
samples, whether washed or not.
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Table II-T2 Reference EA measurements: chemical composition of the raw materials
Na [% wt]
theor
C30B
MT2EtOH
anth
NBAP
RhP
c
n.a.
0.00
0.00
0.00
0.00
exper
0.23
n.a.
n.a.
0.00
0.00
C [% wt]
theor
c
n.a.
73.80
86.43
60.31
61.90
exper
20.18
n.a.
n.a.
57.40 b
61.90 b
N [% wt]
theor
c
n.a.
3.88
0.00
10.00
5.16
exper
1.07
n.a.
n.a.
10.05 b
5.20 b
H [% wt]
c
exper
n.a.
13.41
5.76
4.79
5.71
3.98
n.a.
n.a.
n.a.
n.a.
theor
Si [% wt]
exper norm a
21.39 Ref
n.a.
n.a.
0.00
0.00
-
a
a
The amount of any element estimated by EA is relative and has to be normalized. In the following, the
Si % content detected experimentally in the pristine C30B clay will be considered the reference value for
the normalization of any other EA measurement.
b
Source: Sigma-Aldrich.
c
When possible, the theoretical values of elemental composition have been calculated on the basis of the
chemical formula.
II-4.4
Reference FTIR spectra
Neither 9-anthracenemethanol nor NBAP have been characterized by FTIR
spectroscopy – mainly because of the mechanisms of the chosen photo-functionalization
protocol but also for the selected experimental conditions (further details and the reason
of such a choice are given in the following)3. As expected, the FTIR spectrum of RhP
(Figure II-F7a) exhibits, in the region 3150-2700 cm-1, some bands characteristic of
carbon- and hydrogen-containing species, assigned to various forms of C-H stretching.
It also shows a strong but quite large band above 3000 cm-1 (maximum at 3360 cm-1 ca),
which is typical of unsaturated and/or aromatic compounds. Amino groups are also used
to dominate the region 3650-3250 cm-1 with a broad band of absorption: the presence of
a secondary amino group can also contribute to this portion of the spectrum. The sharp
peak in the region 1800-1690 cm-1 may be assigned to the presence of a C=O (carbonyl
group), but the resonant C=N bond could also interfere. The narrow peak centred at
1650 cm-1 is indicative of some unsaturation – the absorption intensity being intensified
3
At the end of the chapter we’ll realize that the most suitable photo-functionalization method for clays is
the one based on cation exchange processing in the presence of a cationic organic dye (§ II-2.4). In
particular, we’ll show that RhP looks to be more efficient than NBAP (§ II-6.1).
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Lamellar fillers
by the conjugation with the aromatic rings. This observation is further confirmed by the
presence of a pair of weak peaks at 3095 and 3035 cm-1. The two asymmetric doublets
in the region 1615-1495 cm-1 (the first around 1600 cm-1 and the second around 1500
cm-1) are consistent with an aromatic compound and are confirmed by the presence of
the weak absorption bands in the region 3130-3015 cm-1 (aromatic C-H stretching) and
850-670 cm-1 (C-H out-of-plane bending of aromatic rings). The strong double peaks in
the region 1155-950 cm-1 correspond to the typical in-plane C-H bending vibrations of
aromatic compounds. The double peak in the region 1350-1200 cm-1, as well as the
following peak around 1190 cm-1, correspond to the C-O stretching. Finally, the hidden
band corresponding to the region 3300-3030 cm-1, confirmed by the weak hidden bands
between 1430 and 1390 cm-1, are correlated to the fact that the molecule contains an
ammonium ion.
Figure II-F7 Reference FTIR spectra: (a) Rhodamine 6G Perchlorate and (b) pristine
Cloisite ® 30B in KBr pellets.
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Chapter II
The FTIR spectrum of C30B (Figure II-F7b) exhibits a broad absorption band in
the region 3750-3050 cm-1, certainly related to the fact that the clay is organicallymodified (C-H and O-H stretching of the surfactant molecules) but mostly because of
the intrinsic structure of clays (MMTs containing many hydroxyl groups and H-bonded
OH stretching are usually characterized by broad absorption bands in the region 35703200 cm-1). Moreover the presence of hydrogen bonds typically produces a further
significant band broadening: the overlapping of asymmetric and symmetric (H-O-H)
stretching vibrations of H-bonded water is always significant in smectites and any other
expandable clay. The narrower peak centred around 3630 cm-1 may be explained by OH
stretching, as well: in particular, if relatively sharp features occur between 3670 and
3550 cm-1, the compound is likely to contain non-H-bonded OH groups, and it often
corresponds to an alcohol with a sterically-hindered OH group. Anyway, such a feature
is also exhibited by certain inorganics and minerals as an indication of the presence of
“free” OH groups, either on the surface or embedded within the crystal lattice. The
absorption of water is partly hidden under the structural OH stretching band, as well.
The set of two peaks in the region 3000-2800 cm-1 (approximately 2930 cm-1 and 2860
cm-1) is typical of compounds containing long linear aliphatic chains, and MT2EtOH
molecules actually contain a tallow chain. This is confirmed by the sharp but weak peak
at 1471 cm-1. The weak broad peak at 1645 cm-1 could have been due to the fact that
MT2EtOH is a nitro organic compound and C30B surely still contain an excess of the
salt used in the commercial functionalization; indeed, Madejová [30] showed that this
peak actually corresponds to the (H-O-H) bending vibrations of adsorbed water
molecules. The strong peak at 1050 cm-1 is explained by the chemical composition of
MMT, basically a crystalline silicate (Si-O-Si vibrations). The triple set of peaks in the
region 670-400 cm-1 has probably to be assigned to all the possible vibrational modes in
the inorganic crystalline lattice.
114
II-4.5
Lamellar fillers
Reference fluorescence spectra
9-anthracenemethanol hasn’t been characterized by spectrofluorimetry since it is
a neutral molecule and couldn’t undergo cation exchange with inorganic cations: thus, it
doesn’t present any interest in relation to the selected photo-functionalization method4.
By recording the absorption spectrum of NBAP, we found an optimal excitation
wavelength of 627 nm, which is in agreement with the value given by the supplier (λmax
= 628 nm). The fluorescence emission spectrum at the optimal excitation wavelength
for NBAP in ethanol is shown in Figure II-F8a.
Figure II-F8 Reference spectrofluorimetry measurements: absorption and emission
spectra of (a) Nile Blue A Perchlorate and (b) Rhodamine 6G Perchlorate in ethanol.
It is well known that the maximal fluorescence absorption for RhP is around 532
nm: the fluorescence emission spectrum for RhP in ethanol is shown in Figure II-F8b.
RhP molecules have a strong tendency to aggregate: the absorption peak around 490 nm
corresponds to the hypsochromic effect due to the formation of non-fluorescent RhP
4
At the end of the chapter we’ll realize that the most suitable photo-functionalization method for clays is
the one based on cation exchange processing in the presence of a cationic organic dye (§ II-2.4).
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Chapter II
H-aggregates. As expected, pristine C30B doesn’t significantly absorb radiations and,
thus, it doesn’t show any fluorescence emission (result not shown).
II-5
EVALUATION OF THE PERFORMED PHOTO-FUNCTIONALIZATION METHODS
AND CHOICE OF THE PROTOCOL
Since diffusion may be a critical phenomenon for clay functionalization (its
control can be accomplished by adjusting processing temperature [3]), we firstly tried
method (A)5 to exploit the well-known swelling mechanism of clays in a liquid phase –
which, exceptionally in this case, should also be a good solvent for the chosen
fluorescent molecule. In a previous work [2], the dispersion of organophilic clays in
organic solvents has been evaluated by studying the interactions between clay platelets
and solvent molecules at different scales: it has been shown that the best results in terms
of basal spacing increase for a MMT-MT2EtOH are obtained by free swelling in
butyldiglycol, dimethylformamide and benzyl alcohol. Benzyl alcohol is proved to
produce the best results at the nanoscale level with the minimum macroscopic free
swelling (i.e. the minimum solvent uptake), and that’s the reason why we chose benzyl
alcohol for method (A).
Figure II-F9 confirms that benzyl alcohol is a good swelling agent for C30B
( d001 = +21.1 Å) as previously found [2] and shows that the presence of a fluorescent
molecule may even improve the efficacy of swelling mechanism, as indicated by the
raise of the diffraction peak at 38.6 Å. In fact, swelling occurs when the surface tension
of the solvent is higher than the surface energy of the clay (
Burgentzlé et al. [2] reported that
L benzyl alcohol
L solvent
is 39.0 mN/m and
S clay):
S C30B
indeed,
is 34.5 ± 2.0
mJ/m2. Whether in the presence or in the absence of the fluorescent molecule, swelling
resulted in a homogeneous but difficultly dryable gel. Even if some of the drying
procedures we tested seemed to be more efficacious than others, method (A) regularly
caused troubles with the complete evaporation of the residual solvent, mostly because of
the high boiling point of benzyl alcohol (b.p.205°C).
5
Photo-functionalization by clay swelling (§ II-2.1).
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Lamellar fillers
Figure II-F9 XRD evaluation of the results obtained with method (A), with and without
9-anthracenemethanol.
The problems encountered with solvent evaporation pushed us to look for an
alternative procedure to be performed without the use of any solvent. Method (B)6
clearly proved the undeniable practical advantage of avoiding solvents, notwithstanding
it appeared to be unsuitable to our purposes because molecules can’t really diffuse in
the absence of a liquid phase. In addition, a predictable mixing inefficiency (due to the
fact that dry phases can’t really sustain and transfer shear) caused local increases of
temperature and thus a visible degradation of the fluorescent molecule.
Method (C)7 was designed to ameliorate both methods (A) and (B) by taking
advantage of diffusion/swelling mechanisms but still avoiding any solvent, for which
another source of liquid phase was to be found: the melting of the fluorescent molecule.
Figure II-F10a shows that the same good swelling results of method (A) can also be
obtained by method (C) (d001 = 40 Å, i.e. d001 = +22.5 Å). Although this procedure
generally resulted in homogeneous gels (which, once cooled down to room temperature,
could be easily reduced to powder), the need for a liquid phase composed of the sole
molten dye implied the presence of a considerable amount of fluorescent molecule
(Figure II-F10a, diffraction peak at 2 = 6°): we had to find a suitable procedure to
selectively wash out the excess.
6
7
Photo-functionalization by dry compounding (§ II-2.2).
Photo-functionalization by melt compounding (§ II-2.3).
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Chapter II
Figure II-F10 Evaluation of the results obtained with method (C). XRD characterizations
(a) of clay swelling after melt compounding (the excess of 9-anthracenemethanol is
confirmed by the peak at 2 = 6°) followed by washing with THF and acetone,
respectively. These results are substantiated by TGA characterizations of the clay after
melt compounding (b) followed by washing with THF (c) and acetone (d), respectively.
All the washing procedures we tested have already been previously used to
recover clays from a solution [12-17] but centrifugation appeared to be the simplest,
fastest and most efficient technique to separate a solid residue from a liquid phase
[1][18]. The selection of an appropriate solvent to wash photo-functionalized clay has
been done on the basis of its boiling point and of the results of a preliminary solubility
test of 9-anthracenemethanol in acetone, methanol, toluene, dichloromethane and
tetrahydrofurane (THF). We finally chose acetone (b.p.56°C) and THF (b.p.60°C) and
validated our choice by verifying by XRD that these solvents don’t swell the pristine
C30B on rapid immersion (results not shown). We voluntarily avoided drying photofunctionalized clay by oven in order to better preserve the fluorescent molecules.
Unluckily, XRD and TGA show that the washing procedure resulted in a non-selective
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Lamellar fillers
extraction of the fluorescent molecule (Figure II-F10a): both THF and acetone caused a
significant shift of the diffraction peak towards lower values of the interlayer space (20
and 18 Å, i.e. d001 = –20 and –22 Å respectively), nearly back to the d001 value of the
pristine C30B clay (Figure II-F5d). This conclusion has been substantiated by TGA
measurements (Figures II-F10 (b), (c) and (d)): whatever the solvent used for washing,
the derivative curve of the weight loss shows two peaks (273°C and 403°C ca), the
second one being shifted to higher temperatures in comparison to the pristine C30B
peak (360°C) probably exclusively as a result of some molecular rearrangement of the
surfactant into clay galleries during the heating step. 9-anthracenemethanol, which is a
non-ionic small molecule (Dmax = 10 Å, Table II-T1), could have penetrated the network
of the bigger surfactant molecules (Dmax = 29 Å, see Table II-T1) already present into
clay galleries, and eventually be trapped inside it by diffusion hindering or H-bonding
(both 9-anthracenemethanol and MT2EtOH contain hydroxyl groups), but the results
obtained after washing definitely show that these mechanisms cannot assure a stable
photo-functionalization.
Method (D)8 proceeds from the cation exchange process traditionally employed
to characterize [19] and functionalize clays [20][21], and which is still quite popular to
render clays organophilic [7]. More precisely, it has been inspired by some recent works
of Maupin, Bur, Gilman et al., a group of researchers at the National Institute of
Standards and Technology at Gaithersburg, Maryland, USA [22-24]. As ion exchange is
a reversible process, clay functionalization is generally performed in the presence of an
excess of surfactant – typically twice the value of the cation exchange capacity of the
clay (2 CEC) – in order to force the reaction in the desired direction. However, when
the adsorbate is a photo-active chemical and the ion exchange is aimed to the photofunctionalization of the adsorbent, a stricter control on the concentration of surfactant is
required in order to avoid saturation (i.e. fluorescence quenching). Moreover, C30B is
an organically-modified clay which has already undergone a first cation exchange
process: we should better talk of modifier concentration (or residual cation exchange
capacity) instead of cation exchange capacity tout court. This means that most of the
8
Photo-functionalization by cation exchange processing (§ II-2.4).
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Chapter II
Na+ sites originally available are likely to be no longer exchangeable. On the other
hand, it is highly probable that a certain (significant) amount of such sites
(corresponding to the residual CEC) is still represented by inorganic cations, for the
MT2EtOH molecules are surely big enough to be an obstacle to each other because of
sterical hindrance. Indeed, high interlayer packing densities require the aliphatic tails to
overlap, causing their inclination with respect to the silicate surfaces (typical angle
37°) [13]. Since the residual value of CEC for C30B clay is less than the original value
for the inorganic clay, the introduction of an amount of fluorescent molecule equivalent
to the residual valued of CEC into the exchanging medium would be already enough to
be in the presence of an excess: we decided, anyway, to perform the first trials of cation
exchange by method (D) with an amount of fluorescent molecule corresponding to a
value of CEC equal to the modifier (MT2EtOH) concentration (1MC, i.e. 90 meq/100g
as indicated by the supplier). We employed method (D) to perform the photofunctionalization of C30B with two organic cationic dyes (NBAP, RhP) having roughly
the same maximum molecular dimension Dmax, slightly higher than the corresponding
value for 9-anthracenemethanol (15 and 14 Å compared to 10 Å, Table II-T1).
If we take a look at the results obtained by spectrofluorimetry on the washed
photo-functionalized samples, we realize that the photo-functionalization was successful
with both the fluorescent molecules (Figure II-F11). In terms of absorption: NBAP-clay
absorbs in the same range as the raw cationic dyes (no significant metachromasy) while
RhP-clay, as expected, shows a slight metachromatic effect (the main absorption band is
blue-shifted, towards higher energies). Vice versa, in terms of emission: it is NBAPclay emission spectrum which shows a slight shift to higher energies in comparison to
raw NBAP, while RhP-clay emission spectrum is absolutely the same as the raw RhP.
Both NBAP-clay and RhP-clay show a well pronounced photo-activity. RhP-clay
differs from NBAP-clay also because in its absorption spectrum there’s a peak likely
corresponding to the formation of J-aggregates ( 548 nm), which are fluorescent
according to the theory of exciton splitting and which are probably due to a regular
organization of the intercalated dye induced by the presence of a regular array of
MT2EtOH molecules. The presence of a peak probably corresponding to the formation
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Lamellar fillers
of non-fluorescent H-aggregates ( 490 nm, see Figure II-F8) is justified by the strong
tendency of RhP molecules to aggregate – phenomenon which has been reported in the
literature [17] and clearly doesn’t concern NBAP (unless at higher concentrations). A
fluorescence absorption spectrum which indicates the presence of monomers and Jaggregates (dimers) into RhP-clay galleries is in perfect agreement with the results of
molecular modelling recently obtained by Čapková et al. [17]. In the following9 we’ll
show that XRD characterizations of C30B-RhP photo-functional complexes are also in
good agreement with the results previously reported in the literature [15][17] (stable
basal spacing d001 = 22 Å).
Figure II-F11 Evaluation of the results obtained with method (D). Absorption and
emission spectra in ethanol of C30B clay photo-functionalized by method (D) with (a)
NBAP 1MC and (b) RhP 1MC. Samples have been carefully washed with ethanol in
order to assure that the response has to be assigned uniquely to the intercalated dye.
We chose ethanol to wash photo-functionalized samples because, on the basis of
its surface tension (
L ethanol
= 22.8 mN/m <
S C30B),
it shouldn’t produce significant clay
swelling [2]. We further validated our choice by confirming by XRD that ethanol
doesn’t produce interlayer (i.e. nanoscopic) swelling – at least not on rapid immersion
9
See § II-6.
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Chapter II
(results not shown). Some authors wash functionalized clays by water or ethanol-water
mixtures, hot or at room temperature. Indeed, Gates [13] reported that washing with
ethanol or other organic solvents removes exceeding intercalated molecules from
organoclays, whereas washing with water is only partially effective. Actually, the
advantage of ethanol (and ethanol-water mixtures) is that it penetrates the interlayer
spacing of organically modified clays more easily than water – thus it better removes
any excess of surfactant into clay galleries. The choice of pure ethanol instead of an
ethanol-water mixture is justified by the fact that Burgentzlé et al. [2] reported that
ethanol may even act as a degelling agent and, in any case, alcohols with short alkyl
chains (less than five methylene groups) seem to hinder clay interlayer swelling – which
means that washing with ethanol won’t absolutely interfere with the effect strictly due
to the functionalization. Analogously to method (C), we voluntarily avoided drying the
photo-functionalized clays by oven in order to preserve the fluorescent molecules. The
described procedure resulted in a fine powder, which didn’t require further grinding.
In conclusion the most practicable, appropriate and efficient method to photofunctionalize a commercial organoclay is method (D), i.e. cation exchange processing
of the pristine clay with an organic cationic dye followed by washing with ethanol,
recovering by centrifugation and drying under exhaust hood at room temperature. Next
step will be the optimisation of the parameters for the chosen photo-functionalization
protocol, namely the choice of the fluorescent molecule and its concentration.
II-6
OPTIMIZATION OF THE PROTOCOL FOR CATION EXCHANGE PROCESSING (D)
We selected two organic cationic (perchlorate) dyes and we performed the
photo-functionalization of C30B clay following the chosen protocol – cation exchange
processing (D) – with two concentrations of the dye (corresponding to 1 and 0.25 times
the modifier concentration of the organically-modified clay, denoted 1MC and 0.25MC
respectively). The aim was double: we wished to compare two fluorescent molecules in
order to choose the one giving the best results and, at the same time, we intended to find
the optimum value of fluorescent molecule concentration to get the most efficient
photo-functionalization. Indeed, the spatial arrangement of intercalated dyes typically
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Lamellar fillers
depends on the guest concentration in the exchanging medium. Čapková et al. [17]
deduced, by molecular modeling, that fully intercalated samples don’t necessarily need
to be fully ion-exchanged and, more interesting, disordered monomers adsorbed at the
silicate edges may create a sterical barrier to further dye intercalation, which is a reason
good enough to limit the excess of fluorescent dye. Another good reason is that organic
cationic dyes spontaneously aggregate, especially at high concentrations10. Moreover,
the addition of a small amount of adsorbent (clay) to a dilute dye solution causes similar
effects to those observed by increasing the concentration of dye [16] – which means that
in the presence of clay the actual concentration is higher than the expected one. We’ve
always performed cation exchange processing at 80°C since NBPA and RhP could
resist such temperature in solution and, parenthetically, it has been previously proved
that this is the optimal temperature for cation exchange processes [3].
II-6.1
Choice of the fluorescent molecule
NBPA and RhP differ in the wavelength range of absorption (627 and 532 nm
respectively) and fluorescence emission (665 and 553 nm respectively) (Figure II-F8),
the elemental composition (C:N percentage ratio is 6.00 for NBAP and 12.00 for RhP),
the molecular weight (418 and 543 g/mol respectively, see Table II-T1), the crystalline
structure (Figure II-F5) and the thermal degradation behavior (Figure II-F6). They
basically have the same maximum molecular dimension Dmax (15 and 14 Å respectively,
see Table II-T1) and both have a polyaromatic heterocyclic configuration (based on the
benzophenoxazine and xanthene in the case of NBAP and RhP respectively), but the
different XRD fingerprints suggest that their spatial configurations (bonding angles and
possible bendings) are not the same.
10
See considerations about Figure II-F11 – keeping in mind that, according to the theory of exciton
splitting, not all the aggregates are fluorescent.
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Chapter II
Figure II-F12 Spatial configuration of the free fluorescent molecules as visualized by
molecular modeling: NBAP (left) and RhP (right). NBAP is almost bidimensional,
contrarily to RhP molecule. Molecular dimensional values are expressed in ångströms.
In fact, by molecular modeling we can confirm that the two molecules are quite
different in terms of in-plane and out-of-plane spatial arrangement, as shown in Figure
II-F12. This difference in spatial configuration may significantly influence the
capability of each fluorescent molecule to enter clay galleries (in spite of the sterical
hindrance due to the MT2EtOH molecules), to exchange for the Na+ sites corresponding
to the residual CEC, to eventually aggregate and then arrange in crystalline structures.
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Lamellar fillers
Figure II-F13 Evaluation of the results obtained with method (D) and a concentration of
organic cationic dye equivalent to 1MC. Comparison of NBAP and RhP samples in
order to choose the most efficient fluorescent molecule for clay photo-functionalization.
TGA characterizations of (a) NBPA and (b) RhP samples supported by XRD analysis of
(c) NBPA and (d) RhP samples. Bold lines represent the results obtained after washing,
at the very end of the photo-functionalization procedure.
TGA characterizations of C30B clay photo-functionalized with 1MC of NBAP
and RhP are shown in Figure II-F13 (a) and (b) respectively, supported by the relative
XRD paths (c) and (d). Cation exchange processing (D) with 1MC NBAP produces, in
comparison with TGA results for the pristine C30B clay, an additional peak in the area
corresponding to physisorbed chemicals (219°C) (Figure II-F13a). This peak of the
derivative curve is quite high but is easily removed by washing, witnessing that such a
big amount of fluorescent molecule is probably useless to the main objective of photofunctionalization. There’s a relevant difference between this first peak and the second
one (288°C): one could associated both of them to physisorbed chemicals, but it is clear
that washing procedure doesn’t significantly lower it, demonstrating that it corresponds
to chemicals intercalated into clay galleries. The last part of the curve (T>300°C) looks
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Chapter II
more complicated because the permanence of the clay in an exchanging medium at
80°C during cation exchange processing has surely produced a rearrangement of the
chemicals into the galleries. Furthermore, the presence of aromatics always reshapes
TGA curves, giving sometimes rise to a brand new peak at higher temperatures – what
could explain the large shoulder between 400°C and 500°C. A probable molecular
rearrangement into clay galleries could also be a good explanation for the results
obtained by XRD – which are a little controversial with TGA results – taking also into
account the Dmax and the molecular configuration of NBAP (Figure II-F12). Figure IIF13c shows that the d001 spacing for a sample of clay recovered right after cation
exchange processing and simply dried under exhaust hood is 19 Å ( d001 = +1.5 Å) but,
after washing with ethanol and drying again under exhaust hood, it slightly shifts right,
going back to the initial value for pristine C30B (17.5 Å)11. Likely, an excess of NBAP
could enter clay galleries and push further apart the silicate platelets by forming an
additional organic layer which is obviously unstable, since it couldn’t have undergone
cation exchange because of the sterical hindrance due to the presence of the MT2EtOH
molecules. However, TGA assures that photo-functionalization was effective (peak at
288°C): we conclude that XRD measurements cannot detect the photo-functionalization
of C30B clay with NBAP because dye molecules, properly exchanged with Na+ sites
immobilized on silicate platelets, are too small (Dmax = 15 Å) in comparison with the
MT2EtOH molecules (Dmax = 29 Å), even if the paraffinic configuration reduces their
“effective length” to 17.5 Å.
Cation exchange processing (D) with 1MC RhP produces, in comparison with
TGA results for pristine C30B clay, a double additional peak in the area of physisorbed
chemicals (230°C and 258°C) (Figure II-F13b). These two peaks are quite intense –
especially the second one – but have been easily removed by washing, witnessing once
again that such a big amount of fluorescent molecule is absolutely useless to the main
objective of photo-functionalization. The other two peaks (380°C and 425°C)
correspond to the region representative of the organics which entered clay galleries
(T>300°C) and are insensitive to washing – especially the second one, which can be
11
See Figure II-F5d.
126
Lamellar fillers
probably due to the aromatic portion of the intercalated fluorescent molecules.
Contrarily to what observed by XRD in the case of NBAP, photo-functionalization with
RhP (Figure II-F13d) is clearly detectable by this technique (d001 = 22 Å, d001 = +4.5
Å) and the results are surprisingly consistent (even slightly better) with those previously
reported in the literature for the definitely simpler case of pure RhB-MMT complexes
[17]. All the RhP molecules which entered clay galleries seem to have properly
undergone cation exchange, since they’re not removed by washing and the d 001 value
doesn’t decrease. Interlayer spacing could have increased in consequence of a kind of
induced supramolecular organization or, more likely, a rearrangement of the paraffinic
configuration of MT2EtOH molecules induced by the presence of smaller (but still
sterically-hindering) molecules on the absorbent surface. Under the hypothesis that RhP
molecules into clay galleries don’t interact with each other because of their statistical
alternation with MT2EtOH molecules (which masks them to each other) and that
MT2EtOH molecules are the sole responsible for clay interlayer spacing, the presence
of small molecules in addition to bigger molecules bearing long tallow fatty chains
increases the characteristic paraffinic angle between the tallow chains and the silicate
substrate from about 37° to 49° ca, as shown in Figure II-F14. Generally speaking, the
presence of smaller molecules increases the “effective length” of longer molecules in
relation to silicate layers separation, which actually is one of the goals of traditional clay
functionalization in perspective of polymer-based nanocomposite fabrication. A similar
mechanism has been evocated to explain the crystalline swelling of organoclays in
solution: intercalated organics are solvated by solvent molecules, which surround and
support the aliphatic tails, causing a tilt with respect to the silicate surface [13].
Molecular modeling confirmed that small molecules, when intercalated in clay galleries,
lead to the tilting of bigger molecules, as shown by Čapková et al. [17] in the case of
water molecules interacting with the xanthene planes of Rhodamine B (RhB) into MMT
galleries: water molecules can be placed adjacent to the silicate layers and fill the empty
spaces between RhB cations. The analogy with this study is possible if we compare
water to RhP and RhB to MT2EtOH. Iwasaki et al. [16] reported that dye cations into
clay galleries are oriented horizontally ( planes parallel to the silicate surfaces) at low
loadings, tilt towards the perpendicular direction with an increase in dye concentration
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Chapter II
and finally stand vertically at high loadings. In this study, since the chosen pristine clay
is an organically-modified clay, we don’t observe any difference in the orientation of
the dye molecules probably because their configuration is influenced by the presence of
MT2EtOH molecules. Thus, it is possible to vary the concentration of organic cationic
dye and always obtain the same tilted configuration. Anyway, the invariable tilting
angle obtained seems to be directly dependent on the behaviour of the dye cations:
Pospíšil et al. [25] showed by molecular modeling that rhodamine B cations intercalated
in a clay host structure from a diluted solution may organize in a monolayer
arrangement of tilted monomers with a basal spacing within 21-25 Å with a tilting angle
in the range 40-60°, which is in good agreement with the value found in this work (49°).
Fujita et al. [15] reported a similar behaviour for the basal spacing of rhodamine/
taenolite complexes obtained with a molar ratio organic/inorganic varying from 0.086 to
0.46 (d001 nearly constant at 21.7 to 22.2 Å).
Figure II-F14 Possible explication for the increase of pristine C30B interlayer spacing
(a) following photo-functionalization with a cationic organic (b), under the hypothesis of
a synergistic effect due to the presence of both small and bigger organic molecules
cation-exchanged within the silicate platelets.
In conclusion, both NBAP and RhP are suitable to photo-functionalize C30B
organically-modified clay. However, RhP looks to be more efficient (the TGA peak
which is washing insensitive corresponds to a higher temperature) and, additionally,
photo-functionalization with RhP appears to be readily detectable both by TGA and
XRD. Finally, RhP has a wavelength range of absorption and a fluorescence emission
spectrum which is more interesting for an eventual application with Visiovis, the brand
128
Lamellar fillers
new equipment we’re developing in our laboratories12. Thus, we’ll focus our attention
on the results obtained with RhP but, as a matter of fact, we’ll keep comparing them
with the results obtained with NBAP.
II-6.2
Influence of the fluorescent molecule concentration
When dealing with fluorescence phenomena, fluorophore concentration is a key
parameter because of the risk of quenching and the tendency to form supramolecular
assemblies, which occasionally is a desired result but in any case has to be controlled,
somehow. This is the reason why we estimated necessary to evaluate the influence of
the concentration of fluorescent molecule on the results of clay photo-functionalization.
As previously mentioned13, we’ve firstly performed cation exchange processing in the
presence of an amount of fluorescent molecule equivalent to 1MC – which turns out to
surely be an excess of cationic organic dye. We then realized that performing method
(D) with such a concentration of perchlorate is efficacious but not efficient, since TGA
measurements revealed that a large amount of fluorescent molecule doesn’t enter clay
galleries but rather physisorbs on clay platelets, and is readily washed away by ethanol
(see Figure II-F13). Moreover, we’ve already underlined that any excess physisorbed on
clay silicate platelets may create a sterical barrier to further intercalation: therefore, we
decided to diminish the amount of fluorescent molecule to a quarter of the former
amount, i.e. 0.25 times the modifier concentration of C30B (0.25MC).
TGA characterizations of the samples obtained by method (D) with 0.25MC of
NBAP and RhP are shown in Figure II-F15 (a) and (b) respectively, supported by the
relative XRD paths (c) and (d). Cation exchange processing (D) with 0.25MC NBAP
produces, in comparison with the results obtained with 1MC NBAP, a smaller
additional peak in the area of physisorbed chemicals (253°C) (Figure II-F15a). This
peak is lowered by washing but doesn’t completely disappear: the same observation has
already been done for pristine C30B, which undergoes a first weight loss at 253°C
12
13
Visiovis configuration and evolutions will be detailed in § IV-1.
See considerations about method (D) in § II-1.5.
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Chapter II
(Figure II-F6d)14. Therefore, it is confirmed that the small residual peak at 253°C in
washed samples does effectively represent a portion of the MT2EtOH molecules well
intercalated into clay galleries but in a peripheral position – which explains why it is
impossible to completely remove it by washing. The second weight loss occurs at
310°C and has to be assigned to the chemicals intercalated into clay galleries, since the
peak turns out to be perfectly washing insensitive. By the way, this second peak is the
only one to be truly washing insensitive, since the last portion of the derivative curve
(T>325°C) looks complicated, probably because of some molecular rearrangement of
the chemicals into the galleries. The hypothesis of a molecular rearrangement, in this
case, isn’t substantiated by XRD results (Figure II-F15c), because the interlayer spacing
undergoes only an insignificant decrease after washing with ethanol (from 20 to 19 Å,
d001 = –1 Å), unlike the samples obtained with 1MC NBAP (Figure II-F13c). The fact
that the d001 spacing in the 0.25MC NBAP exchanged clay remains almost constant
whereas it was shown that it decreases in the 1MC NBAP exchanged clay is the first
evidence that the concentration of fluorescent molecule influences the efficiency of the
photo-functionalization by method (D). One could expect that, whatever the fluorescent
molecule, an increase of its concentration (from 0.25MC to 1MC) would promote clay
swelling by shifting the XRD peak towards smaller values of 2 (corresponding to
greater value of the interlayer spacing) or by heightening it (corresponding to a greater
fraction of properly photo-functionalized sample, since the intensity of the diffraction
peak for a given crystalline population is directly proportional to its mass fraction in the
sample [8]). Indeed, we can observe that an increased concentration of NBAP seems to
slightly interfere with the cation exchange: when the concentration of fluorescent
molecule lowers, the peak shift is barely higher (20 Å for 0.25MC vs. 19 Å for 1MC),
the consequences of washing are slightly reduced ( d001 = –1 Å for 0.25MC vs. d001 =
–1.5 Å for 1MC, Figure II-F15c vs. Figure II-F13c). The difference is not enormous, but
at least we can conclude that an increase of concentration of fluorescent molecule
doesn’t produce any amelioration.
14
For further details about TGA measurements performed on C30B, go back to § II-4.2.
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Lamellar fillers
Figure II-F15 Evaluation of the results obtained with method (D) and a concentration of
organic cationic dye equivalent to 0.25MC. Comparison of NBAP and RhP samples is
made to confirm the choice of the fluorescent molecule for clay photo-functionalization.
TGA characterizations of (a) NBAP and (b) RhP samples, supported by XRD analysis of
(c) NBPA and (d) RhP samples. Bold lines represent the results obtained after washing, at
the very end of the photo-functionalization procedure.
Cation exchange processing (D) with 0.25MC RhP produces, in comparison with
the results obtained with 1MC RhP, only one additional peak in the area of physisorbed
chemicals (243°C) (Figure II-F15b). This peak is lowered by washing but doesn’t
completely disappear: the same observation as 0.25MC NBAP samples can be done.
The second peak (315°C) may appear ambiguous but it could be explained in a similar
way than the previous one, since its behavior in relation to clay washing is analogous.
Finally, the last weight loss (406°C) corresponds to the intercalated chemicals and is
absolutely washing insensitive. Contrarily to 0.25MC NBAP samples and unexpectedly
with reference to the observations done by Čapková et al. [17], no concentration effects
appear on XRD results when the amount of organic cationic dye in the exchanging
Antonella ESPOSITO
131
Chapter II
medium varies (Figure II-F15d): an increased concentration of RhP neither augments
the d001 spacing (22 Å, the same value obtained with 1MC RhP), nor hinders the cation
exchange mechanism because of the sterical hindrance effects due to the excess of dye
adsorbed on the silicate surfaces. Furthermore, the lowered concentration of dye in the
exchanging solution doesn’t lead to the presence of two broad partially overlapping
diffraction maxima corresponding to two different phases (and, consequently, two
different molecular arrangements of the intercalated organics) [17]: for both values of
concentration, RhP excess doesn’t enter clay galleries and is efficiently removed by
washing, as confirmed by TGA (Figure II-F13b and Figure II-F15b). Clearly, the
intercalation mechanisms are influenced by the presence of the MT2EtOH molecules,
which reduce the possibilities of several molecular arrangements of the dye.
In summary, clay photo-functionalization by cation exchange processing (D)
with high concentrations of fluorescent molecule is possible (no significant fluorescence
quenching seems to occur, as confirmed by spectrofluorimetry results in Figure II-F11)
but unnecessary, since most of the fluorophore excess is washed away and wasted. All
the evidences collected so long indicate that cation exchange processing (D) with a
smaller amount of an organic cationic dye like RhP (e.g. a relatively small percentage of
the modifier concentration, viz. 0.25MC RhP) is sufficient to render the organoclay
photo-active (result not shown). XRD reveals the highest crystalline swelling, moreover
TGA and EA assure that photo-functionalization was successful even in the absence of
a significant surfactant excess. The global shape of the TGA derivative curve for the
0.25MC RhP sample (Figure II-F15b) looks pretty different from the curve obtained
with 1MC RhP (Figure II-F13b) uniquely because of the different ratio of dye to
MT2EtOH molecules.
In the following paragraph we’ll present some complementary characterizations
(FTIR spectroscopy and TGA-FTIR) of C30B clay photo-functionalized 0.25MC RhP.
Such results, obtained before and during the thermal degradation of the exchanged clay,
could eventually support the observations previously made according to XRD and TGA
characterizations.
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II-6.3
Lamellar fillers
Complementary characterizations (C30B 0.25MC RhP)
As the intercalation of RhP cations into clay galleries is based on a simple cation
exchange reaction, the host-guest interactions have a non-covalently-bonded nature
[17][25]: a comparison of FTIR spectra for the host inorganic structure (C30B, reported
from Figure II-F7b), the guest organic dye (RhP, reported from Figure II-F7a) and the
photo-functionalized clay (C30B-RhP) should show that the main absorption bands of
the pristine clay and those of the polyaromatic heterocyclic fluorophore coexist in the
FTIR fingerprint of the photo-functional inorganic/organic complex. Indeed, in the
FTIR spectrum of the clay photo-functionalized 0.25MC RhP (Figure II-F16), one can
distinguish the features of both C30B and RhP.
Figure II-F16 FTIR spectrum of C30B photo-functionalized 0.25MC RhP compared to the
reference FTIR measurements on pristine C30B and RhP (for further details about reference
measurements, see Figure II-F7 and related comments15).
First of all, the broad absorption band in the region 3750-3050 cm-1 (which is
due both to the intrinsic structure of the clay and to the fact that C30B is organicallymodified – please see Figure II-F7b and related comments15) is clearly modified by the
contribution of the sharp peak at 3360 cm-1 due to the polyaromatic dye (see Figure IIF7b and related comments15): in the FTIR spectrum of the photo-functionalized clay,
15
Explanations in § II-4.4.
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133
Chapter II
the peak around 3630 cm-1 (“free” OH groups) is still present, but its intensity is
definitely reduced in comparison with the intensity of the peak around 3360 cm -1
(unsaturated and/or aromatic compounds). As previously reported by Pospíšil et al. [25]
the absorption band at 3360 cm-1 is slightly smoothed and broadened in comparison
with the original band in the FTIR spectrum of the free fluorescent molecule. The set of
two peaks corresponding to the vibrations of long linear aliphatic chains (approximately
2930 and 2860 cm-1, see Figure II-F7b) are also preserved, but their relative intensity is
lowered as well. The two weak peaks at 1645 and 1471 cm-1 previously detected in the
pristine C30B spectrum are hidden – in the spectrum of the photo-functionalized clay –
by the contribution of RhP bands (see Figure II-F7b): RhP fingerprint is clearly
recognizable in the region 1800-1400 cm-1, even if the intensity of all the peaks is
scaled down. Finally, the absorption bands previously assigned to crystalline silicate
(Si-O-Si) and to the vibrations of the inorganic crystalline lattice (Figure II-F7b) remain
unchanged.
Cervantes-Uc et al. [26] have already analyzed the thermal degradation of
commercial Cloisite® 30B by TGA-FTIR, carrying out their measurements at 10°C/min
under dry nitrogen atmosphere. The Gram-Schmidt plot16 showed two main peaks (297
and 427°C) corresponding to the main discharges of volatile products: the first peak
corresponded to a spectrum containing the absorption bands for O–H stretching,
aliphatic C–H stretching, CO2, C–O stretching from alcohol groups and probably
carboxylic acid; the second one (definitely more intense than the first one) corresponded
to a spectrum in which the intensity of the absorption bands for CO2 and aliphatic C–H
stretching increased and the intensity of the signal for carbonyl stretching decreased. By
the way, they also reported the FTIR spectra of the gases evolved at 413 and 500°C,
since therein they found some interesting features. In the spectrum at 427°C they found
that the intensity of the absorption band corresponding to methyl and methylene groups
increased whereas CO2 signal disappeared (no more emission of carbon dioxide), and
some absorption bands which were not clearly defined at 297 and 413°C got a little
16
As previously reported, the Gram-Schmidt plot deals with the total IR absorbance of the evolved gases
in the whole spectral range (§ II-3.5).
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Lamellar fillers
sharper. They also observed that in the spectrum at 500°C almost all the bands
vanished. The only absorption band observed in all the spectra up to 500°C was a band
at 3020 cm-1 ca (C–H stretching in double bonds). In summary, they report that C30B
degradation products showed bands corresponding to water and/or alcohols and/or
carboxylic acid (O–H stretching in the 4000-3300 cm-1 range, O–H bending at 1701 and
1518 cm-1), aliphatic compounds (2990-2800 cm-1) and carbon dioxide (2364 and 2324
cm-1). The band at 1518 cm-1 probably reveals the presence of alkenes (especially if
confirmed by another band at 3020 cm-1): indeed, the presence of alkenes has been
reported by several authors [26][27] and could be justified by some degradation
processes based on the Hoffmann elimination reaction, which is the most probable
degradation mechanism of organically modified clays containing hydroxyl groups [26].
