MILOS IADR

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Polymer Nanocomposites
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Polymer Nanocomposites
Polymer nanocomposites are a new class of
composites, which are particle-filled polymers
for which at least one dimension of the dispersed
particles is in the nanometer range. One can
distinguish three types of nanocomposites,
depending on how many dimensions of the dispersed
particles are in the nanometer range. When the
three dimensions are in the order of nanometers,
we are dealing with isodimensional nanoparticles
(such as spherical silica nanoparticles). When
two dimensions are in the nanometer scale and the
third is larger, forming an elongated structure,
we speak about nanotubes or whiskers (for
example, carbon nanotubes, cellulose whiskers or
electrospun nanofibers). The third type of
nanocomposites is characterized by only one
dimension in the nanometer range. In this case
the filler is present in the form of sheets of
one to a few nanometer thick to hundreds to
thousands nanometers long. This family of
composites can be gathered under the name of
polymer-layered crystal nanocomposites.
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Polymer-Layered Silicate Nanocomposites
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Introduction
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Examples of layered crystals susceptible to
intercalation by a polymer
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Polymer-Layered Silicate Nanocomposites
Amongst all the potential nanocomposite
precursors, those based on clay and layered
silicates have been more widely investigated
probably because the starting clay materials are
easily available and because their intercalation
chemistry has been studied for a long time.
Owing to the nanometer-size particles obtained by
dispersion, these nanocomposites exhibit markedly
improved mechanical, thermal, optical and
physico-chemical properties when compared with
the pure polymer or conventional (microscale)
composites as firstly demonstrated by Kojima and
coworkers for nylon-clay nanocomposites.
Improvements can include, for example, increased
moduli, strength and heat resistance, decreased
gas permeability and flammability.
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Polymer-layered silicate nanocomposites, based on
smectite (water swellable) clays usually rendered
hydrophobic through ionic exchange of the sodium
interlayer cation with an onium (positively
charged) ion (cation, such as alkylammonium
cation), may be prepared via various synthetic
routes comprising exfoliation adsorption, in situ
intercalative polymerization and melt
intercalation. A variety of polymer matrices
(i.e., thermoplastics, thermosets and elastomers)
can be used to prepare polymer-layered silicate
nanocomposites. Generally speaking, two types of
structure may be obtained, namely (1)
intercalated nanocomposites where the polymer
chains are sandwiched in between silicate layers
and (2) exfoliated nanocomposites where the
separated, individual silicate layers are more or
less uniformly dispersed in the polymer matrix.
This new family of materials exhibits enhanced
properties at very low filler level, usually
inferior to 5 wt., such as increased Young's
modulus and storage modulus, increase in thermal
stability and gas barrier properties and good
flame retardancy.
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Structure of Layered Silicates
The layered silicates commonly used in
nanocomposites belong to the structural family
known as the 21 phyllosilicates. Their crystal
lattice consists of 2-D layers where a central
octahedral sheet of alumina or magnesia is fused
to two external silica tetrahedron by the tip so
that the oxygen ions of the octahedral sheet do
also belong to the tetrahedral sheets. The layer
thickness is around 1 nm and the lateral
dimensions of these layers may vary from 300 Å to
several microns and even larger depending on the
particular silicate.
Structure of 21 phyllosilicates
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These layers organize themselves to form stacks
with a regular van der Walls gap in between them
called the interlayer or the gallery. Isomorphic
substitution within the layers (for example, Al3
replaced by Mg2 or by Fe2, or Mg2 replaced by
Li) generates negative charges that are
counterbalanced by alkali or alkaline earth
cations situated in the interlayer. As the
forces that hold the stacks together are
relatively weak, the intercalation of small
molecules between the layers is easy. In order
to render these hydrophilic phyllosilicates more
organophilic, the hydrated cations of the
interlayer can be exchanged with cationic
surfactants such as alkylammonium or
alkylphosphonium (onium). The modified clay (or
organoclay) being organophilic, its surface
energy is lowered and is more compatible with
organic polymers. These polymers may be able to
intercalate within the galleries, under well
defined experimental conditions (as will be
discussed later).
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Montmorillonite, hectorite and saponite are the
commonly used layered silicates. Their chemical
formula are shown in the following table.
Chemical structure of commonly used 21
phyllosilicates M monovalent cation x
degree of isomorphous substitution (between 0.5
and 1.3).
