Why Nanocomposites? Scratch Resistant, Transparent, Filtering

Why Nanocomposites?
 Multi-functionality
“Size does matter”
Increased surface area on nanoparticles
 Small filler size:
 High surface to volume ratio
Interaction Zone
 Small distance between fillers  bulk interfacial material
Particle
 Mechanical Properties
 Increased ductility with no decrease of strength
 Scratching resistance
 Optical properties
 Light transmission characteristics particle size dependent
nanocomposite
Stress
Traditional
a36wt%
b
c
d
polymer
Strain
Scratch Resistant, Transparent, Filtering Coatings
Visible
Ultraviolet
Transmittance rate of 16.7wt.% nanoalumina filled
gelatin films coated on 0.1mm thick plastic substrate
TEM of the 16.7wt% nano
alumina filled gelatin film
Application: Barrier properties
• Imagine a drop of water trying to get through the film made with
nanocomposites. Compared to a film made with conventional
composites, the water drop would face more barrier going through the
film made with nanocomposites because the distance between fillers is
much smaller.
• Uses: Packaging in food, medical and pharmaceutical industry.
Thermal barrier coatings for Hubble
Space Telescope (HST)
Current Problem:
Hubble Space Telescope Imaging Spectrograph
overheats, causing data degradation.
Proposed Solution:
Carbon Nanotube (CNT) may greatly improve
HST’s ability to dissipate excess heat. (2X is the
goal)
Drug delivery
Attributes of nanoparticulate systems:
1.provide a better penetration of the
particles inside the body.
2.can be used for intramuscular or
subcutaneous applications
3.minimizes the irritant reactions at the
injection site.
4.exhibit greater stability, in both longer
shelf storage lives and uptake times.
5.and can be designed to elicit the desired
kinetics, uptake, and response from the
body (i.e. biocompatibility).
Nanocomposite as a Multiscale System
The ability to consider interactions across the multiple scales is a pre-requisite to the
consideration of design decisions that can be made on each scale critical to the performance
of the final product.
 Macroscale composite structures
 Clustering of nanoparticles - micron scale
 Interface - affected zones - several to tens
of nanometers - gradient of properties
 Polymer chain immobilization at particle
surface is controlled by electronic and
atomic level structure
Multiscale Modeling Role
 If we can predict:
Short Functional Groups
R
Matrix
Property
Compatible Enhancing
Block
Block
 Polymer / Particle Interaction (mobility,
conformation, crystallinity) and the chemistry to control it
 Quantum Mechanic  Molecular Dynamic  Coarse grain
Hydrogen Bonding
Long Functional oligomers
 Particle / Particle interaction

Coarse grain discrete  mesoscale continuum
 The effect of these and particle size on local and global mechanical,
thermal, electrical, optical behavior
 Coarse grain  multiscale continuum
 Then we can design and control:
 Extent of the interaction zone – polymer mobility
 Particle dispersion state – transparency, filtering, defect structure
 Glass transition temperature
Interaction Zone
Particle
 Processing
 Optimization of multiple functions
Consider: Carbon Molecules
 Graphite versus Diamond
 Graphite: Used as lubricant and pencil lead is composed of sheets
of carbon atoms in a large molecule. Only weak van der Waals’
forces hold the sheets together. They slide easily over each other.
 Diamond: Carbon atoms stacked in a three-dimensional array (or
lattice), giving a very large molecule. This gives diamond its
strength.
8
Graphite sheets
Diamond structure
 Graphite sheet is a molecule of interlocking hexagonal carbon rings. Each
carbon bonds covalently with three others, leaving one electron unused. The
orbital for these “extra” electrons overlap, allowing electrons to freely move
throughout the sheet. This is why graphite conducts electricity.
Structure of a sheet of graphite
A buckyball
 Buckyballs were discovered by Smalley (Rice University), Kroto and Curl in
1985 by vaporizing carbon with a laser and allowing carbon atoms to condense.
 A buckyball is short for buckmisterfullerene after Buckminster Fuller, an
American architect and engineer, who proposed an arrangement of pentagons
and hexagons for geodesic dome structures.
 It has 60 carbon atoms in a ball shaped with 20 hexagons and 12 pentagons
and has a diameter of about one nanometer.
9

In 1991, carbon nanotubes (CNTs) were discovered by Sumio Iijima of NEC Research
Lab. After taking pictures of buckyballs in an electron microscope, he noticed needle
shaped structures (i.e., cylindrical carbon molecules).

Single-wall carbon nanotubes (SWNTs) versus multiwalled carbon nanotubes (MWNTs)