Table II-T3 Chemical species evolved during TGA analysis of the raw materials and of C30B
photo-functionalized 0.25MC RhP before (bw) and after washing (aw), identified by FTIR
spectroscopy. Comparison with some data from the literature [26][27].
RhP
C30B
C30B [26]
C30B [27]
C30B RhP
0.25MC bw
C30B RhP
0.25MC aw
T [°C]
Chemicals
275
557
290
350
600
297
413
427
H2O, ethyl chloride, oxidized compounds (3738, 3250, 1744, 1515, 1067 cm-1)
H2O, CO2, CO
210
255
375
550
375
560
720
H2O, CO2, CO, aldehyde (C10), ketone (C10) or carboxylic acid (C17)
H2O, CO2, CO
H2O, CO2, alkanes, alkenes, aldehydes, carboxylic acids, amines
H2O, CO2, alkanes, alkenes
H2O, alkanes, alkenes, alcohols
N,N-dimethylacetamide, linear aldehydes (C7-C12), linear and branched
alkanes (C9-C21), alcohols (C8-C16), alkene alcohols (C9-C16), chloro-alkanes
(C14-C16), alkenes (C13-C18)
H2O, CO2, CO, ethyl chloride, aldehydes (C6) or ketones
H2O, CO2, CO, aldehydes or ketones
H2O, CO2, CO
H2O, CO2, CO, N-methylformamide
H2O, CO2, CO
The presence of small signals in the 800-650 cm-1 range would represent some
chlorinated compounds, which could likely come from the thermal decomposition of the
exceeding portion of surfactant which didn’t undergo cation exchange process [26][27].
Antonella ESPOSITO
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Chapter II
In support of TGA results, Edwards et al. [27] rather used solid phase microextraction
techniques to analyse the thermal degradation products evolved during the processing of
organically-modified layered silicates at a given temperature (210°C). The data reported
by Cervantes-Uc et al. [26] and Edwards et al. [27] about the thermal degradation of
Cloisite ® 30B are summarized in Table II-T3 to help comparing with the results we
obtained by TGA-FTIR, listed in the same table.
The parameters of TGA-FTIR analysis chosen for each sample of interest are
listed in Table II-T4, along with the total IR absorbance of CO2 and CO.
Table II-T4 TGA-FTIR parameters (sample initial weights, TGA temperature ranges and the
corresponding weight losses, temperatures at which gases have been discharged and analysed by
FTIR spectroscopy, total amounts of evolved CO2 and CO) used to characterize C30B photofunctionalized 0.25MC RhP before (bw) and after washing (aw), as well as the raw materials.
Initial weight TGA program
[g]
T range [°C]
RhP
12.20
C30B
pristine
16.03
C30B RhP
0.25MC bw
17.58
C30B RhP
0.25MC aw
12.73
a
Weight loss FTIR analysis
T [°C]
[%]
200  320°C
23.0
275
320  640°C
76.6
557
200  450°C
450  800°C
200  280°C
280  450°C
450  800°C
200  450°C
450  620°C
620  850°C
16.0
12.8
9.2
16.9
23.4
11.9
11.1
8.3
290 / 350
600
255
375
550
375
560
720
Total IR
absorbance b
[u/mg]
CO2
CO
1220.0 103.5
315.0
30.4
584.9
56.3
419.5
46.8
a
TGA programs are essentially linear ramps performed with a heating rate of 10°C/min.
b
The total IR absorbance of CO2 and CO has been calculated by integrating the corresponding peaks in
space and time, thus
u
abs
cm s
The thermal degradation of RhP produces almost 4 times more CO2 and 3.5
times more CO than the pristine C30B: this could be justified by the chemical
composition of RhP and pristine C30B determined by EA (Table II-T2)17, since RhP
17
Reference EA measurements are reported in § II-4.3.
136
Lamellar fillers
contains 3 times more carbon per unit of weight than C30B. Therefore, it is obvious that
the amounts of CO2 and CO produced by thermal degradation of C30B photofunctionalized 0.25MC RhP before washing are higher than the same values for pristine
C30B, and that such values slightly decrease after washing (aw).
If we consider, now, the chemical species evolved during TGA analysis (Table
II-T3), we realize that the only compound which could help evaluating the effects of the
photo-functionalization and of the washing procedures is ethyl chloride: any other
product of the thermal degradation of RhP (essentially H2O, CO2 and CO) is a common
combustion product which could not make the difference. Only Edwards et al. [27]
detected, even in commercial C30B, some chloro-alkanes (having much longer aliphatic
chains). Of course, we’re aware that the degradation products of a photo-functionalized
(organically-modified) clay could consist of several complex molecules and that, by the
way, ethyl chloride could react with other degradation products and produce some more
complex compounds before its detection: that’s why we’re presenting these results as
some complementary characterizations. About pristine C30B, TGA-FTIR results are in
perfect agreement with the results previously reported in the literature [26][27] (Table
II-T3): among the degradation products of commercial grade C30B one can find (in
addition to water, carbon dioxide and carbon monoxide) some alkanes and/or alkenes
with several substituting groups (mainly hydroxyl, carboxyl and amine) and always
some C=O bond (characteristic of aldehyde, ketones, acetamide and formamide).
Let’s focus a little longer on ethyl chloride emissions. It’s interesting to observe
that ethyl chloride – which is one of the degradation products of the free fluorescent
molecule – has been detected in the spectrum of the volatile products emitted at 255°C
by C30B photo-functionalized 0.25MC before washing (bw) but not after washing (aw):
this confirms that the first peak of the derivative TGA curve for this sample (Figure IIF15b) effectively corresponds to a portion of physisorbed dye which didn’t enter clay
galleries and, thus, didn’t undergo cation exchange process. The fact that no more
emission of ethyl chloride is detected in the washed sample (aw) assures that the
washing procedure was efficacious and that the sample (aw) doesn’t contain any more
exceeding dye, which excludes that the residual peak at 243°C (still visible after
Antonella ESPOSITO
137
Chapter II
washing) could be assigned to a washable excess. Starting from 550°C only small,
residual organic molecules evolve from the burning samples. Contrarily to what
observed by Cervantes-Uc et al. [26] in the case of pristine C30B, we detected CO2 and
CO emissions from the thermal degradation of C30B photo-functionalized 0.25MC RhP
before (bw) and after washing (aw) up to 850°C. A direct correlation between TGA and
TGA-FTIR results can be done with the help of Figure II-F17.
Figure II-F17 Direct correlation between simple TGA characterizations and TGA-FTIR
results for (a) RhP, (b) pristine C30B, (c) C30B photo-functionalized 0.25MC RhP before
washing (bw) and (d) C30B photo-functionalized 0.25MC RhP after washing (aw).
138
II-6.4
Lamellar fillers
Further general comments about the efficiency of the
photo-functionalization
Let’s take a look at the complete series of TGA curves measured for the samples
obtained by cation exchange processing (D) with both the concentrations (1MC and
0.25MC) of NBAP and RhP (Figure II-F18).
Figure II-F18 Global comparison of the complete series of samples obtained by cation
exchange processing (D) with two different concentrations (1MC and 0.25MC) of (a)
NBAP and (b) RhP. Bold lines represent the thermal behavior of the samples after
washing with ethanol and drying under exhaust hood (bw = before washing, aw = after
washing).
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Chapter II
What’s interesting to observe is that:
 All the washed photo-functionalized clays are more stable than (or at least as
stable as) the pristine C30B clay washed with ethanol, which means that photofunctionalization doesn’t significantly affect thermal resistance;
 All the washed clays (pristine C30B included) are more stable than the
respective unwashed counterparts;
 Both the fluorescent molecules result thermally stabilized for T>300°C in
consequence of their insertion into clay galleries;
 With both NBAP and RhP, the most stable clay is the one which has been
photo-functionalized with 1MC of organic cationic dye and then washed;
 With both NBAP and RhP, the less stable clay is the one which has been
photo-functionalized with 1MC of organic cationic dye but hasn’t yet been washed.
Stabilization by washing has already been highlighted in the literature [28] and is
justified by the removal of the surfactant excess – which typically is organic and, thus,
less stable than clay with respect to temperature. The stabilization of organic chemicals
due to the presence of inorganic clays has been already reported and commented [5][6].
The experimental values of MC obtained for all the photo-functionalized clays
after washing (Table II-T5) confirm once more that both NBAP and RhP are suitable to
photo-functionalize C30B organoclay. Moreover, it is clear that even a smaller
concentration of fluorescent molecule (0.25MC) – whatever the fluorescent molecule –
is sufficient to increase the value of MC and to assure a certain degree of CEC recovery
(167% and 55% for NBAP and RhP, respectively). By the way, in Table II-T5 one can
also observe that, for any given concentration of fluorescent molecule and in the same
cation exchange processing conditions, NBAP looks to be more efficient than RhP in
terms of
MC[ meq / 100 g ] and, thus, in terms of CEC recovery as well.
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Lamellar fillers
Table II-T5 Experimental values of MC (derived from TGA results) and CEC recovery for the
clay photo-functionalized by cation exchange processing (D), with both NBAP and RhP at both
concentrations (1MC and 0.25MC), after washing with ethanol.
Tmax a [°C]
intercalated
chemicals
C30B pristine
MC
MC exper
[g/100g] b
MC exper
[meq/100g] c
CEC recovery d
[%]
30
83
Ref
70
251
+ 202
58
222
+ 167
58
190
+ 129
25
129
+ 55
360
288
345
435
310
400
380
425
C30B NBAP
1MC
C30B NBAP
0.25MC
C30B RhP
1MC
C30B RhP
0.25MC
406
a
Tmax is the temperature corresponding to the max weight loss rate in Derivative ThermoGravimetric
(DTG) curves. Only the peak(s) assigned to intercalated chemicals are considered in order to measure MC
(Modifier Concentration).
b
Each experimental value of MC expressed in [g/100g] has been evaluated by identifying, isolating and
integrating the DTG peak(s) of interest, calculating their area and deducing the % wt as the % area
[reference for % = total amount of organic loss up to 550°C].
c
Each experimental value of MC expressed in [meq/100g] has been calculated on the basis of the
corresponding value expressed in [g/100g] and does already include the contribution due to MT2EtOH
molecules (83 meq/100g). In summary:
MC[ meq / 100 g ]
MC[ g / 100 g ]
1000
Mw
MC[ meq / 100 g ] sample
MC[ meq / 100 g ] RhP, NBAP
MC[ g / 100 g ] RhP, NBAP
1000
M wRhP , NBAP
MC[ meq / 100 g ] C 30 B
83[ meq / 100 g ]
d
CEC recovery corresponds to the variation (expressed as an incremental percentage) of the value of MC
expressed in [meq/100g] in comparison with the reference value for the pristine C30B (83 meq/100g) as a
consequence of the photo-functionalization. Briefly:
CEC re cov ery
Antonella ESPOSITO
MC [ meq / 100 g ]
100
MC [ meq / 100 g ] C 30 B
MC [ meq / 100 g ] sample MC [ meq / 100 g ] C 30 B
100
MC [ meq / 100 g ] C 30 B
141
Chapter II
This observation doesn’t really controvert our previous statement (based on TGA
and XRD results) that RhP looks to be more efficient than NBAP 18: the differences in
MC and CEC recovery for NBAP and RhP can be easily justified by the differences in
molecular configuration of the two organic cationic dyes. Indeed, we estimated by
molecular modeling that RhP molecules occupy a Van der Waals (VdW) volume of 423
Å3, which is almost double in comparison with the VdW volume occupied by NBAP
molecules (297 Å3). The VdW volume of a molecule is the volume within the VdW
surface, which corresponds to the surface that intersects with the VdW radii of the
atoms in the molecular structure. By considering all the experimental evidences –
whether directly collected by XRD and TGA, or deduced by TGA results and then
supported by molecular modeling – we can conclude that the efficacy of the photofunctionalization of an organically-modified clay depends on the processing conditions,
the nature and concentration of the organic cationic dye, but also its molecular
dimension, configuration and occupied volume. Two organic cationic dyes implied in
clay photo-functionalization cannot be compared solely on the basis of their maximum
molecular dimension (Dmax): the volume occupied is also relevant, for it may be the
main parameter responsible for the tilting angle observed by XRD (see Figure II-F14
and related comments)19.
Table II-T6 shows the chemical composition (normalized on the basis of the Si
% detected by EA in the pristine C30B, Table II-T2) of the photo-functionalized clays,
compared to the chemical composition of the pristine C30B clay. The N:C ratio is a
parameter which could help evaluating the extent of MC increase and CEC recovery as
a consequence of the addition of fluorescent molecules into clay galleries during the
cation exchange process. In fact, MT2EtOH has a lower N:C ratio (0.05) in comparison
with NBAP (0.17) and RhP (0.08), meaning that the N:C balance in the pristine C30B is
shifted towards C. By inserting NBAP or RhP molecules into clay galleries, one could
expect that the N:C balance shifts towards N, i.e. increases (particularly in the case of
NBAP-exchanged samples) with respect to the initial value. Analogously, the N:Si ratio
18
19
The choice of RhP has been largely vindicated in § II-6.1
See § II-6.1.
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Lamellar fillers
can be used to assess the addition of fluorescent molecules into clay galleries. Right
after cation exchange processing, the N:Si ratio should increase (Si % content doesn’t
change and fluorescent molecules should bring additional N atoms); after the washing
procedure, it should decrease without recovering the initial value – meaning that the
excess of fluorescent molecule has been removed but a portion of the nitro organic
species inserted into clay galleries is residual. Finally, N:C and N:Si variations at each
processing step let estimating its efficiency at a glance whereas their global values (bold
values, Table II-T6) highlights the effect of the whole photo-functionalization process.
Table II-T6 Chemical composition of the samples photo-functionalized by cation exchange
processing (D), with both NBAP and RhP at both concentrations (1MC and 0.25MC), before
(bw) and after washing (aw) with ethanol.
C30B
(MC)
pristine
C [% wt] norm
N [% wt] norm
H [% wt] norm
Norm Factor (NF) a
N:C b
N:C variation [%]
N:Si
c
N:Si variation [%]
a
C30B NBAP
1MC
bw
aw
C30B NBAP
0.25MC
bw
aw
C30B RhP
1MC
bw
aw
C30B RhP
0.25MC
bw
aw
20.18
1.07
3.98
Ref
23.83
2.39
3.43
0.84
19.05
2.81
2.23
1.06
22.40
1.60
4.00
0.97
18.60
1.46
3.52
1.06
24.43
1.37
3.35
0.74
20.84
1.54
2.91
1.00
23.11
1.33
3.89
0.93
19.83
1.28
3.43
1.03
0.05
0.10
0.15
0.07
0.08
0.06
0.08
0.06
0.06
-
+ 100
+ 200
+ 40
+ 60
+ 20
+ 60
+ 20
+ 20
0.05
0.16
0.12
0.08
0.06
0.11
0.07
0.07
0.06
-
+ 220
+ 140
+ 60
+ 20
+ 120
+ 40
+ 40
+ 20
The Norm Factor (NF) is employed for EA data normalization and is obtained by comparing Si % wt of
each sample with Si % wt of the pristine C30B clay (Table II-T2). NF
Si % wt sample
Si % wt C 30 B
b
N:C ratio reference values (from Table II-T2): MT2EtOH (theoretical 0.05), C30B (experimental 0.05),
NBAP (theoretical 0.16, experimental 0.17), RhP (theoretical and experimental 0.08).
c
N:Si ratio reference values (from Table II-T2): MT2EtOH (theoretical
NBAP (experimental ), RhP (experimental ).
), C30B (experimental 0.05),
From Table II-T6 one can see that N:C variation is positive for all the samples
and diminishes as the concentration of fluorescent molecule decreases (in the case of
NBAP, for example, the N:C variation is +200 % for 1MC and +60 % for 0.25MC). If
Antonella ESPOSITO
143
Chapter II
we compare the two organic cationic dyes, NBAP looks to be more efficient than RhP
in shifting the N:C balance towards N, which definitely agrees with our previsions:
0.25MC of NBAP is sufficient to produce the same N:C balance shift as 1MC of RhP.
Washing shifts N:C balance further towards N if ethanol removes a significant amount
of exceeding surfactant, which may contain not only the free fluorescent molecules, but
also some residual MT2EtOH molecules physisorbed on silicate layers, as previously
demonstrated by TGA (amount of physisorbed MT2EtOH molecules in commercial
C30B
21% wt, Figure II-F6d). Interestingly, the clay photo-functionalized by cation
exchange processing (D) with 0.25MC RhP is the only sample showing no significant
N:C variation before and after washing with ethanol: this means that no significant
excess is removed, thus the photo-functionalization has been performed in its optimum
conditions (minimum concentration of the most efficient fluorescent molecule). About
N:Si variation: it is a positive value for all the samples and diminishes without zeroing
after washing (a global zero N:Si variation would have meant that the whole photofunctionalization process hasn’t produced any modification of the chemical composition
of the pristine C30B clay).
II-7
CONCLUSIONS
The goal of the work presented in this section was to obtain photo-functional (in
other words, photo-active) inorganic-organic complexes from a commercially available
organoclay (Cloisite ® 30B) with the perspective of using them for real-time process
monitoring of polymer-based nanocomposites containing lamellar fillers (essentially
clays) by means of Visiovis, a brand new experimental equipment we’re developing in
our laboratory. Several photo-functionalization methods have been tested with different
concentrations of few fluorescent molecules. All the experimental evidences collected
by XRD, TGA, EA, FTIR spectroscopy, TGA-FTIR and fluorescence spectroscopy
indicate that the best photo-functional inorganic-organic complexes can be obtained by
performing a classical cation exchange process of C30B with 0.25MC RhP (Rhodamine
6G Perchlorate) in a water/ethanol mixture at 80°C, followed by washing with ethanol,
recovering by centrifugation and drying at room temperature under exhaust hood. TGA
144
Lamellar fillers
showed that RhP is the most efficient organic cationic dye: photo-functionalization with
RhP appears to be readily detectable by both TGA and XRD. In addition, RhP-based
complexes have been proved to absorb and emit fluorescence in a wavelength range
which is more interesting for the application we’re developing in our laboratories –
which will be the topic of future works and papers but has recently been overviewed in
a technical journal [29]. Spectrofluorimetry indicates that, whatever the initial
concentration of RhP in the exchanging medium, cations are mostly adsorbed into clay
galleries in the form of monomers and/or fluorescent J-aggregates (probably dimers)
since the presence of the MT2EtOH molecules seems to limit the tendency of RhP
molecules to aggregate and probably induces some spatial regularity. Clay photofunctionalization at higher concentrations of fluorescent molecule is still possible (no
significant fluorescence quenching occurs, as confirmed by spectrofluorimetry for
C30B photo-functionalized 1MC RhP) but unnecessary: most of the fluorophore excess
is washed away and wasted. Cation exchange process with a smaller amount of organic
cationic dye (e.g. a relatively small percentage of the initial modifier concentration MC)
is sufficient to render C30B organoclay photo-active. Indeed, any excess of fluorescent
dye preferentially adsorbs at the silicate edges in a disordered configuration and likely
creates a sterical barrier to further intercalation: these are the reasons why we preferred
to limit the excess of fluorescent dye to 0.25 times the initial modifier concentration –
which completely fulfils the requirements of our study. Finally, XRD showed that the
intercalation of smaller molecules (RhP cations) into organically-modified clay galleries
likely produces a rearrangement of the paraffinic configuration of previously-exchanged
bigger molecules (MT2EtOH cations) and increase the tilting angle from 37° up to 49°.
Antonella ESPOSITO
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Chapter II
II-R
[1]
REFERENCES
Schmidt D.F. Polysiloxane/layered silicate nanocomposites: synthesis, characterization,
and properties. PhD Thesis. Itaka, NY: Cornell University, 2003, 300 p.
[2]
Burgentzlé D., Duchet J., Gérard J.F. et al. Solvent-based nanocomposite coatings I.
Dispersion of organophilic montmorillonite in organic solvents. J. Colloid Interface Sci.
2004, 278, 26-39.
[3]
Le Pluart L., Duchet J., Sautereau H. et al. Surface modifications of montmorillonite
for tailored interfaces in nanocomposites. J. Adhes. 2002, 78 (7), 645-662.
[4]
Xie W., Gao Z., Pan W.-P. et al. Thermal degradation chemistry of alkyl quaternary
ammonium montmorillonite. Chem. Mater. 2001, 13, 2979-2990.
[5]
Bellucci F., Camino G., Frache A. et al. Catalytic charring – volatilization competition
in organoclay nanocomposites. Polym. Degrad. Stab. 2007, 92(3), 425-436.
[6]
Davis R.D., Gilman J.W., Sutto T.E. et al. Improved thermal stability of organically
modified layered silicates. Clays Clay Miner. 2004, 52 (2), 171-179.
[7]
Xi, Y.; Frost, R.L.; He, H. Modification of the surfaces of Wyoming montmorillonite by
the cationic surfactants alkyl trimethyl, dialkyl dimethyl, and trialkyl methyl ammonium
bromides. J. Colloid Interface Sci. 2007, 305, 150-158.
[8]
Shen Z. Nanocomposites of polymers and layered silicates. PhD Thesis. Australia:
Monash University, 2000, 308 p.
[9]
Wilkie C.A. TGA/FTIR: an extremely useful technique for studying polymer
degradation. Polym. Degrad. Stab. 1999, 66, 301-306.
[10]
Cervantes-Uc J.M., Cauich-Rodríguez J.V., Vázquez-Torres H. et al. Thermal
degradation of commercially available organoclays studied by TGAFTIR. Thermochim.
Acta 2007, 457, 92-102.
[11]
Alexandre M., Dubois P. Polymer-layered silicate nanocomposites: preparation,
properties and uses of a new class of materials. Mater. Sci. Eng. 2000, 28, 1-63.
[12]
Pozsgay A., Fráter T., Százdi L. et al. Gallery structure and exfoliation of
organophilized montmorillonite: effect on composite properties. Eur. Polym. J. 2004,
40, 27-36.
[13]
Gates W.P. Crystalline swelling of organo-modified clays in ethanol-water solutions.
Appl. Clay Sci. 2004, 27, 1-12.
146
[14]
Lamellar fillers
Endo T., Sato T., Shimada M. Fluorescence properties of the dye-intercalated smectite.
J. Phys. Chem. Solids 1986, 47 (8), 799-804.
[15]
Fujita T., Iyi N., Kosugi T. et al. Intercalation characteristics of rhodamine 6G in fluortaeniolite: orientation in the gallery. Clays Clay Miner. 1997, 45 (1), 77-84.
[16]
Iwasaki M., Kita M., Ito K. et al. Intercalation characteristics of 1,1'-diethyl-2,2'cyanine and other cationic dyes in synthetic saponite: orientation in the interlayer.
Clays Clay Miner. 2000, 48 (3) 392-399.
[17]
Čapková P., Malý P., Pospíšil M. et al. Effect of surface and interlayer structure on the
Interface Sci. 2004, 277, 128-137.
[18]
Schmidt D.F., Clément F., Giannelis E.P. On the origins of silicate dispersion in
[19]
Lagaly G. Characterization of clays by organic compounds. Clay Miner. 1981, 16, 1-21.
[20]
Bergaya F., Lagaly G. Surface modification of clay minerals. Appl. Clay Sci. 2001, 19,
1-3.
[21]
Cowan C.T., White D. Adsorption by organo-clay complexes. Trans. Faraday Soc. 1958,
54, 691-697.
[22]
Maupin P.H., Gilman J.W., Harris R.H. (Jr.) et al. Optical probes for monitoring
intercalation and exfoliation in melt-processed polymer nanocomposites. Macromol.
Rapid Commun. 2004, 25, 788-792.
[23]
Bur A.J., Roth S.C., Start P.R. et al. Fluorescent probes for monitoring microstructure
of polymer-clay nanocomposites. Proceedings of the Society of Plastics Engineers:
Annual Technical Conference. ANTEC 2004, Vol.1 (Processing), 1315-1318.
[24]
Gilman J.W., Maupin P.H., Harris R.H. (Jr.) et al. High throughput methods for
nanocomposite materials research. Extrusion and visible optical probes. Polym. Mater.
Sci. Eng. 2004, 90, 717-718.
[25]
Pospíšil M., Čapková P., Weissmannová H. et al. Structure analysis of montmorillonite
intercalated with rhodamineB: modeling and experiment. J. Mol. Model. 2003, 9, 39-46.
[26]
Cervantes-Uc J.M., Cauich-Rodríguez J.V., Vázquez-Torres H. et al. Thermal
degradation of commercially available organoclays studied by TGA-FTIR. Thermochim
Acta 2007, 457, 92-102.
[27]
Edwards G., Halley P., Kerven G. et al. Thermal stability analysis of organo-silicates,
using solid phase microextraction techniques. Thermochim. Acta 2005, 429, 13-18.
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Chapter II
[28]
He H., Duchet J., Galy J. et al. Influence of cationic surfactant removal on the thermal
stability of organoclays. J. Colloid Interface Sci. 2006, 295, 202-208.
[29]
Esposito A, Balcaen J, Duchet-Rumeau J, Charmeau JY. Visiovis: monitoring nanofiller
dispersion/distribution in molten polymers. JEC Composites Magazine 2008, 41, 67-71.
[30]
Madejová J. FTIR techniques in clay mineral studies. Vib. Spectrosc. 2003, 31, 1-10.
148
Chapter III
PHOTO-FUNCTIONAL COMPLEXES
A great portion of this chapter corresponds to the content of a paper
recently submitted for publication.
The aim of photo-functionalization is to graft/adsorb a fluorescent molecule onto
clay silicate layers or at their edges, or to introduce it into clay galleries. In the previous
section1 we reported all the details about the set of methods we tested in order to photofunctionalize a given organically-modified clay (Cloisite ® 30B) and we concluded that
the most practicable, appropriate and efficient method is cation exchange processing in
a solution 90/10 permuted water/ethanol at 80°C containing a certain amount of the
organic cationic dye of interest [1]. Then we performed cation exchange processing of
the same organically-modified clay with two different organic cationic dyes (Nile Blue
A Perchlorate, Rhodamine 6G Perchlorate) introduced in two different concentrations in
the exchanging medium (i.e. 1MC and 0.25MC) in order to determine the most efficient
fluorescent molecule and its optimum concentration (Rhodamine 6G Perchlorate in a
concentration equivalent to 25% of the initial modifier concentration, i.e. 0.25MC) [1].
1
Chapter II (PHOTO-FUNCTIONALIZATION – Lamellar fillers).
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Chapter III
In this section we’ll report the results obtained using the procedure previously
optimised to prepare other photo-functional inorganic-organic complexes from four
commercially available clays (Cloisite ® Na+, Cloisite ® 30B, Cloisite ® 10A, Cloisite
® 15A). The results obtained with Cloisite ® 30B have been discussed in the previous
section: we’ll recall them just to make the comparison with the other commercial clays
easier. Among the selected commercial clays we included also Cloisite ® Na+ since the
same photo-functionalization method performed in the absence of organics may help
understanding what’s going on when clay galleries already enfold an organic surfactant:
in other words, we selected Cloisite ® Na+ to get a kind of reference for the other clays.
III-1
MATERIALS
One sodium montmorillonite (Cloisite ® Na+) and three organically-modified
clays (Cloisite ® 30B, Cloisite ® 10A and Cloisite ® 15A) have been purchased from
Southern Clay Products (USA) and used as received. MMT-Na+ (CNa+) is a natural
montmorillonite containing 4-9% ca of moisture and no traces of organic modifiers.
MMT-MT2EtOH (C30B) is a natural montmorillonite organically modified with
MT2EtOH (methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium chloride) in an
initial Modifier Concentration (MC) of 90 meq/100g of clay. MMT-2MBHT (C10A) is
a natural montmorillonite organically modified with 2MBHT (dimethyl, benzyl,
hydrogenated tallow, quaternary ammonium chloride) in an initial MC of 125 meq/100g
of clay. MMT-2M2HT (C15A) is a natural montmorillonite organically modified with
2M2HT (dimethyl, dehydrogenated tallow, quaternary ammonium chloride) in an initial
MC of 125 meq/100g of clay. Both tallow (T) and hydrogenated tallow (HT) fatty
chains have the following composition: 65% C18, 30% C16 and 5% C14. The selected
clays have been photo-functionalized with the organic cationic dye previously chosen,
(Rhodamine 6G Perchlorate, Sigma Aldrich). The raw materials and the corresponding
relevant chemicals, as well as their molecular weight Mw and their most stable free
molecular configuration obtained by energy minimization in a molecular modeling
environment (Materials Studio, Accelrys Software Inc., Module Discover), are listed in
Table III-T1. Permuted water has been ion-exchanged right before using it, assuring a
150
quality of 18 M -cm. Ethanol has been purchased from Sigma-Aldrich and used as
received.
Table III-T1 Raw materials and the corresponding relevant chemicals
CNa+
MC
TRADE or
IUPAC NAME
CHEMICAL
meq
100g
MW
Most stable
configuration in the free
state a
Cloisite ® Na+
MMT-Na+
–
90 b
n.a.
n.a.
Cloisite ® 30B
90
C10A
Cloisite ® 10A
MMT-2MBHT
125
C15A
Cloisite ® 15A
MMT-2M2HT
125
RhP
Rhodamine 6G
Perchlorate
184 d
C30B MMT-MT2EtOH
360.80
average c
382.80
average c
527.60
average c
543.01
a
The most stable molecular configuration of each chemical in its free state has been determined by
minimizing its energy in a molecular modeling environment (Materials Studio v.4.1.0.0, Accelrys
Software – Module Discover – Smart Minimizer, medium convergence level, 5000 max interactions).
b
In the case of CNa+, we report (and we’ve worked referring to) the typical value of Cation Exchange
Capacity (CEC) of a natural sodium MMT (90 meq/100gr of clay).
c
MT2EtOH, 2MBHT and 2M2HT include tallow (T) or hydrogenated (i.e. saturated) tallow (HT) fatty
chains having the following composition: 65% C18, 30% C16, 5% C14.
d
This value has been calculated by considering that the whole amount of cationic organic dye is able to
replace clay inorganic cations [1].
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Chapter III
III-2
PHOTO-FUNCTIONALIZATION PROTOCOL
An efficient photo-functionalization protocol and its optimum conditions have
been established in the previous section of this work: to understand the reasons of our
choices about photo-functionalization protocol, we recommend the lecture of Chapter II
(PHOTO-FUNCTIONALIZATION – Lamellar fillers). Here we just remind that such protocol
(detailed in § II-2.4) was originally designed to exchange organically-modified clays –
which is the reason why we referred to the initial Modifier Concentration (MC) in order
to determine an optimum concentration of Rhodamine 6G Perchlorate. In the following,
however, we are going to use the same protocol to photo-functionalize Cloisite ® Na+, a
clay which is completely inorganic and doesn’t contain any modifier in its pristine state:
in this case, we’ll simply refer to its Cation Exchange Capacity (CEC) – therefore, the
optimum concentration of fluorescent molecule will be 0.25CEC rather than 0.25MC.
III-3
CHARACTERIZATIONS
Analogously to the criterions used for sample characterizations in the previous
section, here again characterizations are mainly aimed to verify whether the fluorescent
molecules adsorbed or not on clay surfaces, and where they are located. Of course, once
again we tested the photo-activity of the samples by spectrofluorimetry.
III-4
REFERENCE MEASUREMENTS
All the pristine commercial clays (CNa+, C30B, C10A, C15A) as well as the free
organic cationic dye (RhP) have been firstly characterized by XRD, TGA, EA, FTIR
and spectrofluorimetry in order to get reference measurements.
III-4.1
Reference XRD measurements
In the previous section2 we reported that RhP shows two marked peaks at 9.8 Å
and 12 Å and we underlined that its peaks couldn’t affect the interpretation of any XRD
measurement performed on the photo-functionalized C30B – which has an initial
interlayer spacing of 17.5 Å. This is factual also for C10A and C15A, which show an
2
See § II-4.1.
152
initial interlayer spacing of 19.2 and 32 Å respectively. On the contrary, a particular
attention will be paid to the interpretation of XRD results obtained for the photofunctionalized CNa+, since the initial interlayer spacing of this hydrophilic clay is 12.7
Å (Figure III-F1). In order to exclude any misunderstanding with the interpretation of
XRD characterizations for the washed photo-functional inorganic-organic complexes,
we judged necessary to include, in the panel of the reference XRD measurements, the
complete set of characterizations for the selected commercial clays (CNa+, C30B, C10A
and C15A) after one step of the same washing procedure employed for the photofunctionalized clays, i.e. rapid immersion in ethanol, manual stirring, recovering by
centrifugation and drying under exhaust hood for several days at room temperature.
Figure III-F1 Reference XRD measurements of the selected commercial clays: (a) CNa+,
(b) C30B, (c) C10A, (d) C15A. Bold lines represent the samples after washing.
This single washing step performed on the pristine clays allows to take into
account the effects due to the immersion in the exchanging medium, independently
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Chapter III
from the effects due to the organic cationic dye. After the washing procedure, the
commercial clays have an interlayer spacing of 12.4 Å (CNa+), 17.5 Å (C30B), 19.2 Å
(C10A) and 25.7 Å (C15A) (bold lines in Figure III-F1): it is interesting to observe that
ethanol has no influence on the interlayer spacing of the hydrophilic clay and, with
reference to the organophilic MMTs, it doesn’t modify the molecular arrangement of
the surfactant into the galleries but in the case of C15A (
d001
= 6.3 Å). These results
will be crucial for the interpretation of any other XRD measurement. XRD reference
measurements for the commercial clays before and after washing are shown in Figure
III-F1. The XRD signature of RhP has been shown in Chapter II (Figure II-F5c) [1].
III-4.2
Reference TGA measurements
TGA (weight vs. temperature) and DTG (weight derivative vs. temperature)
curves for the RhP have been commented in Chapter II [1]: the initial sample weight
halves in the temperature range 200-500°C and the derivative curve shows a single
sharp peak centered at 338°C (Figure II-F6c). CNa+ contains no traces of organics thus
its thermal characterization produces nearly flat TGA and DTG curves (Figure III-F2a),
as expected: the main weight loss would have been the dehydroxilation process, viz. the
loss of structural water from the crystalline lattice at higher temperature. We previously
reported3 that pristine C30B undergoes two weight losses between 150°C and 500°C,
the first one (21% at 253°C) partially corresponding to physisorbed MT2EtOH and the
second one (30% at 360°C) corresponding to intercalated MT2EtOH (Figure III-F2b).
DTG curve for C30B after washing with ethanol helped to realize that the first weight
loss included also a portion of MT2EtOH molecules well intercalated but in a peripheral
position with respect to the clay gallery (15% of the whole amount of intercalated
surfactant). Such observation is also factual for C10A clay (Figure III-F2c) which, in its
pristine state, undergoes a first weight loss (13% of sample weight = 37% of surfactant,
at 207°C) corresponding to a peak considerably (not completely) removed by washing
with ethanol (bold line). After this first weight loss, C10A clay undergoes a second
weight loss (10% of sample weight = 26% of surfactant, at 285°C) – absolutely washing
3
See § II-4.2 and [1].
154
insensitive – and a third weight loss (12% of sample weight = 36% of surfactant, at
368°C), which slightly changes upon washing with ethanol, thus indicating a probable
rearrangement of the surfactant into the clay galleries. This last peak (408°C), better
revealed by washing, corresponds most likely to the aromatic portion of the intercalated
molecules, since it is well known that the presence of aromatics can produce additional
peaks at higher temperatures in DTG curves [1].
(aw)
(bw)
Figure III-F2 Reference TGA measurements of the selected commercial clays: (a) CNa+,
(b) C30B, (c) C10A, (d) C15A. Bold lines represent the samples after washing with ethanol.
Finally, C15A (Figure III-F2d) shows a complex thermal behavior in its pristine
state, since its DTG curve presents a broad peak in the temperature range 150-450°C –
which probably result from the convolution of at least four peaks, as determined by a
multi-peaks fit, giving four shallow humps centered at the following temperature:
252°C, 312°C, 343°C and 413°C. The DTG curve gets much simpler after washing,
since removing the excess of surfactant (indeed only 6% of sample weight, as visualized
by the different weight loss at 500°C for C15A before and after washing, i.e. 41% (bw)
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Chapter III
minus 35% (aw), as shown in Figure III-F2d) allows to better visualize the two weight
losses (23% at 312°C, 12% at 401°C) corresponding to intercalated 2M2HT molecules.
In the case of C15A clay, no significant “edge effects” seem to affect the intercalated
2M2HT molecules: the first peak (252°C) is almost completely removed by washing,
and even a deeper analysis performed by multi-peak fitting and integration of the peak
area confirmed that it entirely corresponds to the excess of surfactant physisorbed at the
platelet edges or on the external clay surfaces (5.4% ca).
It is interesting to observe that the “edge effects” – as visualized by TGA
characterizations – seem to depend on the molecular weight of the surfactant molecules:
anyway, this doesn’t mean that bigger molecules are better confined in between clay
platelets. It simply means that the residual peak assigned to the peripherally-intercalated
surfactant molecules is, in the DTG curve of a washed sample, better masked by the
main intercalation peak if the surfactant molecules have a higher molecular weight –
which actually corresponds to a higher mass of intercalated organic and, thus, to a
higher relative weight loss rate.
III-4.3
Reference EA measurements
Reference EA values are resumed in Table III-T2. The Si % content detected in
each of the pristine clays has been chosen as the reference value to get the
Normalization Factor (NF) for any other EA measurement performed on the photofunctionalized clay samples, whether washed or not.
As expected, the chemical composition of the pristine commercial clays follows
some clear trends. Na % content is relatively high in pristine (unwashed) CNa+ and
diminishes in the commercial organically-modified clays because of the substitution of
Na+ for N+ cations (N % content follows the opposite trend). C % content is not
significant in CNa+ (agreeing with the fact that CNa+ contains no traces of organics) but
dramatically increases in the commercial organically-modified clays, following a trend
which is coherent with the molar weight of the respective surfactants molecules, and
which is obviously opposite to the trend of the relative Si % content. The effects of
washing with ethanol are well recognizable by comparing the C % content of the
commercial organoclays before (bw) and after washing (aw).
156
Table III-T2 Reference EA measurements: chemical composition of the raw materials before
(bw) and after washing (aw).
Na [% wt]
bw
CNa
+
C30B
C10A
C15A
RhP
aw
3.08
0.23
0.21
780 ‰
0.00
C [% wt]
bw
aw
N [% wt]
bw
aw
2.45 < 0.30 < 0.30 < 0.10 < 0.10
< 0.10 20.18 15.36
1.07
0.88
0.13
27.64 21.56
1.20
0.96
0.40
32.64 27.58
0.91
0.82
b
b
n.a. 61.90
n.a.
5.20
n.a.
H [% wt]
Si [% wt] a
bw
aw
bw
aw
1.41
3.98
4.86
6.31
5.71 c
< 0.30
3.32
3.79
5.28
n.a.
25.90
21.39
18.99
17.81
0.00
26.44
22.02
20.18
18.91
n.a.
a
The amount of any element estimated by EA is relative and has to be normalized. In the following,
normalization of any value obtained for each photo-functionalized sample will be done in relation to the
Si % content detected in the corresponding pristine clay.
b
Source: Sigma-Aldrich.
c
Theoretical value calculated on the basis of the chemical formula.
III-4.4
Reference FTIR spectra
The FTIR spectra of RhP and C30B have already been shown and commented in
the previous section (Figure II-F7) [1]. However, we propose once more the spectrum of
C30B, so that it will be easier to make the comparison with the other clays. The FTIR
spectra of the selected commercial clays (Figure III-F3) share some features which are
common to any MMT clay – whether organically-modified or not:
 a broad absorption band in the region 3650-3200 cm-1, assigned to H-bonded
OH stretching, mostly due to the intrinsic structure of clays (hydroxyl groups) and to the
structural water (which hydrates both the exchangeable interlayer cations and the
immobilized cations);
 a strong peak at 1050 cm-1, assigned to the Si-O-Si vibrations, explained by
the chemical composition of MMT clay (basically a framework of crystalline silicates);
 a set of three peaks in the region 670-400 cm-1, assigned to all the possible
vibrational modes in the inorganic crystalline lattice.