This type of clay is characterized by a moderate
negative surface charge (known as the cation
exchange capacity). The charge of the layer is
not locally constant as it varies from layer to
layer and must rather be considered as an average
value over the whole crystal. Proportionally,
even if a small part of the charge balancing
cations is located on the external crystallite
surface, the majority of these exchangeable
cations is located inside the galleries. When the
hydrated cations are ion-exchanged with organic
cations such as more bulky alkyammoniums, it
usually results in a larger interlayer spacing.
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In order to describe the structure of the
interlayer in organoclays, one has to know that,
as the negative charge originates in the silicate
layer, the cationic head group of the
alkylammonium molecule preferentially resides at
the layer surface, leaving the organic tail
radiating away from the surface. In a given
temperature range, two parameters then define the
equilibrium layer spacing the cation exchange
capacity of the layered silicate, driving the
packing of the chains, and the chain length of
organic tail(s).
According to X-ray diffraction (XRD) data, the
organic chains have been long thought to lie
either parallel to the silicate layer, forming
mono or bilayers or, depending on the packing
density and the chain length, to radiate away
from the surface, forming mono or even
bimolecular tilted paraffinic' arrangement.
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A more realistic description has been proposed
based on FTIR experiments (by monitoring
frequency shifts of the asymmetric CH2 stretching
and bending vibrations). The intercalated chains
exist in states with varying degrees of order.
In general, as the interlayer packing density or
the chain length decreases (or the temperature
increases), the intercalated chains adopt a more
disordered, liquid-like structure resulting from
an increase in the gauche/trans conformer ratio.
When the available surface area per molecule is
within a certain range, the chains are not
completely disordered but retain some
orientational order similar to that in the liquid
crystalline state
Alkyl chain aggregation models (a) short alkyl
chains isolated molecules, lateral monolayer
(b) intermediate chain lengths in-plane disorder
and interdigitation to form quasi bilayers and
(c) longer chain length increased interlayer
order, liquid crystalline-type environment
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Structures
Depending on the nature of the components used
(layered silicate, organic cation and polymer
matrix) and the method of preparation, three main
types of composites (phase separated
microcomposite, intercalated nanocomposite, and
exfoliated nanocomposite) may be obtained when a
layered clay is associated with a polymer.

• Scheme of different types of composite arising
  from the interaction of layered silicates and
  polymers
• phase separated microcomposite
• intercalated nanocomposite and
• exfoliated nanocomposite.

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X-ray Diffraction (XRD) is used to identify
intercalated structures. In such nanocomposites,
the repetitive multilayer structure is well
preserved, allowing the interlayer spacing to be
determined. The intercalation of the polymer
chains usually increases the interlayer spacing,
in comparison with the spacing of the organoclay
used, leading to a shift of the diffraction peak
towards lower angle values (angle and layer
spacing values being related through the Bragg's
relation ? 2d sin?, where ? corresponds to the
wave length of the X-ray radiation used in the
diffraction experiment, d the spacing between
diffractional lattice planes and ? is the
measured diffraction angle or glancing angle).
As far as exfoliated structure is concerned, no
more diffraction peaks are visible in the XRD
diffractograms either because of a much too large
spacing between the layers (i.e. exceeding 8 nm
in the case of ordered exfoliated structure) or
because the nanocomposite does not present
ordering anymore.
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XRD patterns of (a) phase separated
microcomposite (organo-modified fluorohectorite
in a HDPE matrix) (b) intercalated nanocomposite
(same organomodified fluorohectorite in a PS
matrix) and (c) exfoliated nanocomposite (the
same organo-modified fluorohectorite in a
silicone rubber matrix)
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Transmission electronic spectroscopy (TEM) is
used to characterize the nanocomposite
morphology. The following TEM micrographs were
obtained for an intercalated and an exfoliated
nanocomposite. Besides these two well defined
structures, other intermediate organizations can
exist presenting both intercalation and
exfoliation. In this case, a broadening of the
diffraction peak is often observed and one must
rely on TEM observation to define the overall
structure.
