Self-assembly Structures of Block Copolymers in Selective Solvents

Self-assembly Structures of
Block Copolymers in Selective
Solvents and of PolysaccharideSurfactant Mixtures
Björn Lindman, Physical Chemistry, Lund University, Sweden
Center of Excellence
Contributions from Lennart Piculell, Tommy Nylander, Per Linse,
Paschalis Alexandridis, Ulf Olsson and other colleagues
Hydrophilic
Hydrophobic
Amphiphilic molecules
ionic and non-ionic surfactants and lipids
block- and graft-copolymers
(polysaccharides and proteins), DNA
Self-Organize
at Interfaces and in Solution
modify interfacial properties
enhance compatibility
compartmentalization
Form the basis of life
Biological membranes
Find widespread
use in industry
Pharmaceuticals, plastics, foods, detergents,
cosmetics, minerals, paper, oil,
remediations, etc.
SDS
CH3(CH2)11SO4- Na+
CTAB
CH3(CH2)15N+(CH3)3 Br -
C12E8
CH3(CH2)11(OCH2CH2)8OH
Surfactant self-assembly
Spherical
Micelle
Lamellar
phase
Reversed
Micelle
Bicontinuous
Structure
Cylindrical
Micelle
Vesicle
Amphiphilic
Lipophilic and Hydrophilic
Combine the properties of polar solutes
like salts with those of hydrocarbons
Ambivalence leads to (in aqueous systems)
And / or
Adsorption at interfaces
(water/air, water/hydrocarbon,
water/solid, water/macromolecule)
Self-assembly
(alone or with low molecular
or macromolecular cosolutes)
Amphiphilic copolymers
Amphiphilic Block Copolymers
EOn-POm-EOn
EOn-BOm-EOn
POn-EOm-POn
EO : ethylene oxide -(CH2-CH2-O)PO : propylene oxide -(CH2-CH(CH3)-O)-
Commercially available
(BASF, ICI, Dow, Hoechst)
Molecular weight range: 2000 – 16000
BO : butylene oxide -(CH2-CH(CH2CH3)-O)- EO composition range: 20 – 80 % (per weight)
Composition, molecular weight and architecture can be tailored to meet specific needs
(control over amphiphilicity)
Surface tension of EO37PO56EO37
70
Pluronic P105
γ / (mN m-1)
60
25°C
50
35°C
40
30
-7
-6
-5
-4
-3
-2
-1
log [Cpolymer/(kg dm-1)]
The arrows denote the location of the cmc:s obtained from dye solubilization.
CMCs of EO block copolymers decrease strongly with T
Block-copolymer self-assembly
CH3
|
HO(CH2CH2O)n(CHCH2O)m(CH2CH2O)nH
PEO
PPO
PEO
Block copolymers: thermal ”gel” is cubic phase
temperature Phase behavior of
amphiphilic block copolymer in water
cloud
point
L1
Lα
I1
H1
V1
water
L1
polymer
I1
H1
V1
Lα
lamellar
increasing amphiphile concentration
Tailoring the molecular packing
Molecular packing imposes the topology of the structural elements
Packing depends on molecular characteristics of amphiphile
(block sequence and architecture, block molecular volume ratio),
but can be adjusted by
solvent quality
(modify relative swelling of blocks)
“cross-linking” of self-assemblies
(adsorb preferentially to the different blocks)
Tailoring the Molecular Packing
solvent quality (modify swelling)
worsen solvent
for red block
Change curvature
cosolutes (adsorb preferentially)
+
Change curvature
worsen solvent
for blue block
Phase diagram
polymer
20
40
wt%
water
- 80
30%
60
- 60
50%
wt%
polymer
- 40
80
- 20
20%
water
20
40
60
wt% oil
80
oil
4 cubic, 2 hexagonal, & 1 lamellar liquid
crystalline phase + 2 isotropic solution phases
in a ternary (isothermal) copolymer – water – oil system
V2
V1
H2
H1
I2
I1
L1
Lα
L2
L44
EO10PO23EO10
Mw = 2200
L64
EO13PO30EO13
Mw = 2900
P84
EO19PO43EO19
Mw = 4200
P104
EO27PO61EO27
Mw = 5900
Experimental
PEO-PPO-PEO
Theoretical A-B
Experimental PI-PS
from Matsen et al
Macromolecules 1996, 29, 1091
from Khandpur et al
Macromolecules 1995, 28, 8796
Block symmetry
Approximately symmetric - (EO)19(PO)43(EO)19
Unsymmetric 80 wt% PEO - (EO)43(PO)16(EO)43
Unsymmetric 10 wt% PEO - (EO)5(PO)68(EO)5
80% PEO