The organically-modified clays (C30B, C10A and C15A) additionally present a
set of two peaks in the region 3000-2800 cm-1 (approximately 2930 and 2860 cm-1)
which is typical of compounds containing long linear aliphatic chains – MT2EtOH
(C30B, Figure III-F3b), 2MBHT (C10A, Figure III-F3c) and 2M2HT (C15A, Figure
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Chapter III
III-F3d) do contain one tallow (or hydrogenated tallow) chain 4. This is confirmed by the
sharp but weak peak around 1470 cm-1.
Figure III-F3 Reference FTIR measurements for the pristine clays in KBr pellets: (a)
CNa+, (b) C30B, (c) C10A and (d) C15A.
The weak peak around 1635 cm-1 is due to the structural water, always present in
natural clays. The narrower peak centred around 3630 cm-1 and present in all the spectra
but the one of C15A (Figure III-F3d) may be explained once again by OH stretching
(non-H-bonded OH groups), often corresponds to an alcohol with a sterically-hindered
OH group (which may be the case of C30B, Figure III-F3b) and could indicate, in some
inorganics and minerals, the presence of “free” OH groups either on the surface or
within the crystalline lattice. A summary of the peaks is given in Table III-T3.
4
The chemical formula of MT2EtOH, 2MBHT and 2M2HT are shown in Table III-T1.
158
Table III-T3 Reference FTIR data for the commercial clays in their pristine state: positions and
probable assignment a of the absorption bands (spectra in Figure III-F3).
Position
[cm-1]
CNa+ C30B C10A C15A
(a)
(b)
(c)
(d)
~ 3633
3445
3436
3429
3417
3065 and 3035
2928
2854
2364 and 2339
1637 or 1630
1473 or 1468
1417
OH stretching of structural hydroxyl groups
(non H-bonded and “free” OH groups) and/or
OH stretch of primary alcohols
OH stretching of structural hydroxyl groups (Hbonded OH groups) and/or OH stretching of Hbonded water (overlapping asymmetric and
symmetric H-O-H)
C-H vibrations (together with 1650 cm-1)
C-H stretching of alkylammonium cations (
asymmetric CH2)
C-H stretching of alkylammonium cations (
symmetric CH2)
Atmospheric CO2
OH deformation of water b
as(C-H) bending of methylene groups in
(CH3)3N+(CH2)nCH3 cations and/or C=C-C
stretching of aromatic rings
885
s((CH3)-N) bending of methyl groups
C-H in-plane bending of aromatic rings and/or
CN stretching of tertiary amines
Si-O-Si stretching (longitudinal mode)
Si-O-Si in-plane stretching and/or C-N
stretching of primary amine
Al2OH deformation and/or OH deformation of
inner hydroxyl groups
AlFeOH deformation
849
AlMgOH deformation
801
Si-O-Si stretching of quartz and silica
726
703
–(CH2)n– rocking (n
Si-O-Si vibrations
625
Coupled Al-O and Si-O out-of-plane vibrations
525
Al-O-Si (octahedral Al) deformation
466
Si-O-Si bending and deformation
1216 or 1203
1118
1046
918
a
Probable assignments
3) of methylene groups
Assignment has been effectuated on the basis of Madejová et al. [2][3] and Coates [4] observations.
b
This peak may also be explained by some olefinic unsaturation, usually corresponding to a relatively
narrow weak-to-moderate peak at 1650 cm-1, whose frequency lowers upon conjugation with another
double bond or an aromatic ring [4].
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Chapter III
III-4.5
Reference fluorescence spectra
It is well known that the maximum absorption for RhP is around 532 nm: the
absorption and fluorescence emission spectra for RhP in ethanol have been reported in
the previous section5 [1]. Unsurprisingly, none of the selected commercial clays absorbs
radiation and produces a fluorescence emission in its pristine state (results not shown).
III-5
CHARACTERIZATION OF THE PHOTO-FUNCTIONAL
INORGANIC/ORGANIC COMPLEXES
In the previous section we could realize that XRD and TGA effectively represent
a set of powerful techniques, which can be easily employed in a complementary way to
characterize natural and organically-modified clays. Therefore, we continue using XRD
coupled to TGA measurements in order to characterize the photo-functional inorganicorganic complexes obtained with the same protocol previously used for Cloisite ® 30B.
III-5.1
Photo-responsive CNa+ 0.25CEC RhP
The double set of characterizations for the photo-functional inorganic-organic
complex CNa+ 0.25MC RhP before and after washing with ethanol are shown in Figure
III-F4. The XRD pattern (Figure III-F4a) indicates that the organic cationic dye has
entered and swollen clay galleries, since the d001 value has considerably increased,
reaching a value of 17.2 Å ( d001 = +4.8 Å). This result is entirely due to intercalated
and properly exchanged molecules, since clay washing with ethanol doesn’t affect the
measured d001 value. TGA curves (Figure III-F4b) confirm that the photo-responsive
CNa+ complexes are absolutely washing insensitive: the whole amount of fluorescent
molecule used for the photo-functionalization process (25% of the CEC of the pristine
clay) entered clay galleries since no excess got physisorbed on clay platelets (no weight
losses occur up to 300°C).
5
See § II-4.5.
160
Figure III-F4 Characterizations of the photo-functional inorganic-organic complex CNa+
0.25CEC RhP, before and after washing with ethanol. XRD (a) is substantiated by TGA
results (b). Bold lines represent washed samples.
III-5.2
Photo-responsive C30B 0.25MC RhP
XRD and TGA results for the photo-functional inorganic-organic complex C30B
0.25MC RhP before and after washing with ethanol have already been reported and
commented in the previous section6 and in a submitted paper [1]. Here we just remind
that, in agreement with some other results found in the literature [5][6], right after the
photo-functionalization we measured an interlayer spacing d001 = 22 Å ( d001 = +4.5 Å),
which resulted to be affected neither by the RhP concentration in the exchanging
medium, nor by the following clay washing (Figure III-F5a). We also observed that
cation exchange processing of C30B with 0.25MC RhP produced a peak in the area of
physisorbed chemicals (243°C) (Figure III-F5b), which is lowered by washing but
6
§ II-6.2.
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Chapter III
doesn’t completely disappear because it partially corresponds to 2M2EtOH molecules
properly exchanged but intercalated in a peripheral position, therefore less protected
from thermal degradation. In spite of its maximum temperature (315°C), we could
explain the second peak in a similar way, since its behavior in relation to washing is
analogous. We finally concluded that the last weight loss (406°C) corresponds to the
peak assigned to intercalated chemicals, since it is washing insensitive.
Figure III-F5 Characterizations of the photo-functional inorganic-organic complex
C30B 0.25MC RhP, before and after washing with ethanol. XRD measurements (a) are
substantiated by TGA results (b). Bold lines represent washed samples.
III-5.3
Photo-responsive C10A 0.25MC RhP
XRD and TGA results for the photo-functional inorganic-organic complex C10A
0.25MC RhP before and after washing with ethanol are shown in Figure III-F6. The
photo-functionalization process of C10A with 0.25MC RhP produces, in comparison
with the TGA results for the pristine clay, an additional peak in the area of physisorbed
162
chemicals (194°C), which is efficaciously removed by washing and surely corresponds
to an excess of RhP molecules – since it has never been observed in the pristine
organoclay, whether before or after one-step washing (Figure III-F2c). The following
peak (235°C) is lowered by washing but it doesn’t completely disappear, analogously to
the peak observed at 207°C (and lowered to 225°C by washing) for the pristine C10A
(Figure III-F2c). This analogy confirms that such peak must be assigned to the 2MBHT
molecules properly exchanged but intercalated in a peripheral position.
C10A 0.25MC RhP, before and after washing with ethanol. XRD measurements (a) are
The peak at 292°C (Figure III-F6b) is also considerably lowered by washing and
it reasonably corresponds to the peak already observed in the pristine organoclay at
285°C (Figure III-F2c) and previously assigned to the first weight loss of intercalated
chemicals. The main difference is that the peak for the photo-functional complex isn’t
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Chapter III
exactly shaped as the peak of the pristine C10A – it rather shows a certain tendency to
split up in a double peak after washing. By the way, on the basis of a simple analogy
with the peak observed in the pristine C10A, one could hypothesize that a significant
portion of the RhP molecules entered clay galleries but wasn’t able to undergo cation
exchange process, probably because of the sterical hindrance effect due to 2MBHT
molecules. Indeed, Paul et al. [7] showed that the absence of OH groups in the
molecular structure of the surfactant reduces interlayer packing and molecular density,
leaving more vacancies which could be occupied by smaller molecules (RhP molecules,
in this case). However, we haven’t observed any shift of the XRD peak (22.6 Å before
and after clay washing as shown in Figure III-F6a, d001 = +3.4 Å with respect to the
pristine clay), however this would have been the sole evidence confirming that some
molecules entered clay galleries without undergoing cation exchange (we remind that an
efficient washing procedure is supposed to remove any unexchanged excess, whether
unconfined or confined, and that removing a confined excess usually causes a decrease
of the interlayer spacing) [1]. With the help of the evidences collected after the photofunctionalization we can conclude that the previous assignment of the peak at 285°C for
the pristine C10A is probably inexact and should be rather assigned exclusively to the
aliphatic portions of the surfactant molecules properly exchanged but in a peripheral
position, i.e. partially confined. Consequently, the last peak at 374°C (Figure III-F6b),
which looks less finely-shaped than the corresponding peak in the pristine organoclay
(Figure III-F2c) probably because of a higher complexity of the molecular arrangement
into clay galleries, is the peak corresponding to intercalated aromatic chemicals.
III-5.4
Photo-responsive C15A 0.25MC RhP
XRD and TGA results for the photo-functional inorganic-organic complex C15A
0.25MC RhP before and after washing with ethanol are shown in Figure III-F7. The
photo-functionalization of C15A seems to produce the most complex XRD patterns,
with two populations of crystallites having different interlayer spacing before washing
and a single population having an interlayer spacing d001 = 25 Å after washing (Figure
III-F7b). Indeed, these results shouldn’t surprise, since the cation exchange process has
been performed on the pristine commercial clays in their as-received state (i.e. without
164
further purification and/or washing): we’ve already shown that even a single-step
washing procedure with ethanol may considerably change the molecular arrangement of
the surfactant into clay galleries by removing any excess (whether unconfined or
confined), which sometimes correspond to a significant change of the XRD pattern as
highlighted in the case of the pristine C15A (Figure III-F1d). Thus, it isn’t surprising
that the photo-functionalization process increased the interlayer spacing of both the
crystalline phases detected in the pristine commercial organoclay, shifting the first peak
from d001 = 32 Å to 37 Å ( d001 = +5 Å) and the second peak from d001 = 20.3 Å to 22 Å
( d001 = +1.7 Å), the latter value (22 Å) being in agreement with the d001 values
obtained with C30B (Figure III-F5a) and C10A (Figure III-F6a).
C15A 0.25MC RhP, before and after washing with ethanol. XRD measurements (a) are
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Chapter III
Apart from these observations, it is noteworthy that the unmodified but washed
C15A showed an initial value d001 = 25.7 Å (Figure III-F1d, bold line) and that such
value hasn’t really changed upon the cation exchange process followed by washing (d001
= 25 Å, bold line in Figure III-F7a). It is in such kind of situations that complementary
characterization techniques (namely TGA) become useful to better understand what’s
going on. Figure III-F7b shows that the photo-functionalization of C15A with 0.25MC
RhP produces, in comparison with the TGA results for the pristine C15A (Figure IIIF2d), a strong additional peak at 285°C in the area of physisorbed chemicals. Such peak
decreases upon washing and surely corresponds to an excess of RhP molecules. After
washing, the shape of the DTG curve (bold line in Figure III-F7b) looks quite complex
and in any case different from the corresponding curve for the pristine C15A after the
single-step washing procedure (bold line in Figure III-F2d). The sample undergoes four
weight losses whose peaks can’t be easily resolved. Photo-functionalized C15A presents
such a complex structure that neither XRD nor TGA can univocally detect whether the
fluorescent molecules are intercalated or not into the clay galleries and, consequently,
none of these techniques can explain the molecular configuration of the surfactant and
of the cationic dye into the sample. Spectrofluorimetry will prove that all the inorganicorganic complexes we obtained are effectively photo-active and will likely inform about
the molecular configuration of the photo-functionalized clays – at least of each complex
compared to the others.
III-5.5
Comparison of the photo-responsive complexes
As previously observed in Chapter II, EA completes the set of characterizations
which can be made on functionalized clays. Indeed, this technique allows estimating the
relative amount of any specific element (but the oxygen) present in the sample. By
performing EA of each photo-functionalized sample before and after washing, and by
comparing these results to the ones obtained for the relative pristine commercial clay, it
is possible to get some other information about the effects of the photo-functionalization
and those of the washing procedure. The limiting factor is that EA can give only relative
chemical compositions, meaning that the amount of any detected element has to be
166
“normalized”: normalization of EA data has been done with reference to the Si % wt
content detected in each of the pristine commercial clays.
Table III-T4 EA measurements: chemical composition of the photo-active inorganic-organic
complexes (0.25MC RhP) compared to the composition of the pristine clays.
Na a [% wt]
pristine
+ RhP
+
C a [% wt]
pristine + RhP
2.45
0.84
< 0.30
C30B < 0.10 ≤ 0.5 ‰ 15.36
C10A 0.13
< 0.1 ‰ 21.56
C15A 0.40
< 0.1 ‰ 27.58
CNa
9.07
19.25
24.23
27.73
N a [% wt]
pristine
< 0.10
0.88
0.96
0.82
H a [% wt]
Si [% wt]
+ RhP pristine + RhP pristine + RhP
0.60
1.24
1.34
1.13
< 0.30
3.32
3.79
5.28
1.80
3.32
3.70
4.86
26.44
22.02
20.18
18.91
23.68
22.12
20.71
19.54
a
All the values are normalized on the basis of a Norm Factor (NF), obtained by comparing the Si %
content of each washed sample with the Si % content of the corresponding pristine clay after washing:
NF
Si % wt sample ( aw)
Si % wt pristine ( aw)
The chemical composition of all the photo-active inorganic-organic complexes,
compared to the composition of each of the pristine commercial clays after washing
with ethanol, is listed in Table III-T4. Once again, EA values follow some clear trends.
Na % content further decreases after the second cation exchange process, becoming less
than few hundreds parts per million in the washed photo-active complexes (whatever
the pristine commercial organoclay). C % content dramatically increases for CNa+,
since the photo-functionalization is the very first organic modification of the sodium
MMT. By the way, C % content slightly increases also for the organoclays – to an
extent which depends on several factors, e.g. the initial surface coverage, the sterical
hindrance effect due to the surfactant, its molecular arrangement, the interlayer packing
and molecular density into clay galleries – following the expected trend.
It is worthy to observe that the variation of C % wt adsorbed by the organoclays
seems to be inversely proportional to the molar weight of the surfactant ( C% = 3.89,
2.67 and 0.15 for C30B, C10A and C15A, respectively). Interestingly, N % content
increases as well, and not exclusively for CNa+ (which is in all circumstances supposed
to show the most remarkable variations): this proves that performing a second exchange
process of an organoclay with some organic cationic molecules smaller than the first
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Chapter III
organic surfactant allows to better take advantage of the intrinsic CEC value of the
organic host (CEC recovery) and to increase the surface coverage. We could even think
to estimate the extent of CEC recovery by evaluating such increase of the N % content:
for all the organoclays we observe an increase of about 40%, with a slight trend
inversely proportional to the molar mass of the surfactant ( N% = 0.36, 0.38 and 0.31
for C30B, C10A and C15A, respectively) meaning that the CEC recovery is probably
less efficacious in the presence of highly sterically-hindering surfactant molecules.
By comparing the FTIR7 fingerprint of each photo-functional inorganic-organic
complex to the spectrum obtained for the corresponding commercial clay in its pristine
state, it is possible to detect the presence of any additional chemical and, eventually, its
surroundings (i.e. whether it is adsorbed onto silicate layers or it is a free excess).
As already highlighted in the previous section8, the intercalation of RhP cations
into clay galleries is based on a simple mechanisms of cation exchange reaction, thus
the host-guest interactions have a non-covalently-bonded nature: a comparison of the
FTIR spectra for the host inorganic structures (pristine commercial clays, Figure III-F3),
the guest organic chemical (RhP, Figure II-F7) and the photo-functional inorganicorganic complexes (bold lines in Figure III-F8) should show that the main absorption
bands of the pristine clays and those of the organic cationic dye coexist in the FTIR
fingerprints of the photo-active clays. Indeed, in the FTIR spectra of the photofunctional inorganic-organic complexes, one can distinguish the features of the pristine
commercial clays (thin-lined curves in Figure III-F8) but one can also recognize some
absorption bands characteristic of RhP in the region 1800-1400 cm-1. The global shape
of the absorption band associated to the O-H stretching doesn’t significantly change, but
the relative intensities of the two peaks therein seem to inverse. In the case of C15A
0.25MC RhP (Figure III-F8d), the photo-functionalization followed by washing acts
also as a purifying procedure since the inorganic-organic complex shows, in comparison
with the pristine commercial clay, a signature in which the features typical of clays (i.e.
the absorption bands assigned to Si-O-Si and to the inorganic crystalline lattice) are
7
8
The FTIR spectroscopy technique has been briefly introduced in § II-3.4.
See § II-6.3.
168
better recognizable. This is not surprising, as we previously showed by XRD (Figure
III-F1d), TGA (Figure III-F2d) and EA (Table III-T2) that C15A is the organoclay with
the highest excess of surfactant in its pristine commercial state. Of course, the FTIR
spectrum collected for CNa+ (Figure III-F8a) is the one in which the effects of the
photo-functionalization are the most evident – one can even detect the presence of weak
absorption bands due to RhP around 3000 cm-1, which can’t be seen in the spectra of the
photo-functionalized organoclays because of the double set of peaks assigned to the
long linear aliphatic chains.
Figure III-F8 FTIR spectra of the washed photo-active inorganic-organic complexes in
KBr pellets: (a) CNa+ 0.25CEC RhP, (b) C30B 0.25MC RhP, (c) C10A 0.25MC RhP and
(d) C15A 0.25MC RhP.
As already stressed in Chapter I, fluorescent molecules are fickle but versatile
tracers, sensitive to environmental changes concerning chemical composition of the host
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Chapter III
matrix, atomic arrangement and molecular configuration of the dye molecules, physical
confinement, temperature, etc. and rhodamines are known to be a probe well adapted to
study heterogeneous systems, thanks to the strong dependence of their absorption and
emission spectra on the host material. Indeed, apart from a change in the chemical
composition of the environment, the presence of a host matrix may produce a physical
confinement of the dye molecules and induce the formation of a particular type of
aggregates (monomers, dimers, H-aggregates or J-aggregates) [5].
Figure III-F9 Spectrofluorimetry characterizations (absorption and emission spectra) of
the washed photo-functionalized inorganic-organic complexes in ethanol: (a) CNa+
0.25CEC RhP, (b) C30B 0.25MC RhP, (c) C10A 0.25MC RhP and (d) C15A 0.25MC
RhP. Thin lines represent the spectra of pure RhP. Prior to perform spectrofluorimetry,
samples have been carefully washed with ethanol in order to detect only the response of
intercalated dye molecules.
Spectrofluorimetry is threefold useful to characterize photo-functional inorganicorganic complexes, because:
170
 absorption spectra can inform on the configuration and the supramolecular
arrangement eventually induced by the host inorganic structure on the guest organic
moieties (fluorophore-fluorophore interactions);
 fluorescence emission spectra are good revealers of host-guest interactions
(which can be expected to produce some shift or reshape of the emission band);
 fluorescence calibration (intensity of fluorescence emission vs. concentration)
may help understanding which kind of interactions, whether fluorophore-fluorophore or
host-guest, are prevalent (a balance on the side of the latter could mean that there are
less fluorophore-fluorophore interactions, thus preventing fluorescence quenching even
at the highest concentrations).
Figure III-F9 clearly proves that all the inorganic-organic complexes processed
with the described method are photo-active, meaning that photo-functionalization was
successful for all the selected commercial clays. The shape of RhP absorption and
emission bands doesn’t significantly change upon intercalation of the cations into clay
galleries, with the exception of C10A inorganic-organic complexes (Figure III-F9c).
Both the absorption and emission spectra of C10A 0.25MC RhP show a slight
hypsochromic shift with respect to pure RhP, and the absorption spectrum of the photofunctional complex doesn’t contain any peak indicating the presence of non-fluorescent
H-aggregates – contrarily to pure RhP, whose molecules have a strong tendency to form
aggregates, as previously reported and commented [1]. The specificity of C10A
0.25MC RhP complexes may be due to the chemistry of C10A surfactant (2MBHT,
Table III-T1). 2MBHT contains a benzyl group, which could favorably interact with the
polyaromatic heterocyclic structure of RhP cations and favor the absorption of RhP in
form of monomers rather than dimers or higher-order aggregates. The fact that the
surfactant bears an aromatic group could slightly modify the fluorescence mechanisms
also because fluorescence phenomena are characteristically associated to the presence of
aromatic compounds. However, such an investigation of the fluorescence mechanisms
(absorption of photons and relaxation phenomena) would require the utilization of more
specific and advanced analytical tools, as well as a deeper knowledge and some skills
that the authors haven’t yet developed. Moreover, a detailed investigation of the physics
responsible for the optical behavior (absorption of radiation and fluorescence emission)
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Chapter III
of the photo-functional inorganic-organic complexes presented in the previous and the
present chapters would be out of target with respect to the main objective of the work,
i.e. monitoring distributive and dispersive flow in a transparent screw/barrel system
(Visiovis) during polymer-clay melt compounding.
III-6
CONCLUSIONS
The goal of the work presented in this chapter was to obtain some other photoactive inorganic-organic complexes from commercially available organoclays (Cloisite
® 10A and Cloisite ® 15A) and one natural (unmodified) clay purchased from the same
provider (Cloisite ® Na+), using the same photo-functionalization protocol established
in the previous chapter and already (successfully) used to prepare photo-active Cloisite
® 30B: cation exchange process with 0.25MC (or 0.25CEC, in the case of natural clay)
RhP. The main application we envisioned for these additional photo-active inorganicorganic complexes is always the same (being also the reason why we started looking for
a photo-functionalization protocol): the real-time process monitoring of polymer-clay
nanocomposites by Visiovis, the equipment we are developing in our laboratory and
which is accurately described in Chapter IV [9]. However, there are several potential
applications for this class of photo-functional inorganic-organic complexes: in addition
to a diversified and extensive use for real-time monitoring of any other process for claybased nanocomposites (we discussed the potentialities of fluorescence techniques in
Chapter I), these photo-active clay minerals could be used to produce microlasers (as
Vietze et al. [8] reported about zeolite), to trace pollution in soils, to obtain a new class
of pigments which, exploiting the same advantages of nanocomposites, could have the
same performances of traditional microsized pigments but at much lower contents, etc.
In this chapter we reported and commented the results of the characterizations (XRD,
TGA, EA, FTIR and fluorescence spectroscopy) done on the processed clay minerals, as
well as a rapid comparison of the four photo-functional inorganic-organic complexes
currently available for Visiovis experiments. All the collected experimental evidences
show that our photo-functionalization protocol with RhP seems to be well adapted to
any clay mineral, whether organically-modified or not, and whatever the nature and the
amount of the pre-existing surfactant (if present). The comparison of the photo-active
172
Cloisite ® Na+ with the photo-active organoclays witnesses the difficulties encountered
to characterize the photo-functional inorganic-organic complexes when some organic
surfactant is already present into clay galleries – especially if its excess is considerable,
as in the case of Cloisite ® 15A. Indeed, even if the interactions of clay minerals with
organic cationic dyes (and, in particular, the adsorption of the dye molecules into clay
galleries by cation exchange process) represent a topic developed in the literature since
a long time (see Chapter I for a hint of state of the art), few studies have dealt with the
interactions of dyes with clay minerals already modified by some other organic species.
Nonetheless, the behavior of organoclays is attracting more and more attention by the
industry and, now more than ever, the possibility of tracing organoclays and monitoring
their behavior appear of great interest.
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Chapter III
III-R
[1]
REFERENCES
Esposito A, Raccurt O., Charmeau JY, Duchet-Rumeau J. Photo-functionalization of an
organically-modified clay. J. Colloid and Interface Sci. Submitted for publication, 2008.
[2]
Madejová J., Komadel P. Baseline studies of the Clay Minerals Society source clays:
infrared methods. Clays Clay Miner. 2001, 49 (5), 410-432.
[3]
Madejová J. FTIR techniques in clay mineral studies. Vibrat. Spectr. 2003, 31, 1-10.
[4]
Coates J. Interpretation of infrared spectra, A practical approach, in Encyclopedia of
Analytical Chemistry, R.A. Meyers (Ed.), Chichester: John Wiley & Sons Ltd, 2000, pp.
10815-10837.
[5]
Čapková P., Malý P., Pospíšil M. et al. Effect of surface and interlayer structure on the
Interface Sci. 2004, 277, 128-137.
[6]
Fujita T., Iyi N., Kosugi T. et al. Intercalation characteristics of rhodamine 6G in fluortaeniolite: orientation in the gallery. Clays Clay Miner. 1997, 45 (1), 77-84.
[7]
Paul D.R., Zeng Q.H., Yu A.B. et al. The interlayer swelling and molecular packing in
organoclays. J. Colloid Interface Sci. 2005, 292, 462-468.
[8]
Vietze U., Krauß O., Laeri F. et al. Zeolite-dye microlasers. Phys. Rev. Lett. 98, 81 (21),
4628-4631.
[9]
Esposito A, Balcaen J, Duchet-Rumeau J, Charmeau JY. Visiovis: monitoring nanofiller
174
Chapter IV
PROCESSING
The experimental device presented in this chapter has been recently overviewed
in an article came out in June 2008 (JEC Composites Magazine n°41) [1].
Polymer-clay nanocomposites became truly marketable only some time after the
registration of the first industrial patents [2]: the ability to properly and reproducibly
disperse nanofillers in a polymer was, and still is, considered to be a strict requisite for
their profitable commercialization. Although inorganic moieties are generally required
to perfectly disaggregate, disperse and distribute into the organic matrix, clays are often
affected by distribution and/or dispersion problems of their elementary and primary
particles [3]. We’ve already insisted on the fact that a method capable of monitoring the
evolution of the morphology during processing would greatly help the development of
polymer-clay nanocomposites. Indeed, as previously said (§ I-2.1), if filler aggregation
depends essentially on the physico-chemical interactions of its particles, filler dispersion
and/or distribution into the matrix is directly influenced by processing efficiency. The
morphology of polymer composite materials changes throughout the manufacturing
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Chapter IV
process but seems primarily influenced, in the case of thermoplastic matrices, by the
extrusion of formulated composite pellets and the injection molding of final objects.
Such steps of the manufacturing chains are often performed by means of devices which
have a complex geometry and, thus, are particularly difficult to monitor.
In this chapter we’ll present a novel and innovating equipment entirely designed,
assembled and developed in our laboratories over a period of about 5 years – including
a little more than two years of a previous PhD project (Maël Moguedet) and the whole
duration of three years of the following PhD project (Antonella Esposito). As suggested
by the name itself, Visiovis is a tool exclusively devoted to processing visualization.
Why processing? Because it consists essentially of a screw/barrel system (French vis =
English screw) in which – and this is the real innovation – the barrel is entirely made of
a transparent material, which permits to continuously visualize (visio-) the flow from all
the possible directions of observation. This property encouraged the exploration of all
the potentialities of such a novel tool – and that’s the topic of this chapter. In particular,
we slightly changed its original configuration to adapt it to the real-time investigation of
mixing between a polymer matrix and a lamellar inorganic filler and, thus, to in-line
monitoring of nanocomposite processing which still is, as previously observed, the
crucial factor for polymer-clay nanocomposites. The photo-functional inorganic/organic
complexes (which are the topic of Chapters II and III) have been prepared expressly to
be used as photo-active lamellar fillers for Visiovis experiments.
IV-1
VISIOVIS
Visiovis has been entirely assembled in our laboratories for the first time in 2003.
This tool was originally designed to visualize the 3D trajectories of a single fluorescent
particle plunged in a transparent fluid [4][5]. During these last three years, we further
developed such an original equipment in order to adapt it to the analysis of nanofiller
dispersion/distribution in viscous media (e.g. molten thermoplastic polymers or uncured
thermoset resins) flowing in a geometrically complex system which is comparable, in
the case of Visiovis, to the meter section of typical screw/barrel systems of industrial
devices for extrusion and injection molding.
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IV-1.1
Original configuration
Visiovis is an experimental device designed and assembled by Maël Moguedet
during his PhD at the Ecole Supérieure de Plasturgie in Oyonnax1 (2002-2005) with the
precious help of the technician Jean Balcaen. The “nucleus” of Visiovis is the assembly
composed of a transparent barrel hosting a screw, but the real advantage – essential for
a complete in-line 3D visualization of the speed cartogram of a fluid evolving in the
space in-between a helicoidal screw and its cylindrical envelop – is the transparency of
the barrel. Indeed, transparency is the property which chiefly rendered the Visiovis
project innovative and interesting for in-line monitoring of the processing of polymers
and polymer-based composites.
The total transparency of Visiovis barrel allowed Moguedet and coworkers [4] to
employ – in an absolutely innovative way – a technique which is relatively common in
the field of fluid dynamics. They specifically designed Visiovis with the perspective of
using it for Particle Tracking Velocimetry (PTV), a technique allowing to originate the
speed cartogram by following the discrete movements of few particles plunged in a
fluid over a finite period of time – under the hypothesis of low particle concentration,
which is the condition permitting to follow the movements of each single particle. The
principle of PVT is schematized in Figure IV-F1 [4].
The main objective of Moguedet’s work was to find a new method to visualize
the entire 3D trajectory of a single particle plunged in a fluid and then to compare the
results obtained by Visiovis with the results obtained by computer simulation. Indeed,
computer simulation is very common in fluid dynamics since it is capable of deducing
information generally not accessible via traditional experiments. On the other hand, a
considerable effort has been done by several groups of researchers to find a practical
method capable of collecting “real” data (although concerning model materials on pilot
implantations) to be compared to the “predicted” ones and to be later “extrapolated” to
1
In February 2005 the Ecole Supérieure de Plasturgie (which up to that moment was a private institution)
was integrated by INSA Lyon and, nowadays, the whole center in Oyonnax (laboratories and educational
facilities) represents the Site de Plasturgie INSA – in particular, the laboratory and all the technical staff
and researchers working therein are now labelled Laboratoire des Matériaux Macromoléculaires (LMM).
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the real, industrial systems. In particular, Moguedet and coworkers had been inspired by
some works in which industrial extruders were equipped with one or more windows and
(at least) one camera: they found the idea interesting but underlined the limits of such
systems and looked for a better solution. Acquiring photos and videos on industrial
systems equipped with one or more windows is certainly a good method to visualize the
melting mechanisms of polymer pellets during processing – particularly because such
observations can be directly correlated to other information acquired by conventional
captors for temperature and pressure; however, Moguedet and coworkers were looking
for something more complete (3D rather than 2D), more precise (quantitative rather
than qualitative), more specific (the ultimate aim was to position a particle in the space
and then to reconstruct 3D speed cartograms). Doubtless, that was a first hard challenge.
t1
z
x
tn
Lighting
for tracer
excitation
z
x
y
x
t1
Camera shifting to follow the
particles continuously in time
y
x
tn
Flow direction
Figure IV-F1 Scheme of the principle of PVT technique, chosen by Moguedet and coworkers
to design and develop Visiovis [4].
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IV-1.1.1
Components and utilization
In its original configuration (Figure IV-F2), Visiovis is composed of a squarepitched2 screw adjusted in a transparent barrel made of poly methyl methacrylate
(PMMA), deliberately treated against UV radiation3 and assuring a transparency of
about 80% for
44 rpm, torque
> 380 nm [4]. The screw is actuated by an electrical motor (speed max =
max
= 9 N·m). An aperture allows to fill the system with the fluid of
choice, to introduce the tracing particle and to empty the circuit at the end of the
experiments; a tube connected in close circuit permits to virtually prolong the duration
of each experiment by making the fluid continuously circulate into the system. The
screw/barrel system is surrounded by a mobile framework supporting all the other
components, in particular four CCD cameras (Basler A301F), equipped with yellow
filters4 and able to record up to 80 images/s with a resolution of 640 × 480 pixels and a
depth of 8 bits (256 grey levels). Two of the cameras are aligned horizontally and two
vertically, face-to-face on opposite sides of the screw/barrel system – configuration
which, thanks to the mobility of the framework, allows to follow the tracing particle at
each instant with at least two cameras, meaning that at any time it is possible to deduce
its 3D coordinates. Four UV diodes (emission 400 ± 5 nm) are placed around the
screw/barrel system, assuring a 3D lightening. A calibrator, consisting of two
perpendicularly crossing matrices of perpendicular filaments made of fluorescent nylon
(5mm × 5mm), is fixed at one end of the screw/barrel system and allows to measure and
then adjust any difference in the orientation, translation and/or rotation, zoom and/or
focus of the four cameras. The images are recorded thanks to an acquisition circuit
composed of two PCs and an external clock (to assure the synchronization of the
cameras and to set the acquisition time). Visiovis geometrical parameters [4] are listed
in Table IV-T1 and compared to the typical design parameters of the meter section of
industrial screw/barrel systems for extrusion and injection molding [6] (Figure IV-F3).
2
A screw is square-pitched when its pitch is fairly similar to its external diameter (Table IV-T1).
The choice of PMMA treated against UV radiation is justified by the original external light source (four
UV diodes).
4
The yellow filters cut-off the excitation light coming from the diodes and make the cameras visualize
only the fluorescence emission of the tracing particle.
3
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UV diodes
Electrical
motor
Aperture
PMMA barrel
Calibrator
CCD
camera
Screw flight
Tube for
closed circuit
Mobile
framework
Figure IV-F2 Visiovis in its original configuration – as it was designed, assembled and used by
Moguedet and coworkers [4].
Table IV-T1 Visiovis geometrical parameters [4] compared to the typical design parameters of
the meter section of industrial screw/barrel systems for extrusion and injection molding [6].
Geometrical parameters
Visiovis
Meter section
Barrel diameter
Db
40 mm
120 mm
Screw root diameter
Ds
30 mm
110 mm
40 mm
120 mm
5 mm
5 mm
34.6 mm
109 mm
20°
18.37°
6.9
21.8
Curvature
0.44
0.15
Torsion
0.16
0.05
Tan( )
0.33
0.33
250 mm
n.a.
Screw pitch
Channel depth
Channel width
Screw angle
(measured in the middle of the channel)
Aspect ratio
Screw length
2
Ph
H
W
L
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Ds
H
2
Ph
Db
Figure IV-F3 Schematic of a typical screw/barrel system and the parameters which characterize
its geometry [1].
Moguedet and coworkers [4] chose poly dimethylsiloxane (PDMS) as the model
fluid – a transparent silicone oil having a viscosity of about 100 Pa·s and characterized
by a Newtonian rheological behavior. As a tracing particle, they used a small fragment
of the same filament employed for the calibrator, made of fluorescent nylon (diameter
0.4 mm). This small particle has a density similar to those of the PDMS fluid, which has
allowed to presuppose that the particle was going to perfectly follow the flow lines.
The acquisition of data in Moguedet’s work was simplified by the fact that it was
aimed to the detection of a single point showing the highest luminosity in comparison
with the deep black of the background – and of course this single point corresponded to
the particle of fluorescent nylon. For this reason, Moguedet and coworkers designed an
in-line image processing which noticeably reduced the amount of collected data: once
realized that the fluorescent point representing the detected particle occupied an area of
4×4 pixels on each image acquired by the cameras, they designed a simple software
(C++ environment) capable of acquiring exclusively the image corresponding to an area
of 60×60 pixels centered on the most luminous point visualized by the cameras.
Successively this image, the 3D coordinates of the most luminous point and the date and
time of the acquisition (reduced to a binary code) were recorded in a bitmap format. The
performances of such acquisition system were reduced to the acquisition of 7 images/s
on average. A sample of the data acquired with the Visiovis in its original configuration
is shown in Figure IV-F4.
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Figure IV-F4 Typical format of the data acquired by Moguedet and coworkers [4] with the
Visiovis in its original configuration. This example represents a series of five images, acquired
consecutively by a single camera. One can see, in each picture, the detected fluorescent particle
and (encircled at the bottom) the date and time of acquisition encoded in a binary format.
If the acquisition of data in Moguedet’s work was facilitated by the detection of
the brightest point associated to the single fluorescent particle, the processing of such
data was definitely more complicated. In addition to a double real-time data processing
executed in a C++ environment (selection of the 60×60 pixels area centered around the
brightest point and reduction of the noise intrinsically associated to the CCD cameras),
a labor-intensive processing of the acquired data was necessary to finally reconstruct the
3D trajectories of the single fluorescent particle. This second part of data processing
was realized in the Matlab environment and included image filtering, image adjusting
on the basis of the information given by the calibrator, position adjusting on the basis of
the refraction index of the air (atmosphere), the PMMA (barrel) and the PDMS (model
fluid), as well as any other correction imposed by the optical effects due to the fact that
the barrel represents a cylindrical surface (which interfere with any optical acquisition).
More details about the whole data processing can be found in the PhD manuscript by
Moguedet [4].
IV-1.1.2
Advantages and limitations
As already mentioned, since the beginning the Visiovis project presented a great
interest in the field of fluid dynamics and, indirectly, of polymer processing. However,
Moguedet and coworkers had to cope with several problems before achieving their
objectives. If the filament made of fluorescent nylon solved several of their problems
(individuation of a proper tracing particle and realization of the calibrator for the correct
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alignment and regulation of the cameras), the idea of a totally transparent barrel hosting
a rotating screw rapidly became a problem much more difficult to be solved. Indeed, the
barrel couldn’t have been made of glass because glass stops UV radiation; couldn’t have
been made of some special glass (e.g. quartz) because, no matter how good its optical
properties would be, it’d always have been too fragile to support the radial pressure
originated by the movement of the screw and by the presence of the fluid (moreover, a
barrel entirely made of quartz would have been excessively expensive). They realized
soon that the only cheap and practicable solution was a barrel made of plastic.
PMMA seemed to represent a rapid and cheap solution, primarily thanks to its
transparency – but, of course, PMMA cannot tolerate high temperature (even if the glass
transition temperature of PMMA is around 100°C, any temperature increase may render
the barrel enough malleable to get susceptible of deformation under the action of the
internal pressure). In addition, the fact that the barrel is made of plastic imposes to pay a
particular attention to the chemical compatibility of any fluid introduced into the system
(or any solvent used to wash it) with the material used for the barrel: only inert oils can
be chosen as the model fluid, and only solvents which neither swell nor dissolve PMMA
can be used for cleaning.
By the way, the barrel wasn’t the only source of problems. If a tracing particle
was already available, Moguedet had still to find a suitable model fluid, which should
have had the following properties: (1) being a macromolecular fluid; (2) resisting UV
radiation without degrading; (3) being optically transparent (to UV radiation but also to
visible radiation); (4) being inert and, more specifically, being compatible with PMMA;
(5) being capable of flowing at room temperature (since any experience with Visiovis
had to be realized at room temperature). After having tried (unsuccessfully) to fill the
closed circuit of Visiovis with a solution of water and poly ethylene oxide (PEO), they
finally found a compromise in poly dimethylsiloxane (PDMS), a transparent and inert
silicone oil, commercially available in different molar weight and, thus, different
viscosities. At least, for the purposes of Moguedet’s work, an advantage of the fact that
PDMS is Newtonian is that this property authorized to perform experiments at relatively
low rotational speeds, as Newtonian fluids aren’t sensitive to shear and, consequently,
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their speed cartogram is supposed to stay unchanged even when the screw speed is
increased. The independence of the flow speed profile from the rotational speed induced
Moguedet to choose the slowest speed available for the screw (1 rpm), in order to
optimize the acquisition procedure which, in addition, was slowed down by real-time
data processing5.