TEM micrographs of poly(styrene)-based
nanocomposites (a) intercalated nanocomposite
and (b) exfoliated nanocomposite
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Preparation
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Method 1 Exfoliation-Adsorption
The layered silicate is exfoliated into single
layers using a solvent in which the polymer (or a
prepolymer in case of insoluble polymers such as
polyimide) is soluble. It is well known that
such layered silicates, owing to the weak forces
that stack the layers together can be easily
dispersed in an adequate solvent. The polymer
then adsorbs onto the delaminated sheets and when
the solvent is evaporated (or the mixture
precipitated), the sheets reassemble, sandwiching
the polymer to form, in the best case, an ordered
multilayer structure. Under this process are
also gathered the nanocomposites obtained through
emulsion polymerization where the layered
silicate is dispersed in the aqueous phase.
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This technique has been investigated in attempts
to produce nanocomposites with nitrile-based
copolymer and polyethylene-based polymer. To
produce the nitrile copolymer-based
nanocomposite, the copolymer was dissolved in
dimethylformamide in the presence of 15 wt.
modified clay. After solvent evaporation, the
film recovered was characterized by both XRD and
TEM. XRD reveals a broad diffraction peak that
has been shifted towards a higher interlayer
spacing (21.5 Å, unmodified clay 11.8 Å,
modified clay 16.5 Å, ). The large broadening of
the peak may indicate that partial exfoliation
has occurred, as corroborated by TEM analysis
where both stacked (intercalated) and isolated
(exfoliated) silicate layers can be observed.
I individual silicate layer S stacked
silicate layers
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Method 2 In situ Intercalative Polymerization
In this technique, the layered silicate is
swollen within the liquid monomer (or a monomer
solution) so as the polymer formation can occur
in between the intercalated sheets.
Polymerization can be initiated either by heat or
radiation, by the diffusion of a suitable
initiator or by an organic initiator or catalyst
fixed through cationic exchange inside the
interlayer before the swelling step by the
monomer.
Many interlamellar polymerization reactions were
studied in the 1960s and the 1970s using layered
silicates, but it is with the work initiated by
the Toyota research team that the study of
polymer-layered silicate nanocomposites came into
vogue about 15 years ago.
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Examples
Schematic representation of the montmorillonite
modification with nitroxyl-based organic cation
and its subsequent use to produce PS-based
exfoliated nanocomposite
TEM micrograph of Na-montmorillonite exfoliated
in HDPE after in situ intercalative
polymerization of ethylene.
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Method 3 Melt Intercalation
The layered silicate is mixed with the polymer
matrix in the molten state. Under these
conditions and if the layer surfaces are
sufficiently compatible with the chosen polymer,
the polymer can crawl into the interlayer space
and form either an intercalated or an exfoliated
nanocomposite. In this technique, no solvent is
required.
The thermodynamics that drives the intercalation
of a polymer inside a modified layered silicate
while the polymer is in the molten state has been
studied. In general, the outcome of polymer
intercalation is determined by an interplay of
entropic and enthalpic factors. In fact, although
the confinement of the polymer chains inside the
silicate galleries results in a decrease in the
overall entropy of the macromolecular chains,
this entropic penalty may be compensated by the
increase in conformational freedom of the
tethered alkyl surfactant chains as the inorganic
layers separate, due to the less confined
environment.
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Example
TEM micrograph of organo-modified montmorillonite
exfoliated in a ethylene-vinyl acetate copolymer
matrix.
Schematic of the morphology of organo-modified
montmorillonite
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Method 4 Template Synthesis
A last technique reported for preparing layered
silicate-based nanocomposites implies the in situ
hydrothermal crystallization of the clay layers
(hectorite) in an aqueous polymer gel medium
where the polymers often act as template for the
layers formation.
This method is particularly adapted to water
soluble polymers such as PVP and others. The
typical method for in situ hydrothermal
crystallization of a polymer/hectorite
nanocomposite consists in refluxing for 2 days a
2 wt. gel of silica sol, magnesium hydroxide
sol, lithium fluoride and the desired polymer in
water. XRD patterns attest for the formation of
a polymer/hectorite intercalated nanocomposite.
The interlayer spacing linearly depends upon the
wt. of polymer incorporated
Correlation between d spacing from XRD patterns
and weight percent polymer in synthetic
PVP-hectorite clay nancomposites
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Properties
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Layered silicate nanofillers have proved to
trigger a tremendous properties improvement of
the polymers in which they are dispersed. Amongst
those properties, unexpected large increase in
moduli (tensile or Young's modulus and flexural
modulus) of nanocomposites at filler contents
sometimes as low as 1 wt. has drawn a lot of
attention. Thermal stability and fire retardancy
through char formation are other interesting and
widely searched properties displayed by
nanocomposites. Those new materials have also
been studied and applied for their superior
barrier properties against gas and vapor
transmission. Finally, depending on the type of
polymeric materials, they can also display
interesting properties in the frame of ionic
conductivity or thermal expansion control.