40% PEO
10% PEO
Effect of copolymer architecture on
self-assembly
EO13PO30EO13
(L64)
PO19EO33PO19
(25R4)
3 cubic, 2 hexagonal, & 1 lamellar liquid
crystalline phases + 2 isotropic solution phases
in a ternary (isothermal) block copolymer – water – oil system
Lα
H1
I1
L1
V2
H2
I2
L2
Amphiphilic graft
copolymer self-assembly
Hydrophobically modified water soluble polymers: HM-P
Polymer
backbone
Hydrophobic
group
Features:
Applications:
• can form inter- and intrachain
hydrophobic aggregates
•
Rheology modifiers and
thickeners in paint formulations
cosmetics/skin care products,
detergents, oil recovery
•
Drug delivery systems
•
Dispersing/stabilizing agents
• exhibit unique rheological
properties
• have strong tendency to associate
with surfactants and other polymers
The associative (hydrophobically modified)
water-soluble polymers
Hydrophobe
Polymer chain
grafted
end-capped
Associated structures in aqueous solutions of hydrophobically
modified polymers (HMPs)
Hydrophobically modified water soluble polymers (HMP)
Polymer – modified surfactant
A Slightly Hydrophobic Cellulose Derivative
The degree of ethyl and hydroxyethyl substitution determines
the hydrophobicity of polymers in the EHEC family.
Viscosity /Pa.s
Hydrophobic modification of a water-soluble
polymer increases viscosity
Cross-links in HM-EHEC
entanglements of polymer chains
physical bonds of associating hydrophobic tails
physical bonds of associating segments of the EHEC backbone
Surfactant like behavior
polar head
hydrophobic tail
surfactant micelle
polymer aggregate
Reversible inter-chain association
Hydrophobe
Polymer chain
Crosslink
transient
network
viscosity
Cyclodextrin
O
O
O
O
O
O
OH HO
HO
Hydrophilic exteriorO
O
O
OH
O
O
O
O
HO
HO
O
O
O
O
H
OH
HO
O
H
O
O
O
O
OC
C O
H
O
O
H C
H
H
C
H
C
C
H
O
H
Hydrophobic cavity
Hydrophobic molecules can hide
inside the cavity of a CD
water molecule
CD breaks the hydrophobic
associations
cCD
η/η0
Too large hydrophobe
Surfactant-polymer systems.
General aspects
Polymer-Surfactant Association: pearl-necklace model
Polymer-Surfactant Interaction
• Cooperativity
• Surfactant micellization induced by polymer
Polymer-Surfactant Complexes
micelle
alginate
+ C12TAB
mixed micelle
Nonic cellulose
derivative + SDS
hydrophobically modified cellulosics + SDS
When do surfactants bind to polymers?
• Ionic Surfactants
Oppositely charged polymers
Non-ionic polymers
self-assembly induced by polymer
• All Surfactants
Hydrophobically modified polymers
mixed micellization
Hydrophobic association is always essential to the interaction
The structure of Associative
Polymers
Hydrophobic group
Water soluble backbone
Viscosity /Pa.s
Hydrophobic modification of a water-soluble
polymer increases viscosity
The influence of surfactant
concentration on viscosity
Viscosity
log cSurfactant
Hydrophobically modified
polymer
Polymer
Viscosity and hydrophobic microdomains
hydrophobically modified 1% w/w EHEC
Addition of:
• Na+ Cl-
Increase micelles size
• DoTA+ Cl-
Increase viscosity
-Broaden the area of high viscosity
Screening electrolyte (NaCl)
Oppositely charged surfactant
(DoTAC)
Decrease in viscosity depends on the stoichiometry between polymer
hydrophobic side-chains and mixed micelles.
One way to alter the stoichiometry is to: Decrease the number of
micelles by increasing their size.
The Viscosity and the Mixed micelles Concentration in
Mixtures of 1 w/w% HMHEC and 30 mm surfactant
(SDS+DoTAC) versus the molar ratio of DoTAC
The viscosity can increase with
addition of a surfactant that
changes the shape of the micelles
DoTAC
Polymer-modified surfactant mixed micelles
Oil droplet
Polymer-surfactant systems.