Height (mm)
Length (mm)
Width (mm)
Figure IV-F5 Three-dimensional trajectory of a single fluorescent particle in the channel of the
screw/barrel system of Visiovis (screw rotational speed 1 rpm) [4]. The fact that points are
missing for all the positions close to the cylindrical surface of the barrel witnesses the
limitations due to refraction.
The experiments performed with this first version of Visiovis have been positive
in terms of concretization of the project and demonstration that this original tool works
as expected. These first measurements could produce the approximate 3D trajectory of a
particle plunged in a transparent Newtonian fluid evolving into the screw/barrel system
(Figure IV-F5). In particular, they revealed the presence of two speed comportments –
slow close to the barrel surface and rapid close to the screw surface. These observations,
along with the trajectory of the particle, have been successfully confirmed by computer
5
Some information about the real-time data processing during acquisition can be found in § IV-1.1.1. For
more details, please refer to Moguedet’s work [4].
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simulations (Figure IV-F6). More in detail, a particle introduced into the system and
starting its progression close to the axis of the system, proceeds rapidly and then moves
towards the surface of the barrel, thus its progression slows down [4]. By the way, the
system still presents several optical problems (once more due to the refraction at the
cylindrical surface of the barrel – coupled to the fact that the four UV diodes create an
isotropic-like lightening which is prone to reflection and diffraction phenomena). These
residual optical problems caused an intensive loss of observed data any time that the
particle got close to the surface of the barrel. The first results obtained by Visiovis and
confirmed by computer simulations have been published in 2004 [7] and the same group
of researchers actually keeps developing the initial model [8].
Experimental radial position
Simulated final radial position
Extrapolation of the simulation
Barrel internal surface
Radial position
(mm)
Screw root surface
Time (s)
Figure IV-F6 Evolution of the radial position of the particle in the channel of the screw/barrel
system of Visiovis (screw rotational speed 1 rpm) [4]. Experimental observations correlate well
with the results of simulation.
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IV-1.2
Evolutions of the configuration
It was Moguedet himself who firstly proposed, at the end of his PhD manuscript
[4], some possible modifications of Visiovis:
 the replacement of the four CCD cameras with some more recent “intelligent
cameras”, capable of performing an automatic image processing for the detection of the
brightest point without employing the resources of the computers’ processors – which
could ameliorate the performances of the acquisition of data;
 the realization of an additional computer-controlled system for the automatic
translation of the mobile framework, so that the cameras could follow the advancement
of the tracing particle – which could allow to follow more than one single particle;
 the replacement of the cylindrical barrel by another barrel with an internal
cylindrical surface and an external rectangular shape – which would significantly reduce
the problems caused by refraction.
Our objective is to adapt Visiovis to the real-time monitoring of the dispersion
and distribution of lamellar fillers in a polymer matrix during nanocomposite
processing. To achieve our objective, we unavoidably had to modify Visiovis original
configuration because of some additional difficulties linked to the multiplicity of the
tracing particles to be followed and to their reduced dimensions, as well. Anyway, we
haven’t necessarily followed the lines suggested by Moguedet and coworkers – meaning
that many other possibilities of evolution, ameliorations and diversification are still left.
In fact, we passed through several changes of the configuration, trying to adapt step-bystep this tool to our objectives. The easiest and cheapest changes we could attempt on
Visiovis concerned: (1) the position of the CCD cameras, (2) the form of the lightening
for fluorescence excitation and (3) the addition of the most sensitive instrument for the
characterization of the fluorescence behavior of the photo-functional fillers previously
described6 – a spectrofluorimetry relied to the system by an optical fiber.
6
See Chapter II for more details about the photo-functionalization of lamellar mineral fillers and Chapter
III for the description of the photo-functional inorganic/organic complexes realized with the perspective
of using them with Visiovis.
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IV-1.2.1 From 3D lighting to 2D laser plan
At the real beginning of our experimental work – even before the development of
a photo-functionalization method for lamellar fillers – we were quite determined to keep
Visiovis in its original configuration, thus we tried to find a suitable photoluminescent
nanofiller to continue investigating the flow behavior of the same Newtonian matrix
(PDMS) filled with several particle (not just a single tracing particle) evolving in the
same geometrical system (i.e. without modifying the screw profile, since we already had
some information about the existing one).
Indeed, we found a rather good solution for our problems: a cheap, commercially
available phosphorescent pigment (GT5700, GloTech Inc., New Zealand) consisting of
an alkaline rare-earth aluminate, easily excitable by UV radiation, white or any visible
light (240-440 nm), emitting in the yellow/green range (520 nm), with an intense and
persistent emission (>12 hours, certified by the provider on the basis of the measuring
protocol DIN 67510). Two images of this pigment taken by light microscopy are given
in Figure IV-F7 and their excitation and phosphorescence spectra are shown in Figure
IV-F8 (data provided by GloTech Inc.).
100 m
100 m
Figure IV-F7 Two images of GT5700 particles taken by light microscopy (magnification 20x).
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Figure IV-F8 Absorption and phosphorescence spectra of the pigment GT5700.
GT5700 chemical, physical and luminescent properties are resumed in Table IVT2. This photoluminescent pigment is employed for many applications, including brush
painting, spray painting, candle making and glass moulding.
Table IV-T2 Main chemical, physical and luminescent properties of GT5700 pigment.
Chemical properties
Composition Alkaline Rare-Earth Aluminate
Insoluble in Water, Alkalis and Organic Solvents
Decomposition Acids
Physical properties
Appearance
Specific Gravity
Particle Size Distribution
(Laser Granularity)
Yellowish
3.6 g/cm3
45-55 m
(D50)
Luminescent properties
Excitation
Excitation Wavelength
Peak Value
Glow Color
Glow Duration
UV radiation, white or any visible light
240-440 nm
520 nm
Yellow-Green
> 12 hours
We performed a test of visualization of the GT5700 pigment by Visiovis and we
actually got some interesting results: the intense brightness due to the phosphorescence
emission is effectively sufficient to visualize the particles with the CDD cameras. We
could easily distinguish the dark zones (pure PDMS) from the zones containing the
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phosphorescent particles. In addition, we could clearly visualize any difference in the
spatial distribution of the particles during mixing, as shown by Figure IV-F9.
barrel
differences
in focus
screw flight
pure PDMS
GT5700
particles
barrel
barrel
Figure IV-F9 Some examples of the test of visualization of the GT5700 pigment by Visiovis.
Isotropic-like lightening and excitation by the four UV diodes.
However, the test of visualization of the GT5700 pigment highlighted once more
the great limitations of the system in its actual configuration and of the chosen pigment,
as well. As previously reported, Moguedet and coworkers [4] were aware of the optical
problems created by the cylindrical PMMA barrel and by the isotropic-like lighting of
the four diodes. Their acquisitions and measurement greatly suffered from something
they mainly recognized as a refraction phenomenon. The images collected during our
test of visualization of the GT5700 pigment (Figure IV-F9) clearly show that refraction
isn’t the only optical phenomenon affecting Visiovis in its original configuration: most
of the visualization problems come from a strong reflection of the light by the surface of
the barrel – to a point that no particle can be visualized in the upper and lower portions
of the screw/barrel system, i.e. in the channel sections perpendicular to the cameras.
Moreover, as the particles are phosphorescent, once they’re excited they all emit at the
same time, in any point of the visualized portion of the screw/barrel system: this means
that the cameras should properly (viz. correctly focused) visualize all the particles, even
if they’re placed on different focalization plans. This is obviously impossible, as shown
by the differences in focus visible on the examples in Figure IV-F9.
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The same images shown in Figure IV-F9 are presented again in Figure IV-F10,
but in their negative version: the negative version of such images, in which the only
contrast is given by a bright phosphorescence emission on a dark background, greatly
helps the evaluation of the visualization limits of Visiovis in its original configuration.
Two other negative images are added to support the observations done on the basis of
the first two images. It is unambiguous that the zones close to the barrel surface are
“critical” for visualization – besides, this area is probably the most interesting, since it
could help understanding the dependency of the particle distribution on the radial
position, i.e. on the distance from the axis of the screw/barrel system (relative distance
from the barrel surface and/or from the screw root surface).
barrel
differences
in focus
screw flight
pure PDMS
GT5700
particles
barrel
barrel
barrel “critical”
zones
Figure IV-F10 Some examples of the test of visualization of the GT5700 pigment by Visiovis.
The negative version greatly helps the evaluation of the test images. The first two images (upper
side) are the same images already shown in Figure IV-F9.
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Taking into account the problems encountered by Moguedet and coworkers in
relation with isotropic-like lightening and the refraction phenomena at the cylindrical
surface of the barrel, and being aware of the fact that changing the shape of the barrel
would have required a deep reorganization of Visiovis configuration, we estimated that
the first thing to do was rather to modify the lightening system. Therefore, we planned
to fabricate an optical system which, coupled to a laser source, would create a thin sheet
of light – a virtual, optical section of Visiovis screw/barrel system. In the presence of
such a bidimensional lightening, the fact that GT5700 pigment is phosphorescent not
only has no more interest, but appear even inappropriate: indeed, the main interest of a
2D lightening is the possibility of visualizing exclusively the particles included in the
thin sheet of light – which requires that the particles are excited by the laser sheet and
produce a fluorescent response exclusively when excited, whereas the glow duration of
the phosphorescent pigment means that the particles emit even if they aren’t anymore
excited.
Although GT5700 pigment has good performances when visualized by Visiovis,
abandoning its usage hasn’t been too much disappointing for several reasons:
 the pigment isn’t a lamellar nanofiller – the particles have an almost unitary
shape factor, which means that they are spherical (see the D50 value in Table IV-T2);
 the pigment has a density (3.6 g/cm3, Table IV-T2) which is inadequate for
the PDMS (0.97 g/cm3) – a filler which is much denser than the matrix is more prone to
sedimentation by gravity;
 the pigment is phosphorescent – a property which, as previously explained, is
no more appropriate for the lightening system we were planning to assemble.
The new lightening system will have to be chosen on the basis of several criteria:
(1) the commercial availability and the price of the laser source; (2) the wavelength of
laser emission – for the optimum excitation of the photo-functionalized filler; (3) the
fact that the PMMA barrel must be transparent (but also resistant) to the light emitted by
the laser. More details about the new lighting system will be given in § IV-1.3.
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IV-1.2.2
Position of the CCD cameras
We’ve already highlighted that the zones close to the barrel surface are “critical”
and may be particularly interesting for the visualization of particle distribution during
mixing. We’ve also stressed, when presenting Visiovis and its geometrical parameters,
that our screw/barrel system may be compared to the meter section of the industrial
screw/barrel systems for extrusion and injection molding – thanks to the fact that most
industrial devices has a meter zone characterized by a square-pitched screw and a
shallow channel (see Figure IV-F3 for a schematic representation of the meter section of
a screw/barrel system, as well as Table IV-T1 for a comparison of Visiovis geometrical
parameters with the typical design values of the meter section of industrial devices).
Once understood that the screw profile adjusted in our transparent barrel has a shallow
channel (5mm deep), one realizes how much important is an accurate visualization of
the zones close to the barrel surface. The choice of a new lighting system, consisting of
a thin laser sheet creating an optical slice of the system, is absolutely coherent with this
new criterion of visualization. By making the laser sheet pass exactly by the axis of the
screw/barrel system, we’re planning to visualize the longitudinal section of the channel,
in which the fluid and its filler are supposed to mix up (see Figure IV-F11).
laser sheet
Figure IV-F11 Schematic representation of a laser sheet passing by the axis of the screw/barrel
system of Visiovis. Only the particles lying on the optical plan lightened by the laser sheet are
excited and have a detectable fluorescence emission (similarly to PIV, see Figure I-F18).
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It is obvious that, in such a configuration, the camera placed on the same side of
the laser source and the one placed on the opposite side (viz. the cameras lying onto the
optical plan created by the laser sheet) become totally useless. Only one camera – those
placed on the plan perpendicular to the laser sheet – continues accomplishing its task of
visualization (the camera on the opposite side being useless as well, because the data
collected by this second, perpendicular camera would be redundant). In other words, the
fact that we changed the lightening system forced us to change the position of the CCD
cameras, as well. Since the only interesting position for visualization was, at the present,
the plan perpendicular to those traced by the laser sheet, we estimated that the best thing
to do was to align the four cameras axially, alongside the screw/barrel system, right in
face of the visualized longitudinal section of Visiovis channel (Figure IV-F12).
laser source
mobile framework
cameras
laser sheet
Figure IV-F12 Position of the CCD cameras in relation to the laser sheet passing by the axis of
the screw/barrel system of Visiovis.
IV-1.2.3
Optical fiber and in-line spectrofluorimetry
The first test of visualization performed with the luminescent GT5700 pigment
and the original configuration of Visiovis allowed us to estimate the accuracy of our
system – limits that for the moment we could not ameliorate, since we weren’t planning
to change either the CCD cameras or the screw/barrel system itself. Thanks to the
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images acquired during the test of visualization, we calculated that the CCD cameras
can detect approximately 83
m per pixel – a sensitivity which could be probably
acceptable for the phosphorescent pigment (D50 = 45-55 m as reported in Table IV-T2)
but surely not enough for lamellar fillers, which have a multiscale structure (Figure IF23) with a minimum dimension of 1 × 100 nm (isolated clay platelets) and a maximum
dimension of about 10
m. More specifically, the typical dry particle size of the
commercial clays selected to perform photo-functionalization is described by the
following distribution7: 10% of the particles measure less than 2 m, 50% less than 6
m and 90% less than 13 m. It is evident that the sole cameras aren’t adequate and
anymore sufficient for a proper visualization of the complex phenomena occurring
during nanocomposite processing. This is the reason why, in addition to the visual
detection performed by the CCD cameras, we decided to equip Visiovis with another
detection system – a spectrometer connected to an optical fiber probe able to collect the
intensity of fluorescence emission during polymer/clay mixing. Spectrofluorimetry is a
technique sensitive to phenomena occurring at a different scale in comparison with the
CCD cameras – the latter performing a global in-line monitoring of mixing, the former
providing more specific and space-restrained information. The main advantage of this
additional technique is that fluorescence is extremely sensitive to several properties of
the environment in which the tracing molecule is positioned – including temperature,
pH, chemical composition, molecular arrangement and physical confinement – as
previously stressed in Chapters II and III.
Of course, we had to check the visualization limits and the sensitivity of both the
CCD cameras and the spectrometer with the photo-active complexes previously
prepared by photo-functionalization of commercial clays. These tests of visualization
will also be useful to calibrate both the detection systems (cameras and spectrometer)
for any future experience with Visiovis. The results of calibration will be discussed in
the following paragraph.
7
These values are provided by the supplier. Percentages are expressed by volume.
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IV-1.2.4
Calibration of the detection systems
Before performing any significant experience with Visiovis, we had to assure a
satisfactory calibration of the old and new detection systems with the photo-active
lamellar fillers prepared ad hoc for our visualization tool. The best calibration would be
enough accurate to allow a direct correlation between the concentration of fluorescent
molecule and the luminosity detected for each pixel of the CCD cameras, or the
intensity of the fluorescence emission detected by the spectrometer. Such a scheme of
correlation would allow to obtain a real-time concentration cartogram of the optical
sections created by the laser sheet longitudinally with respect to the screw/barrel
system. Unluckily, such a fortunate calibration would require many measurements and
would surely be complicated for the following reasons: (1) when penetrating the fluid,
the intensity of the planar laser sheet decreases (the higher the concentration of tracer,
the quicker it decreases); (2) the initial concentration of the injected masterbatch is well
known, but unfortunately there’s no way to predict its spatial and temporal evolution
during mixing; (3) we haven’t yet designed an efficient method to get some samples of
the fluid evolving in the screw/barrel system, in order to confirm by some other
technique the results obtained by the cameras and the spectrometer.
Thus, for the moment we could only perform a kind of “qualitative calibration”
of the cameras and of the spectrometer with the first photo-functional inorganic/organic
complex prepared by cation exchange process: the photo-active filler based on Cloisite
® 30B (C30B 0.25MC RhP)8. Even though any calibration is quantitative by nature, we
hazarded called it “qualitative” simply because, in our case, we performed calibration
just to understand whether the photo-active fillers are detected or not, and which is the
optimum concentration to be used for any future experience made on Visiovis.
The calibration of the detection systems required the preparation of a certain
amount of mixtures of the photo-active lamellar filler with the transparent model fluid.
We prepared 11 mixtures having a controlled concentration (0%, 0.001%, 0.0025%,
0.005%, 0.01%, 0.025%, 0.05%, 0.1%, 0.25%, 0.5%, 1%, where percentages are to be
8
More information about the preparation of this photo-active lamellar filler can be found in Chapter II.
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intended by weight) starting from an initial 1% wt mixture and proceeding by dilution.
The initial mixture has been prepared by a mixer equipped with a 60 mm
dilacerator
disk – which is supposed to facilitate mixing by breaking the eventual aggregates and
better dispersing the filler into the matrix – rotating at 1000 rpm for 20 min (Disperser
TurboTest Rayneri 33/300P). We mixed the 10 Pa·s PDMS9 (Siliconöl M10000, Carl
Roth, Germany) with a proper amount of the photo-active lamellar filler C30B 0.25MC
RhP10. These controlled mixtures have then been poured in 4.5 mL disposable PMMA
cuvettes11 (Rotilabo ® Plastibrand Elumal-Küvetten) specific for spectrofluorimetry,
assuring a perfect optical permeability (wavelength range 300-900 nm, std dev ≤ 0.004
extinction units starting from 320 nm). All the mixtures are shown in Figure IV-F13.
Figure IV-F13 Controlled-concentration mixtures (C30B 0.25MC RhP in PDMS) prepared for
the “qualitative calibration” of Visiovis detection systems.
9
We used this silicone oil for all our experiences with Visiovis. The reasons why we chose a 10 Pa·s
PDMS instead of a 100 Pa·s PDMS (as Moguedet and coworkers did [4]) deal only with practical aspects
and will be given in the following.
10
We decided to firstly focus on C30B 0.25MC RhP because it was the first photo-active lamellar filler
ready to be used and, parenthetically, it showed a good photo-activity since the first tests of visualization.
11
We purposely chose such special cuvettes since they are made of the same material as Visiovis barrel.
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The calibration procedure for the spectrometer essentially consisted in recording
a fluorescence emission spectrum for each mixture (Figure IV-F14) and the calibration
of the CCD cameras in taking an image of each cuvette (Figure IV-F15).
Figure IV-F14 “Qualitative calibration” of the spectrometer connected to the optical fibre: test
performed with 11 mixtures (C30B 0.25MC RhP in PDMS) having different concentrations
(from 0% to 1%). Concentration is expressed in percentages by weight. Integration time 3s.
The results of the calibration for the spectrometer (fluorescence emission spectra,
Figure IV-F14) are unsurprising and, somehow, reassuring. Pure PDMS isn’t excited by
the laser sheet and doesn’t produce any parasite fluorescence phenomenon, as expected.
A concentration of 0.001% wt of C30B 0.25MC RhP doesn’t show any significant
fluorescence response, as well. This could seem weird if compared to the extremely low
concentrations of fluorescent dye used for traditional tracing experiments (only some
parts per million); indeed, we should keep in mind that Visiovis experiences are quite
different from traditional tracing experiments, since:
 we’re not using a pure fluorescent dye to trace an homogeneous fluid in which
the dye is perfectly soluble – we’re rather introducing a photo-functional lamellar filler
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in a macromolecular fluid, we’re not sure that mixing will be successful and to which
extent and, in any case, the fluorescent molecules are supposed to be intercalated into
clay galleries, thus clay platelets could engender a barrier effect for fluorescence
excitation and emission, analogously to what happen during thermal degradation or gas
permeation;
 the device we coupled the spectrometer to is pioneering – we’re not using a
fluorescence microscope or any other commercially available equipment to observe the
fluorescence behavior of the photo-active lamellar filler in its environment, thus we risk
to have to cope with some additional optical limitations probably unknown to the people
who perform traditional tracing experiments.
As the concentration reaches a value of 0.0025% wt of C30B 0.25MC RhP, we
observe a first slight fluorescence response around 600 nm – a value of wavelength
which is not exactly the same observed for pure RhP (553 nm, Figure II-F8b) and for
the photo-functional complex (553 nm, Figure II-F11b) in ethanol. This shift may be
due to the medium in which the spectra have been collected (PDMS for the calibration
and ethanol for the other characterizations) but also to the global environment in which
the measurements have been performed, viz. to the fact that the spectra in Figure IV-F14
have been recorded by a spectrometer and collected by an optical fiber placed in front of
the PMMA tub, whereas the spectra in Figure II-F8b and II-FB11b have been recorded
by a commercial spectrometer in standard measurement conditions.
For the values comprised between 0.01 and 0.1% wt, the fluorescence response
seems to be perfectly proportional to the concentration of C30B 0.25MC RhP. Starting
from the concentration value equal to 0.1% wt, a saturation-like phenomenon seems to
occur and the intensity of the main emission peak stop increasing (on the other hand, it
doesn’t start decreasing either, as usually observed in case of fluorescence saturation).
The results obtained for the calibration of the CCD cameras (images taken in the
dark, planar laser source irradiating from top to bottom Figure IV-F15) could a priori
confirm the results obtained for the calibration of the spectrometer, but the eventuality
of obtaining different (i.e. complementary and/or less precise) results shouldn’t be
surprising since, as previously explained, the two detection systems aren’t sensitive to
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the same phenomena and cover two different length scales (macro-scale global view for
the CCD cameras, micro- and/or nano-scale local view for the spectrometer). First of
all, the images confirm that pure PDMS isn’t excited by the laser sheet and doesn’t
produce any parasite fluorescence phenomenon. On the other hand and unsurprisingly,
the CCD cameras look less sensitive to fluorescence emission than the spectrometer –
observation which justifies our choice of adding another detection system to Visiovis12.
Indeed, nothing appears on the collected images until the concentration reaches a value
of at least 0.01% wt, afterward the quality of the acquired images gets better and better
up to a concentration value of 0.1-0.25% wt. When the concentration gets higher than
0.25% wt, the local saturation of the fluorescence emission causes a remarkable
degradation of the image quality – as well as a considerable loss of data, particularly
evident for the highest concentration value (1% wt). Besides, another optical problem
arises when the concentration is too high: the mixture becomes less and less transparent
and the penetration depth of the laser sheet rapidly decreases: a further good reason to
be cautious about concentration issues. It’s worthy underline that the optimum
excitation requires the planar laser source to penetrate to a depth at least equal to the
maximum flight depth: as the channel of Visiovis profile is constant and swallow like
most meter sections (5 mm), this requirement is easily fulfilled (Figure IV-F15). This
constraint would have been certainly stricter if Visiovis had a profile similar to the feed
section (constant but deeper channel) or to the transition section (variable channel
depth)13 – one more reason to be careful about acquiring and interpreting data on the
very first portion of Visiovis – containing the connection between the transition and the
meter sections. Indeed, this portion is primarily concerned by concentration and laser
penetration issues: the highest concentration (the lowest laser penetration depth) is
observed right after the injection of the tracing masterbatch, viz. where screw channel is
variable (and surely deeper).
12
13
See § IV-1.2.3.
The typical screw profile and its different sections are described in Chapter I (see Figure I-F2).
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L
A
S
E
R
pure PDMS
45 mm
0.001 %
0.0025 %
0.005 %
0.01 %
0.025 %
0.05 %
0.1 %
0.25 %
0.5 %
L
A
S
E
R
1%
L
A
S
E
R
critical
laser
depth
Figure IV-F15 “Qualitative calibration” of the CCD cameras: test performed with 11 mixtures
(C30B 0.25MC RhP in PDMS) having different concentrations (from 0% to 1%). Laser source:
from top to bottom. Percentages are expressed by weight.
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In conclusion, the choice of the correct concentration value of the photo-active
lamellar filler to be mixed with PDMS by Visiovis is quite complicated and must be
done on the basis of several parameters – but typically the main criteria for choosing are
the detection limits of the CCD cameras and of the spectrometer, as well as the laser
penetration depth. An acceptable compromise must be found between an insufficient
concentration for the visualization by the CCD cameras and an excessive concentration
which would cause a saturation of the fluorescence emission and, therefore, a loss of
information. The best compromise would be to accept that the data recorded by the
spectrometer are slightly under their optimum of detection so that the images are
acquired by the cameras in their optimum conditions of detection. Therefore, the best is
to assure a concentration of about 0.05-0.1% wt. By the way, as previously underlined,
concentration issues about Visiovis are always more complicated than expected: indeed,
even if the initial concentration of the injected masterbatch is well known, it is
impossible to predict its spatial and temporal evolution during mixing: one can only
predict that, after the injection of a tracing masterbatch into the pure PDMS used to fill
the closed circuit of Visiovis screw/barrel system, mixing will be accompanied by a
dilution of the initial concentration – but such dilution won’t be constant in time and,
even worse, it will have a complicated spatial dependence reflecting the mixing
efficiency of the system. This means that: (1) injecting an initial masterbatch which is
too concentrated will surely hinder the visual detection by the CCD cameras at the very
first moments of the experience because of luminosity saturation, as shown in Figure
IV-F15 (whereas the fluorescence detection won’t be affected because Figure IV-F14
doesn’t show any decrease of the fluorescence emission intensity, but rather a plateau at
the highest concentration values); (2) injecting an initial masterbatch which isn’t enough
concentrated will almost certainly make the fluorescence detection less clear (the
emission peak won’t be at its maximum intensity, as shown in Figure IV-F14) and, as a
consequence of the dilution, will force us to stop the experience sooner than in the
previous case (the concentration would drop sooner under the detection limits), but at
least the absence of an initial luminosity saturation (Figure IV-F15) would allow us to
collect all the visual data by the cameras.
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On the basis of some early tests of visualization with the photo-active lamellar
fillers, we observed that the efficiency of visualization by Visiovis seemed to decrease
with the degree of homogeneity of the mixture evolving into the screw/barrel system –
in other words, the most interesting moments of any experience performed by Visiovis
seem to be the very first ones. For such reason, we judged necessary to assure a correct
visualization of the first part of any future experience and, after several trials, we found
that the best compromise is to inject (in pure PDMS) 10 mL of an initial masterbatch
having a concentration of photo-active lamellar filler of 0.25% wt. More details about
the experimental protocol will be given in § IV-2.
IV-1.3
Actual configuration
After the modifications we operated, Visiovis consists of some old components
in their previous configuration, some old components in a new configuration and some
new components. The screw/barrel system (which can be considered as the “nucleus” of
Visiovis) is the main old component kept in its original configuration. Analogously, the
electrical motor, the aperture (to introduce the fluid and the tracers), the tube (for close
circuit) and the mobile framework – all these components stay unchanged in their initial
configuration. On the other hand, the CCD cameras are now disposed differently on the
mobile framework (they’re aligned axially, alongside the screw/barrel system)14, their
yellow filters have been replaced by new filters (better adapted to the new light source),
the diodes have been substituted by a green laser source ( = 532 nm, nominal power 20
mW CW15, Figure IV-F16 (a) and (b)), the acquisition circuit has been simplified (no
more external clock and D-latch memories). The new components are: an optical system
which creates, from the linear laser source, a bidimensional laser sheet (Figure IV-F16
(c) and (d)); an electromechanical output transducer (a trigger, basically a switch),
which couples the image acquisition to the screw rotation (the cameras are controlled by
the movement, since they acquire one image per screw revolution); the spectrometer
(USB2000+ miniature, Oceanoptics), interfaced to the screw/barrel system by an optical
14
15
More details and the reasons for such change of configuration are available in § IV-1.2.2.
Continuous Wave.
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fiber (600 m
with a resolution of 2.5 nm) positioned in front of the channel of the
screw/barrel system, on the opposite side of the cameras and perpendicularly to the laser
sheet; a tap in the middle of the tube for close circuit, facilitating the draining of the
system after each experiment. The calibrator has been simply removed.
(a)
(c)
(b)
(d)
Figure IV-F16 Thin laser sheet ( = 532 nm, nominal power 20 mW CW) passing by the axis
of the screw/barrel system.
A global picture of Visiovis in its actual configuration, accompanied by some
more detailed pictures of its actual components, is shown in Figure IV-F17 [1].
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Laser sheet @ 532 nm
Aperture
Electrical motor
CCD cameras
Screw/barrel system
Tube for
close circuit
Spectrometer
Trigger
Optical fiber
Figure IV-F17 Visiovis in its actual configuration – after the modifications we made to the
lighting system, the change of position of the CCD cameras and the addition of the optical fiber
and the spectrometer [1].
IV-1.3.1
Objectives
Once the modifications made and the detection systems calibrated, we could plan
how to perform experiments on Visiovis in its actual configuration and, in particular, we
could finally consider the following questions:
 Which model materials would it be better to use to perform the visualization
experiences (in other words, which model fluid and which photo-active lamellar filler)?
 How should we prepare the masterbatches to be injected into the screw/barrel
system from the apposite aperture?
 Which method should we use to inject the masterbatches?
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 How will we exploit the experimental data collected by the cameras and the
spectrometer?
 Shouldn’t we conceive a system which would allow us to validate the results
eventually obtained by Visiovis (e.g. sampling and coupling to other characterization
techniques)?
Aware of the multiplicity of problems to be solved and questions to be answered
to, we realized that the objectives with the highest priority were, at present: (1) planning
a correct experimental protocol able to give some interesting results and (2) performing
some early visualization tests on the freshly-reconfigured Visiovis, in order to prepare
the way to the future experiments. With these targets in mind, we tried to answer to as
many questions as possible – anyway, some of them will rather remain a perspective –
and we attempted to suggest some realistic solutions.
IV-2
EXPERIMENTAL PROTOCOL
As previously announced, our optimization vocation starts with finding a first,
realistic and practicable experimental protocol which would allow exploiting the freshly
reconfigured Visiovis to obtain some interesting and – of course – interpretable results.
We’ll firstly describe the experimental protocol used to perform the visualization tests:
the interpretation of the experimental results is an issue to be considered soon after.
With reference to the model materials to be used for the visualization test (model
fluid and photo-active lamellar filler): as we haven’t yet found an appropriate substitute
for the transparent PDMS, we decided to continue using such macromolecular viscous
fluid as the model matrix (Siliconöl M1000016, Carl Roth, Germany); about the photoactive lamellar filler, we decided to perform some rapid visualization tests of the four
16
We used Siliconöl M10000 (10 Pa·s) rather than Siliconöl M100000 (100 Pa·s) because, contrarily to
Moguedet and coworkers [4], the way we were going to use Visiovis required to change the model fluid
after each experiment – whereas Moguedet and coworkers could use the same fluid for longer time. The
need for frequently changing the fluid is a key parameter for the feasibility of a given experimental
protocol: the steps of filling and empting the screw/barrel system are the slowest and the most delicate, as
all the air bubbles must be carefully evacuated before performing any visualization activity – indeed, the
presence of air bubbles significantly affects optical phenomena.
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photo-functional inorganic/organic complexes prepared by cation exchange process 17 of
commercial clays (Cloisite ® Na+, Cloisite ® 30B, Cloisite ® 10A and Cloisite ® 15A)
to be sure that they’re correctly detected by the cameras and the spectrometer. The
results of the visualization test performed on the four photo-functional complexes is a
little bit surprising (not all the photo-active lamellar fillers can be efficiently visualized
by the cameras) but, after all, it shouldn’t astonish that much: as previously commented,
fluorescence is a sensitive but fickle technique, about which one can never be confident
of getting for sure some good results. Indeed, several reasons could explain the fact that
three of the four photo-active lamellar fillers can be perfectly detected (C30B 0.25MC
RhP, C10A 0.25MC RhP and C15A 0.25MC RhP) whereas one cannot (CNa+ 0.25CEC
RhP). A first reason could be the chemical composition of the commercial clays used to
prepare the photo-functional complexes: the photo-active lamellar fillers which can be
easily visualized by both the cameras and the spectrometer are the ones prepared from
organoclays – contrarily to the one which doesn’t show any visible fluorescence, rather
prepared from a natural clay. However, such explication doesn’t really persuade, since
the characterizations of the dry photo-functional inorganic/organic complexes by
spectrofluorimetry didn’t reveal any relevant difference in the fluorescence emission of
the four samples. Another possible reason must be searched, then, in the interactions of
the photo-active lamellar fillers with the silicone oil: the presence (or, we should better
say, the absence) of an organic surfactant in clay galleries could play a significant role
in the formation of a positive (negative) interaction of the photo-active filler with the
PDMS, assisting (hindering) the fluorescence emission by the RhP cations.
Definitely, finding an explication isn’t that easy. We’ll just observe that some
difference has been visually detected even before performing the visualization test by
Visiovis: when preparing the masterbatches with the four photo-active lamellar fillers,
we noticed that the aspect of the mixture prepared with CNa+ 0.25CEC RhP was
different (visibly less homogeneous, meaning a less intimate mixture) with reference to
17
More details about the photo-functionalization method are available in Chapter II. On the other hand,
Chapter III deals with the characterization of the four photo-functional inorganic/organic complexes.
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the other mixtures18, as shown by Figure IV-F18. In any case and whatever the reason
for such behavior, this supplementary visualization test made us exclude one of the four
photo-active fillers: no further experiments will be performed with CNa+ 0.25CEC RhP.
zoom
zoom
Figure IV-F18 Visual comparison of the masterbatches prepared mixing Siliconöl M10000
with two of the four photo-active lamellar fillers (CNa+ 0.25CEC RhP and C30B 0.25 MC RhP)
to perform a preliminary visualization test by Visiovis. The aspect of the mixture prepared with
CNa+ 0.25CEC RhP (left) is visibly different from the aspect of the mixture prepared with C30B
0.25MC RhP (right) at the same concentration (0.1% wt).
With reference to the method used to prepare the masterbatches: all the mixtures
have been prepared by a mixer equipped with a 60 mm
dilacerator disk rotating at
1000 rpm for 20 min (Disperser TurboTest Rayneri 33/300P) – exactly the same method
used to prepare the controlled mixture for the calibration of the detection systems. We
mixed the selected PDMS (Siliconöl M10000) with a proper amount of the photo-active
lamellar filler in order to assure a concentration of photo-active lamellar filler of 0.25%
wt, as determined by the calibration of the detection systems (§ IV-1.2.4).
It is worthy to observe that, after a certain time, the masterbatches prepared with
the described method undergo decantation and a significant portion of (not all) the filler
sediments. Undoubtedly, this is not a good sign – a suspension of nanosized particles is
18
Parenthetically: as we’ve also prepared some photo-functional inorganic/organic complexes by cation
exchange processing with Nile Blue A Perchlorate (Chapter II), we tried to prepare some mixtures of
PDMS with CNa+ 0.25CEC NBAP and CNa+ 1CEC NBAP and we compared them to the mixtures of
PDMS with C30B 0.25MC RhP and C30B 1MC RhP (the homologue complexes, but functionalized with
Rhodamine 6G Perchlorate). We noticed the same differences shown in Figure IV-F18.
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supposed to be stable, since gravity effects should be negligible for very fine particles.
The dry photo-active lamellar fillers are certainly characterized by a size distribution19
including more or less fine particles – this would explain the fact that only a portion of
filler undergoes sedimentation. In reality, when a lamellar filler is mixed with a polymer
matrix and if these two components of the mixture have a high affinity, a certain degree
of “spontaneous” exfoliation of the filler (and, thus, an induced reduction of the average
particle size) can be observed. In this case, a partial sedimentation of the masterbatches
made us early foresee that no driving forces exist for our photo-active lamellar fillers to
spontaneously exfoliate into the selected PDMS – neither related to the chemistry of the
mixture, nor produced by the shearing effects of mixing. Such forethought will be later
proved by rheology measurements20. For the moment we just underline that, if GT5700
pigment was too dense (3.6 g/cm3, Table IV-T2) to avoid sedimentation when mixed up
with PDMS (0.97 g/cm3), our photo-active lamellar fillers are less denser but certainly
not perfectly compatible – at least in terms of density – with the selected model fluid.
Indeed, the clays used to prepare the photo-active lamellar fillers have a density of 2.86
g/cm3 (Cloisite ® Na+), 1.98 g/cm3 (Cloisite ® 30B), 1.90 g/cm3 (Cloisite ® 10A) and
1.66 g/cm3 (Cloisite ® 15A)21. Therefore, their sedimentation is unavoidable (maybe
just slower) in the absence of a massive exfoliation.
With reference to the method used to inject the masterbatch into the screw/barrel
system: we conceived two different modes of injection but only one practical procedure
(the only available at the moment).
The first mode of injection could be used to model an extrusion step – mixing up
pure polymer pellets (which gradually melt) with the dry filler, to formulate composite
pellets to be successively used for the fabrication of the final objects. This first injection
mode requires the preparation of a three-layer “unmixed masterbatch” composed of a
layer of dry filler stacked between two layers of PDMS (total volume 10 mL, equivalent
concentration 0.25%wt as previously indicated), to be injected into the system via a 20
19
We couldn’t characterize their particle size distribution because of the tiny amounts of sample we could
produce by cation exchange process with the fluorescent molecule (rather expensive).
20
See § V-4.1.
21
Even if the photo-functionalization process could have slightly changed such values, they still represent
a good reference for comparisons.
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mL syringe (previously cut at its extremity to avoid shearing). We tested this injection
mode only once, then we had to suspend it because of some problems due to the model
materials (poorly compatibles) and the geometry of the screw/barrel system (the actual
profile of Visiovis screw has a poor mixing efficiency – unsurprisingly, as we showed
that it rather models a meter zone22). Briefly, the filler couldn’t properly be mixed up
with the PDMS and, consequently, showed a strong tendency to sediment during – and
particularly after – the experiment. Sedimentation resulted in serious problems about the
complete purge of the system.
The second mode of injection is more adapted to model a portion of the injection
molding devices (the meter zone, of course) – melting again the composite pellets
previously formulated by extrusion and using the molten mixture for the fabrication of
the final objects. This second injection mode requires the preparation of a masterbatch
having a good quality of mixing (following the method of the disperser) and a volume
of 10 mL, to be injected in the system via the same 20 mL syringe previously described
(Figure IV-F19). This is the mode of injection we focused our attention on.
Figure IV-F19 Method used to inject the masterbatch (here, the second injection mode is
shown) into the screw/barrel system: via the appropriate aperture, using a 20 mL syringe
previously cut at its extremity to avoid shearing.
In summary, the experimental protocol to test the novel configuration of Visiovis
is the following:
22
The geometrical parameters of Visiovis screw/barrel system are listed in Table IV-T1.
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Preparation of the masterbatch. PDMS (Siliconöl M10000) is mixed up with a
suitable amount of photo-active filler (target concentration 0.25% wt) then intensive
mixing is performed by a mixer equipped with a 60 mm
dilacerator disk rotating at
1000 rpm for 20 min (Disperser TurboTest Rayneri 33/300P). We prepared a masterbatch
for all the photo-active lamellar fillers that looked suitably detectable (C30B 0.25MC
RhP, C10A 0.25MC RhP and C15A 0.25MC RhP).
Preparation of the syringe for the masterbatch injection. A standard 20 mL
syringe is previously cut at its extremity to avoid shearing while injecting, then it is filled
up with 10 mL of the previously prepared masterbatch.
Injection of the masterbatch into the system. Once the syringe prepared, the
screw/barrel system filled up with neat PDMS (Siliconöl M10000) and purged of all the
air bubbles, the room light switched off, the spectrometer zeroed for the black
background, the screw rotation set at about 20 rpm (corresponding to a voltage of 15V)
and the laser sheet switched on, the syringe is plunged vertically in the aperture and the
injection is rapidly achieved in the lowest accessible point.
IV-2.1
Acquisition of data
Once the injection of the masterbatch into the system executed as previously
described, the experiment is officially started and experimental data are automatically
acquired: one image per screw revolution is recorded by each CCD camera thanks to the
trigger, and a fluorescence emission spectrum is regularly recorded every three seconds
by the spectrometer thanks to the optical fiber. The images are stocked sequentially in a
*.bmp file named by a code composed by the date and the hour of data recording. At the
end of the experiment, each sequence of images can be used to reconstruct the
corresponding video. The fluorescence spectra are recorded individually in *.txt files.
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IV-2.1.1
Images
Figure IV-F20 shows a series of consecutive images (top to bottom) acquired by
one CCD camera (the first in the progression of the fluid, i.e. the closest to the point of
injection – which parenthetically is the camera showing the portion of screw profile in
which there’s the transition from the compressing to the pumping zone23) in the dark,
after the injection of a masterbatch of C30B 0.25MC RhP (left), C10A 0.25MC RhP
(middle), C15A 0.25MC RhP (right). Only the first images are shown, thus these series
represent just the beginning of mixing (6 screw revolutions, corresponding to a lapse of
time of 18 seconds ca).