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Mechanical Properties
The Young's modulus (or tensile modulus),
expressing the stiffness of a material at the
start of a tensile test, has shown to be strongly
improved when nanocomposites are formed. Nylon-6
nanocomposites obtained through the intercalative
ring opening polymerization of ?-caprolactam,
leading to the formation of exfoliated
nanocomposites, show a drastic increase in the
Young's modulus at rather low filler content.
Dependence of tensile modulus E at 120ºC on clay
content for organo-modified montmorillonite and
saponite based nanocomposites
Effect of clay content on tensile modulus,
measured at room temperature, of organomodified
montmorillonite/nylon-6-based nanocomposite
obtained by melt intercalation
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Stress at break. In thermoplastic-based
(intercalated or exfoliated) nanocomposites, the
stress at break, which expresses the ultimate
strength that the material can bear before break,
may vary strongly depending on the nature of the
interactions between the matrix and the filler.
Tensile stress evolution for nanocomposites based
on various thermoplastic matrices
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Elongation at break. The effect of nanocomposite
formation on the elongation at break has not been
widely investigated. When dispersed in
thermoplastics such as for intercalated PMMA and
PS or intercalated-exfoliated PP, the elongation
at break is reduced. In the last case, the
decrease is very important, dropping from 150 and
105 for a pure PP matrix and a 6.9 wt.
nonintercalated clay microcomposite,
respectively, down to 7.5 in the better case for
a PP-based nanocomposite filled with 5 wt.
silicate layers. Interestingly enough, such a
loss in ultimate elongation does not occur in
elastomeric epoxy or polyol polyurethane
matrices. Rather, the addition of a nanoclay in
cross-linked matrices triggers an increase of the
elongation at break.
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Thermal Stability and Flame Retardant Properties
Another highly interesting property exhibited by
polymer-layered silicate nanocomposites concerns
their increased thermal stability but also their
unique ability to promote flame retardancy at
quite low filling level through the formation of
insulating and incombustible char.
Oxygen plasma surface erosion rate of neat nylon
6 (?), nylon 6/5.0 wt layered silicate
nanocomposite (?), and nylon 6/7.5 wt layered
silicate nanocomposite (?).
TGA traces for PDMS (solid line) and PDMS
nanocomposite (dashed line) containing 10 wt.
organo-modified montmorillonite
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Gas Barrier Properties
The high aspect ratio characteristic of silicate
nanolayers in exfoliated nanocomposites has been
found to highly reduce the gas permeability in
films prepared from such nanomaterials.
CO2 permeability of polyimide clay composites
prepared by curing CH3(CH2)17NH3
montmorillonite-poly(amic acid) films at 300ºC.
Curves A and C are calculated for filler with an
aspect ratio of 20 and 2000, respectively, Curve
B was generated by least squares fitting of the
permeability equation to the experimental data.
The inset illustrates a possible self-similar
aggregation mechanism for the clay plates.
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Ionic Conductivity
Nanocomposites have been also considered to tune
ionic conductivity of PEO. An intercalated
nanocomposite obtained by melt intercalation of
poly(ethylene oxide) (40 wt.) into
Li-montmorillonite (60 wt.) has shown to enhance
the stability of the ionic conductivity at lower
temperature when compared to more conventional
PEO/LiBF4 mixture. This improvement is explained
by the fact that PEO is not able to crystallize
when intercalated, hence eliminating the presence
of crystallites, non-conductive in nature. The
conductivity of PEO/Li-montmorillonite
nanocomposite is 1.6x10-6 S/cm at 30ºC and
exhibits a weak temperature dependence with an
activation energy of 2.8 kcal/mol. The higher
ionic conductivity at ambient temperature
compared to conventional LiBF4/PEO electrolytes
combined with a single ionic conductor character
makes those nanocomposites new promising
electrolyte materials.
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Other Properties
Coefficient of Thermal Expansion (CTE) Due to
the high aspect ratio of the exfoliated silicate
layers, the coefficient of thermal expansion
(CTE) of poly(imide)-based nanocomposites with
hexadecylammonium cation exchange montmorillonite
can be strongly reduced, going from 3.6 x 10-5
K-1 to values as low as 1.55 x 10-5 K-1 when 10
wt. of nanofiller is dispersed. A noticeable
decrease of 45 (1.96 x 10-5 K-1) was observed
with only 1 wt. of nanoclay.