Phase separation
Segregating Polymer/Surfactant mixtures
Mixtures of oppositely charged polyelectrolyte + surfactant:
Associative phase separation
In a mixed solution
Interactions between cosolutes are:
Repulsive (most common)
or
Attractive (electrostatic, hydrophobic)
Depending on interaction
Segregation
Association, or
Miscibility
POLYELECTROLYTE EFFECTS
• A polyelectrolyte in aqueous solution dissociates
into 1 polyion and n counterions; typically n >> 1
• a large no. of particles: large ∆Smix
• If the counterions mix into a phase, the polyion has
to follow (condition of electroneutrality)
POLYELECTROLYTE
EFFECTS
• Dissociating counterions on one of the polymers increases miscibility
tremendously
• Added salt facilitates demixing in both cases.
Polymer – Surfactant
Phase Diagrams
SEGREGATING POLYMER/SURFACTANT MIXTURES
• In general (i.e,. in absence of electrostatic or hydrophobic attractions), effective
repulsion between a polymer and a surfactant micelle is expected
• Since a surfactant micelle is effectively a polymer, repulsion should lead to a
segregative phase separation, as for mixtures of dissimilar polymers
Nonionic polymer + nonionic surfactant
Segregation
MIXTURES OF OPPOSITELY CHARGED
POLYELECTROLYTE + SURFACTANT:
ASSOCIATIVE PHASE SEPARATION
• For intrinsically hydrophilic polyions, the association is
driven only by electrostatic interactions
• Close analogy to polyelectrolyte complexes
Association
Anionic polysaccharide + Cationic surfactant
Nature of conc phase: conc soln/gel, liq crystal, solid crystal
Effect of salt on polyelectrolyte + ionic surfactant
Low salt
Association
Intermediate salt
Miscibility
High salt
Segregation
Problem with conventional
approach
• Concentrated phase generally contains 4 ions in
unknown proportions => complex system!
OPPOSITELY CHARGED MIXTURES: TWO
ALTERNATIVE REPRESENTATIONS
• Left hand diagram – conventional mixing plane
• Right hand diagram – alternative mixing plane
• Stoichiometric mixtures belong to both mixing planes
Segregation in a P-S- systems
40°C NaHy Mw = 90000
------
NaHy – SDS – H2O – 1M NaBr
NaHy – SDS – H2O
Network formation and gelation
• A gel contains at least two components,
one solid-like and one liquid-like, where
both are continuous throughout the gel.
What are polyelectrolyte gels?
• Polymer network with charged groups
Gel Swelling Experiment: How
&
Why
• Make gel pieces of cross-linked polymer
• Immerse gel pieces in series of solutions with increasing conc of additive
water
water
+ additive
=> Potential ”responsive gels” (drug delivery, water retention…)
=> Info on interactions between gel & additive
water
+ more
additive
General Swelling Isotherm
for ”Weakly Hydrophobic”
Nonionic Gel with Ionic
Surfactant
35
30
V/V 0
25
20
15
10
5
0.1
0
1
cac 10
C f,SDS
100
Gel Swelling Experiments Detect
Surfactant Binding
CMC:
V / m (ml/g)
200
SHS
STS
SDS SDeS SOS
150
100
=> HEC binds
alkyl sulfates with
> 8 carbon tails
50
0
HEC gels swollen in
alkyl sulfate solutions
Sjöström & Piculell
Langmuir 17(2001)3836
0
0.1
1
c (mM)
10
100
Cat-HEC Gels + Different
Anionic Surfactants
STS
SDS
SD(EO)2S
CMC:
Sjöström & Piculell
Colloids Surf A
183-185 (2001) 429
V / m (ml/g)
1000
• Collapse & redissolution
100
• Two CAC:s!?