It is clear that any interpretation of such images would be extremely subjective –
unless a suitable image processing is found to “translate” the qualitative information
given by the serial images in, at least, some quantitative trends. Obtaining quantitative
and absolutely reliable results won’t probably be easy – maybe won’t even be possible –
and our present objective is actually to extract some general but provable information.
By the way, the possibility of a direct visualization of the mixing progression in any
point of the screw/barrel system is innovative and certainly original.
IV-2.1.2
Videos
As previously said, the series of consecutive images shown in Figure IV-F20 can
be also used to reconstruct videos. The same reflections made about the single images
(any interpretation would be extremely subjective and certainly biased, somehow) can
be transposed to the videos reconstructed from the complete sequences of images.
However, no processing algorithms are available to get some quantitative information
directly from a video – one would rather use the sequence of images which compose it
to perform any quantitative analysis – therefore talking about video processing is here
meaningless, as our first source of data are the images.
23
More details about a typical screw profile for injection moulding devices – melting zone, compressing
zone and meter zone – are available in Chapter I.
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C30B 0.25MC RhP
0.25% wt
C10A 0.25MC RhP
0.25% wt
C15A 0.25MC RhP
0.25% wt
Figure IV-F20 Three series of consecutive images (top to bottom) acquired by one of the CCD
cameras in the dark after the injection of a masterbatch containing C30B 0.25MC RhP (on the
left), C10A 0.25MC RhP (in the middle) and C15A 0.25MC RhP (on the right).
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IV-2.1.3
In-line fluorescence spectra
Figure IV-F22 shows some examples of in-line fluorescence-spectra regularly
recorded by the spectrometer, interfaced with Visiovis by means of an optical fiber
which, as previously explained24, is placed perpendicularly to the laser sheet, in front of
the screw/barrel series but on the opposite side of the CCD cameras. The emplacement
of the optical fiber is better shown in Figure IV-F21.
Figure IV-F21 Two pictures illustrating the position of the optical fiber used to interface the
spectrometer with Visiovis and, thus, to regularly collect in-line fluorescence spectra.
The fluorescence spectra shown in Figure IV-F22 have been recorded during the
visualization tests performed by injection of a masterbatch of C30B 0.25MC RhP (top),
C10A 0.25MC RhP (middle), C15A 0.25MC RhP (bottom). The fluorescence emission
– whose spectrum is recorded every 3s (integration time 3s) by the spectrometer 25 –
cannot support the information supplied by the images and the videos (the phenomena
associated to the detection are absolutely different and occur at a totally different length
scale) but could eventually complete it. In fact, as previously argued in § IV-1.2.3, the
sole CCD cameras are neither adequate nor sufficient for a correct visualization of the
phenomena occurring during nanocomposite processing. Spectrofluorimetry, on the
other hand, is a technique capable of giving some information which are, surely, spacerestrained, but which could imply a deeper assessment of clay exfoliation mechanisms.
24
25
The actual configuration of Visiovis has been described in § IV-1.3.
See § IV-1.2.3.
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Photo-active
lamellar
filler
Laser 532 nm
Photo-active
lamellar
filler
Laser 532 nm
Photo-active
lamellar
filler
Laser 532 nm
Figure IV-F22 In-line fluorescence spectra acquired by Visiovis (details in the text). From top
to bottom: C30B 0.25MC RhP, C10A 0.25MC RhP and C15A 0.25MC RhP.
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IV-3
PROCESSING OF THE ACQUIRED DATA
The serial images acquired by each CCD camera (an example is shown in Figure
IV-F20) are useful to evaluate visually the temporal evolution of nanofiller distribution
in the volume of fluid comprised in between the screw flights, the screw root surface
and the barrel surface – in other words, they are useful to estimate the efficiency of the
visualized screw profile in terms of distributive mixing. Moreover, images can be used
to reconstruct videos which straightforwardly show such temporal evolution – certainly,
videos represent a pure qualitative result, but somehow they could help understanding
mixing dynamics. We’ve previously avowed that talking about some processing method
to get quantitative information from a video is actually meaningless, whereas for serial
images (time-related sequence of images) it is possible to conceive some procedure to
“translate” the qualitative information in a quantitative trend. With the precious and
irreplaceable help of Jean Balcaen we implemented two Matlab programs which process
the serial images acquired in presence of the photo-active lamellar fillers on the basis of,
respectively, (1) the integral standard deviation of the luminosity of the images, and (2)
the Fourier transform of textured images. All the Matlab functions we developed are
available in the Appendix.
In relation to the collected fluorescence emission spectra, we estimated that their
simple visualization as a function of processing time is already a valuable information,
thus for the moment we haven’t searched through the possibility of further processing.
IV-3.1
Images
The images shown in Figure IV-F20 as an example of the data acquired by
Visiovis have been taken by one of the four CCD cameras – more specifically by the
first camera in the progression of the fluid, i.e. the closest to the point of injection of the
masterbatch. Indeed, as previously explained, this first camera shows the portion of the
screw profile in which the transition from the compressing to the pumping zone occurs.
This zone is certainly interesting; nevertheless, the corresponding visualized volume has
a shape which is too complex and inadequate for the image processing we conceived –
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Chapter IV
which is actually more adapted to the analysis of a regular rectangular area. Thus, the
images in the field of the first camera have been collected, but they won’t be used for
any further image processing other than the reconstruction of the videos. The first
channel section useful for image processing (i.e. entirely visualized by a single camera)
falls in the field of the second camera and, just to begin, we decided to focus on one
channel section – as a result, the first and second cameras will be largely enough to
perform preliminary visualization experiences by Visiovis.
When introducing the topic of the acquisition of data, we said that the images are
stocked sequentially in a single *.bmp file named by a code composed by the date and
the hour of data recording26. The preliminary procedure for image processing is, thus, to
extract each single image from the unique *.bmp file recorded by Visiovis acquisition
circuit. This step can be executed by using the Matlab function decoupe.m (available in
the Appendix). After that, as a biggest portion of each image is completely black 27 and
we’re rather interested to the channel section, a further cutback of the serial images is
necessary, in order to reduce the area of the images to be processed just to the rectangle
around the channel section: the Matlab function performing this operation is fenetre.m
(available in the Appendix). Right after these two basic steps, Visiovis images are ready
to undergo the two aforementioned processes28. Figure IV-F23 shows an example of the
input (on the left) and output (on the right) images for the function fenetre.m.
fenetre.m
Figure IV-F23 Example of an input (left) and output (right) image for the function fenetre.m.
26
See § IV-2.1.
The black portions of the images correspond to the flight silhouette and the body of the screw.
28
See § IV-3.
27
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PROCESSING
IV-3.1.1
Standard deviation of image luminosity
The image processing performed by the Matlab function ecrtype.m (the M-file is
available in the Appendix) is based on the integral standard deviation of the luminosity
of Visiovis images. For each picture of a given sequence (input), this function calculates
the local values of the luminosity29 and their standard deviations, integrates the values
of standard deviation on the whole surface of the picture and then plots the logarithm of
the integral standard deviation of the luminosity as a function of the image number in
the sequence (i.e. the number of screw revolutions, as each camera acquires one image
per screw revolution). Of course, the temporal dependence can be deduced by the
rotational speed of the screw – in other words, since the screw turns at 20 rpm ca, one
revolution takes 3s and, consequently, the cameras acquire one image every 3s.
Interpretation’s key. In theory, the smaller the integral standard deviation of the
luminosity is, the more homogeneous the mixture and the more efficient the distributive
mixing will be.
Limitations. The integral standard deviation will never be smaller than a certain
value because of to the difference of the mixture (never completely black) in comparison
with the screw profile (always completely black).
Moreover, this image processing is not morphology sensitive: two images with
different textures may give the same results in terms of standard deviation of luminosity.
An example of the plot which can be obtained by this image processing based on
the integral standard deviation of the luminosity of the images is shown in Figure IVF24 [1]. Several successive passages of the masterbatch containing the photo-active
filler in the field of the camera are easily recognizable. The fact that the first peak
rapidly lowers indicates that the masterbatch is gradually diluted by the neat PDMS and
29
We just remind, here, that Visiovis cameras acquire 8bit images, i.e. Visiovis images are represented by
a colormap having 28=256 grey levels. Each pixel can assume one of these 256 grey levels, and each level
corresponds to a different light intensity (luminosity).
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Chapter IV
mixes up with the matrix. In this example, the fluid evolving into the screw/barrel
system effectuates 2.5 recirculations in the closed circuit, as it passes three times in the
visual field of the acquiring camera.
Figure IV-F24 Typical result obtained by the image processing based on the integral standard
deviation of the luminosity of Visiovis images – output of the Matlab function ecrtype.m. As the
screw rotational speed is 20 rpm ca, the temporal dependence can be derived by multiplying the
number of screw revolutions by 3 (one screw revolution every 3s).
IV-3.1.2
Discrete Fourier Transform (DFT)
of textured images
The image processing performed by the Matlab function normft.m (the M-file is
available in the Appendix) is motivated by the fact that the Fourier transform (FT) can
codify a textured image by the frequencies of repetition of its elementary textural units
(e.g. spirals or twisted fluorescent volutes on a homogeneously black background). As
any image can be expressed as a function of two discrete spatial variables, the FT of an
image is the sum of complex exponentials having different amplitudes, frequencies and
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phases, and indeed such representation plays a critical role in a broad range of image
processing applications (including enhancement, analysis, restoration and compression).
If f ( x, y ) is a function of two spatial variables x and y , then we can define the
two-dimensional Fourier Transform of f ( x, y ) by the following relationship:
F(
where (
x
F(
x
,
,
y
x
y
)
x
j
f ( x, y)e
y
xx
e
j
yy
dxdy
(IV-E1)
) are frequency variables associated to the spatial variables ( x, y ) .
,
y
) , which can be called the frequency-domain representation of f ( x, y ) ,
is a complex-valued function periodic in both
usually only the range
x
,
x
and
(period 2 ); for such reason,
y
of the function is displayed. F (0,0) is the sum
y
of all the values of f ( x, y ) and, thus, is often called the constant component of the FT.
The inverse two-dimensional FT is given by
f ( x, y)
1
4
2
F(
x
x
,
y
)e j
xx
e
j
yy
d
x
d
y
(IV-E2)
y
Roughly speaking, this expression proves that f ( x, y ) can truly be represented as
a sum of an infinite number of complex exponentials (i.e. sinusoids) having different
frequencies. The amplitude and the phase spectra of the contributions at the frequencies
(
x
,
y
) are given by F (
x
,
y
).
Just to give an example: let’s consider a function f (m, n) which equals 1 within a
rectangle and 0 everywhere else in the plan (Figure IV-F25a): Figure IV-F25b shows
the plot of the magnitude spectrum of its FT F (
plot is F (0,0) . The plot shows that F (
m
,
n
m
,
n
) . The peak at the centre of the
) has more energy at high horizontal than at
high vertical frequencies: this means that f (m, n) horizontal cross sections are narrower
pulses than vertical cross sections (narrow pulses have more high-frequency content
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Chapter IV
than broad pulses). Please note that small dimensions in the Euclidean space correspond
to high frequencies in the Fourier space, and vice versa (m vs. n rectangle dimensions).
(a)
(b)
Figure IV-F25 A simple rectangular function (a) and the amplitude of its Fourier transform
represented as a mesh plot (b).
Another common way to visualize the FT is to display the log F (
m
,
n
) as an
image (Figure IV-F26): the fact of using the logarithm helps to bring out more details of
the FT in regions where F (
m
,
n
) is very close to 0.
Figure IV-F26 The logarithm of the Fourier transform of a simple rectangular function (Figure
IV-F25a) represented as an image.
Figure IV-F27 shows two additional examples of the FT amplitude spectra for
simply-shaped functions (a tilted rectangle, on the left, and a circle, on the right). These
additional examples show that the FT is sensitive not only to the eventual presence of
image textures, but also to their position and orientation (i.e. to the image symmetry).
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Figure IV-F27 Two additional examples of Fourier transform for simply-shaped functions. The
FT is sensitive to the symmetry of the image, i.e. to the position and orientation of its textural
features.
Actually, the amplitude spectra given by the FT are invariant to translation – a
simple translation of a given textural feature doesn’t modify the amplitude but only the
phase of the sinusoidal contributions. On the other hand, the TF is sensitive to rotation.
Let’s consider a sinusoid of period T having a given initial orientation (Figure
IV-28a): its FT is represented by two peaks aligned in the same direction, corresponding
to the frequencies 1 T and 1 T . A rotation of an angle
makes the FT analogously
rotate, without changing its global appearance (Figure IV-28b).
Figure IV-F28 Sinusoidal surface of period T parallel (left top) and tilted (left bottom) with
respect to the x axis, and the corresponding FT (on the right).
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Evidently, we took into account these properties (which are typical of the FT)
when we conceived the Matlab function normft.m for Visiovis image processing.
Working on a computer with digital images (i.e. constituted by pixels) requires
using a form of the transform which is known as the Discrete Fourier Transform (DFT).
There are two principal reasons for using this form: (1) the input and output of the DFT
are both discrete, which makes it convenient for computer manipulations; (2) there is a
fast algorithm for computing the DFT, known as the Fast Fourier Transform (FFT). The
DFT is defined for a discrete function f (m, n) that is nonzero only over a finite region
0 m
M 1 and 0 n
N 1 (which is exactly the case of a digital image). The two-
dimensional M-by-N DFT and inverse M-by-N DFT relationships are given by:
M 1N 1
F ( p, q)
f (m, n)e
j
2
pm
M
e
j
2
qn
N
where
m 0n 0
2
f (m, n)
2
qn
j
pm j
1 M 1N 1
F ( p , q )e M e N
where
MN p 0 q 0
p 0,1,..., M 1
(IV-E3)
q 0,1,..., N 1
m
0,1,..., M
1
n
0,1,..., N 1
(IV-E4)
The Matlab built-in functions fft, fft2 and fftn implement the FFT algorithm for
computing the one-dimensional, two-dimensional and N-dimensional DFT respectively.
As we were going to deal with images (two-dimensional discrete functions), we were
mainly interested in the 2D-DFT Matlab built-in function. The DFT of a digital image
gives a spectrum of all the frequencies comprised between a maximum frequency
and a minimum frequency
30
m in
MAX
. The maximum frequency which can be associated to a
textured digital image is the frequency intrinsically generated by the regular presence of
pixels (maximum of details), whereas the minimum frequency is related to the physical
image dimensions31 (minimum of details). The scheme in Figure IV-F29 clarifies the
idea of image maximum and minimum frequencies.
30
In other words, the DFT associates a power value (i.e. a value of squared amplitude) at each frequency
comprised between the maximum and the minimum frequency.
31
If the image is rectangular, it is related to its biggest dimension.
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MAX
480 pixels
m in
640 pixels
Figure IV-F29 The maximum and minimum frequencies which can be associated to a digital
image (the dimensions of Visiovis images are 640 × 480 pixels and each pixel visualizes an area
having a maximum dimensions of about 83 m, as previously said in § IV-1.2.3).
Knowing the nature of the information that a FT can give about a textured digital
image, the image processing performed by the Matlab function normft.m is aimed to the
codification of the Visiovis images by the frequencies of their elementary textural units
(namely, spirals or twisted fluorescent volutes in a homogeneously black matrix). The
higher the main coding frequencies, the finer the texture of the digital image: an image
which is finely textured is coded by high frequencies, since the highest frequencies are
able to represent the finest details. We saw how the FT highlights any regular, repetitive
structure which appears in a digital image considered as a 2D signal, function of the two
spatial coordinates; we underlined, as well, that the FT is sensitive to image symmetry
and that the minimum frequency depends on the maximum physical dimension of the
image. Indeed, Visiovis images are rectangular (their biggest dimension is the diagonal
of the rectangle, whereas their smallest dimension is their height, i.e. 480 pixels) and
only circles are perfectly symmetric in all the directions, which means that only circles
can be frequentially isotropic. These observations let guess that a preliminary
processing step is necessary to get rid of any dimensional incongruity before calculating
the FT of our images. This is the reason why, before doing any other action, the
anamorphosis of each picture of a given series is operated, thus eliminating any problem
of aspect ratio of the rectangular images and “forcing” Visiovis images to be
frequentially isotropic. The Matlab function performs then the FT of each anamorphous
image, calculates the logarithm of the squared norm log F ( p, q) , subtracts the noise
previously calculated on a reference image, calculates the average amplitude for each
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Chapter IV
frequency and plots the averaged values as a function of the frequency. Finally it
associates, to each image of a given sequence, the most representative frequency (i.e.
the mean frequency weighted by the intensities, in other words the frequency
statistically most probable) and plots such values as a function of the number image in
the sequence (i.e. the number of screw revolutions, as each camera acquires one image
per screw revolution). The temporal dependence is deduced by the screw rotational
speed – as the screw turns at 20 rpm ca, one revolution takes 3s and, thus, the cameras
acquire one image every 3s.
Interpretation’s key. Theoretically, a zero frequency should correspond to the
perfect mixture homogeneity; on the contrary, high frequencies reflect the presence of
regular and fine textural units.
In case of perfect miscibility. The lower the representative frequency, the more
homogeneous the mixture, the more efficient the distributive mixing will be.
In case of reduced miscibility. The higher the representative frequency, the finer
the texture, the more efficient (though incomplete) the distributive mixing.
Limitations. It is actually impossible to obtain a truly zero frequency since
images have, by definition, finite dimensions (indeed, only an infinite image could have a
zero minimum frequency). Hence, if a zero frequency is obtained, it should be certainly
interpreted as a relative value.
Moreover, the contrast between the mixture (never completely black) and the
screw profile (always completely black) produces a Heaviside (i.e. a step-like) function,
which corresponds to an artifactual permanent texture of the image.
An example of the plot which can be obtained by this image processing based on
the Discrete Fourier Transform of textured images is shown in Figure IV-F30 [1]. Once
again, several successive passages of the masterbatch containing the photo-active filler
in the field of the camera are easily recognizable, but the physical meaning of the
observed phenomena is not the same as the previous image processing (§ IV-3.1.1). By
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PROCESSING
the way, the fact that the first peak rapidly lowers still indicates that the masterbatch is
gradually diluted by the neat PDMS and mixes up with the matrix.
Figure IV-F30 Typical result obtained by the image processing based on the Discrete Fourier
Transform (DTF) of textured Visiovis images – output of the Matlab function normft.m. As the
screw rotational speed is 20 rpm ca, the temporal dependence can be derived by multiplying the
number of screw revolutions by 3 (one screw revolution every 3s).
IV-3.1.3
Validation of data processing
As Moguedet and coworkers [4] developed an analytical model to describe the
flow of a viscous Newtonian fluid in the helical rectangular channel of a screw [7]
(validated by three-dimensional finite elements calculations in the Matlab environment),
we solicited Yves Béreaux to collaborate to validate our methods for data processing.
By adapting the Matlab code previously developed, Yves Béreaux generated a sequence
of images showing the deformation of a blob of tracing masterbatch into the helical
rectangular channel of Visiovis screw/barrel system. Under the hypothesis that the blob
is initially spherical, Béreaux represented, in the longitudinal section of the screw/barrel
system, a white circle on a black background: as the model is supposed to reproduce the
trajectories of a particle in the screw channel, it also allows to trace its position in a
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Chapter IV
given longitudinal section of the screw channel as a function of time. By modeling the
initial spherical blob as a given number of particles disposed to form a filled circle,
Béreaux could reconstruct the deformation of the blob reproducing by computer
simulation the trajectories of the particles but rather tracing their reciprocal distances in
conditions of pure recirculation, i.e. under the hypotheses that:
 molecular diffusion can be neglected32;
 the global flow rate is nil and the mixing of the blob of tracing masterbatch
with the neat fluid occurs in a single screw channel, viz. in the space confined between
two adjacent flights, the screw root surface and the barrel surface33. Some of the images
obtained by this method are shown in Figure IV-F31 (reproduced with the permission of
the author). We validated only the second method for data processing – the one based
on the DFT of textured images – since the first one, based on the standard deviation of
image luminosity, is more intuitive and its principle is easier to understand. In addition,
the hypothesis made about the absence of molecular diffusion makes the images
generated by computer simulation rather inadequate to be compared with the images
acquired by Visiovis: a comparison of Figure IV-F20 and Figure IV-F31 reveals that the
obtained images cannot represent the gradients of luminosity (in terms of brightness and
fuzziness of the white spirals on the black background) which, on the contrary, are
unavoidable in any image acquired during the experiments. Indeed, this difference could
affect also the validation of the second method (it actually does, as we’ll show later on),
but certainly to a lesser extent. The result obtained by the DFT method on the numerical
images designated as the reference is shown in Figure IV-F32 (a): this curve has to be
compared to the first part of the graph in Figure IV-F30 (reported in Figure IV-F32 (b)
to ease the comparison), as the curve in its entirety actually represents an experiment
which includes three passages of the tracing blob in front of the camera (recirculation
operated by the tube for closed circuit34).
32
This hypothesis is deduced from the absence of fuzziness from numerical images: Figure IV-F20 shows
that this hypothesis is unrealistic for the system observed by Visiovis. Even if slow, molecular diffusion
should not be neglected in real systems. Besides, high concentrations of the tracing masterbatch produce a
significant diffusion of the laser source, which definitely is the main cause of dizziness (see § IV-4.2).
33
These conditions could be approximately reproduced experimentally by duly tuning the back pressure:
indeed, back pressure facilitates the recirculation flow and decreases the global flow rate (§ IV-4.3).
34
A description of the actual configuration of Visiovis has been given in § IV-1.3 (Figure IV-F17).
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PROCESSING
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
…30
…35
…40
…45
…50
Figure IV-F31 Sequence of numerical images generated by Yves Béreaux (by means of a code
previously developed [7] and suitably adapted to our requirements), showing the deformation of
a spherical blob of tracing masterbatch in Visiovis screw channel, under the hypothesis of pure
recirculation and absence of molecular diffusion. These images validated the data processing
based on the DFT of textured images. Images reproduced with the author’s permission.
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Chapter IV
(a)
(b)
Figure IV-F32 Comparison of the typical results obtained by the processing based on the DTF
method (Matlab function normft.m) applied to (a) the images generated by numerical simulation
(Figure IV-F31) and (b) the images acquired during Visiovis experiments (Figure IV-F20). The
comparison validates data processing but shows some slight divergences – surely based on the
different origin of the processed images (more details in the text).
The only difference in Figure IV-F32 is that the average representative frequency
as a function of the image number for the images generated by computer simulation (a)
is characterized by two slope values, whereas the homologous curve obtained for the
images acquired by Visiovis (b) presents a slope which varies continuously and assumes
at least three different values. By the way, one could deliberately decide to fit the curves
obtained by processing acquired images (Figure IV-F32 (b)) with the model revealed by
the curve obtained with numerical images (double-slope curve, Figure IV-F32 (a)) – so
that two main parameters35 could be deduced and used to compare different systems.
A further validation of the DFT method for Visiovis data processing is provided
by the frequency distribution obtained for each numerical image of the reference series
(Figure IV-F33)36. A visual explication of the features appearing in the graph of Figure
IV-F33 is given in Figure IV-F34. At the beginning, the peak represents the minimum
35
Indeed, it is the results obtained applying the DFT data processing to the ideal case of numerical images
that proved that mixing could be probably described by two parameters: the slope values of the curve
showing the average representative frequency as a function of the number of screw revolutions.
36
The color scale is just an arbitrary scale which shows only the relative “importance” of all the possible
frequencies present in each image of the reference series.
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PROCESSING
frequency which could ever be due to morphological features – related to the maximum
dimension of the blob (i.e. initial diameter) (see Figure IV-F34 (a)). Since the blob gets
stretched and thinner, the white circle is gradually transformed in a lengthening lamella
and its characteristic dimension decreases (as expected for a laminar flow, the thickness
of the striations decreases): the width of the features in the frequency distribution graph
increases (Figure IV-F34 (b)) as shown by the boundary identified by line C. Moreover,
as the lamellae stretch and form spirals because of recirculation, the statistical number
of intersections observed in any cross section increases and their frequency of repetition
increases as well: an additional component in the frequency distribution appears (Figure
IV-F34 (c)) producing the peak represented by line B. Indeed, any regular distribution
of white features (spiral intersections) on a black background produces, in the frequency
domain, a Dirac impulsion whose position varies with the distance (and thus, indirectly,
with the number and thickness) of the spirals – in particular, as their reciprocal distance
decreases (i.e. as their number increases and their thickness decreases), the DFT gives a
peak which shifts towards higher frequencies (Figure IV-F34 (d)): line B bends towards
higher frequencies. Concurrently, a further phenomenon occurs: the augmentation and
thinning of the lamellae gradually expand the percentage of occupation of the black
background by the white features. A better occupation of the space by repeating features
is visualized, in the DFT frequency distribution, by an increased intensity of the peak
associated to the minimum frequency (see Figure IV-F35), corresponding to line A. This
phenomenon helps understanding why, in the graphs of Figure IV-F32, the average
frequency diminishes in spite of the appearance of higher-frequency features in the DFT
domain and of their further shifts towards higher frequencies: like any averaged value,
the representative frequency depends on the whole distribution. An increased intensity
of line A (Figures IV-F33 and IV-F35) indicates an improvement of global distributive
mixing (repartition of the striations), whilst the appearance and gradual shift of peaks at
higher frequencies (lines B and C) represent an improvement of local distributive
mixing (striation thickness).
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Chapter IV
A
B
C
Figure IV-F33 Frequency distribution obtained for the numerical images (Figure IV-F31) by
the DFT method (§ IV-3.1.2). The explications given in the text are illustrated in Figure IV-F34.
1 intercept
initial blob
(a)
1 intercept
thinner
striation
decreasing width
increasing width
(b)
2 intercepts
(c)
7 inter.
increasing frequency
shift towards
higher frequencies
(d)
Figure IV-F34 Visual explication of the features in the graph of Figure IV-F33.
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PROCESSING
A
B
C
low
frequencies
image number
high
Figure IV-F35 Three-dimensional visualization of the frequency distribution shown in Figure
IV-F33. Note that line B undergoes just a frequency shift, whereas line A increases in intensity.
IV-3.2
Videos
Once the single images extracted and isolated from the sole *.bmp file recorded
by Visiovis acquisition circuit (§ IV-3.1), the obtained images can be considered (since
they are!) as the frames of a temporal sequence and, thus, used to reconstruct a video.
This operation can be executed by running the Matlab function video.m (available in the
Appendix).
This function also generates a montage of the video, i.e. a panel visualizing at a
glance all the frames used for the reconstruction. The montage is particularly interesting
when the video is reconstructed from the windows cutback around the channel section,
as shown on the example in Figure IV-F36.
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Figure IV-F36 Example of montage of a video reconstructed from the windows cutback around
the channel section – one of the output of the Matlab function video.m. Acquisition done by the
second camera in the dark after the injection of a masterbatch containing C30B 0.25MC RhP.
IV-3.3.
Fluorescence spectra
The easiest way to visualize the regularly acquired in-line fluorescence emission
spectra as a function of processing time is to convert the temporal sequence of (intensity
of fluorescence emission vs. frequency) curves in a 3D shaded surface plot to show the
results on a rectangular region delimited by the processing time [s] and the wavelength
[nm], as shown in Figure IV-F22. The Matlab function used for the visualization of the
fluorescence spectra is called spctr.m and is available in the Appendix.
If the images (and the results of image processing) can be used to evaluate the
distributive mixing into the screw/barrel system, the fluorescence spectra may rather
inform about dispersive mixing – since the emission signature of the fluorescent dye
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used to photo-functionalize clay is sensitive to any change of its molecular environment
due to clay intercalation and/or exfoliation, and such morphological changes are a direct
consequence of dispersive mixing. Manifestly, all the detection systems the Visiovis is
equipped with are complementary and useful to characterize nanocomposite processing.
IV-4
SOME EARLY RESULTS
Conscious that the development of a tool for the visualization of nanocomposite
processing, along with the conception of brand new detection and/or characterization
systems, are far from being easy, we kept testing Visiovis with different systems and in
different conditions (when and if possible), hopeful to recognize its advantages and its
limitations. Keeping in mind that any test, by definition, may give negative results (or,
even, may not give any result), we performed some experiments in order to evaluate:
 the behavior of three different photo-active lamellar fillers – namely C30B
0.25MC RhP, C10A 0.25MC RhP and C15A 0.25MC RhP37;
 the possibility of using Visiovis to perform conventional tracing experiences –
that is, injecting a masterbatch containing a given amount of the pristine commercial
clay and a smaller amount of the same clay previously rendered photo-active;
 the influence of the back pressure on the visualization executed by Visiovis –
thanks to an additional van capable of regulating the flow in the closed circuit.
IV-4.1
Comparison of different photo-active lamellar fillers
While dealing with the modification of Visiovis configuration and planning how
to perform the first visualization tests, we dwelled on clay modification since we needed
to prepare a suitable photo-active lamellar filler to be used with Visiovis – coherently
with the main objective of our work. In Chapter II we detailed the actions we went
through to establish an efficacious (and efficient) photo-functionalization protocol and,
in Chapter III, we characterized four inorganic/organic complexes that we succeeded to
37
More details about the preparation and the characterization of the three photo-active lamellar fillers are
available in Chapters II and III.
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Chapter IV
render photo-functional by cation exchanging four commercial clays with a rhodamine
dye. However, we remind that during the tests performed to establish an appropriate
experimental protocol for Visiovis in its new configuration38, we realized that CNa+
0.25CEC RhP couldn’t be used in the visualization conditions imposed by Visiovis. In
conclusion, for the moment we disposed of three different photo-active lamellar fillers,
namely C30B 0.25MC RhP, C10A 0.25MC RhP and C10A 0.25MC RhP. It is with
these photo-active lamellar fillers that we performed our first visualization experiences
– respecting the experimental protocol previously established. The videos showing the
three experiences are available in the multimedia CD-Rom accompanying the PhD
manuscript.
As expected, images and videos are captivating but really can’t help interpreting
the results of the visualization experiences. Figure IV-F37 and Figure IV-F38 show the
trends revealed by the image processing previously described and based, respectively,
on the integral standard deviation of the image luminosity (§ IV-3.1.1) and on the DFT
of textured images (§ IV-3.1.2).
In all the curves of Figure IV-F37 the passages of the masterbatch containing the
photo-active filler (0.25% wt) in the field of the camera are easily recognizable. During
the first passage (which occurs more or less at the same time after the injection of the
masterbatch – unsurprisingly, since the pumping effect of the screw is the same for all
the samples) the C30B 0.25MC RhP appears less homogeneously distributed than the
other samples, as its integral standard deviation is higher meaning that there are intense
variations of luminosity in the visualized channel section. We can also observe that the
first peak for all the samples is, actually, a double peak: this reflects the fact that the
fluorescent masterbatch doesn’t proceed as a block but is stretched, plied and deformed
by the action of the screw, thus at the moment of passing in the field of the acquiring
camera (which is the second one, thus closer to the middle of the screw rather than to
the point of injection) several of its portions arrive with a slight temporal shift. Anyway,
this curve doesn’t inform about the actual morphology of the mixture at the right instant
of the acquisition (the other image processing will probably do). After this first passage,
38
See § IV-2.
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PROCESSING
the standard deviation of luminosity of course decreases, then increases again at the
moment of the second passage in front of the camera, after a complete recirculation. As
expected, the second peak is lower than the first one, witnessing the homogenization of
the masterbatch with the neat PDMS. One could notice that at the first passage the most
similar behaviors were those of C10A 0.25MC RhP and C15A 0.25MC RhP; at the
second passage, on the other hand, C30B 0.25MC RhP got closer to C10A 0.25MC RhP
whereas C15A 0.25MC RhP makes the difference. First of all, in the case of C30B and
C10A fillers, even the second peak seems to be double while the second peak of C15A
is clearly single and broad; besides, the whole curve of C15A is constantly lower than
the other two curves. These early results don’t show any major difference in distributive
mixing for different photo-active lamellar fillers, even if some slight details can anyway
be perceived.
Figure IV-F37 Processing based on the integral standard deviation of luminosity. Comparison
of different photo-active fillers (C30B 0.25MC RhP, C10A 0.25MC RhP, C15A 0.25MC RhP)
following the experimental protocol previously established (rotational speed 20 rpm ca).
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Chapter IV
Figure IV-F38 Processing based on the DFT of textured images. Comparison of different
photo-active fillers (C30B 0.25MC RhP, C10A 0.25MC RhP, C15A 0.25MC RhP) following
the experimental protocol previously established (rotational speed 20 rpm ca). Inside: y-zoom of
the background plot. 75 is the arithmetical mean value of frequency (reference max value 150).
In all the curves shown in Figure IV-F38 the two passages of the masterbatch in
the visual field of the camera are easily recognizable as well, and occur at the same
moment as it has been previously detected (Figure IV-F37). In the frequency-domain,
the image processing produces an additional feature whose explanation is in the
processing itself. A point having a frequency of 75.00 is present in all the curves and is
the same for all the photo-active fillers: indeed, it doesn’t mean anything as it just
represents the image chosen as the reference (i.e. the image just before the arrival of the
very first portion of the fluorescent masterbatch in front of the acquiring camera) and,
thus, it corresponds to the sole image which is really, completely black (for the
background numerical noise is spotlessly subtracted). Therefore, we zoomed to the
upper portion of the curves and we showed them again in the inner graph in Figure IV-
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F38. It is rather upsetting (even if probably expected) to find, in these curves, most of
the features previously observed and commented about Figure IV-F37. We’ve already
explained, however, that the physical reasons of such observed phenomena aren’t the
same – and this can be directly deduced from the fact that the two image processing
don’t derive from the same intuition. We preferred think, then, that no significant
difference exist between the mixtures processed with the three different photo-active
lamellar fillers – at least not in the conditions offered by Visiovis (matrix, screw profile,
speed, temperature). At least, these early results have the merit of illustrating Visiovis
potentiality.
With reference to the in-line fluorescence spectra: the data acquired during these
experiences with the three photo-active lamellar fillers have been previously shown in
Figure IV-F22. Effectively, no significant differences can be detected by comparing the
fluorescence emission spectra either – apart the behavior of C15A 0.25MC RhP, which
looks slightly different. Finally, we could just observe that the three photo-active fillers
seem to be more similar in terms of “morphology of the mixture” (Figure IV-F38) than
in terms of “distribution into the channel section” (Figure IV-F37).
IV-4.2
Comparison of different amounts of filler
In preparation of the “qualitative calibration” of Visiovis detection systems 39 we
announced that we were planning to focus essentially on the first photo-active lamellar
filler produces by the photo-functionalization protocol described in Chapter II. The
results obtained by comparing the three different photo-active lamellar fillers we
disposed of convinced us to continue testing the system only (at least for the moment)
with C30B 0.25MC RhP. Manifestly, the factors influencing these early results are
several and not always easy to control. We should remind, here, that owing to the
calibration, we found that the optimum concentration of the photo-active lamellar filler
in the masterbatch for the injection in Visiovis screw/barrel system is 0.25% wt. We
decided, however, to test the visualization capability of Visiovis with masterbatches
39
The calibration of the detection system has been detailed in § IV-1.2.4.
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Chapter IV
containing different amounts of filler. As we didn’t want to change the optimum
concentration of photo-active filler, we simply prepared three masterbatches following
the procedure previously described but adding, besides the photo-active filler, a given
amount of the corresponding pristine clay. Apart from this, we didn’t change the
experimental protocol. Briefly, we tested the system with the following masterbatches:
 Siliconöl M10000 + C30B 0.25MC RhP (0.25% wt)
= total amount of filler 0.25% wt
 Siliconöl M10000 + C30B 0.25MC RhP (0.25% wt) + C30B (0.75% wt)
= total amount of filler 1% wt
 Siliconöl M10000 + C30B 0.25MC RhP (0.25% wt) + C30B (2.75% wt)
= total amount of filler 3% wt
We must admit that these tests were risky: the calibration of the system already
revealed that an excessively high concentration of filler (whether photo-active or not)
can cause some optical problems, since as the concentration increases the mixture with
PDMS becomes less and less transparent and the penetration depth of the laser sheet
rapidly decreases. Nevertheless, we thought that it could be worthy trying. Figure IVF39 shows the trends revealed by both our image processing approaches.
The integral standard deviation of luminosity doesn’t show any particular trend
in relation with the presence of the pristine clay and to the fact that we made its amount
vary. The only remarkable points would be the shape of the first peak obtained for the
injection of C30B/C30B 0.25MC RhP 2.75/0.25% wt (i.e. the masterbatch containing
the highest total amount of filler), as well as the shape of its second peak, which is more
similar to a single than to a double peak. Otherwise, adding an amount of pristine clay 3
times higher than the amount of the photo-functionalized clay (C30B/C30B 0.25MC
RhP 0.75/0.25% wt) doesn’t significantly change the shape of the curve – which is a
rather good conclusion in the future eventuality of using Visiovis to perform real tracing
experiences, viz. experiences in which only a fraction of the filler acts as a tracer.
The same observations can be made about the results of the image processing
based on the DFT of textured images (Figure IV-F39, bottom).
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Figure IV-F39 Processing based on the integral standard deviation of luminosity (top) and on
the DFT of textured images (bottom). Comparison of three masterbatches containing different
total amounts of filler (0.25%, 1% and 3% wt) but always the same amount of the photo-active
filler C30B 0.25MC RhP (0.25% wt). Injection executed following the standard experimental
protocol (rotational speed 20 rpm ca).
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Chapter IV
We previously stated that no significant differences could be perceived upon the
addition of different amounts of pristine clay to the initial masterbatch (0.25% wt photoactive filler). Indeed, the fact that nothing seems to change (or even any little change we
could discover) may have an explanation which cannot be found uniquely in the curves.
This is a typical example of situation in which, even if we’re aware that Visiovis images
and videos can’t really provide quantitative information on our experiences, we realize
that they efficiently support the interpretation, e.g. by suggesting the causes which could
possibly be attributed to ambiguous features present on the curves.
1st camera
2nd camera
(a)
(b)
(c)
Figure IV-F40 Selected images acquired by the first (on the left) and the second camera (on the
right) after the injection of C30B/C30B 0.25MC RhP 0.00/0.25% wt (a), 0.75/0.25% wt (b) and
2.75/0.25% wt (c), respectively. A zoom is made on the processed image window.
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The images and the videos reconstructed for the visualization experiences we’re
trying to interpret actually reveal the significant consequences of adding an increasing
amount of pristine clay to the standard masterbatch – containing the optimum amount of
filler. Here we reported some selected frames (Figure IV-F40) to support our comments,
but the videos are also available in the CD-Rom accompanying the PhD manuscript.
The uncertainties claimed at the beginning of this paragraph (and justified by the
observations made during the calibration of the detection systems) concretized, and with
the help of the visualization experiences here presented we could finally proved them.
The concentration of filler in the tracing masterbatch has to be attentively controlled and
reduced, when possible, for the highest the concentration of filler, the worst the optical
clarity of the system will be.
(a)
(b)
(c)
Figure IV-F41 In-line fluorescence spectra acquired after the injection of C30B/C30B 0.25MC
RhP 0.00/0.25% wt (a), 0.75/0.25% wt (b) and 2.75/0.25% wt (c), respectively.
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Chapter IV
By the way, at least theoretically, the in-line fluorescence spectra aren’t affected
by such concentration issues: when a tracing experiment has to be performed, in which
a higher concentration of filler is supposed to be used, spectrofluorimetry is a valuable
alternative to the image acquisition and processing (Figure IV-F41).
IV-4.3
Regulation of the back pressure
The last visualization test we performed on Visiovis was aimed to verify whether
our tool is capable or not to detect the effect of the application and regulation of a back
pressure to the system. In reality, any screw/barrel system is subjected to an intrinsic
back pressure due to the presence of a restrained section at the exit of the system. The
back pressure represents an obstacle to the flow – it increases the residence time of the
fluid into the screw/barrel system and is partially responsible of the recirculation within
the screw channel. Sometimes, the back pressure has to be increased to accentuate these
phenomena and consequently ameliorate the quality of mixing.