Finally, nanocomposites have been used in highly
technical domains such as the potentiality to use
polyaniline-based nanocomposite as
electrorheological sensitive additive or the
combination of dispersed layered silicates in a
liquid crystal medium for the production of
stable electro-optical devices exhibiting a
bistable and reversible electro-optical effect
between a light scattering opaque state and a
transparent state
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Polymer-Fibrillar Silicate Nanocomposites
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Fibrillar Silicates
Fibrillar silicate (FS) is composed of
nano-scaled single crystals (fibers). FS is a
class of hydrated Mg/Al silicate, and there are
several types of FS minerals found in nature.
The most abundant type is known as
attapulgite/palygorskite, which is found mostly
in the United States and China. This lecture will
primary discuss attapulgite, and its chemical
formula is Mg5AlSi8O20(HO)2(OH2)44H2O. The
primary structural units of FS are the silicate
single crystals that are 100-3000 nm in length
and 10-25 nm in diameter, and these single
crystals stack/agglomerate into particles with
sizes in microns.
Representative SEM images of the FS powder
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Unlike layered silicates such as montmorillonite,
which are difficult to completely exfoliate into
nano-scaled silicate layers and to uniformly
distribute in polymer matrices, FS is relatively
easy to separate into nano-scaled single crystals
and to distribute uniformly in matrices. This is
because the spacing among the aggregated single
crystals in FS is much larger than that of the
silicate layers in montmorillonite. As a result,
the interaction of the single crystals in FS is
considerably weaker than that of the silicate
layers in montmorillonite. Therefore, without
chemical substitution of metal ions with
surfactants such as alkyl amine ions. FS can be
readily separated into nano-scaled single
crystals by simply dispersing FS
agglomerates/particles in polar solvents like
ethanol, followed by vigorously mechanical
stirring. Additionally, the interfacial bonding
between the silanized FS nano-scaled single
crystal filler and the resin matrix can be
reasonably strong since there are abundant SiOH
groups on the surface of FS single crystals, and
these groups can react with silane coupling
agents such as 3-methacryloxypropyltrimethoxy
(MPTMS).
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FS Nano-Scaled Single Crystals
The FS nano-scaled single crystals possess a high
degree of structural perfection and superior
mechanical properties. For example, the strength
of a FS nano-scaled single crystal is over 50 GPa
TEM images of separated and silanized FS
nano-scaled single crystals
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Mechanical Properties
Representative fracture surfaces of three-point
flexural specimens (a) neat/unfilled
Bis-GMA/TEGDMA (b) Bis-GMA/TEGDMA filled with
2.5 (mass faction) nano FS (c) Bis-GMA/TEGDMA
filled with 50 (mass faction) glass filler (d)
Bis-GMA/TEGDMA filled with 2.5 (mass faction)
nano FS and 50 (mass faction) glass filler
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The bridging mechanism has been proposed to
explain the reinforcement by the nano-scaled
fibrillar silicates. If a micro-crack is
initiated in a matrix under contact wear and/or
other stresses, the fibrillar silicates remain
intact across the crack planes and support the
applied load. Crack-opening is therefore
resisted by the bridging fillers and the matrix
is reinforced. Requirements for the fillers to
achieve the effective bridging reinforcement
include high mechanical properties (e.g.,
strength and modulus) and a large aspect ratio.
Impregnation of the fibrillar silicates into
Bis-GMA/TEGDMA dental resins/composites could
result in two opposite effects the reinforcing
effect due to the highly separated and uniformly
distributed nano FS single crystals, and the
weakening effect due to the formation of FS
agglomerates/particles. The simultaneous
accomplishment of a high degree separation and
uniform dispersion of nano FS in dental
resins/composites is still a technical
bottleneck.