• Both correlate with CMC
=> both reflect surfactant
self-assembly
10
0
0.0001
0.001
0.01
0.1
c (mM)
1
10
Adsorption of EHEC
CH3
CH3
CH2
CH2
O
O
CH3
O
OH
HO
HO
O
O
O
CH2
CH2
CH2
O
O
O
HO
O
CH2
O
CH2
CH2
CH2
HO
CH2
O
CH2
O
CH2
O
CH2
CH3
CH2
CH2
OH
n
Solvent
Polymer
Surface
Aqueous systems:
Adsorption since water interacts unfavorably with polymer
(clouding polymer) or surface (hydrophobic surface)
Polar/nonpolar surfaces
T dependence: Solvency
The poorer the solvent
the better the adsorption
The influence of the solvent
Poor solvent
Good solvent
Adsorption of EHEC on SiO2:
Solvency effects due to cosolutes
7
Na2SO4
6
NaCl
5
4
3
2
Increase in
adsorption
NaSCN
1
NaI
0
0
0.2
0.6
0.4
0.8
1.0
Decrease in
adsorption
Concentration (M)
Decrease
in CP
45
Increase in
CP
NaI
NaSCN
40
35
Na2SO4
NaCl
30
25
0
0.2
0.4
Concentration (M)
0.6
0.8
Interfacial behavior of polymersurfactant mixtures
Polymer-Surfactant at Interfaces
General Swelling Isotherm
for ”Weakly Hydrophobic”
Nonionic Gel with Ionic
Surfactant
35
30
V/V0
25
20
15
10
5
0.1
0
1
cac 10
C f,SDS
100
EHEC/SDS on Hydrophobized Silica
• Substrate:Silanol groups reacted with dimethyloctylchlorosilane
• EHEC preadsorbed from 0.01 wt% solution. (Intermediate
adsorption)
• SDS adsorbs on hydrophobized silica; Competitive adsorption!
Turbidity of bulk solution
100 ppm cat-HECCl (LR30M) + SDS
0.1
2φ
0.08
Absorbance
Absorbance
0.06
1φ
Increase in turbidity due to
precipitation in the bulk solution.
1φ
0.04
0.02
0
0.001
0.01
0.1
1
10
100
SDS concentration (mM)
SDS concentration (mM)
polycation and anionic surfactant
Polymer-Surfactant applications: Shampoo
S-
--- --- - -P+SH2O
H2O
-- - + ----P S--H2O
-- - + ---P ---
P+
+
SS-
CATIONIC CELLULOSE DERIVATIVES
CH3
(CH2CH2O)2
+
N
CH2CHCH2
O
OH
HO
O
O
HO
R
CH3
OH
CH2
Cl
O
OH
CH2OCH2CH2OH
τ
1−τ
JR-400: R=CH3
τ =29mol%
Mw =400000
LM-200: R=C12H25
τ =3mol%
Mw =100000
1.8
100
1.6
90
1.4
80
70
1.2
60
2Φ
1.0
50
0.8
40
0.6
30
0.4
cac
0.0
0.001
20
cmc
0.2
Polar surface
Thickness [nm]
Adsorbed amount [mg/m2]
The effect of SDS addition to pre-adsorbed JR-400 layers
10
0
0.01
0.1
1
10
SDS [mM]
>5mM SDS
0.1-0.6mM SDS
P+-S -stoichiometric complex
adsorption
- solubility of complex decreases
because of charge neutralization
- conformation of complex becomes
compact, since intramolecular
electrostatic repulsion is screened
desorption
-- -- - - -
- - - -- - -- --
- solubility of complex increases
because of cooperative SDS
binding
- complex expands due to the
increase of the net negative charge
of complex
Desorption process was too slow to be followed
Rinsing of adsorbed JR-400/SDS layers on silica
Effect of rinsing (10mM NaCl) on adsorption
Reference
1.4
Adsorption of JR-400/SDS complexes
from pre-mixed solutions
Rinsing was started (t=1000)
1.8
5mM SDS
1.6
2
1.0
adsorbed am ount [m g/m ]
adsorbed amount [mg/m2]
1.2
10mM SDS
0.8
0.6
2φ
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.06mM SDS
0.0
0.4
0.001
0.01
0.1
1
SD S [m M ]
0.006mM SDS
Rinsing was started after adsorption
of the complex from pre-mixed solution
reached steady state
0.2
0.0
0
1000
2000
3000
4000
5000
time [sec]
- For the complexes which were formed in post-precipitation region,
the adsorbed amount jumped up by rinsing
10
Effect of rinsing on adsorbed JR-400/SDS layers
on hydrophobized silica
4.0
3.5
3.0
a
2.5
b
2.0
1.5
1.0
0.5
c
0.0
0
1000
2000
3000
4000
5000
6000
Time [sec.]
The complexes adsorbed from mixed polymer (100 ppm)/surfactant (5 mM) solutions and rinsing was
started at t = 1000 sec.
(a) adsorption was carried out in water followed by rinsing with water
(b)
adsorption was carried out in 10 mM NaCl followed by rinsing with water
(c) adsorption was carried out in 10 mM NaCl followed by rinsing with 10 mM NaCl.
+
+
+
+ ++
+
+
+
+
+
+
polycation adsorption
General trends of co-adsorption of cationic
cellulose
+
derivatives with SDS
+
adsorption
phase
separation
desorption
added anionic surfactant
no SDS
low SDS
rinsing
high SDS