Keeping in mind the relevance of such processing parameter, we performed three
tests to visualize the effects of an eventual variation of the back pressure on mixing. We
preferred varying this parameter (instead of the rotational speed of the screw and/or the
viscosity of the model fluid) since, for several reasons, we estimated that it was the only
factor capable of influencing the mixing process in the conditions imposed by Visiovis.
Indeed, PDMS is a Newtonian fluid40 and the screw profile we dispose of41 isn’t the
best for mixing.
To increase the back pressure (and also to be able to regulate it), we equipped the
existing tube for closed circuit of an additional van, which doesn’t alter the circuit if
fully opened but can almost completely arrest the flow when fully closed. Besides, any
intermediate position is also possible. With such additional van, we could perform three
tests in the presence of different back pressures, namely: (1) a test in which the van is
fully open (100% OPEN), (2) a test in which the van is half open (50% OPEN) and (3) a
40
Newtonian fluids are characterized by a value of viscosity which doesn’t depend on the shear rate.
We remind, here, that Visiovis geometrical parameters are comparable to the typical design parameters
of the meter section of industrial devices. The meter section of a screw isn’t, by definition, the most
suitable for mixing.
41
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PROCESSING
test in which the van is fully closed (100% CLOSE). All these tests have been carried
out, as usually, by injecting a masterbatch (PDMS Siliconöl M10000 + 0.25% wt C30B
0.25MC RhP) prepared as previously described42.
Figure IV-F42 show the results of the tests 100% and 50% OPEN. We couldn’t
show the results of the three tests all together, for the test 100% CLOSE lasted 9 times
longer than the other tests. The longer duration of the last test is quite obvious, as 100%
CLOSE means that the van almost stops the flow and accentuate to the maximum extent
the recirculation within the screw channel, as the corresponding video shows (available
in the CD-Rom accompanying the PhD manuscript). It’s interesting to observe that,
when the van is half open (50% OPEN), the first peak of the curve (log integral std dev
of luminosity vs. time) doesn’t really change, but the second one looks smoothed and
the whole curve is markedly lowered. However, the fact that the van is half opened must
have produced only a slight increase of the back pressure, because the residence time of
the fluid into the screw barrel system is basically the same (the position of the peaks
hasn’t changed). The same comments can be made for the curve (average representative
frequency vs. time).
On the contrary, when the van is fully closed (100% CLOSE) (Figure IV-F43)
the back pressure is at its greatest value and the prevalent phenomenon is the
recirculation within the screw channel: the fluid proceeds very slowly and has enough
time to mix up with the neat matrix before arriving in the field of the acquiring camera.
This is the reason why the curves for the last test are completely different than the
curves for the former ones. The limits of mixing are achieved when the curves reach
their asymptote. Note that, apart from the time scale, these graphs have the same scale
as the graphs shown in Figure IV-F42 to facilitate the comparison.
For all the performed tests, the fluorescence spectra reflect the same behaviors
observed by the cameras.
42
See § IV-2.
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Chapter IV
Figure IV-F42 Processing based on the integral standard deviation of luminosity (top) and the
DFT of textured images (bottom), respectively. The masterbatch contains 0.25% wt of a photoactive filler (C30B 0.25MC RhP). The position of the van for the regulation of back pressure is:
fully open (100% OPEN) and half open (50% OPEN). Rotational speed 20 rpm ca.
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PROCESSING
Figure IV-F43 Processing based on the integral standard deviation of luminosity (top) and the
DFT of textured images (bottom), respectively. The masterbatch contains 0.25% wt of a photoactive filler (C30B 0.25MC RhP). The position of the van for the regulation of back pressure is:
fully close (100% CLOSE). Rotational speed 20 rpm ca. Note the duration of the test (45 min).
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Chapter IV
IV-5
CONCLUSIONS
In this chapter we presented Visiovis, an original and innovative tool suitable for
visualizing viscous fluids flowing in a geometrically complex system – more precisely a
screw/barrel system. After a brief summary of Visiovis origins – who assembled it, for
which applications and in which initial configuration – we went through all the steps we
had to traverse to suitably modify its configuration in order to adapt it to our new needs.
Indeed, we were planning to adapt the existing tool to the analysis of nanofiller
dispersion/distribution mechanisms in molten thermoplastic polymers or, eventually, in
uncured thermoset resins. To attain our objectives, we certainly had to change Visiovis
original configuration – but we decided to do it steadily for both practical and economic
reasons. The choice of gradually but incessantly change Visiovis configuration made us
work in a situation in constant evolution, being ceaselessly faced to new and unexpected
problems to be solved. Notwithstanding, we dared developing two detection systems
and tried to exploit the acquired data as much as possible. In this chapter we described
how we performed the visualizations, how the CCD cameras and the spectrometer are
integrated on Visiovis, how we collected and processed the experimental data. All the
results shown in this chapter required a lot of work and sometimes didn’t result as
expected. Our main objective was, realistically, to increase Visiovis potentialities and to
suggest a further, possible employment for a tool which has already demanded large
efforts. Have we fulfilled such requirements?
In the next and last chapter we’ll rapidly summarize the technical progresses
already achieved on Visiovis and we’ll discuss of some possible further ameliorations.
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IV-R REFERENCES
[1] Esposito A, Balcaen J, Duchet-Rumeau J, Charmeau JY. Visiovis: monitoring nanofiller
[2] Gao, F. Clay-polymer composites: the story. Materials Today 2004, November, 50-55.
[3] Liu, J., Boo W.-J., Clearfield A. et al. Intercalation and exfoliation: a review on
morphology of polymer nanocomposites reinforced by inorganic layer structures.
Mater. Manuf. Processes 2006, 20, 143-151.
[4] Moguedet M. Développement d'un outil d'aide à la conception et au fonctionnement
d'un ensemble vis-fourreau industriel – Application à l'injection de thermoplastiques
[5] Esposito A, Charmeau JY, Duchet-Rumeau J. Analyse des mécanismes de dispersion de
nanocharges dans un polymère fondu. Conséquences sur la morphologie de
nanocomposites obtenus par injection. CR des 15èmes Journées Nationales AMAC sur
les Composites (JNC15), 361. Marseille, 2007, 1216 p. ISBN: 978-2-87717-090-1.
[6] Agassant J.-F., Avenas P., Sergent J.-P. et al. La mise en forme des matières plastiques.
3rd Ed. Paris: Tec & Doc Lavoisier, 1996, 640 p. ISBN: 9782743000165.
[7] Béreaux Y., Moguedet M., Raoul X. et al. Series solutions for viscous and viscoelastic
fluids flow in the helical rectangular channel of an extruder screw. J. Non-Newtonian
Fluid Mech. 2004, 123 (2-3), 237-257.
[8] Béreaux Y., Charmeau J.Y., Moguedet, M. A simple model of throughput and pressure
development for single screw. J. Mater. Process. Technol. 2009, 209 (1), 611-618.
[9] Ottino J.M. The kinematics of mixing: stretching, chaos, and transport. 1st Ed.
Cambridge: Cambridge University Press, 1989, 364 p. ISBN 0-521-36878-2.
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Chapter V
In the previous chapter we presented Visiovis – a visualization tool thoroughly
designed, assembled and developed in our laboratories to monitor, model and optimize
polymer melt processing in screw/barrel systems. Lately, we intended adapt such a tool
to monitor mixing during compounding of molten polymers with inorganic fillers –
namely, to monitor polymer-clay nanocomposite processing. We have amply stressed
how real-time monitoring techniques would facilitate the work of materials engineers –
since in situ characterizations are generally less time-consumptive, less labor-intensive
and more cost-efficient than ex situ characterizations. Nonetheless, the intensive work
required to conceive, develop, test and then validate any new characterization technique
shouldn’t be underestimated. Sometimes, the way which leads to the set up of a new
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characterization method isn’t exactly a highway, but does rather look like a tortuous
path which requires the collaboration of several experts from different domains and,
manifestly, more than a few years of hard and meticulous working. From time to time, it
is required and may be useful to make a point about the progresses accomplished
(ameliorations or just simple evolutions) – which doesn’t exempt from recognizing any
possible mistake or identifying the limitations of the system in its actual configuration.
The main objective of this chapter is explicitly to make a point on the technical
progresses made during the last three years on Visiovis – giving an instant picture of the
actual situation but also some suggestions for further amelioration, whenever a problem
has been encountered.
V-1
MATERIALS
As previously underlined, real-time process monitoring often constrains to use
pilot equipments and model materials – in particular, when the objective is to visualize
the flows in geometrically-complex equipments, process engineers frequently have to
choose model fluids. As our purpose was to monitor polymer-clay melt processing, we
had to found both a model fluid and some model fillers having specific and suitable
optical properties (optical inertness for the fluid and optical activity for the fillers).
Model fluid. The choice of poly dimethylsiloxane (PDMS) as the model fluid
has been largely vindicated in Chapter IV. Indeed, the first reflex was of course to make
profit of the choices previously made by Moguedet and coworkers [1] and eventually
adapt them by taking into account the new requirements. This is the fundamental reason
why we chose PDMS: the same kind of fluid had been previously and successfully used
for Visiovis experiments. However, silicone oil with lower viscosity (10 Pa·s rather than
100 Pa·s) would have been easier to handle (that is to say, it could make it easier to fill
up and empty the screw/barrel system).
Nevertheless, whilst PDMS represented an acceptable compromise for Moguedet
and coworkers, it didn’t really fulfill all the additional requirements we needed to obtain
polymer-clay nanocomposite morphologies by melt compounding. We have previously
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Chapter V
called attention to some problems encountered during the preparation of masterbatches
(PDMS + photo-active lamellar fillers) for the calibration of Visiovis detection systems
(§ IV-2): the photo-active lamellar filler CNa+ 0.25CEC RhP couldn’t be visualized by
the CCD cameras, and the aspect of the mixture prepared with such filler was different
in comparison with the other mixtures (Figure IV-F18). We then evocated two possible
reasons for these evidences: (1) the chemical composition of the commercial clays used
to prepare the photo-functional complexes, and (2) the interactions of the photo-active
lamellar fillers with the silicone oil. As the first explication didn’t really persuade us,
we got more and more convinced that the problem is essentially related to the chemical
composition and molecular arrangement of the chosen PDMS fluid (Figure V-F1). To
confirm our suspicions, we carried out some rheological measurements, whose results
will be shown soon after (§ V-4.1). Meanwhile, we searched through the literature to
find out any similar observation reported about the interactions of PDMS and lamellar
mineral fillers.
Figure V-F1 Chemical formula of poly dimethylsiloxane (PDMS), the silicone oil we used as
the transparent, viscous model fluid for Visiovis experiments. The macromolecular backbone
consists of alternating Si and O atoms (contrarily to the majority of organic polymers, based on
C atoms) and the only accessible groups are methyl groups (–CH3).
Schmidt [2] and Paquien [3] dealt with filled polysiloxanes during their PhD
research work: the former synthesized, characterized and evaluated the properties of
polysiloxane/layered silicate nanocomposites, whereas the latter investigated mostly the
rheological properties and the filler dispersion of PDMS/silica slurries. Paquien et al.
[4] published soon after the results obtained by dynamic mechanical measurements and
TEM on fumed silica/PDMS suspensions and focused their work on: (1) the modulation
of the interactions between silica particles and PDMS through a controlled silylation of
filler surface; (2) the effect of the procedure used to graft the silanol groups on the silica
surface; (3) the effect of silica volume fraction on dispersion. They finally established a
250
relation between the silica grafting ratio, the aggregate size and the rheological
properties of the suspensions. Indeed, it is well known that silica particles can establish
favorable interactions with polysiloxane macromolecular networks: Schaer et al. [5]
have even developed a model for the description of silica particles dispersion in silicone
polymers, taking into account the particle structure (porosity and density as a function
of size), the penetration of PDMS into the silica particles, the bound formation between
PDMS and accessible silanol sites, as well as the erosion of silica agglomerates – first in
intermediate fragments, then in aggregates of a few hundreds of nanometers. Some
works are also available about poly dimethylsiloxane reinforced by other fillers, e.g.
barium titanate particles [6] and mica flakes [7]. Recently, lamellar mineral fillers
(clays) have become quite popular also as a reinforcing agent of polysiloxane polymers,
coherently with the general trend registered about other carbon-based polymers (§ I-2).
Burnside and Giannelis [8] reported the first melt-processed layered silicate/poly
dimethylsiloxane nanocomposites synthesized by delamination of the silicate particles
in the PDMS matrix, followed by cross-linking. Afterwards, the number of works about
the synthesis and properties of silicone rubber/clays nanocomposites in the literature
noticeably augmented [9-18]. Wang and coworkers [19] openly proposed organic MMT
as a substitute for aerosilica in liquid silicone rubber systems. However, if compounding
lamellar mineral fillers to polysiloxane networks is nowadays interesting several
research groups, it is worthy highlighting that nobody has ever reported about a PDMS
matrix with the molecular structure of an inert silicone oil (Figure V-F1). Most of the
time clays (whether natural or organically-modified) are added to silanol-terminated
(hydroxyl-terminated) PDMS [8-10][17], vinyl-terminated PDMS [10][13], –NH2 and –
PEO terminated PDMS [17] and some other PDMS matrices containing at least one
reactive site in their repeating unit, for example SiCH3CH=CH2O [18]. The principal
objective is, apparently, to introduce the filler in the liquid precursor, disperse it, then
add the cross-linking agent (e.g. TEOS and/or tin 2-ethylhexanoate or whatever else)
and cure the rubber composite at room [8-10][12] or higher temperatures [13][18][19].
Contrarily to Burnside and Giannelis [8], clay intercalation is typically accomplished in
appropriate solutions [13][14] or with the addition of a dispersing aid, such as small
amounts of chloroform [9] or even distilled water [10] (only for PDMS–OH). Though,
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this strategy doesn’t always work as expected1. Moreover, even if probably performed,
the results of rheological measurements on such systems are rarely reported: the authors
rather perform mechanical tests [9][10][12][13][15][18][19] in order to evaluate the
performances of cured samples (as the main purpose is the reinforcement of the polymer
matrix), supported by XRD2, swelling tests, TGA, TEM and sometimes SEM, AFM,
permeability measurements, IR spectroscopy.
The control of the interactions between polymer macromolecules and the surface
of some filler is, in the case of polysiloxanes, more complex than in the case of common
carbon-based polymers. Several authors have faced the difficulty of pointing out which
are the factors influencing the extent of clay exfoliation in polymer composites (whether
the compatibility of clay modification with polymer chemistry, or the processing route
and parameters, or both of them, and in which proportion) [20-26]. Takeuchi and Cohen
[10] showed that the reinforcement in PDMS elastomers can be attributed to the
anchoring of the hydroxyl end-group of polymer chains to the silicate surface of the
fillers and that the mechanical properties of the obtained networks cannot be superseded
without further chemical modification of the system. They could enhance the properties
of their PDMS elastomers only if the networks were formed from the hydroxylterminated precursor. LeBaron and Pinnavaia [12] confirmed that organoclay could
readily intercalate linear PDMS molecules terminated by hydroxyl groups, even though
they observed little or no intercalation with analogous molecules terminated by methyl
groups: this would mean that the interactions of terminal silanol groups with the internal
surface of clay galleries represent an essential step of swelling and intercalation
mechanisms of clays. Kaneko and Yoshida [18] recently highlighted that, hitherto, few
researchers have reported that different kinds of clay have different behaviors when
compounded to PDMS matrices. Even if they assure that exfoliation can be achieved in
high-molar-mass PDMS matrices without solvent assistance or high shearing, they also
admit that reinforcing PDMS rubbers by layered silicates is far more complicated than
1
Takeuchi and Cohen [10] observed that attempted network synthesis using just water as a dispersing aid,
whilst efficacious for hydroxyl-terminated PDMS, was unsuccessful for vinyl-terminated PDMS. In this
latter case, they rather added a buffer solution (pH=7) since the addition of water is suspected to prevent
the hydrosilylation cross-linking reaction from occurring.
2
Kaneko et al.[17], for instance, evaluated the morphology of silicone/clay slurries by SAXS and WAXS.
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reinforcing them by silica or carbon black, mostly because of their unique morphology
and interactions with the matrix. They observed that the main driving force for polymer
intercalation into clay galleries results from the enthalpic contribution generated by the
establishment of many favorable polymer-filler surface interactions and that, in the case
of PDMS networks, essentially depends on the insertion of terminal segments of PDMS
chains containing specific groups (e.g. Si(CH3)OH groups) into the interlayer spacing.
Such end-groups could effectively interact with the silanol sites present on clay platelet
surfaces (especially at the platelet edges, indeed) by hydrogen bonds. This is the reason
why some authors found that the addition of small amounts of water occasionally helps
clay exfoliation in PDMS–OH matrices [10]. However, in the majority of conventional
organoclays, most of the –OH polar sites aren’t anymore available after cation exchange
process: that’s the reason why Kaneko and Yoshida reported that clay agglomeration
was more evident in PDMS-organoclay composites than in the composites containing
unmodified clay. This observation contradicts somehow our reflections about PDMS
masterbatches containing CNa+ 0.25CEC RhP: at least theoretically, the masterbatches
containing the photo-active filler produced from natural clay should present a better
dispersion than all the other masterbatches prepared from C30B 0.25MC RhP, C10A
0.25MC RhP, C15A 0.25MC RhP. Indeed, rheological measurements (§ V-4.1) showed
no significant differences between the four photo-active lamellar fillers: none of the
masterbatches showed the typical behavior of a properly dispersed polymer-clay
nanocomposite (the principles of morphological characterization by rheology have been
reported in § I-2.2.1). PDMS-clay interactions are maybe more complex than expected.
The evidences discussed so far let presume that, in the case of polysiloxane-clay
composites, a simple compatibilization of the polymer and the filler particles based on
the degree of hydrophilicity/hydrophobicity of the compounded ingredients isn’t enough
to assure the formation of nanocomposite morphologies. This difficulty has been only
recently pointed out and, unluckily, represents the main limitation of our visualization
tool and the associated characterization techniques. The problem is that Visiovis in its
actual configuration (§ IV-1.3) isn’t compatible with reactive fluids, liquid precursors,
solvents used as dispersing aids, etc. because of the material used to fabricate the barrel
(PMMA) and, for the reasons evocated in the previous chapter, it cannot sustain high
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temperatures. As a result, a better choice of the model fluid would be possible only if a
PDMS containing different terminal or lateral groups could be synthesized, so that the
polymer chains could favorably interact with the photo-active lamellar filler but remain
inert with respect to the PMMA internal surface of Visiovis barrel. Actually, the quest
for such an ideal transparent fluid could be quite difficult, as Schmidt et al. [16] recently
published a paper about the origins of silicate dispersion in polysiloxane/layered nanocomposites and clearly showed that the factors influencing clay dispersion are numerous
and not always simple to control. They prepared polysiloxane composite samples from a
variety of matrices (with respect to both chemical functionality and molecular weight)
and lamellar mineral fillers (natural and synthetic, hydrophilic and hydrophobic) just to
determine the origins of silicate dispersion in a generic polysiloxane matrix. They found
that, in the case of organoclays, the presence of an appropriate number of long (C12-C18)
ammonium-bound alkyl chains is essential, as well as the presence of sufficient amounts
of polar functional groups. They also concluded that, generally speaking, an otherwise
incompatible polymer can be made compatible with a given filler by the inclusion of the
appropriate number of dispersion-enhancing functional groups either at the chain-ends
or elsewhere in the polymer. It is worthy to report the observation they made that, to the
date of their contribution (i.e. 2006), polymer-clay nanocomposites had generated over
a thousand publications, whilst only a handful dealt with polysiloxane. The publication
by Kaneko et al. [17] came out soon after (2007) and roughly confirmed Schmidt and
coworkers’ results. In particular, Kaneko and coll. reminded that, as previously found
by LeBaron and Pinnavaia [12], the comparison of PDMS–SiOH with PDMS–Si(CH3)3
showed that, with an equivalent molar mass and in the presence of the same organoclay,
the first matrix produced intercalation while the latter did not.
Photo-functional fillers. On the basis of the previous considerations, if a proper
functionalization of PDMS (modification of its terminal groups or synthesis of a novel
repeating unit containing dispersion-enhancing lateral groups) is obtained, as a result of
the experimental activity of the last three years four photo-active lamellar fillers are
nowadays available to perform visualization experiments by Visiovis: CNa+ 0.25CEC
RhP, C30B 0.25MC RhP, C10A 0.25MC RhP, C15A 0.25MC RhP. Any choice about
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both the procedure and the parameters used for clay photo-functionalization has been
amply discussed and vindicated in Chapter II, while the results of the characterizations
performed on the photo-functional inorganic-organic complexes have been reported and
interpreted in Chapter III.
If no good alternatives for the choice of the model fluid can be found (since the
requirements imposed to the polymer matrix are numerous and quite strict), another
possibility would be to purposely prepare a new class of photo-functional complexes –
eventually but not necessarily by cation exchange process – in which the photo-activity
would be once again assured by the RhP moieties inserted into clay galleries3, but the
chemical compatibility with the PDMS matrix would be guaranteed by a specifically
designed surfactant. This solution is inspired by the analogous problem encountered by
some researchers who wanted to obtain exfoliated PP-clay nanocomposites by melt
processing without making use of any dispersion-enhancing additives. Wang et al. [27],
for instance, observing that many research efforts had been focused on dispersing MMT
in PP (and that such efforts failed because of the absence of strong interactions between
clays and highly apolar polyolefins), proposed to overcome the hurdle of formulating
complex compounds by a compatibilization method which can be certainly generalized
to other systems: synthesizing ammonium-terminated polymer chains (belonging to the
same family but eventually shorter than those present in the polymer matrix) and using
them to perform cation exchange processing of clay mineral fillers. This idea has been
afterwards proposed by Schmidt [2] to obtain PDMS-modified clays by cation exchange
processing a MMT-Na+ with short ammonium-terminated PDMS chains resulting from
the acidification of a commercial amino-terminated PDMS. In such a situation – that is,
when the surfactant for clay modification is insoluble in most of the solvents capable of
swell clays – some other procedure for cation exchange processing must be used and the
choice of the experimental protocol can be long and labor-intensive: Schmidt developed
a melt exchange technique, but the work of Ma et al. [14] let also guess that some other
method can be found, for instance choosing a different solvent (or a mixture of solvents)
3
This condition must be assured because, if another fluorescent dye is selected, the lighting source should
be consequently changed.
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Chapter V
as the exchanging medium4. The same idea has been developed by Li et al. [15]: they
modified clay by siloxane surfactants and dispersed it into polymethylsilsesquioxane
(PMSQ) by solution intercalation. Afterwards, they compared the system with the same
matrix compounded with a commercial organoclay (Cloisite ® 15A). As a confirmation
of what previously reported, they concluded that organoclays modified by carbon-based
surfactants are not suitable to prepare PMSQ-clay nanocomposites.
If no good alternatives for the choice of the fluid or the photo-functionalization
of the lamellar mineral fillers are found, there’s yet one alternative: synthesizing photoactive silica fillers. The photo-functionalization of silica particles could be envisaged
as: (1) a photo-functionalization ex situ, including a modification of the silica particles
right after their synthesis (and typically introducing the fluorescent cationic dye on their
external surface) or (2) a photo-functionalization in situ, viz. performed at the same time
of the synthesis (which could eventually be designed to obtain a core-shell morphology,
in which the silica shell would enclose a fluorescent RhP core). Ow and coll. [28], for
instance, recently described highly fluorescent and photo-stable core-shell nanoparticles
(size range 20-30 nm) obtained by a modified Stöber synthesis5, which are
monodisperse in solution and resulted 20 times brighter and more photo-stable than
their constituent fluorophore. They synthesized the particles for biological applications
(labeling of macromolecules for bioimaging experiments), but nothing lets imagine that
such a class of particles couldn’t be used for applications in the material field. However
it is obvious that, if we consider using photo-active core-shell silica particles (whether
the fluorophore encloses or is enclosed by silica) for Visiovis experiments, even tough
4
To be honest, during the last year of PhD research activity, we started performing some trials of cation
exchange process of Cloisite ® Na+ with a commercial low molecular weight NH2+-PDMS-NH2+. We first
tried to reproduce the experimental protocol proposed by Schmidt [2] (melt cation exchange process) with
and without the fluorescent cationic dye but then, as we didn’t estimate this procedure sufficiently “clean”
for fluorescence applications (Schmidt assumed that the excesses of hydrochloric acid and/or surfactant
were negligible, but fluorescence doesn’t tolerate the presence of contaminants or excesses which could
affect absorption and emission), we rather started adapting the protocol set up in Chapter II to a surfactant
insoluble in water (such as the NH2+-PDMS-NH2+ provided by Degussa). We tested the cation exchange
process protocol described in § II-2.4 with Cloisite ® Na+ and a proper amount of NH2+-PDMS-NH2+ in
toluene, with and without RhP: however, we won’t present any preliminary result here, since we haven’t
yet optimized the parameters of the protocol and we haven’t yet completed the required characterizations.
5
The Stöber synthesis of colloidal silica was first described in 1968 and is nowadays largely used to
obtain monodisperse nano- to micro-sized silica particles. Van Blaaderen et al. first reported the covalent
incorporation of organic fluorophores into Stöber colloidal silica and the synthesis of fluorescent silica
nanoparticles in the hundreds of nanometers size range [28].
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the interactions between PDMS and filler particles will be probably enhanced, the
information retrieved would be less rich than in the case of photo-active lamellar fillers.
We amply discussed about clay morphology and the differences between distributive
and dispersive mixing during polymer-clay compounding (Chapter I): we also proposed
an experimental protocol which, performed on Visiovis, could inform about distribution
(thanks to the CCD cameras and the image processing) and probably dispersion (thanks
to spectrofluorimetry) of photo-active clays in the transparent fluid modeling a molten
polymer. As the core-shell silica particles are spherical and would likely be stabilized
by the favorable interactions established with the matrix, only distributive mixing could
be visualized.
V-2
EQUIPMENT
Configuration. As described in Chapter IV, the actual configuration of Visiovis
is the result of some reasoned changes justified by:
 the complex geometry of the screw/barrel system;
 the selection of the area of interest (the volume of fluid comprised between
two adjacent screw flights, the internal surface of the barrel and the screw root surface);
 the set up of a planar light source for fluorescence excitation (laser sheet);
 the intrinsic difficulties associated to data acquisition and processing.
All these factors have equally contributed to the modification of configuration detailed
in Chapter IV. Some of them are strictly correlated to each other and interdependent. As
several critical problems have been encountered about the compatibility of the model
fluid with the photo-active lamellar fillers (§ V-1) and would need to be promptly fixed,
we believe that Visiovis configuration shouldn’t be considered as a main concern.
Light source. We mentioned that some of the factors which determined Visiovis
actual configuration are strictly correlated to each other and result interdependent: light
source is one of the constrained parameters. The first aspect limiting the choice of the
light source is, obviously, economic: powerful, monochromatic and perfectly collimated
laser sources can be quite expensive. In our case, the choice has been helped by the fact
that the cationic dye selected to perform clay photo-functionalization (Rhodamine 6G
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Chapter V
Perchlorate) is one of the most common fluorophores, well known by several groups of
researchers and regularly used by biologists (see § I-4): this consideration shouldn’t be
underestimated, as it obviously implies that the laser sources adapted for RhP excitation
(as well as the required filters to separate the emission from the excitation source) are
commercially available and (only) reasonably expensive. We’ve repeatedly stressed that
RhP represents an ideal probe to study heterogeneous systems thanks to the dependence
of its absorption and fluorescence emission on the properties of the matrix: therefore,
we suggest to persevere as long as possible with such fluorophore – a choice which will
be automatic in case the photo-active clays (whose photo-functionalization protocol has
been described and optimized in Chapter II, and whose characterizations are discussed
in Chapter III) will be used as they are, but which should be rather imposed if any other
solution is found (e.g. Stöber synthesis of core-shell colloidal silica particles, § V-I).
Barrel. The transparent PMMA barrel represents, undoubtedly, the main core of
Visiovis. Its transparency symbolizes the practical interest of process visualization and,
therefore, corresponds to the aspect which should be absolutely preserved. On the other
hand, the fact that the barrel is made of plastic have represented, since the beginning,
one of the most serious limitations of Visiovis (§ IV-1.1.2). Of course, the best solution
would be to find a material (probably a special glass) totally transparent and capable of
tolerating high temperatures and high radial pressures; in the absence of such an ideal
solution, we suggest to replace the actual barrel with another PMMA barrel having the
same internal surface (cylindrical) but a parallelepipedic external surface. This wouldn’t
fix the problems due to temperature and pressure, but at least would ameliorate the
optical quality of the visualization. Some accesses for real-time sampling would also
represent a valuable amelioration and would allow a further exploitation of the actual
screw/barrel system (see § V-4).
Screw. Far from being a problem, the screw profile rather offers a big margin of
modification. In Chapter I we stressed that the industry of plastic gradually developed
its own equipments and progressively multiplied the number of applications requiring
specific processing tools. Nowadays, the trend of the market concerning the processing
tools for the industry of plastic tremendously developed the relation supply-demand, to
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a point that the possible configurations of the same processing tool are as various as its
possible applications. Screw profiles are customizable – sometimes several units having
specific functions (mixing, homogenizing, etc.) can be composed to form a single screw
profile perfectly adapted to a specific application. In relation to the choice of the screw
profile, Visiovis doesn’t limit imagination – the only feature directly influenced by the
choice of the screw profile is the data acquisition and processing.
Feeding. We previously detailed the protocol for the experiments performed by
Visiovis and, in particular, we described the procedure used to inject the masterbatch
into the system: “the syringe is plunged vertically in the aperture and the injection is
rapidly achieved in the lowest accessible point” (§ IV-2). At the moment we performed
the experiments, we had no alternatives to inject the tracing masterbatch into Visiovis
screw/barrel system. Indeed, as reported by Cassagnau et al. [29]6 and confirmed by our
own experience, the feeding system may have a deep influence on the results of tracing
experiments: furthermore, the actual feeding system of Visiovis (Figure IV-F19) is far
from being practical and handy. The utilization of a syringe allowed us to perform
experiments and collect some early results – that’s surely a good point – however we
suggest, as a perspective easy to implement and probably rather inexpensive, to equip
Visiovis with a permanent, firmly positioned feeding system (eventually shaped as a
syringe) which should finally reduce any manipulation inaccuracy inevitably produced
by the operator and, thus, assure a better reproducibility of the observed phenomena.
V-3
PROCESSING OF THE ACQUIRED DATA
In Chapter IV we related Visiovis evolutions and its actual configuration, as well
as the possibilities of data acquisition and processing. We described both the categories
of experimental data provided by Visiovis (i.e. the images and the fluorescence spectra)
but we rather focused on the images, viz. we set up two methods for image processing:
6
Cassagnau et al. [29] developed a UV-fluorescence monitoring device to evaluate in situ the mixing
efficiency of an internal batch mixer (§ I-4). They observed that the fluorescence curves recorded by this
device strongly depended on the experimental conditions of injection of the tracer, thus they decided to
simply drop the tracer on the molten flow stream.
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Chapter V
the first based on the standard deviation of image luminosity (§ IV-3.1.1), the second
based on the Discrete Fourier Transform of textured images (§ IV-3.1.2). We justified
our choices and we validated the selected methods for image processing (in particular
the second one) by computer simulation (§ IV-3.1.3), nonetheless a direct and precise
correlation between observed and simulated phenomena has yet to be established7, and
the data acquisition and processing methods set up during the last three years represent
a tremendously interesting starting point. Moreover, if we take into consideration the
problems encountered with the model materials (in particular, the lack of compatibility
between the model fluid and the photo-active lamellar fillers prepared from commercial
clays), it is clear that Visiovis (and the described data collection and processing) has not
yet been completely exploited – one more reason to suggest looking for a more suitable
set of model fluid and photo-active fillers before changing Visiovis configuration8 or
searching for new methods for data processing. In addition, the choice of a set of model
materials more prone to clay intercalation and or exfoliation would finally allow to take
advantage of the complementary information provided by spectrofluorimetry.
V-4
INTERPRETATION AND VALIDATION OF THE RESULTS
The interpretation and validation of the results obtained by Visiovis is certainly
priority – if compared to any possible change of configuration, or to the quest of some
new method for data processing. After having introduced the aforementioned methods
for data processing (§ IV-3.1.1 and IV-3.1.2), we provided a preliminary validation of
the results obtained by such methods, as well as of their most probable interpretation (§
IV-3.1.3). Indeed, data processing and the interpretation of the ensuing results cannot be
considered complete without an accurate and systemic experimental validation, attained
by correlating Visiovis experimental results with the evidences provided by other (more
conventional) characterization techniques such as XRD, TEM and rheology (§ I-2.2.1).
We are absolutely aware of the importance of such validation and, actually, we planned
to perform it but, as previously stressed, the choice of methyl-terminated PDMS as the
7
8
We’ll come back on this topic in a following paragraph (§ V-4.2).
About this topic, please refer to § V-2.
260
model fluid restricted considerably our prospects – directly (electron microscopy cannot
be performed on oils or uncured resins) or indirectly (the properties of the model fluid
aren’t specifically restraining with respect to XRD and rheology which, on the contrary,
contributed to the assessment of a visible lack of compatibility of the photo-active fillers
with the PDMS matrix). There’s no need to spell out that the best way to correlate
Visiovis results (in situ) with the information provided by traditional techniques (ex
situ) would be to perform a systemic real-time sampling of the compound evolving into
the screw/barrel system and characterize it by XRD, TEM and rheology. Manifestly,
real-time sampling represents something priority for Visiovis development, as well.
V-4.1
Real-time sampling
As explained when discussing about the calibration of Visiovis detection systems
(§ IV-1.2.4), we haven’t yet designed a practical and handy method to get some samples
of the compound evolving in the screw/barrel system – nonetheless, we acknowledge its
priority. For the moment, just to get a hint of the possible correlation between Visiovis
and the other conventional techniques for morphological characterization, we performed
some measurement tests on the masterbatches prepared for Visiovis experiments (§ IV2) in their initial state by XRD and rheology. We couldn’t even consider performing
electron microscopy on such masterbatches because, as previously reported, electron
microscopy imposes a few but strict conditions on the nature and physical aspect of the
samples (§ I-2.2.1). One of such conditions – probably the strictest for silicone oil – is
that the samples have to be observed in vacuum as the air molecules would significantly
scatter the electrons. Environmental electron microscopy could probably represent a
possible alternative to conventional (under high vacuum) electron microscopy (it allows
even hydrated samples to be viewed in low-pressure wet environments) but, unluckily,
we didn’t dispose of an environmental electron microscope during this PhD research
activity and, in any case, the characterization would have been surely not conventional.
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Chapter V
XRD9. Schmidt et al. [16] pointed out that, because of the lack of X-ray contrast
(i.e. differences in electron density) between the silicone and the silicate (as compared
to layered silicate nanocomposites with carbon-based polymers), the XRD pathways of
PDMS-clay nanocomposites often present low intensities of the diffraction peaks and
low signal-to-noise ratios – which limits the use of XRD to materials containing at least
10% wt of clay, even if smaller amounts of clay are likely to produce better dispersion.
We definitely confirm the observations reported by Schmidt et al. [16]: indeed,
we performed XRD measurements on a number of masterbatches prepared with all the
available photo-active lamellar fillers, alone or with the corresponding pristine clays, by
different methods of compounding (manual stirring, mixing by a disperser TurboTest
Rayneri 33/300P, ultrasound probe) and with different clay contents (1%, 3%, 5% wt)…
whatever the clay content, whatever the nature of the filler, whatever the procedure used
for compounding, we got silent XRD pathways. Two examples of silent XRD pathways
are shown in Figure V-F2 (bold lines): they have been both obtained for a masterbatch
containing 0.25% wt of the photo-functional complex C30B 0.25MC RhP and 0.75% wt
of the pristine C30B (total amount of clay 1% wt). In particular, Figure V-F2 (a) shows
the pathways for the masterbatch obtained by manual stirring, whereas Figure V-F2 (b)
shows the pathways for the same masterbatch prepared by the disperser (20 min at 1000
rpm). Probably, we would have erroneously concluded that clay particles were perfectly
exfoliated into the polymer matrix (see considerations previously made about erroneous
interpretations of silent XRD pathways § I-2.2.1) if we didn’t dispose of the following
evidences – rather supporting the fact that clays were distributed into the PDMS, but no
dispersive mixing had occurred:
 the masterbatches appeared initially (i.e. right after mixing) homogeneous, but
after some time started settling – meaning that a considerable amount of clay particles
remained agglomerated and, under the action of gravity, drifted down to the bottom of
9
The experimental protocol for XRD measurements is the same used for the photo-active lamellar fillers,
described in § II-3.1.
262
the pan, whereas a minor fraction of smaller clay particles 10 appeared stably suspended
just because insensitive to gravity11;
 to confirm our conjecture, we centrifuged the aforementioned masterbatches
(5 min at 10000g, i.e. 15500 tr/min) in order to remove as much PDMS as possible, so
that the sediment (the sample) got enriched of silicate filler to the maximum extent. In
the case of the centrifuged samples (containing essentially the clay powder wetted by an
insignificant amount of PDMS) we got an XRD pathway showing a single low-intensity
peak roughly corresponding to the interlayer spacing of the dry filler (Figure V-F2, thin
lines). This evidence represents a further confirmation of the fact that methyl-terminated
PDMS may have some advantageous properties in terms of visualization (transparency,
inertness) but certainly isn’t a suitable fluid to model carbon-based molten polymers
compounded with clays.
Figure V-F2 XRD pathways of a masterbatch containing 0.25% wt of photo-functional
complex (C30B 0.25MC RhP) and 0.75% wt of the corresponding pristine clay (C30B),
before and after centrifugation (bold and thin lines, respectively). Two methods having
different compounding efficiencies are compared: (a) manual stirring and (b) mixing by a
disperser TurboTest Rayneri 33/300P (20 min at 1000 rpm).
10
Clay fillers can be characterized by more or less large size distributions.
We affirm that a minor fraction of smaller clay particles appeared stably suspended because, in spite of
decantation, the masterbatches kept their colored appearance and we never observed a clear separation of
colored (red/rose photo-active filler) sediment from a clear (transparent PDMS) supernatant.
11
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Chapter V
Rheology further confirmed the inadequateness of methyl-terminated PDMS, as
we are going to show later on.
Rheology. The literature largely confirms that rheology is a valuable tool for the
analysis of filler dispersion in molten polymers, as reported in § I-2.2.1. Undoubtedly,
rheology can provide useful information in a variety of different measurement modes,
e.g. flow, dynamic, transient measurements. As we had already collected some negative
evidences from the XRD characterization of the masterbatches (see previous section),
we estimated unnecessarily time-consumptive to perform a complete set of rheological
characterizations just to confirm several times the same, negative result. Therefore, we
proceeded following only the most simplistic approach: we performed some dynamic
measurements in order to check whether the complex viscosity at low frequencies got
increased by the presence of a percolating filler and, additionally, whether the moduli G’
and G’’ crossed in correspondence of a percolation threshold – indicating a transition
from the purely viscous liquid behavior (typical of unfilled methyl-terminated PDMS)
to a solid-like behavior as the frequency decreases. The behavior of the samples (that is,
the behavior of methyl-terminated PDMS) is particularly exasperating for rheological
characterizations: the specific properties of methyl-terminated PDMS [3] make of it a
fluid which flows incredibly easy and can perfectly wet most of the surfaces12 – namely,
the metallic plate surface of a cone-plate geometry of a rheometer. These extraordinary
properties represent a serious obstacle to the formation of a regular and stable meniscus
in the gap between the cone and the plate of the rheometer – indeed, the fluid gradually
flows and the amount of measured sample diminishes, producing erroneous results. This
is the reason why the first measurement tests, performed on a stress-controlled AR1000
rheometer with the largest available cone-plate geometry (35 mm
), were unsuccessful
– in spite of the remarkable sensibility of the equipment (even to low-viscosity fluids)
and of an optimized choice of the geometry. Resolute to get at least one measurement
for confirmation, we performed few further tests on a strain-controlled ARES rheometer
with a Couette geometry (in order to prevent the sample from escaping the measurement
volume) – unsurprisingly, regardless of the reduced sensibility of the equipment with
12
Paquien [3] offered a clear and complete summary of the typical properties of polysiloxanes.
264
respect to the stress-controlled rheometer, these measurements were at least correct.