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Polymer-Carbon Nanotube Nanocomposites
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Since the discovery of carbon nanotubes by Iijma,
researches related to the carbon nanotubes and
their composite materials have been dramatically
increased. The single-walled and multi-walled
carbon nanotubes are supposed to be formed by
rolling a graphene sheet and graphene sheets,
respectively. The carbon nanotubes have been
regarded as one of the ultra-strong materials in
the world. Additionally, the thermal and
electrical conductivities of carbon nanotubes are
also supposed to be high due to the highly
conjugated graphene structure. The arguments for
the true mechanical and electrical/thermal
properties of both single-walled and multi-walled
nanotubes never cease. Whether chemical bonding
between the nanotubes and their surrounding
polymer-based matrix in the composites exits or
not, is another disputable topic. More
importantly, how to effectively separate the
aggregated (bundled) carbon nanotubes (especially
single-walled) and uniformly distribute them into
matrices is another challenging task that
researchers have to solve before applying the
materials for making composites.
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Structure of Carbon Nanotubes
The bonding in carbon nanotubes is sp², with each
atom joined to three neighbors, as in graphite.
The tubes can therefore be considered as
rolled-up graphene sheets (graphene is an
individual graphite layer). There are three
distinct ways in which a graphene sheet can be
rolled into a (single-walled) carbon nanotube.
The first two of these, known as armchair (top
left) and zig-zag (middle left) have a high
degree of symmetry. The terms "armchair" and
"zig-zag" refer to the arrangement of hexagons
around the circumference. The third class of
tube, which in practice is the most common, is
known as chiral, meaning that it can exist in two
mirror-related forms. An example of a chiral
nanotube is shown at the bottom left.
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Quick Facts of Carbon Nanotubes
Average Diameter of SWNT's 1.2 -1.4 nm Distance
from opposite Carbon Atoms (Line 1) 2.83
Å Analogous Carbon Atom Separation (Line 2)
2.456 Å Parallel Carbon Bond Separation (Line 3)
2.45 Å Carbon Bond Length (Line 4) 1.42 Å C - C
Tight Bonding Overlap Energy 2.5 eV Lattice
(Bundles of Ropes of Nanotubes) Triangular
Lattice (2D) Lattice Parameter (Armchair 16.78
Å Zigzag 16.52 Å Chiral 16.52 Å) Density
(Armchair 1.33 g/cm3 Zigzag 1.34 g/cm3
Chiral 1.40 g/cm3 Interlayer Spacing (Armchair
3.38 Å Zigzag 3.41 Å Chiral 3.39
Å) Fundamental Gap For (n, m) n-m is divisible
by 3 (Metallic, 0 eV) For (n, m) n-m is not
divisible by 3 (Semi-Conducting, 0.5 eV) Young's
Modulus (SWNT) 1 TPa Young's Modulus (MWNT)
1.28 TPa Tensile Strength 100 to 500 GPa
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Ropes are bundles of tubes packed together in an
orderly manner. The individual SWNTs packed into
a close-packed triangular lattice with a lattice
constant of 17 Å.
Cluster of nanotubes and nanoparticles
(a) High resolution image showing typical carbon
nanotube in the tip region. (b) Carbon nanotubes
embedded in carbon support film.
Purified carbon nanotubes
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Chemical Functionalization of Carbon Nanotubes
The surfaces of CNTs have to be chemically
functionalized to achieve good dispersion in
polymer/CNT composites and strong interface
adhesion between surrounding polymer chains.
CNTs are assembled as ropes or bundles, and there
are some catalyst residuals, bucky onions,
spherical fullerenes, amorphous carbon,
polyhedron graphite nano-particles, and other
forms of impurities in the as-grown CNTs. Thus,
purification, cutting or disentangling, and
activation treatments are needed before chemical
functionalization.
After purification, CNTs can be oxidized by many
oxidizing agents (including oxygen, air,
concentrated sulfuric acid, nitric acid, hydrogen
peroxide, and their mixtures) or by being treated
with plasma to introduce carboxylic acid and/or
hydroxyl groups on the surface or open ends. The
carboxylic acid and/or hydroxyl groups can be
further converted to a variety of other
functional groups (including halo, vinyl, epoxy
and others).
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Composite Processing Method 1 Solution Processing
Perhaps the most common method for preparing
polymer nanotube composites has been to mix the
nanotubes and polymer in a suitable solvent
before evaporating the solvent to form a
composite film. One of the benefits of this
method is that agitation of the nanotubes powder
in a solvent facilitates nanotube de-aggregation
and dispersion. Almost all solution processing
methods are variations on a general theme which
can be summarized as 1. Dispersion of
nanotubes in either a solvent or polymer
solution by energetic agitation. 2. Mixing of
nanotubes and polymer in solution by energetic
agitation. 3. Controlled evaporation of solvent
leaving a composite film. In general, agitation
is provided by magnetic stirring, shear mixing,
reflux or, most commonly, ultrasonication.