The length of CNTs vary, but the smallest
diameter seen in SWNTs is about one nm.
Strength, stiffness (E modulus), and density of
common materials
Materials
6061 Aluminum
(bulk)
4340 Steel (bulk)
Tensile
Strength
(MPa)
Tensile
Modulus
(GPa)
Density
(g/cm3)
310
69
2.71
1,030
200
7.83
Nylon 6/6 (polymer)
75
2.8
1.14
Polycarbonate
(polymer)
65
2.4
1.20
E-glass fiber
3,448
72
2.54
S-2 glass fiber
4,830
87
2.49
Kevlar 49 aramid
fiber
3,792
131
1.44
T-1000G carbon
fiber
6,370
294
1.80
10
Carbon
nanotubes
30,000
1,000
1.90
A single-walled carbon nanotube
CNT
A scanning electron microscope (SEM) image of a CNT hanging
off the tip of an atomic force microscope (AFM) cantilever.
 Nanofibers and MWNTs: hollow tubular geometries with aspect ratios
(L/d) ranging in the thousands.
Material
Diameter
(nm)
Length
(nm)
Young’s
Modulus
(GPa)
Tensile
Strength
(GPa)
Vapor-grown
carbon
nanofibers
10-200
30,000100,000
400-600
2.7-7.0
~ 1.3
500-40,000
320-1470
13-52
SWNT
10 m
Scanning electron microscope image of
vapor-grown carbon nanofibers in a
polypropylene matrix
11
300 nm
Image of MWNTs in a polystyrene matrix
 Challenge: Unlike fibers in conventional laminates, waviness of the nanotubes and
nanofiber reinforced materials complicates the material property calculations.
Representative volume elements (RVEs) may be modeled as shown below:
 Waviness is defined by the waviness factor,
12
w
A
LNT
Predictions of the Young’s modulus of elasticity:
 The modulus of the RVE2 (the right diagram in the previous page),
Ex= ERVE2, and the effective modulus for randomly oriented
nanotubes, E3D-RVE2, have complex formulas, but are both are
functions of the waviness factor. E3D-RVE2 as functions of nanotube
volume fraction and w, is shown below.
13
 Strength prediction: In general, relations for predicting strength are
complex. However, for randomly oriented fibers, an approximate
equation may be used to estimate the tensile strength, as follows:
~x 
S S 
2S LT  ST
 ln T 2 mf 
1 
  S mf
S LT 
Where,
 x  composite tensile strenght
S LT  shear strenght
Smf  matrix stress corresponding
to the fiber failure strain
ST  transverse tensile strength
14
Failure modes of nanofiber-reinforced composites
Fracture mechanisms in carbon
nanotube-reinforced composites
Paving the way to stronger materials
By H. Wagner, Nature 2007
The properties of materials reinforced by nanoparticles often fall far short of those
predicted by theory, but now a layer-by-layer assembly approach offers a way in
which nanocomposite materials could begin to realise their true potential.
• There are fundamental differences between manmade composites at the micrometer-scale
and those with nanoscale reinforcement. In particular, the paucity of structural defects in
high-aspect-ratio nanoparticles means that their strength and stiffness are much closer to
the theoretically predicted values.
• The comparatively large surface-area-to volume ratio of elongated nano-objects promises
materials with much improved mechanical properties.
• If there are strong interactions at the interface between the matrix and nanofiller, efficient
stress transfer from one to the other leads to increased strength and stiffness of the
composite.
• In the case of a weaker interface, the friction and energy dissipation associated with
extensive pull-out (that is, extraction) of the nanoparticles from the surrounding matrix in an
expanding crack translates to substantial toughness.
Problem of Nanocomposites:
• Aggregation of the nanosized components
because of the relatively strong interparticle forces
between them
• Leads to particle–matrix mixtures with
high viscosities which can make the
processing of these materials quite
challenging.
• Most existing nanocomposites have
disappointingly low fractions of particle
content, and relatively weak mechanical
properties when compared with those predicted by
theory.
• The addition of even tiny amounts of dispersed
phase to matrices leads to outstanding properties,
at least up to a‘critical mixing threshold’ of
particle content above which aggregation effects
become significant and the mechanical properties
cease to improve.
Optimal Nanocomposites requirement:
(1) The particle aspect ratio  high-aspect-ratio particles
(2) Particle dispersion
(3) Particle packing (or alignment)
(4) Polymer-to-particle interfacial stress transfer  the adhesion was
strengthened in a number of different ways
The evolving architecture of nanocomposites
(a) Poorly dispersed nanoparticles (blue)
form aggregates in a polymer (red)
nanocomposite.
(b) The bilayer nanocomposites made by
Kotov and co-workers comprising clay
nanoplatelets (blue) embedded in a
poly (vinyl alcohol) matrix (red).
(c) Basic ‘unit cells’ of nacre and bone the latter of which has a more regular
staggered organization - have densely
packed structures and are seen as the
archetypes of biological
nanocomposites, possessing high
modulus, strength and - at larger
length scales - toughness.
Critical issues in nanocomposites
1.
Dispersion
Uniform dispersion of nanoparticles, and nanotubes
against their agglomeration due to van der Waals bonding
is the first step in the processing of nanocomposites.
 SWCNTs tend to cluster into ropes and MWCNTs
produced by chemical vapor deposition are often
tangled together like spaghettis.
 The separation of nanotubes in a solvent or a matrix
material is a prerequisite for aligning them.
2. Alignment
Because of their small sizes, it is exceedingly difficult
to align the nanotubes in a polymeric matrix material in a
manner accomplished in traditional short fiber composites.
The lack of control of their orientation diminishes the
effectiveness of nanotube reinforcement in composites,
whether for structural or functional performance.
3. Volume and rate
High volume and high rate
fabrication is fundamental to
manufacturing of nanocomposites as
a commercially viable product.
4. Cost effectiveness
Besides high volume and high rate
production, the cost of nanocomposites also
hinges on that of the nanoreinforcement
material, particularly, nanotubes. It is
anticipated that as applications for nanotubes
and their composites increase the cost will be
dramatically reduced.