Nonetheless (and unsurprisingly as well), such measurements confirmed that the filler
didn’t get dispersed into the matrix (no increase of the complex viscosity has ever been
observed for any of the aforementioned masterbatches13) and distribution was probably
inhomogeneous and certainly unstable (G’ and G’’ never crossed). The results obtained
by dynamic rheological characterizations performed on the same masterbatches chosen
to show XRD silent pathways (Figure V-F2), as well as the homologous curves for the
neat PDMS, are shown in Figure V-F3.
Figure V-F3 Dynamic rheological behavior of the neat PDMS (a) and of the masterbatch
containing 1% wt of filler (0.25% wt C30B 0.25MC RhP and 0.75% wt C30B) prepared
by (b) manual stirring and (c) mixing by the disperser (20 min at 1000 rpm).
13
Just for rheological measurements, we even prepared a masterbatch containing 10% wt of clay!
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Chapter V
V-4.2
Computer simulation
As a final point, we would like to express a few considerations about computer
simulation. In Chapter IV we affirmed having used computer simulation to justify and
partially validate the image processing applied to Visiovis experimental data. We hope
we have been sufficiently clear to convince about the pertinence of supporting the real
experiments with the results of simulations. Here we would only like to stress that, in
order for such correlation to be correct and complete, the experimental conditions and
the simulation equations and parameters must be as close as possible to each other (or,
better, as coherent as possible with each other): this is possible only if both experiments
and simulations are developed at the same time and, in particular, in the total respect of
the limitations imposed by the counterpart. The perspective of fixing Visiovis feeding
system (§ V-2), for instance, will surely help the correlation with computer simulation.
The experimental protocol could then be adjusted for the results to be as repeatable as
possible and as coherent as possible with the results of the simulation (§ IV-3.1.3). On
the other hand, computer simulation could be adapted to reproduce the flow behavior
exactly in the same geometrical plan created by Visiovis laser sheet 14, and the images
obtained by computer simulation could be rendered a little more “realistic” by applying
a standard image treatment to increase fuzziness before applying the image processing
procedures described in § IV-3.1.1 and IV-3-1-2.
A lot of work left, quite a lot of courage needed!
14
Indeed, the computer simulation performed by Yves Béreaux (§ IV-3.1.3) represents the screw channel
as it appears if observed in the direction perpendicular to the screw flight surface, whereas Visiovis laser
sheet enlightens a plan which passes by the axis of the screw/barrel system, thus is parallel to the axis and
form an angle equal to the screw helix angle of the screw (§ IV-1.2.1). So far, computer simulation and
experiments show the same phenomena – if the screw helix angle is neglected in terms of visualization.
266
V-R
REFERENCES
[1] Moguedet M. Développement d'un outil d'aide à la conception et au fonctionnement
d'un ensemble vis-fourreau industriel – Application à l'injection de thermoplastiques
[2] Schmidt D.F. Polysiloxane/layered silicate nanocomposites: synthesis, characterization,
and properties. PhD Thesis. Cornell University, 2003, 300 p.
[3] Paquien J.N. Etude des propriétés rhéologiques et de l’état de dispersion de suspensions
PDMS/silice. Thèse. Lyon: INSA de Lyon, 2003, 270 p.
[4] Paquien J.N., Galy J., Gérard J.-F. et al. Rheological study of fumed silica-polydimethyl
siloxane suspensions. Coll. Surf. A: Physicochem. Eng. Aspects 2005, 260, 165-172.
[5] Schaer E., Guizani S., Choplin L. Model development for the description of silica
particles dispersion in silicone polymer. Chem. Eng. Sci. 2006, 61, 5664-5677.
[6] Khastgir D., Adachi K. Rheological and dielectric studies of aggregation of barium
titanate particles suspended in polydimethylsiloxane. Polymer 2000, 41, 6403-6413.
[7] Osman M.A., Atallah A., Müller M. et al. Reinforcement of poly(dimethylsiloxane)
networks by mica flakes. Polymer 2001, 42, 6545-6556.
[8] Burnside S.D., Giannelis E.P. Synthesis and properties of new poly(dimethylsiloxane)
nanocomposites. Chem. Mater. 1995, 7 (9), 1597-1600.
[9] Wang S., Long C., Wang X. et al. Synthesis and properties of silicone rubber/organo
montmorillonite hybrid nanocomposites. J. Appl. Polym. Sci. 1998, 69, 1557-1561.
[10] Takeuchi H., Cohen C. Reinforcement of poly(dimethylsiloxane) elastomers by chainend anchoring to clay particles. Macromol. 1999, 32, 6792-6799.
[11] Burnside S.D., Giannelis E.P. Nanostructure and properties of polysiloxane-layered
silicate nanocomposites. J. Polym. Sci. B: Polym. Phys. 2000, 38, 1595-1604.
[12] LeBaron P.C., Pinnavaia T.J. Clay nanolayer reinforcement of a silicone elastomer.
Chem. Mater. 2001, 13, 3760-3765.
[13] Osman M.A., Atallah A., Kahr G. et al. Reinforcement of poly(dimethylsiloxane)
networks by montmorillonite platelets. J. Appl. Polym. Sci. 2002, 83, 2175-2183.
[14] Ma J., Xu J., Ren J.-H. et al. A new approach to polymer/montmorillonite
nanocomposites. Polymer 2003, 44, 4619-4624.
[15] Li B.-Y., Ma J., Liu H.-Y. et al. Siloxane surfactant-modified clay and its effect in
reinforcing the laminate of polymethylsilsesquioxane. J. Appl. Polym. Sci. 2006, 100,
3974-3980.
Antonella ESPOSITO
267
Chapter V
[16] Schmidt D.F., Clément F., Giannelis E.P. On the origins of silicate dispersion in
[17] Kaneko M.L.Q.A., Torriani I.L., Yoshida I.V.P. Morphological evaluation of
silicone/clay slurries by small-angle/wide-angle X-ray scattering. J. Braz. Chem. Soc.
2007, 18 (4), 765-773.
[18] Kaneko M.L.Q.A., Yoshida I.V.P. Effect of natural and organically modified
montmorillonite clays on the properties of polydimethylsiloxane rubber. J. Appl. Polym.
Sci. 2008, 108, 2587-2596.
[19] Wang J., Chen Y., Jin Q. Organic montmorillonite as a substitute for aerosilica in
addition-type liquid silicone rubber systems. Macromol. Chem. Phys. 2005, 206, 25122520.
[20] Dennis H.R., Hunter D.L., Chang D. et al. Effect of melt processing conditions on the
extent of exfoliation in organoclay-based nanocomposites. Polymer 2001,42,9513-9522.
[21] Zhang Q., Wang Y., Fu Q. Shear-induced change of exfoliation and orientation in poly
propylene/montmorillonite nanocomposites. J. Polym Sci B: Polym Phys 2003, 41, 1-10.
[22] Rhoney I., Brown S., Hudson N.E. et al. Influence of processing method on the
exfoliation process for organically modified clay systems. I. Polyurethanes. J. Appl.
Polym. Sci. 2004, 91, 1335-1343.
[23] Wang K., Liang S., Du R. et al. The interplay of thermodynamics and shear on the
dispersion of polymer nanocomposites. Polymer 2004, 45, 7953-7960.
[24] Homminga D., Goderis B., Hoffman S. et al. Influence of shear flow on the preparation
of polymer layered silicate nanocomposites. Polymer 2005, 46, 9941-9954.
[25] Fedullo N., Sclavons M., Bailly C. et al. Nanocomposites from untreated clay: a myth?
Macromol. Symp. 2006, 233, 235-245.
[26] McAlpine M., Hudson N.E., Liggat J.J. et al. Study of the factors influencing the
exfoliation of an organically modified montmorillonite in methyl methacrylate/poly
(methyl methacrylate) mixtures. J. Appl. Polym. Sci. 2006, 99, 2614-2626.
[27] Whang Z.M., Nakajima H. Manias E. et al. Exfoliated PP/clay nanocomposites using
ammonium-terminated PP as the organic modification for montmorillonite. Macromol.
2003, 36, 8919-8922.
[28] Ow H., Larson D.R., Srivastava M. et al. Bright and stable core-shell fluorescent silica
nanoparticles. Nano Lett. 2005, 5 (1), 113-117.
[29] Cassagnau P., Melis F., Bounor-Legare V. UV fluorescence monitoring of the mixing of
molten polymers in a batch mixer. Polym. Eng. Sci. 2003, 43 (4), 923-932.
268
CONCLUSIONS
The interest for polymer/clay nanocomposites blew a few decades ago, but the
topic is still having a great vogue, since clay nanocomposites offer greatest hopes for a
dramatic improvement of several properties. Definitely, it’s time to adapt the know-how
developed about real-time monitoring to the processing of polymer nanocomposites!
During the last three years we worked on a project which was started in 2003 and which
is far from being over: it is well known that the work necessary to conceive, develop,
test and validate new characterization techniques is intensive and time-consumptive. We
are conscious that there are some difficulties which haven’t yet been overcome, but we
are also glad to affirm that we found an adequate solution to more than one problem.
We solved a first technical problem by establishing and optimizing a protocol for
the photo-functionalization of commercially available organically-modified clays: we
selected one of the most commonly used organoclays and, on the basis of several tests
performed with different fluorescent molecules in different experimental conditions, we
concluded that the best photo-functional inorganic-organic complexes can be obtained
by cation exchange processing with an amount of Rhodamine 6G Perchlorate equal to
Antonella ESPOSITO
269
CONCLUSIONS
25% of clay surfactant, in a 90/10 water/ethanol mixture at 80°C, followed by washing
with ethanol, recovering by centrifugation, drying at room temperature under exhaust
hood. Clay photo-functionalization with higher concentrations of fluorescent molecule
is still possible, but unnecessary and trespassing the main objective of the work: just
rendering clays photo-active. Any variation of the concentration of fluorescent molecule
in the exchanging medium could modify the mechanisms of absorption (e.g. formation
of aggregates and adsorption at silicate edges, preventing any further intercalation). The
intercalation of smaller molecules into the galleries of a clay previously modified by
bigger molecules of surfactant produces a rearrangement of the paraffinic configuration
and increases the tilting angle from 37 to 49°. Thanks to the aforementioned protocol,
today we dispose of four photo-functional inorganic-organic complexes which, of
course, have been prepared to be used with Visiovis, but could be much more versatile.
Moreover, we observed that the photo-functionalization followed by careful washing
doesn’t affect thermal resistance – on the contrary, a second exchange process with
smaller molecules produces some CEC recovery and improve surface coverage.
We succeeded in adapting an existing tool (Visiovis) to the real-time monitoring
of polymer/clay distributive and dispersing mixing in the molten state, which could help
understanding the key factors for the processing of polymer/clay nanocomposites. We
set up and calibrated two complementary detection systems (cameras and spectrometer),
we showed how to collect experimental data (images, videos and fluorescence emission
spectra), we implemented some algorithms for data processing (two image processing
based on the integral standard deviation of image luminosity and the Discrete Fourier
Transform of textured images), we succeeded in performing some preliminary tests and
suggested a possible interpretation of the results – trying to identify the limitations of
the systems and to justify and/or explain the problems encountered. Finally, we made a
point about the progresses accomplished, giving an instant picture of the actual situation
but also some suggestions for further amelioration – a better choice of the model fluid
or a better reciprocal compatibilization of the selected fillers and matrix, a slight change
of the barrel external surface, quite a lot of opportunities about the screw profile, a more
accurate feeding system and the conception of a real-time sampling method which
would allow a direct correlation with more conventional characterization techniques.
270
APPENDIX
---------------------------------------------------------------------function []=decoupe(fichvis,name0,deb)
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Compilation by Jean Balcaen
Revision by Antonella Esposito
INSA de Lyon © 2008
This function extracts a sequence of isolated frames from the images
stocked in a single *.bmp file recorded by Visiovis and named by a
code composed by the date and the hour of data recording.
Example of command line:
decoupe('Tue_Jul_24_17h36m00s2007_87','Manip16\Cam87\16-87',0);
fichvis = name of the *.bmp file (input)
name0 = path to be given to the extracted frames (output)
deb = first number of the sequence (output index)
name=['C:\MATLAB6p5\',fichvis,'.bmp'];
aref=imread(name);
s=size(aref)
s0=s(1)/480
it0=s0;
for it=1:s0
it0=it0-1;
a0=aref((it-1)*480+1:it*480,:);
nameit0=['C:\MATLAB6p5\',name0,'_',num2str(it0+deb+1),'.bmp']
imwrite(a0,nameit0,'bmp');
end
---------------------------------------------------------------------function []=fenetre(imdec,numdeb,numfin,x0,y0,x1,y1);
% Compilation by Jean Balcaen
% Revision by Antonella Esposito
Antonella ESPOSITO
Appendix 1/6
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This function operates a cutback of the sequential images output of
the function decoupe.m in order to reduce the area of interest to a
rectangle around the channel section.
fenetre('Manip16\Cam87\16-87_',1,80,240,370,585,445);
imdec = path previously given to the extracted frames (input)
numdeb = first number of the sequence (input index)
numfin = last number of the sequence (input index)
x0 = abscissa of the top left corner of the rectangle
y0 = ordinate of the top left corner of the rectangle
x1 = abscissa of the bottom right corner of the rectangle
y1 = ordinate of the bottom right corner of the rectangle
for x=numdeb:numfin
x
name=['C:\MATLAB6p5\',imdec,num2str(x),'.bmp'];
a1=double(imread(name))/255;
a=a1(y0:y1,x0:x1);
name=[imdec,'ftr',num2str(x),'.bmp'];
imwrite(a,name,'bmp');
end
---------------------------------------------------------------------function []=ecrtype(ftr,nameref,deb,fin);
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This function performs the image processing based on the integral
standard deviation of the luminosity of Visiovis images, previously
isolated by the function decoupe.m and cutback by the function
fenetre.m
ecrtype
('Manip16\Cam87\16-87_ftr','Manip16\Cam87\16-87_ftr13',14,80);
ftr = path previously given to the extracted cutback frames (input)
nameref = path of the image chosen as a reference (image completely
black at the beginning of the sequence)
deb = first number of the sequence (input index)
fin = last number of the sequence (input index)
namer=['C:\MATLAB6p5\',nameref,'.bmp'];
a0deb=double(imread(namer));
a0deb=a0deb/max(max(a0deb));
a0deb=1-a0deb;
for x=1:fin-deb
X=x+deb
Antonella ESPOSITO
Appendix 2/6
name=['C:\MATLAB6p5\',ftr,num2str(X),'.bmp'];
a0=double(imread(name));
a0=a0.*(1-a0deb)+mean(mean(a0.*(1-a0deb)))*a0deb;
a1=a0-mean(mean(a0));
a1=(a1.^2/mean(mean(a0))^2).^0.5;
a(x)=mean(mean(a0));
end;
figure (1)
plot(a,'o-')
figure(2)
imagesc(a0)
dlmwrite([ftr,'ecrtype.txt'],a,'\t')
---------------------------------------------------------------------function []=normft(ftr,nameref,deb,fin,freq);
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This function performs the image processing based on the Discrete
Fourier Transform (DFT) of the textured numerical images acquired by
Visiovis, and previously isolated by the function decoupe.m and
cutback by the function fenetre.m
normft
('Manip16\Cam87\16-87_ftr','Manip16\Cam87\16-87_ftr13',14,80,150);
ftr = path previously given to the extracted cutback frames (input)
nameref = path of the image chosen as a reference (image completely
black at the beginning of the sequence)
deb = first number of the sequence (input index)
fin = last number of the sequence (input index)
freq = value of frequency estimated adapted to the processed images
and used for the calculation of the Fourier Transform (150)
name=['C:\MATLAB6p5\',ftr,num2str(deb),'.bmp'];
adeb=double(imread(name));
namer=['C:\MATLAB6p5\',nameref,'.bmp'];
a0deb=double(imread(namer));
a0deb=a0deb/max(max(a0deb));
a0deb=1-a0deb;
A0=fftshift(fft2(adeb));
nA0=(A0.*conj(A0));
%.^0.5;
sn=size(nA0);
xx=zeros([sn(1)*sn(2),1]);
yy=zeros([sn(1)*sn(2),1]);
a=zeros([freq,fin-deb]);
nomb=zeros([freq,fin-deb]);
Antonella ESPOSITO
Appendix 3/6
for x=1:fin-deb
X0=x+deb
name=['C:\MATLAB6p5\',ftr,num2str(X0),'.bmp'];
a0=double(imread(name));
a0=a0.*(1-a0deb)+mean(mean(a0.*(1-a0deb)))*a0deb;
A=fftshift(fft2(a0));
nA1=(A.*conj(A));
%.^0.5;
[cx,cy]=find(nA1==max(max(nA1)));
nA=nA1-nA0;
s=size(nA);
for x1=1:s(1)
for y1=1:s(2)
X=2*((x1-cx(1))/s(1))+0.01;
Y=2*((y1-cy(1))/s(2))+0.01;
R=min(s)/2;
if (abs(X)>=abs(Y))
t=Y/X;
X=X*cos(atan(t));
Y=Y*sin(atan(t));
end;
if (abs(Y)>=abs(X))
t=X/Y;
X=X*sin(atan(t));
Y=Y*cos(atan(t));
end;
r=round(R*(X^2+Y^2).^0.5)+1;
vie(x1,y1)=r;
if (r<freq)
a(r,x)=a(r,x)+nA(x1,y1);%*r;
nomb(r,x)=nomb(r,x)+1;
end;
end;
end;
end;
figure(4)
imagesc(vie)
af=a./(nomb+1);
saf=size(af)
rx=1:saf(1);
af=flipud(af);
laf=log(af-min(min(af))+1);
laf=laf-min(min(laf));
for x=1:fin-deb
maf(x)=mean(laf(:,x).*rx')/mean(laf(:,x));
end;
laf=laf-mean(mean(laf))+2;
laf=laf.*(sign(laf)+1)/2;
af=flipud(af);
figure(1)
surf(laf)
Antonella ESPOSITO
Appendix 4/6
shading interp
colormap jet
hold on
contour3 (laf,20,'r')
figure(2)
pcolor(laf)
shading interp
colorbar
dlmwrite([ftr,'normft.txt'],maf,'\t');
figure(3)
plot(maf(1:end),'o-')
---------------------------------------------------------------------function []=video(nameim,num);
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Compilation and revision by Antonella Esposito
INSA de Lyon ® 2008
This function reconstruct a video from a sequence of frames (whether
cutback by the function fenetre.m or not) and generates a montage of
the video – a panel visualizing at a glance all the frames used for
reconstruction (the montage is useful only for the windows cutback
around the channel section).
video('Manip16\Cam87\16-87_',80);
nameim = path of the frames to be used for reconstruction (input)
num = total number of frames
nameref=['C:\MATLAB6p5\',nameim,'1.bmp'];
for x=1:num
x
name=['C:\MATLAB6p5\',nameim,num2str(x),'.bmp'];
[multim(:,:,:,x),map]=imread(name,'bmp');
imshow(multim(:,:,:,x),map);
tmp=getframe;
vid(:,x)=tmp;
end
m=montage(multim)
saveas(m,'montage','emf')
movie2avi(vid,'video','fps',10,'compression','none','colormap',gray)
info=aviinfo('video')
Antonella ESPOSITO
Appendix 5/6
---------------------------------------------------------------------function [a]=spctr(nom,deb,fin)
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INSA de Lyon ® 2008
This function visualizes the fluorescence emission spectra regularly
acquired by Visiovis as a 3D shaded surface plot on a rectangular
region delimited by the processing time [s] and the wavelength [nm].
spctr('exp1_00',1,112);
nom = path and filename of the sequence of spectra (input)
deb = number of the first spectrum
fin = number of the last spectrum
for x=1:fin-deb
nm=[nom,num2str(deb+x),'.txt'];
a0=dlmread(nm,'\t',[18,1,2065,1]);
xaxis(1,x)=x*3;
a(:,x)=a0(:,1);
end
yaxis=xlsread('yaxis');
surf(a,'XData',xaxis,'YData',yaxis)
shading interp
xlabel('Time [s]')
set(gca,'XDir','reverse')
ylabel('Wavelength [nm]')
zlabel('Fluorescence emission intensity [a.u.]')
----------------------------------------------------------------------
Antonella ESPOSITO
Appendix 6/6
Résumé détaillé en français
MISE EN ŒUVRE DE NANOCOMPOSITES. Mélange en voie fondue.
Les propriétés des matériaux polymères dépendent directement de leur chimie. Le seul
moyen de changer les propriétés d’un matériau polymère sans changer sa nature chimique la
plus intime est le compoundage – l’introduction d’additifs chimiquement et/ou physiquement
hétérogènes et leur dispersion dans la matrice polymère d’accueil, qui se fait typiquement en
voie fondue. Les facteurs responsables de la majorité des problèmes liés au compoundage sont :
 la chimie de la matrice polymère d’accueil ;
 les propriétés physico-chimiques des additifs ;
 le taux d’additifs dans le mélange ;
 le procédé de mise en oeuvre choisi pour le compoundage.
Antonella ESPOSITO
Thèse INSA de Lyon (2008)
Résumé détaillé en français 1/20
Visualisation de l’écoulement dans un système vis/fourreau. Suivi en ligne du mélange polymère/nanocharges.
La qualité et les performances des matériaux composites à matrice polymère dépendent
certainement des propriétés intrinsèques des matériaux dont ils sont constitués, mais aussi de la
capacité du procédé de mise en œuvre à obtenir la meilleure dispersion possible de tous les
composants du mélange dans la totalité du volume du polymère transformé – afin que l’on
puisse assumer que toute échantillon de matériau composite, quelle que soit son origine, ait les
mêmes propriétés souhaitées.
Traditionnellement, le compoundage s’articule en trois étapes, toutes essentielles pour
obtenir de bons résultats : l’alimentation, le mélangeage et la granulation. On remarquera que
l’extrusion et l’injection – les procédés de mise en œuvre des matériaux plastiques les plus
diffusées – comportent toutes les deux une étape de mélange en voie fondue : l’objectif de
l’extrusion est la formulation des granulés composites, qui seront ensuite refondus lors de
l’injection moulage pour enfin obtenir des pièces ayant la forme souhaitée. On remarquera
également que l’extrusion et l’injection partagent la même complexité géométrique des outils de
transformation, car toutes les deux sont réalisées à l’aide de systèmes vis/fourreau ayant des
profils de vis adaptés à chaque application et à chaque matériau. La question que l’on se pose
est donc la suivante : comment se comportent les additifs renforçants (les charges) quand ils
sont mélangés à un polymère fondu transformé dans un système vis/fourreau ?
Pour que le procédé de mise en œuvre soit performant, le mélange en voie fondue doit
l’être aussi – ce qui signifie que les performances globales sont influencées aussi bien par le
choix de l’outil de transformation (conception et sélection du profil de la vis) que par son
utilisation (optimisation des paramètres de mise en œuvre). Le meilleur choix repose sur la
définition des résultats que l’on veut obtenir et de comment peut-on les obtenir – autrement dit,
le meilleur choix dérive de la compréhension des phénomènes (c’est-à-dire les mécanismes de
mélange, dispersion et distribution des charges) mais aussi de la connaissance et de la maîtrise
des outils de transformation dont on dispose à l’heure actuelle (les systèmes vis/fourreau). Les
systèmes vis/fourreau pour la transformation des matériaux polymères et de leurs composites
sont identifiés par le diamètre nominal de la vis (D) et par la longueur de sa portion filetée (L),
cette dernière étant composée de trois sections géométriquement bien distinctes : la section
d’alimentation (chenal à profil constant et plutôt profond), la section de transition (chenal à
profil variable et profondeur décroissante) et la section de pompage (chenal à profil constant et
profondeur réduite). Nous nous sommes intéressés seulement à la section de pompage, dans
laquelle le polymère est entièrement fondu et qui transporte et mélange davantage la matière
pour en assurer l’homogénéité physique, chimique et thermique jusqu’à son injection dans le
Antonella ESPOSITO
moule. Mélanger consiste à utiliser l’énergie mécanique pour développer un champ de vitesse et
induire ainsi du mouvement au sein du fluide transformé, de façon à ce que tout champ de
concentration à gradient initialement élevé soit homogénéisé. Malgré le caractère universel du
mélange, les phénomènes qui le dominent sont encore méconnus et pas tout à fait compris et
maîtrisés. S’il est vrai que les aspects cinématiques et les différentes échelles (temporelle et
spatiale) du mélange de deux fluides ont été analysés et décrits, on ne peut tout de même pas
ignorer que les récents progrès réalisés par la science et le génie des matériaux n’arrêtent pas de
lancer des défis de plus en plus ardus aux ingénieurs procédés. Les nanocomposites à matrice
polymère et à base d’argile attirent encore (et depuis quelque temps, désormais) une attention
croissante, tant de la part du monde académique que de l’industrie, imposant ainsi des critères
d’homogénéité du mélange plus stricts et étendus jusqu’aux plus petites échelles. Pour que ces
avancements technologiques soient également intéressants du point de vue économique,
l’objectif à se donner aujourd’hui est la réalisation de mélanges homogènes sur plusieurs
échelles et, surtout, jusqu’à l’échelle moléculaire – si possible en se servant des outils de
transformation traditionnels, opportunément optimisés.
Traditionnellement les écoulements sont étudiés grâce à des expériences de visualisation
– plus précisément, depuis que Reynolds (1883) découvrit l’existence des différents régimes
d’écoulement (laminaire, turbulent et de transition) en visualisant le comportement d’un traceur
coloré injecté isocinétiquement au centre d’un cylindre transparent dans lequel un fluide
transparent s’écoulait avec un débit connu. Le régime d’écoulement laminaire, en particulier, est
observé lorsque les fluides s’écoulent à faible vélocité, et d’autant plus facilement s’ils sont
aussi hautement visqueux. Comme les polymères (thermoplastiques) à l’état fondu possèdent
généralement des viscosités élevées, et comme le volume mis à disposition pour l’écoulement
dans les systèmes vis/fourreau est réduit, le seul régime d’écoulement que l’on puisse observer
pour un polymère fondu dans le chenal d’une vis d’extrusion ou d’injection est le régime
d’écoulement laminaire : les mécanismes responsables du mélange distributif, donc, se réduisent
à une séquence d’étirement, découpage et recombinaison de lamelles de fluide qui glissent les
unes sur les autres sans se croiser et qui, en s’étirant, deviennent plus minces et facilitent la
diffusion moléculaires entre couches voisines. Pour que le mélange soit efficace, l’approche
traditionnelle exige que les lamelles de fluide soient assez fines et reparties de façon homogène
dans la totalité du volume de matière transformée : évidemment, ces critères ne suffisent plus
lorsque l’on s’attaque aux nanocomposites à matrice polymère et à base de charges lamellaires
(la diffusion des macromolécules de polymère à l’intérieur des galeries étant considérée comme
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un des mécanismes responsables de l’exfoliation, c’est-à-dire du mélange dispersif). D’ailleurs,
si les expériences de visualisation à l’aide de traceurs colorés ont – depuis désormais plus d’un
siècle et suite à l’exemple donné par Reynolds – contribué à la compréhension des mécanismes
d’écoulement et de mélange entre deux fluides, rares sont les applications de telles méthodes
(ou d’autres méthodes similaires) à l’étude de la distribution et dispersion de charges au sein
d’un fluide. Cette observation est d’autant plus légitime car il s’agit, ici, d’étudier le mélange
d’un polymère fondu avec des charges lamellaires (qui ont une structure complexe multi échelle
et aucune propriété optique native remarquable) dans un système ayant une géométrie complexe
(système vis/fourreau). La question que nous nous sommes posée ensuite est donc la suivante:
comment visualiser le procédé de mise en œuvre des nanocomposites à matrice polymère et à
base d’argile, sachant que ni les outils de transformation traditionnels (à parois opaques) ni les
argiles (optiquement inertes) ne permettent la visualisation ?
Le fait que les particules du renfort puissent se briser ou s’agréger davantage pendant le
mélange, ne fait que rendre l’analyse de l’écoulement encore plus complexe. Lorsqu’il s’agit
d’étudier simultanément l’écoulement d’un polymère fondu et son mélange avec des charges
lamellaires, le mélange dispersif (fortement lié aux propriétés physico-chimiques des particules
inorganiques, c’est-à-dire à leur taille, leur morphologie et leur compatibilité avec la matrice
organique) devient, évidemment, aussi important que le mélange distributif (dépendant plutôt de
la géométrie de l’outil de transformation, ainsi que des paramètres de la mise en œuvre). Les
notions d’agrégation et d’agglomération (et aussi de dispersion et de distribution) sont plutôt
génériques, certes, mais suffisamment précises pour décrire les macro- et micro-composites,
dans lesquels les particules du renfort sont assez grandes et ont une morphologie assez simple.
En revanche, la morphologie des nano-composites à base d’argile peut être complexe et
certainement plus variée – de ce fait, une utilisation équivalente des mêmes notions crée souvent
de la confusion et se révèle parfois erronée. La littérature montre clairement que des
morphologies nanocomposites peuvent être obtenues par mélange en voie fondue de polymères
avec des charges lamellaires : nombreux sont les travaux qui ont été consacrés à la modification
de la chimie de surface des argiles (compatibilisation) mais, mises à part les prédictions d’une
probable influence des paramètres de mise en œuvre sur les morphologies obtenues, aucun
travail n’a encore propose de conclusions univoques à ce sujet. En relation aux avantages des
nanocomposites par rapport aux composites traditionnels : la littérature d’aujourd’hui abonde de
revues et travaux expérimentaux plus ou moins pointus célébrant les améliorations présumées
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que l’on obtiendrait grâce à des morphologies nanostructurées. Oui, mais à quelle échelle de
production arrive-t-on à assurer le contrôle d’une morphologie nanocomposite ?
La révolution opérée par les morphologies nanocomposites repose sur la création d’une
interface exceptionnellement grande que les nanocharges mettent à disposition pour établir des
interactions avec la matrice – d’où la présence de fortes interactions entre les particules mêmes
et, donc, la tendance à former des agrégats. Les premières utilisations des argiles avaient pour
objectif de renforcer la matrice organique par introduction d’agrégats inorganiques de taille
micrométrique. Les résultats obtenus, pourtant, n’étaient pas manifestement meilleurs par
rapport aux résultats déjà garantis par les composites traditionnels : les argiles sous forme
d’agrégats n’ont pas d’avantages par rapport aux charges micrométriques traditionnelles, car
l’intérêt majeur leur dérive seulement des feuillets élémentaires, et ces derniers ne peuvent être
exploités que si l’on disperse parfaitement les argiles au sein de la matrice polymère. C’est pour
cela que les nanocomposites polymère/argile n’ont fait leur apparition sur le marché que bien
après les premiers brevets sur leur fabrication ; pour cette même raison, ces matériaux innovants
peinent à trouver leur marché de niche… une nouvelle méthode pour le suivi en ligne des
évolutions morphologiques des mélanges polymère/argile pendant le procédé de mise en œuvre
pourrait contribuer de façon significative au développement des nanocomposites à base de
charges lamellaires.
La morphologie des nanocomposites à base de charges lamellaires est difficile à
caractériser et, comme le montrent les discordances parfois présentes dans la littérature, presque
autant difficile à décrire. Le niveau technique atteint de nos jours mets à notre disposition de
nombreuses techniques de caractérisation, aussi bien ex situ que in situ. Les techniques ex situ
les plus communes sont la Diffraction des Rayons X (DRX) et la Microscopie Electronique à
Transmission (MET), mais d’autres techniques ont été utilisées – la Microscopie Electronique à
Balayage (MEB), la Calorimétrie Différentielle à Balayage (DSC), la rhéologie, la Microscopie
à Force Atomique (AFM), les techniques de diffusion des rayons X (Wide-Angle et Small-Angle
X-ray Scattering, WAXS et SAXS), la Résonance Magnétique Nucléaire (RMN) à l’état solide.
Chacune de ces techniques possède des avantages et des désavantages ; le principal désavantage
commun à toutes les techniques de caractérisation ex situ reste, en tous cas, l’impossibilité de
corréler de manière directe et univoque la morphologie obtenue au procédé de mise en œuvre
par lequel il a été possible de l’obtenir. Bref, les techniques ex situ ne démystifient pas vraiment
le procédé de mise en œuvre qui, au contraire, demeure inconnu, tel qu’une « boite noire ».
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Les techniques in situ ont été traditionnellement développées pour des études de
dynamique des fluides, plutôt que pour des analyses morphologiques de fluides mélangés à des
additifs (ou bien à des charges). Comme cela avait été le cas pour Reynolds, c’est le fait d’avoir
visualisé ce qui se passe pendant le procédé de mise en œuvre (Maddock, 1959) qui a indiqué le
chemin aux chercheurs. Mais Maddock venait tout juste de lancer une idée, car son système ne
permettait pas vraiment le suivi en ligne : le procédé devait être stoppé pour pouvoir récupérer
les informations. Les techniques in situ telles que le moulinet, la Vélocimétrie Doppler par
Laser (LDV), la Vélocimétrie par Imagerie de Particules (PIV), la Vélocimétrie Doppler par
Ultrasons (UDV) ont été généralement développées à l’échelle des laboratoires de recherche,
qui disposent d’équipements et de savoir-faire adaptés. Mais d’autres techniques in situ ont été
développées, et ont rencontré un discret succès même en dehors des laboratoires de recherche
fondamentale : c’est le cas des techniques qui se servent de sondes locales comme les pigments
(colorimétrie), les fluorophores, les traceurs ayant une signature infrarouge reconnaissable ou
des propriétés spécifiques de conductivité, les sondes sensibles aux variations de pH, etc. Ces
techniques (et en particulier la Fluorescence Induite par Laser, LIF) sont typiquement utilisées
pour mesurer des valeurs locales (moyennées dans le temps) de concentration, afin d’estimer le
temps de mélange, ou encore pour calculer la Distribution des Temps de Résidence (RTD) du
matériau à l’intérieur de l’outil de transformation. Les sondes employées représentent rarement
des additifs intéressants pour la qualité du mélange : en général elles n’apportent aucune
propriété – mise à part, évidemment, celle qui permet le suivi. On est donc obligé d’accepter le
risque que la présence d’un matériau sonde puisse modifier les écoulements et, par conséquent,
fausser la visualisation…
Une dernière possibilité pour la caractérisation morphologique des écoulements lors des
procédés de mise en œuvre des polymères est issue de la simulation numérique effectuée grâce
aux ordinateurs, qui ont une puissance de calcul de plus en plus développée. La simulation
numérique permet, en effet, de « visualiser » les zones de mélange chaotique et d’estimer, de
façon quantitative, le mélange laminaire, tout en évitant la difficulté des parois opaques des
outils de transformation… ceci dit, toute simulation devrait être validée par l’expérience.
Ce travail de thèse s’inscrit dans le contexte que nous venons de décrire : les récents
progrès accomplis par les ingénieurs et les chimistes poussent le marché à se tourner vers les
matériaux nanocomposites polymère/argile, mais aucun marché significatif ne pourra s’ouvrir si
aux avantages technologiques n’est associé aucun avantage économique. Le seul moyen de
rendre cela économiquement avantageux est d’adapter les outils de transformation déjà
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disponibles à la mise en œuvre de cette nouvelle famille de composites – étape qui ne pourra pas
être franchie à moins de passer par la compréhension des phénomènes responsables des résultats
que l’on voudrait obtenir, la profonde connaissance des outils dont on dispose actuellement et la
capacité de prévoir des modifications qui pourraient les adapter aux nouveau objectifs que l’on
se donne. Motivés par ce contexte, nous avons essayé de répondre tout d’abord aux questions
techniques que la problématique soulève : comment visualiser des charges lamellaires
inorganiques ayant une structure multi-échelle et optiquement inertes (1) pendant leur mélange
avec un polymère fondu visqueux réalisé à l’aide d’outils à parois opaques (2) ? Nous avons
surmonté la première difficulté technique en rédigeant et puis en mettant en place un protocole
de photo-fonctionnalisation d’argiles commerciales. La différence principale par rapport aux
méthodes classiques de traçage (LDV, PIV) et aux méthodes qui se servent de sondes locales
plus traditionnelles (LIF) est que, dans notre cas, ce sont les charges elles-mêmes qui font office
de sondes – autrement dit, tous les ingrédients contribuent activement aux propriétés finales du
mélange. De plus, en contrôlant le comportement en fluorescence des charges lamellaires photofonctionnalisées et, surtout, en corrélant leur émission aux possibles morphologies (agglomérée,
intercalée, exfoliée), le suivi en ligne du mélange polymère/argile aurait un avantage double :
 les dimensions réduites des argiles photo-fonctionnalisées (même si agrégées, elles ne
mesurent pas plus d’une dizaine de microns) en feraient des traceurs optimaux pour visualiser le
mélange distributif sans perturber l’écoulement ;
 les propriétés optiques des argiles photo-fonctionnalisées pourraient être, selon la
nature de la molécule fluorescente sélectionnée, sensibles aux changements de l’environnement
et fournir par spectrofluorimétrie des renseignements à propos du mélange dispersif.
Par rapport à la deuxième difficulté technique : nous avons repris une maquette froide
(Visiovis) qui avait été assemblée auparavant dans nos laboratoires et qui présentait l’avantage
considérable d’être constituée d’un système vis/fourreau dont le fourreau est entièrement
transparent, mais qui avait été initialement conçue pour d’autres objectifs (Moguedet, 2005), et
nous l’avons adaptée et développée davantage pour qu’elle nous permette de visualiser le
mélange d’un polymère fondu avec les argiles précédemment photo-fonctionnalisées.
PHOTO-FONCTIONNALISATION. Charges lamellaires.
Le but principal de cette première partie du travail de thèse étant de trouver une méthode
de photo-fonctionnalisation qui puisse rendre les charges lamellaires optiquement actives, nous
avons d’abord sélectionné une argile commerciale organiquement modifiée (Cloisite ® 30B)
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parmi les plus citées dans la littérature et trois molécules fluorescentes (9-anthracenemethanol,
Nile Blue A Perchlorate et Rhodamine 6G Perchlorate), et nous avons testé plusieurs méthodes
de photo-fonctionnalisation : (A) le gonflement libre de l’argile dans une solution contenant la
molécule fluorescente (9-anthracenemethanol), (B) le mélange en voie sèche de l’argile avec la
molécule fluorescente (9-anthracenemethanol) à l’aide d’un malaxeur, (C) le mélange en voie
fondue de l’argile avec la molécule fluorescente (9-anthracenemethanol) et enfin (D) l’échange
cationique de l’argile avec une molécule fluorescente ionique (Nile Blue A Perchlorate et
Rhodamine 6G Perchlorate). Les échantillons ainsi modifiés ont été caractérisés (avant et après
lavage, si possible) par Diffractométrie des Rayons-X (DRX), Analyse ThermoGravimétrique
(ATG), Analyse Elémentaire (AE), Spectroscopie InfraRouge par Transformée de Fourier
(IRTF), ATG couplée IR (ATG-IR) et spectrofluorimétrie. La détermination de la configuration
des molécules fluorescentes suite aux interactions avec la structure inorganique qui les accueille
est indispensable pour comprendre le mécanisme de photo-fonctionnalisation (évaluer s’il s’agit
de l’adsorption de monomères, dimères ou agrégats d’ordre supérieur), pour juger la qualité des
complexes inorganiques-organiques photo-fonctionnels obtenus (c’est-à-dire pour vérifier qu’ils
soient effectivement photo-actifs et comprendre quelles informations peut-on déduire à partir
des mesures de fluorescence) et utiliser les charges photo-fonctionnalisées de manière efficace
et pertinente pour le suivi en ligne du mélange polymère/argile. En général, l’objectif de la
photo-fonctionnalisation est de greffer/adsorber une molécule fluorescente (espèce optiquement
active) sur les feuillets de silicate, aux bords de ces mêmes feuillets, ou encore de les introduire
à l’intérieur des galeries comprises entre les feuillets. Les techniques de caractérisations ont été
utilisées pour : vérifier que la photo-fonctionnalisation a bien eu lieu ; essayer de comprendre où
est-ce que les molécules fluorescentes ont été adsorbées (à l’extérieur ou bien à l’intérieur des
espaces interfoliaires) ; prouver l’efficacité du lavage suivant la photo-fonctionnalisation ; tester
l’activité optique des complexes inorganiques-organiques obtenus.