Sonication can be provided in two forms, mild
sonication in a bath or high-power sonication
using a tip or horn.
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Composite Processing Method 2 Melt Processing
Solution processing is completely unsuitable for
many polymers that are insoluble. Melt
processing is a common alternative, which is
particularly useful for dealing with
thermoplastic polymers. Amorphous polymers can
be processed above their glass transition
temperatures while semi-crystalline polymers need
to be heated above their melt temperatures to
induce sufficient softening. Advantages of this
technique are its speed and simplicity, and its
compatibility with standard industrial
techniques. In general, melt processing involves
the melting of polymer pellets to form a viscous
liquid. Any additives, such as carbon nanotubes
can be mixed into the melt by shear mixing. Bulk
samples can then be fabricated by techniques such
as compression molding, injection molding or
extrusion. However it is important that
processing conditions are optimized, not just for
different nanotube types, but for the whole range
of polymernanotube combinations. This is
because nanotubes can affect melt properties such
as viscosity, resulting in unexpected polymer
degradation under conditions of high shear rates
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Composite Processing Method 3 Thermoset Polymer
Composite Processing
The most common thermosetting polymers used in
the formation of polymer nanotube composites have
been epoxy resins. Generally these are polymers
that cure when mixed with a catalyzing agent or
hardener. In most cases the epoxy begins life in
liquid form, facilitating nanotube dispersion.
In the simplest cases nanotubes have been
dispersed by sonication in a liquid epoxy such as
Shell EPON 828 epoxy. This blend is then cured
by the addition of hardener such as triethylene
tetramine. After having been left to gel
overnight the composite was cured at 100 ºC for 2
h. While this is the simplest case, a number of
variations have been described such as improving
the nanotube distribution by dispersing the
nanotubes in a solution of a block co-polymer
(Disperbyk-2150) in ethanol a liquid epoxy was
then added and the ethanol removed by
evaporation subsequently, the hardener was added
and the composite was poured into a mould and
cured in a vacuum oven at 25 ºC for 18 h.
50
Composite Processing Method 4 In situ
Polymerization
Over the last 5 years, the in situ polymerization
in the presence of carbon nanotubes has been
intensively studied for the preparation of
polymer grafted nanotubes and processing of
corresponding polymer composite materials. The
main advantage of this method is that it enables
grafting of polymer macromolecules onto the walls
of carbon nanotubes. In addition, it is a very
convenient processing technique, which allows the
preparation of composites with high nanotube
loading and very good miscibility with almost any
polymer type. This technique is particularly
important for the preparation of insoluble and
thermally unstable polymers, which cannot be
processed by solution or melt processing.
Depending on required molecular weight and
molecular weight distribution of polymers, chain
transfer, radical, anionic, and ring-opening
metathesis polymerizations can be used for in
situ polymerization processing.
51
Properties and Applications of Polymer/CNT
Nanocomposites
CNTs have been proposed for many potential
applications including conductive and high
strength composites energy storage and energy
conversion devices sensors field emission
displays and radiation sources hydrogen media
and nanometer-sized semi-conductor devices,
probes, and interconnects, etc. Continuing
advances on dispersion and alignment of CNTs in
polymer matrices will further promote
developments in and expand the range of
applications of these nanocomposites.
52
Mechanical Properties
Incorporation of CNTs into a polymer matrix can
potentially provide structural materials with
dramatically increased modulus and strength. For
example, adding 1 wt. MWCNTs in the PS/MWCNT
composite films by the solution-evaporation
method, results in 40 and 25 improvements in
tensile modulus and break stress, respectively.
It has been observed that a monotonic increase of
resistance to indentation (Vickers hardness) by
up to 3.5 times on adding 2 wt. SWCNTs in epoxy
resin. It was also found that adding 1 wt.
MWSNTs to polyvinyl alcohol (PVA) increased the
modulus and hardness by 1.8 times and 1.6 times,
respectively. The uniform distribution and
alignment of CNTs in polymer matrices are
significant to enhance the effectiveness of
reinforcement. For example, for PMMA/MWCNT
composites containing 1 wt. MWCNT, the storage
modulus at 90 ºC is increased by an outstanding
1135 due to the uniform distribution enhanced by
in situ polymerization.