La méthode (A) était censée faciliter la diffusion des molécules fluorescentes dans les
galeries – grâce à la tendance prononcée que les argiles ont à gonfler dans un milieu aqueux ou
solvaté (exceptionnellement, dans notre cas, le milieu responsable du gonflement devait être
aussi un bon solvant pour la molécule fluorescente). Que ce soit avec ou sans la molécule
fluorescente, les gels que nous avons obtenus étaient à chaque fois bien gonflés et homogènes,
mais difficiles à sécher : même si quelques unes des techniques de récupération/séchage que
nous avons essayées étaient plus efficaces que d’autres, nous ne sommes jamais parvenus à
éliminer complètement le solvant résiduel.
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Les problèmes rencontrés lors de l’élimination complète du solvant résiduel nous ont
forcés à chercher une méthode alternative, qui ne demande aucun solvant. En effet, la méthode
(B) nous a clairement montré tous les avantages de l’absence de solvants ; pourtant, elle n’est
pas adaptée à la photo-fonctionnalisation des charges lamellaires, car en absence d’une phase
liquide (quelle qu’elle soit sa nature) les molécules ne peuvent pas vraiment diffuser – d’ailleurs
le mélange n’est pas non plus efficace, car une phase sèche n’est pas bien capable de transférer
le cisaillement imposé par le malaxeur.
La méthode (C) a été donc mise en place en prenant inspiration des avantages des deux
autres méthodes : nous voulions profiter des mécanismes de diffusion, sans pour autant utiliser
un milieu aqueux ou solvaté – nous avons donc choisi de remplacer les solvants par la phase
liquide obtenues par fusion de la molécule fluorescente. Bien que cette méthode nous ait donné
des gels équivalents (en termes de gonflement) à ceux obtenus par la méthode (A), le besoin
d’un excès remarquable de molécule fluorescente ne correspond pas à des critères économiques.
Nous avons constaté que, malheureusement, une modification réalisée avec une molécule neutre
(9-anthracenemethanol) n’est pas stable – au contraire, elle est tout à fait réversible et ne résiste
pas aux lavages.
La méthode (D) dérive des techniques d’échange cationique traditionnellement utilisées
pour caractériser et fonctionnaliser les minéraux argileux et qui, en particulier, sont souvent
adoptées pour modifier le caractère hydrophile des argiles en organophile. L’idée d’effectuer un
deuxième échange avec une molécule fluorescente ionique est inspiré des travaux récents d’un
groupe de chercheurs du National Institute of Standards and Technology (NIST) à Gaithersburg,
Maryland (USA). Cette méthode – que nous avons réalisée avec deux molécules fluorescentes à
deux différentes concentrations dans le milieu d’échange – s’est révélée encore une fois la plus
efficace pour la fonctionnalisation des charges lamellaires gonflantes – que ce soit avec le Nile
Blue A Perchlorate ou la Rhodamine 6G Perchlorate.
Nous avons ensuite décidé d’optimiser les paramètres pour l’échange cationique (D) et
de définir la nature et la concentration de la molécule fluorescente la plus efficace. Pour ce faire,
nous avons tout d’abord comparé les résultats obtenus en échangeant la même argile avec deux
concentrations différentes de la même molécule fluorescente – correspondantes à 100% et 25%
du taux de surfactant présent dans les galeries de la Cloisite ® 30B (dénommées 1MC et
0.25MC, respectivement). Effectivement, lorsqu’il s’agit de fluorescence, la concentration de
fluorophore devient vite un factor primordial à cause du risque de quenching (diminution, voir
disparition totale de l’intensité émise) et de la tendance spontanée des molécules fluorescentes
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(surtout ioniques) à s’assembler en structures supramoléculaires, lesquelles peuvent parfois être
désirables mais doivent être tout de même maitrisées. Nous avons montré que l’échange
cationique en présence d’un excès de molécule fluorescente est possible mais pas nécessaire, car
une fraction considérable de molécules fluorescentes ne pénètre pas dans les galeries et, même
si c’est le cas, ne subit l’échange et est donc éliminée lors des lavages. En conclusion, nous
avons retenu une concentration optimale correspondante à 25% du taux de surfactant présent
dans les galeries de l’argile organiquement modifiée.
Par rapport aux choix de la molécule fluorescente: sur la base de plusieurs évidences
expérimentales, nous nous sommes plutôt orientés vers la Rhodamine 6G Perchlorate – laquelle
s’est révélée non seulement plus efficace en termes d’échange cationique et d’expansion des
galeries des argiles, mais aussi mieux détectables par les différentes techniques d’analyse
employées (DRX et ATG, plus particulièrement). En plus, les plages d’absorption et d’émission
de la Rhodamine 6G Perchlorate nous sont apparues mieux adaptées aux caractéristiques
techniques de l’outil de visualisation pour lequel nous avions entrepris cette démarche de photofonctionnalisation. Nous avons aussi constaté que, indépendamment de sa concentration, la
Rhodamine 6G Perchlorate est responsable d’un gonflement qui correspond toujours à une
distance interfoliaire de 22 Å : nous avons émis l’hypothèse (supportée par des cas similaires
présents dans la littérature et concernant l’absorption de molécules de solvant dans les galeries
d’argiles organiquement modifiées) que les molécules fluorescentes diffusent dans les galeries
et s’intercalent entre les chaînes carbonées initialement présentes, en remplissant ainsi le sites
ioniques qui n’ont pas été occupés à cause de l’encombrement stérique, de façon à entourer la
base des chaînes aliphatiques, à les supporter et à forcer l’angle initial de la configuration
paraffinique à subir une augmentation (de 37° à 49°). Le remplissage des sites ioniques vacants
par les molécules fluorescentes accroît la fraction de matière organique stablement adsorbée
dans les galeries et, par conséquent, rend le « recouvrement » des surfaces inorganiques plus
efficace, améliorant ainsi l’efficacité globale de l’échange cationique et regagnant une Capacité
d’Echange Cationique (CEC) plus importante. Ce phénomène se vérifie chaque fois que les
galeries d’argile se trouvent à accueillir deux espèce chimiques de taille différente : l’efficacité
d’occupation du volume globalement disponible au sein des galeries en est améliorée. Le
recouvrement de la CEC est le seul aspect qui pourrait éventuellement rendre le Nile Blue A
Perchlorate préférable par rapport à la Rhodamine 6G Perchlorate – du moins il a le mérite de
nous avoir révélé que l’efficacité des échanges cationiques ne dépend pas seulement de la
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concentration de surfactant et de la température, mais aussi de la nature chimique, de la taille
maximale (Dmax), de la configuration et du volume occupé par ses molécules.
Enfin, en comparant par ATG les différents échantillons (avant et après lavage) obtenus
avec les deux molécules fluorescentes et les deux concentrations, nous avons remarqué que :
 toute argile photo-fonctionnalisée lavée est plus stable (ou, du moins, autant stable)
que la Cloisite ® 30B lavée – ce qui signifie que la photo-fonctionnalisation ne diminue pas la
stabilité thermique des argiles ;
 toute argile photo-fonctionnalisée est stabilisée par le lavage (y comprise la Cloisite
® 30B commerciale) ;
 les deux molécules fluorescentes sont stabilisées, pour T>300°C, grâce à la protection
thermique (propriété barrière) des feuillets de silicate ;
 dans tous les cas, l’argile la plus stable est celle qui a été échangée avec 1MC de
molécule fluorescente, et puis lavée ;
 dans tous les cas, l’argile la moins stable est celle qui a été échangée avec 1MC de
molécule fluorescente, mais pas lavée.
COMPLEXES PHOTO-FONCTIONNELS. Echange cationique.
Nous avons donc mis au point et optimisé un protocole de photo-fonctionnalisation pour
des charges lamellaires commerciales organiquement modifiées (Cloisite ® 30B) sélectionnées
parmi les plus utilisées : il s’agit d’un deuxième échange cationique réalisé dans une solution
90/10 eau permutée/éthanol à 80°C en présence d’une quantité de molécule fluorescente
ionique égale à 25% du taux de surfactant présent initialement dans les galeries de l’argile. En
prévision des applications que nous comptions faire des complexes photo-fonctionnels ainsi
obtenus, nous avons préféré la Rhodamine 6G Perchlorate au Nile Blue A Perchlorate.
En se basant sur le protocole précédemment rédigé, nous avons ensuite réalisé d’autres
complexes photo-fonctionnels et, ici, nous présentons et comparons les résultats obtenus à partir
de quatre argiles commerciales (Cloisite ® Na+, Cloisite ® 30B, Cloisite ® 10A et Cloisite ®
15A). En particulier, dans cette sélection nous avons inclus aussi une argile commerciale 100%
inorganique (Cloisite ® Na+) parce que nous estimions que le même procédé réalisé en absence
de molécules organiques dans les espaces interfoliaires ferait une bonne référence pour mieux
comprendre certains mécanismes d’adsorption et d’échange cationique. Les autres argiles ont
été sélectionnées sur la base de leur fréquence d’utilisation (on les retrouve souvent dans les
publications scientifiques) et de leur surfactant : la Cloisite ® 30B contient une longue chaîne
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aliphatique et deux courtes chaîne terminées –OH, la Cloisite ® 10A contient une longue chaîne
aliphatique et un cycle aromatique, la Cloisite ® 15A contient deux longue chaîne aliphatiques.
L’argile hydrophile et ces argiles organophiles présentent des distance interfoliaire variables :
12.4 Å (Cloisite ® Na+), 17.5 Å (Cloisite ® 30B), 19.2 (Cloisite ® 10A), 25.7 (Cloisite ® 15A).
Après avoir caractérisé les matériaux dans leur état initial (les argiles et la Rhodamine 6G
Perchlorate), nous avons comparé les résultats obtenus pour la série de quatre argiles photofonctionnalisées. L’analyse élémentaire nous a fourni toutes les tendances attendues par rapport
aux pourcentages atomiques de Na, C, N, H et Si. En particulier, pour les argiles commerciales
pas encore fonctionnalisées mais lavées, nous avons observé que la variation en pourcentage de
C adsorbé est inversement proportionnelle à la masse molaire du surfactant – autrement dit, le
pourcentage de sites Na+ remplacés lors d’un échange cationique diminue d’autant plus que
l’encombrement stériques des molécules de surfactant augmente. Cette observation peut paraître
logique mais devient essentielle pour comprendre la notion de « recouvrement » et pour une
meilleure exploitation de la CEC d’une argile. En effet, l’analyse élémentaire nous montre que
le pourcentage atomique de N augmente pour toutes les argiles après photo-fonctionnalisation :
cette augmentation prouve qu’un deuxième échange cationique avec une petite molécule (plus
petite que le surfactant utilisé pour le premier échange cationique) permet de mieux exploiter la
CEC de l’argile et améliore donc le recouvrement de la surface inorganique par les molécules
organiques. Par spectrofluorimétrie nous avons vérifié que les quatre complexes inorganiquesorganiques sont effectivement photo-actifs. Nous avons remarqué un léger déplacement
hypsochromique seulement dans les spectres d’absorption et d’émission du complexe obtenu à
partir de la Cloisite ® 10A – ce qui pourrait être lié à la présence d’un cycle aromatique dans la
structure des molécules de surfactant. D’autres caractérisations en fluorescence, plus
développées et mieux adaptées, seraient nécessaires pour comprendre ce genre de phénomènes ;
l’objectif de la photo-fonctionnalisation dans ce travail de thèse était principalement de rendre
les argiles optiquement actives, et nous l’avons atteint.
PROCEDE DE MISE EN ŒUVRE. Suivi en ligne du mélange.
Les procédés de mise en œuvre tels que l’extrusion et l’injection moulage sont réalisés à
l’aide d’outils de transformation ayant des géométries plutôt complexes et, par conséquent, leur
suivi en ligne est particulièrement difficile. Nous présentons, ici, un outil original et innovant (la
Visiovis) intégralement conçu, assemblé et développé dans nos laboratoires sur une période
totale d’environ cinq ans – un peu plus que deux ans dans le cadre d’un précédent travail de
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thèse (Maël Moguedet, Thèse de Doctorat, 2005) et les trois ans prévus pour le travail de thèse
qui fait l’objet de ce manuscrit (Antonella Esposito, Thèse de Doctorat, 2008). Comme son nom
l’indique, la Visiovis est un outil consacré à la visualisation des procédés de mise en œuvre.
Pourquoi ? Parce que cette maquette comprend un système vis/fourreau qui a la particularité
d’être constitué d’un fourreau transparent – ce qui permet de visualiser les écoulements en
temps réel et dans toutes les directions. La Visiovis avait été initialement conçue pour visualiser
les trajectoires 3D d’une seule particule fluorescente plongée dans un fluide transparent : il
s’agissait, donc, d’un simple problème de dynamique des fluides. Au cours de ces trois
dernières années, nous avons légèrement modifié sa configuration initiale pour l’adapter au
suivi en ligne du mélange polymère/nanocharges (plus en particulier, pour la caractérisation de
la dispersion et distribution de charges lamellaires dans un polymère fondu) – ce qui pourrait
nous renseigner sur le procédé de mise en œuvre des nanocomposites par mélange en voie
fondue. Il faut dire que Moguedet et ses collaborateurs conçurent le système vis/fourreau de la
Visiovis pour que ses paramètres géométriques soient semblables aux valeurs caractéristiques
des sections de pompage des vis d’extrusion et d’injection industrielles.
Après avoir évalué les avantages et les désavantages du système dans sa configuration
initiale, nous l’avons modifié et y avons intégré un spectromètre afin d’en augmenter davantage
les potentialités.
En résumé, la Visiovis se compose actuellement de :
 un système vis/fourreau : une vis à pas carré (pas 40 mm, diamètre à cœur 30 mm) est
ajustée dans un fourreau en PMMA (diamètre 40 mm) avec une profondeur du chenal de 5 mm ;
 un moteur électrique qui actionne la vis (vélocité max 44 tr/min, couple max 9 N m) ;
 une ouverture qui permet d’introduire le fluide et le mélange maître avec le traceur ;
 un tuyau qui connecte la sortie à l’entrée et réalise un circuit fermé ;
 un support mobile autour du système pour installer ce qu’il faut pour la visualisation ;
 quatre caméras CCD (Basler A301F) alignées le long du fourreau : enregistrement de
80 images/s, résolution 640
480 pixels avec une profondeur de 8 bits (256 niveau de gris) ;
 une source laser pulsée ( 532 nm, puissance nominale 20 mW CW) ;
 un système optique qui crée une nappe laser (bidimensionnelle) à partir de la source
laser pulsée (monodimensionnelle) ;
 un transducteur électromécanique (essentiellement un interrupteur) qui connecte la
rotation de la vis aux caméras et pilote ainsi l’acquisition d’images (une image par tour de vis) ;
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 un spectromètre (USB2000+, Oceanoptics) interfacé avec le système vis/fourreau via
une fibre optique (diamètre 600 m, résolution 2.5 nm) positionnée à l’opposée des caméras ;
 un robinet situé au milieu du tuyau de recirculation, qui facilite la vidange.
Une fois que la configuration de la Visiovis a été modifiée et que les charges lamellaires
ont été photo-fonctionnalisées, nous avons effectué une calibration « qualitative » des systèmes
de détection (caméras CCD et spectromètre) – le but étant de vérifier d'abord que les complexes
photo-fonctionnels aient une émission en fluorescence suffisamment intense pour être captée et,
ensuite, d’estimer la concentration d’argile photo-fonctionnalisée nécessaire pour une détection
optimale. La calibration des systèmes de détection a été effectuée avec le complexe à base de
Cloisite ® 30B. Pour ce faire, nous avons réalisé plusieurs mélanges maîtres à base de Siliconöl
M10000 (huile silicone transparente – essentiellement du PDMS terminé méthyle – viscosité 10
Pa s) et ayant des concentrations d’argile photo-fonctionnalisée connues et contrôlées (0%,
0.001%, 0.0025%, 0.005%, 0.01%, 0.025%, 0.05%, 0.1%, 0.25%, 0.5%, 1% en poids). Les
mélanges maîtres ont été préparés avec un disperseur TurboTest Rayneri 33/300P équipé d’un
disque dilacérateur (diamètre 60 mm) tournant à 1000 tr/min pendant 20 min. Le choix de la
concentration optimale d’argile photo-fonctionnelle à introduire dans le mélange maître (qui
sera ensuite injecté dans le système vis/fourreau préalablement rempli de PDMS propre) n’est
pas simple et doit se baser essentiellement sur les limites de détection des caméras CCD et du
spectromètre, mais aussi sur la profondeur de pénétration de la nappe laser (qui doit être au
moins égale à la profondeur du chenal de la vis, c’est-à-dire au moins 5 mm). Sur la base de
toutes les évidences collectées lors de la calibration, nous avons trouvé que le meilleur protocole
pour effectuer les expériences Visiovis est d’injecter, dans le système vis/fourreau de la Visiovis
préalablement rempli de fluide modèle transparent, 10 mL d’un mélange maître préparé avec
0.25% en poids d’argile photo-fonctionnalisée et du même fluide modèle. N’ayant pas d’autres
fluides macromoléculaires transparents à proposer en alternative au PDMS, nous avons continué
à utiliser l’huile silicone comme matrice modèle. En ce qui concerne les charges photo-actives :
nous avons testé les quatre complexes photo-fonctionnels pour vérifier leur activité optique et,
curieusement, nous avons constaté que tous les complexes étaient correctement visualisées par
les caméras CCD sauf celui à base de Cloisite ® Na+. Pour cette raison, nous avons poursuivi
les expériences seulement avec les complexes photo-fonctionnels obtenus à partir des argiles
organiquement modifiées (Cloisite ® 30B, 10A et 15A). Par rapport à la modalité d’injection du
traceur dans le système vis/fourreau : nous avons conçu deux méthodes, mais nous en avons
utilisé seulement une (la seule possible, pour l’instant). La première méthode est plus adaptée à
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la modélisation du procédé d’extrusion (mélange en voie fondue de granulés de polymère pur
avec une charge sous forme de poudre, afin de formuler des granulés composites) et demande la
préparation d’un « mélange maître non homogénéisé » composé de trois couches : une couche
de poudre sèche entre deux couches de PDMS pur (volume total 10 mL, taux de poudre 0.25%
en poids). La seconde méthode (celle que nous avons utilisée) est plus adaptée à la modélisation
de la phase de pompage du procédé d’injection moulage (deuxième fusion et homogénéisation
des granulés composites, précédemment formulés par extrusion) et demande la préparation d’un
mélange maître ayant une bonne qualité (en termes de distribution et dispersion des charges),
d’un volume total de 10 mL, avec un taux de charges de 0.25% en poids. Toutes les expériences
ont été réalisées en injectant le mélange maître dans le système vis/fourreau via l’ouverture et
grâce à une seringue de 20 mL, coupée à son extrémité de manière à éviter tout cisaillement non
contrôlé. La vitesse de rotation de la vis a été réglée à 20 tr/min et les expériences ont été
réalisées dans le noir. Une fois le mélange maître injecté dans le système, l’acquisition des
données est automatique : pilotées par l’interrupteur, les caméras acquièrent une image par tour
de vis ; le spectromètre, de son côté, est programmé pour enregistrer, via la fibre optique, un
spectre d’émission de fluorescence toutes les trois secondes. Les deux systèmes de détection
sont indispensables, car seules les caméras ne suffisent pas à obtenir une caractérisation correcte
et complète du procédé de mise en œuvre des nanocomposites : la spectrofluorimétrie, sensible
à des phénomènes qui intéressent une échelle spatiale plus petite par rapport aux caméras,
pourrait donner quelques informations complémentaires – notamment concernant la dispersion
(intercalation et/ou exfoliation) des charges lamellaires.
Les images acquises avec la Visiovis peuvent être utilisées pour reconstruire des vidéos
(information qualitative qui permet d’évaluer directement et visuellement l’évolution temporelle
de la distribution des nanocharges dans le volume de fluide compris entre deux filets, la surface
de la vis et la paroi du fourreau), mais nous avons aussi essayé d’en extraire une information
quantitative en mettant au point deux traitements d’image qui peuvent être exécutés dans un
environnement Matlab. Les deux traitements se basent, respectivement, sur (1) l’intégrale de
l’écart type de luminosité des images et (2) la Transformée de Fourier (TF) d’images texturées.
Pour chaque image d’une séquence donnée, le traitement (1) calcule les valeurs locales de
l’écart type de la luminosité sur toute la surface de l’image et trace la courbe du logarithme de
l’intégrale de cet écart type en fonction du numéro qui identifie les images de la séquence.
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(1) INTEGRALE DE L’ECART TYPE DE LA LUMINOSITE
Interprétation. Théoriquement, plus faible est l’intégrale de l’écart type de la
luminosité, plus homogène est le matériau visualisé et donc plus efficace est le mélange
distributif.
Limites. L’intégrale de l’écart type de la luminosité ne sera jamais inférieure à
une valeur seuil à cause de la différence entre le mélange visualisé (jamais complètement
noir) et le profil de la vis (toujours complètement noir).
De plus, ce traitement ne détecte pas la morphologie des images texturées : deux
images ayant différente textures pourraient donner les même résultats en termes d’écart
type de luminosité.
La Transformée de Fourier (TF) est un algorithme qui peut codifier une image texturée
en utilisant les fréquences avec lesquelles ses unités texturales élémentaires se répètent (dans
notre cas, il s’agit de spirales ou volutes claires – produites par fluorescence – sur un fond noir).
Comme nous avions à faire avec des séries d’images numériques (constituées de pixels), nous
avons naturellement utilisé la Transformée de Fourier dans sa version Discrète (DFT). La DFT
d’une image numérique donne le spectre de toutes les fréquences comprises entre une fréquence
maximale
max
et une fréquence minimale
min.
Ces deux fréquences peuvent être attribuées à
deux propriétés intrinsèques de toute image numérique : la présence d’une matrice régulière de
pixels (niveau de détail max) et la dimension maximale de l’image (niveau de détail min),
respectivement. Une image finement texturée sera codifiée par des hautes fréquences dans le
domaine de Fourier, puisque les plus hautes fréquences peuvent décrire les plus petits détails.
Pour chaque image d’une séquence donnée, le traitement (2) effectue l’anamorphose de l’image,
calcule le logarithme de la norme au carré, soustrait le bruit d’une image vide choisie comme
référence, calcule l’intensité moyenne de chaque fréquence entre la minimale et la maximale et
trace la courbe des valeurs moyennes en fonction de la fréquence. Ensuite, à chaque image il
associe la fréquence la plus significative (autrement dit, la fréquence la plus probable du point
de vue statistique, que nous appellerons fréquence représentative) et trace la courbe de ces
dernières valeurs en fonction du numéro qui identifie les images de la séquence.
Si le premier traitement d’image est intuitif et immédiat, nous avons préféré valider le
deuxième en l’appliquant à une série d’images numériques synthétiques obtenues avec l’aide de
Yves Béreaux, un des collaborateurs de Maël Moguedet à l’époque de son travail de thèse. Nous
avons ainsi rapproché la modélisation numérique aux expériences de laboratoire – ce qui ouvre
d’autres perspectives pour les deux secteurs.
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(2) TRANSFORMEE DE FOURIER D’IMAGES TEXTUREES
Interprétation. Théoriquement, une fréquence nulle indiquerait un mélange
parfaitement homogène, tandis que des hautes fréquences indiqueraient la présence
d’unités texturales fines et régulières.
Si la miscibilité est parfaite… Plus la fréquence représentative est basse, plus
homogène est le matériau visualisé et donc plus efficace est le mélange distributif.
Si la miscibilité est partielle… Plus la fréquence représentative est haute, plus la
texture est fine et donc plus efficace (encore que incomplet) est le mélange distributif.
Limites. En réalité, on ne pourra jamais atteindre des fréquences nulles car toute
image numérique, par définition, a des dimensions finies (seulement une image infinie
peut avoir une fréquence nulle). Par conséquent, toute éventuelle fréquence nulle devrait
être interprétée en tant que valeur relative.
De plus, le contraste entre le matériau visualisé (jamais complètement noir) et le
profil de la vis (toujours complètement noir) génère une fonction de Heaviside (c’est-àdire une fonction marche) qui correspond à une texture artificielle toujours présente dans
les images.
Conscients du fait que le développement d’un outil pour la visualisation des procédés de
mise en œuvre des nanocomposites – surtout quand il est associé à la conception de nouveaux
systèmes de détection et caractérisation en ligne – n’est pas une mince affaire, nous avons
continué à tester le système en ayant comme objectif d’en reconnaître les limites mais aussi les
potentialités. Nous avons réalisé quelques expériences pour avoir des résultats préliminaires qui
puissent aider à développer davantage cet outil. En particulier, nous avons cherché à évaluer :
 les différences de comportement des trois charges lamellaires photo-fonctionnelles,
préalablement modifiées par échange cationique à partir de Cloisite ® 30B, 10A et 15A ;
 la possibilité d’utiliser la Visiovis pour des expériences de traçage conventionnelles
(c’est-à-dire en injectant un mélange maître chargé principalement d’argile non modifiée, mais
aussi d’une petite quantité de la même argile photo-fonctionnalisée) ;
 la façon dont la Visiovis visualise un changement de contre-pression.
Les résultats préliminaires n’ont révélé aucune différence majeure en termes de mélange
distributif entre les différents complexes photo-fonctionnels testés, et l’absence de différences
(observée par tous les système de détection et avec tous le traitements d’images) nous a fait
émettre l’hypothèse (successivement confirmée par DRX et rhéologie) qu’aucune des argiles ne
peut établir des interactions fortes avec la matrice PDMS – ce qui se traduit en une réduction de
l’efficacité du mélange distributif et dispersif, aggravée par le fait que le profil de la Visiovis
n’est pas le profil le plus adapté pour le mélange.
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De ce fait, nous avons continué à travailler de préférence avec le complexe photo-actif à
base de Cloisite ® 30B, et notamment nous avons préparé trois mélange maître pour essayer de
vraies expériences de traçage :
 un mélange maître contenant 0.25% en poids d’argile photo-fonctionnalisée et 0% en
poids d’argile non modifiée (taux total d’argile 0.25% en poids) ;
 un mélange maître contenant 0.25% en poids d’argile photo-fonctionnalisée et 0.75%
en poids d’argile non modifiée (taux total d’argile 1% en poids) ;
 un mélange maître contenant 0.25% en poids d’argile photo-fonctionnalisée et 2.75%
en poids d’argile non modifiée (taux total d’argile 3% en poids).
Nous avons constaté un problème majeur lié à l’augmentation du taux d’argile : le fait
que le fluide visualisé devienne de plus en plus opaque. Par conséquent, nous avons observé que
la profondeur de pénétration de la nappe laser diminue en dessous du seuil critique, et que les
particules d’argile diffusent beaucoup plus la lumière d’excitation et diminuent ainsi la clarté
optique du système de visualisation. Ce genre de phénomènes est invisible aux traitements
d’images – d’où l’intérêt à coupler tous les systèmes de détection. En revanche, les spectres en
fluorescence ne sont pas affectés par l’augmentation de concentration et continuent à détecter
seulement la portion d’argile photo-fonctionnalisée.
Pour évaluer les effets de la contre-pression, nous avons installé une vanne au milieu du
tuyau qui réalise le circuit fermé, à la sortie du système vis/fourreau. Nous avons utilisé cette
vanne pour régler le débit et, plus en particulier, nous avons effectué trois expériences avec :
 la vanne complètement ouverte (ce qui correspond au système non perturbé) ;
 la vanne ouverte à moitié ;
 la vanne complètement fermée (ce qui correspond à un arrêt complet du débit).
Tout système vis/fourreau est sujet à une contre-pression intrinsèque, due simplement au
fait que la sortie soit représentée par une section réduite par rapport au diamètre du fourreau. La
contre-pression agit contre le débit et, donc, fait obstacle à l’écoulement – ce qui fait augmenter
le temps de résidence et facilite la recirculation dans le chenal de la vis. Parfois, on joue sur la
contre-pression pour améliorer la qualité du mélangeage. Nous avons observé que le fait de
fermer à moitie la vanne de la contre-pression ne change pas la forme globale des courbes
obtenus avec les traitements d’image, mais produit quand même des changements remarquables
en termes de lissage et décalage verticales des courbes. En revanche, quand la vanne de la
contre-pression est complètement fermée, les courbes changent radicalement d’aspect et les
temps s’allongent considérablement à cause de la recirculation.
Antonella ESPOSITO
BILAN DE LA MISE AU POINT DE LA VISIOVIS.
Problèmes rencontrés et suggestions d’amélioration.
Pour conclure, nous avons fait le bilan des progrès déjà accomplis et des améliorations
qui devraient être encore apportées au système pour que la Visiovis soit vraiment un outil utile à
la visualisation et, ensuite, à la compréhension et à la maîtrise du procédé de mise en œuvre des
nanocomposites par mélange en voie fondue.
Fluide modèle. Nous avons constaté que le PDMS terminé méthyle n’est pas forcement
compatible avec les argiles commerciales organiquement modifiées – censées être compatibles
avec des polymères carbonées (plus nombreux et donc plus répandus que les polysiloxanes).
Dans la littérature nous avons trouvé quelques confirmations du fait que disperser des charges
dans n’importe quel polysiloxane est certainement un défi plus difficile à vaincre par rapport
aux polymères carbonées. Equilibrer la balance hydrophilie/hydrophobie n’est pas suffisant
pour rendre une charge compatible avec un polysiloxane : un contrôle plus strict de la chimie de
surface des charges, mais aussi de la chimie des macromolécules qui constituent la matrice, est
essentiel. D’autre part, la fourreau en PMMA n’est pas compatible avec des fluides réactifs, des
liquides précurseurs, des solvants (même en très faibles quantités), et ne peut pas être chauffé :
le cahier des charges concernant le fluide modèle idéal pour la Visiovis est lourd et pas facile à
respecter. Nous suggérons tout de même de songer à une modification des chaînes PDMS afin
d’améliorer la compatibilité entre les charges et la matrice sans pourtant interagir avec la
surface en PMMA du fourreau.
Charges photo-fonctionnelles. Dans l’hypothèse de trouver un fluide modèle mieux
adapté que le PDMS terminé méthyle, à l’heure actuelle on disposerait déjà de quatre charges
lamellaires photo-fonctionnelles – préparées à partir de la Cloisite ® Na+, de la Cloisite ® 30B,
de la Cloisite ® 10A et de la Cloisite ® 15A. Si aucun autre fluide n’est adapté pour remplacer
le PDMS terminé méthyle, nous suggérons de préparer une nouvelle série de complexes photofonctionnels contenant la Rhodamine 6G Perchlorate comme espèce optiquement active, et un
surfactant spécifiquement conçu pour assurer la compatibilité avec la matrice PDMS. Nous
avons déjà commencé à travailler en cette direction, mais notre travail n’a pas encore abouti par
manque de temps. Enfin, une dernière possibilité est offerte par la synthèse de billes de silice
ayant une morphologie cœur-écorce et qui pourraient envelopper la rhodamine. Dans ce cas, la
Visiovis visualiserait seulement le mélange distributif et le spectromètre ne serait plus essentiel.
Antonella ESPOSITO
Configuration générale de la Visiovis. Nous n’envisageons pas de modifications de la
configuration générale de la Visiovis. Nous n’avons pas remarqué de sérieux problèmes par
rapport à la position des caméras, à la position de la fibre optique, à la forme et position de la
nappe laser… La source d’excitation devrait être changée seulement si un autre fluorophore est
sélectionné pour remplacer la Rhodamine 6G Perchlorate – mais nous suggérons vivement de ne
pas changer de fluorophore. Le fourreau en PMMA représente en même temps une grosse limite
mais aussi le plus grand avantage de la Visiovis : nous souhaiterions suggérer de reconstruire le
fourreau (toujours en PMMA si nécessaire) avec des parois externes carrées – ce qui faciliterait
la visualisation et éliminerait tout problème optique lié à la réflexion sur une surface courbée.
Par rapport au profil de la vis : nous invitons à tester plusieurs profils de vis afin d’en évaluer
les performances. Enfin, nous conseillons de concevoir un dispositif fixe pour une alimentation
plus fiables du traceur à l’intérieur du système vis/fourreau et pour une meilleure répétabilité
des phénomènes observés.
Exploitation des données acquises. Le rapprochement que nous avons opéré, avec la
collaboration de Yves Béreaux, entre la simulation numérique et les expériences de laboratoire,
représente un point de départ extrêmement intéressant pour d’autres futurs développement des
deux secteurs. Néanmoins, la Visiovis n’a pas encore été exploitée à la hauteur de toutes ses
potentialités, principalement à cause des problèmes de compatibilité rencontrés entre le fluide
modèle et les complexes photo-fonctionnels à base d’argile. Nous encourageons à poursuivre le
travail en utilisant les mêmes systèmes de détection et les mêmes traitements d’image.
Prélèvement d’échantillons et validation des résultats. Toute nouvelle technique de
caractérisation se doit d’être validée par les résultats obtenus sur un même échantillon avec des
techniques de caractérisation traditionnelles. Pour ce faire, il est nécessaire de pouvoir effectuer
des prélèvements d’échantillons pendant les expériences de visualisation. Nous n’avons pas
encore réalisé un dispositif qui permette le prélèvement en ligne d’échantillons, mais nous en
reconnaissons l’importance. Pour l’instant, nous avons caractérisé les mélanges maître (avant
l’injection dans la Visiovis) par DRX et rhéologie : nous avons rencontré un certain nombre de
problèmes qui nous ont empêché d’effectuer une corrélation directe et fiable entre les résultats
obtenus avec la Visiovis et ceux obtenus avec les techniques de caractérisation traditionnelles.
Nous en avons déduit seulement une confirmation des remarques faites auparavant à propos du
manque de compatibilité entre les charges photo-fonctionnelles et la matrice PDMS. Cet aspect
de validation est sans doute prioritaire pour le futur développement de la Visiovis.
Antonella ESPOSITO
FOLIO ADMINISTRATIF
THESE SOUTENUE DEVANT L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON
NOM : ESPOSITO
(avec précision du nom de jeune fille, le cas échéant)
DATE de SOUTENANCE : 5 décembre 2008
Prénoms : Antonella
TITRE : Visualisation de l’écoulement dans un système vis/fourreau. Suivi en ligne du mélange polymère/nanocharges.
NATURE : Doctorat
Numéro d'ordre : 2008-ISAL-0124
Ecole doctorale : Matériaux de Lyon
Spécialité : Matériaux Polymères et Composites
Cote B.I.U. - Lyon : T 50/210/19
/
et
bis
CLASSE :
RESUME :
L’addition de nanocharges aux polymères à l’état fondu semble pouvoir en améliorer considérablement les propriétés, pourvu
que cette deuxième phase soit parfaitement désagglomérée, dispersée et distribuée dans la matrice. Les charges lamellaires
(argiles) et leur nanocomposites attirent depuis quelque temps une attention croissante, tant de la part du monde académique
que de l’industrie, imposant ainsi des critères d’homogénéité du mélange polymère/charges plus stricts et étendus sur plusieurs
échelles. D’autre part, la morphologie des nanocomposites à base de charges lamellaires est difficile à caractériser et presque
autant difficile à décrire, étant souvent affectée par des problèmes de dispersion et/ou distribution. Une nouvelle méthode pour
le suivi en ligne des évolutions morphologiques des mélanges polymère/argile au cours du procédé de mise en œuvre pourrait
contribuer de façon significative au développement et à la commercialisation de cette catégorie de nanocomposites.
L’objectif de ce travail de thèse est de poser les bases pour le développement d’une méthode de caractérisation morphologique
en temps réel qui puisse permettre de mettre en évidence et comprendre les mécanismes de dispersion/distribution de charges
dans un milieu visqueux (polymère thermoplastique fondu ou résine thermodurcissable non réticulée) en écoulement dans un
système ayant une géométrie complexe (zone de pompage des ensemble vis/fourreau pour l’extrusion et l’injection), en
démystifiant enfin les outils de transformation. Nous avons repris une maquette froide (Visiovis), assemblée auparavant dans le
laboratoire du Site de Plasturgie INSA à Oyonnax (Maël Moguedet, Thèse INSA, 2005), présentant l’avantage considérable
d’être constituée d’un fourreau entièrement transparent, et nous l’avons adaptée et développée davantage afin de visualiser le
mélange d’un polymère modèle (PDMS) avec des argiles préalablement rendues photo-actives. Nous nous sommes donnés les
moyens pour de telles expériences de visualisation en entreprenant plusieurs essais originaux de photo-fonctionnalisation des
argiles avec différentes fluorophores, et en mettant finalement au point un protocole de photo-fonctionnalisation par échange
cationique de montmorillonites avec la Rhodamine 6G Perchlorate ; ce protocole a été ensuite utilisé pour rendre optiquement
actives quatre argiles commerciales parmi les plus récurrentes dans la littérature. Nous avons caractérisé les complexes photoactifs ainsi obtenus par DRX, ATG, analyse élémentaire, spectroscopie FTIR, ATG couplée FTIR, spectrofluorimétrie. Après
avoir calibré les systèmes de détection dont nous avons équipé la Visiovis (caméras CCD et spectromètre, ce dernier interfacé
avec le système vis/fourreau par fibre optique), nous en avons exploré le potentialités et les limitations de visualisation dans
une configuration d’éclairage planaire par nappe laser, similairement aux méthodes déjà largement utilisées pour le suivi en
ligne des écoulements (PLIF, PVT, PIV). Des méthodes pour le traitement des données acquises ont été suggérées et évaluées.
MOTS-CLES : polymère fondu, PDMS, systèmes modèles, compoundage, mélange, argiles, fonctionnalisation, extrusion,
injection moulage, suivi en ligne, nanocomposite, morphologie, dispersion, distribution, fluorescence, laser, visualisation des
écoulements
Laboratoire(s) de recherche : Laboratoire des Matériaux Macromoléculaires (LMM)
Ingénierie des Matériaux Polymères (IMP) – UMR CNRS #5223 – INSA de Lyon
Directeur(s) de thèse: Jean-Yves CHARMEAU
Jannick DUCHET-RUMEAU
Président de jury :
Composition du jury :
Rapporteurs : José Maria KENNY, Serge BOURBIGOT
Examinateurs : Jean-Jacques FLAT, Jean-François GERARD
Directeur(s) de thèse : Jean-Yves CHARMEAU, Jannick DUCHET-RUMEAU
RAW MATERIALS
RAW MATERIALS
M w = 208.26
M w = 208.26
g/mol
Dm ax = 10 Å
g/mol
Dm ax = 10 Å
9-anthracenemethanol
9-anthracenemethanol
M w = 417.84
M w = 417.84
g/mol
Dm ax = 15 Å
g/mol
Dm ax = 15 Å
Nile Blue A Perchlorate
M w = 543.01
M w = 543.01
g/mol
Dm ax = 14 Å
g/mol
Dm ax = 14 Å
Rhodamine 6G Perchlorate
M w = 360.80
M w = 360.80
g/mol
MC = 90
g/mol
meq
100 g
MC = 90
Dm ax = 29 Å
Dm ax = 29 Å
MT2EtOH Cloisite ® 30B
MT2EtOH Cloisite ® 30B
M w = 382.80
M w = 382.80
g/mol
meq
=
125
MC
g/mol
meq
=
125
MC
100 g
100 g
2MBHT Cloisite ® 10A
2MBHT Cloisite ® 10A
M w = 527.60
M w = 527.60
g/mol
MC = 125
g/mol
meq
100 g
MC = 125
2M2HT Cloisite ® 15A
SAMPLE NOMENCLATURE
2M2HT Cloisite ® 15A
SAMPLE NOMENCLATURE
Example:
C30B 1MC RhP
Example:
C30B 1MC RhP
pristine
clay
concentration of the
fluorescent molecule
(fraction)
pristine
clay
concentration of the
fluorescent molecule
(fraction)
fluorescent
molecule
C30B
C10A
C15A
CNa+
MC
CEC
NBAP
RhP
meq
100 g
Cloisite ® 30B
Cloisite ® 10A
Cloisite ® 15A
Cloisite ® Na+
Modifier Concentration
Cation Exchange Capacity
fluorescent
molecule
C30B
C10A
C15A
CNa+
MC
CEC
NBAP
RhP
Cloisite ® 30B
Cloisite ® 10A
Cloisite ® 15A
Cloisite ® Na+
Modifier Concentration
Cation Exchange Capacity
meq
100 g

Visualisation de l`écoulement dans un système vis/fourreau. Suivi en

Transcription

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