53
Thermal Properties
The addition of CNTs could increase the glass
transition, melting and thermal decomposition
temperatures of the polymer matrix due to their
constraint effect on the polymer segments and
chains. Adding 1 wt. CNTs to epoxy increases
the glass transition temperature from 63 to 88
ºC. Similarly, with 1 wt. well-dispersed
SWCNTs, the glass transition temperature of PMMA
is increased by 40 ºC. It was also found that
the thermal decomposition temperature of
polypropylene (PP) at peak weight loss in
nitrogen was increased by 12 ºC with 2 vol.
MWCNTs, and that MWCNTs significantly reduced the
PP heat release rate making it as effective a
fire-retardant as PP/PP-g-MA/clay. Additionally,
the incorporation of CNTs could improve the
thermal transport properties of polymer
composites due to the excellent thermal
conductivity of CNTs. This offers an opportunity
for polymer/CNT composites for usages as printed
circuit boards, connectors, thermal interface
materials, heatsinks, lids and housings, and
high-performance thermal management from
satellite structures down to electronic device
packaging. It was found that the thermal
conductivity of epoxy increased by up to 300
with 3 wt. SWCNTs.
54
Electrical and Electrochemical Properties
The first realized major commercial application
of carbon nanotubes is their use as electrically
conducting components in polymer composites. It
has been reported that the GE Plastics has been
using CNTs in a poly(phenylene oxide)
(PPO)/polyamide (PA) blend for automotive mirror
housings for Ford to replace conventional
micron-size conducting fillers, which would
require loadings as high as 15 wt. to have a
satisfactory anti-static property but which would
impart poor mechanical properties and a high
density to the composite. Recently,
super-capacitors are attracting great attention
because of their high capacitance and potential
applications in electronic devices. It has been
reported that the performance of supercapacitors
with MWCNTs deposited with conducting polymers as
active materials is greatly enhanced compared to
electric double-layer super-capacitors with CNTs
due to the Faraday effect of the conducting
polymer. Besides these, polymer/CNT
nanocomposites could have applications in
electrochemical actuation, electromagnetic
interference shielding, wave absorption,
electronic packaging, self-regulating heater, and
others.
55
Optical and Photovoltaic Properties
Nonlinear optical organic materials, such as
porphyrins and dyes, provide optical limiting
properties for photonic devices to control light
frequency and intensity in a predictable manner.
However, these are narrow band optical materials.
Carbon nanotubes have been found to be able to
significantly enhance the materials
performance. Another potentially important
application of CNTs is in polymer-based
light-emitting devices. The advantages for
organic light-emitting diodes (OLEDs) based on
conjugated polymers are low cost, low operating
voltage, excellent processability and
flexibility. However, their low quantum
efficiency and stability have limited their
applications and developments. CNTs
(particularly the soluble polymer grafted CNTs)
could improve the materials efficiency and
stability. Besides, CNTs are also widely used in
organic photovoltaic devices. Doping with 6 wt.
chemical functionalized MWCNTs by grafting
dodecylamine chains, the photosensitivity of
oxotitanium phthalocyanine (TiOPc) is five-fold
higher than that of un-doped TiOPc when exposed
to 570 nm wavelength
56
Supplemental Reading Materials

• Pinniavaia TJ, Beall G. Eds. (2001). Polymer Clay
  Nanocomposites, Wiley and Sons, New York.
• Alexandre M and Dubois P (2000). Polymer-Layered
  Silicate Nanocomposites Preparation, Properties
  and Uses of a New Class of Materials.  Materials
  Science and Engineering, 28, 1-63.
• Tian M, Gao Y, Liu Y, Liao Y, Hedin NE. and Fong
  H (2007). Fabrication and Evaluation of
  BIS-GMA/TEGDMA Dental Resins/Composites
  Containing Nano-Fibrillar Silicate. Dental
  Materials, 24, 235-243, 2008.
• Lau K, Gu C, Hui D (2006). A Critical Review on
  Nanotube and Nanotube/Nanoclay Related Polymer
  Composite Materials. Composites Part B 37,
  425436.
• Coleman JN, Khan U, Blau WJ, Gunko YK (2006).
   Small but Strong A Review of the Mechanical
  Properties of Carbon NanotubePolymer Composites.
  Carbon 44, 16241652.
• http//www.pa.msu.edu/cmp/csc/nanotube.html