2. Methods
Carbon nanotubes, with their unique shapes and characteristics, are
being considered for various applications. A carbon nanotube has a
tubular shape of one-dimensional nature which can be grown through a
nano metal particle catalyst. More specifically, carbon nanotubes can be
synthesized by arc discharge or laser ablation of graphite. In addition,
carbon nanotubes can be grown by a chemical vapor deposition technique.
With the chemical vapor deposition technique, there are also variations
including plasma enhanced and so forth. Carbon nanotubes can also be
formed with a frame synthesis technique similar to that used to form
fumed silica. In this technique, carbon atoms are first nucleated on the
surface of the nano metal particles. Once supersaturation of carbon is
reached, a tube of carbon will grow. Regardless of the form of
synthesis, and generally speaking, the diameter of the tube will be
comparable to the size of the nanoparticle. Depending on the method of
synthesis, reaction condition, the metal nanoparticles, temperature and
many other parameters, the carbon nanotube can have just one wall,
characterized as a single walled carbon nanotube, it can have two walls,
characterized as a double walled carbon nanotube, or can be a
multi-walled carbon nanotube. The purity, chirality, length, and defect
rate can vary. Very often, after the carbon nanotube synthesis, there
can occur a mixture of tubes with a distribution of all of the above,
some long, some short. Some of the carbon nanotubes will be metallic and
some will be semiconducting.
One aspect of the present study is directed to mechanical
functionalization of carbon fibers processed in situ with molten
polymers to create reactive bonding sites at the ends of the fibers. The
reactive sites react with the polymer to chemically bond the carbon
fibers to the polymer. This can be achieved with a variety of carbon
fibers, including single or multi-walled carbon nanotubes and standard
micron sized carbon fibers. It works well in conjunction with a variety
of polymers that possess chemical groups having double bonds or various
tertiary carbon bonds. Similar observations of good bonding at sites of
broken covalent graphite and graphene bonds have been made while
mechanically exfoliating graphite into graphene in situ with polymers.
The fibers are broken or cut while in molten polymers during melt
processing, and this can be achieved either by having a specially
designed cutting tool in the melt processing equipment, or through high
shear in the melt processing, or by a combination of the two. The
opening up of new fiber ends by breaking or cutting the fibers while
surrounded by liquid polymers introduces dangling bonds having unfilled
valences which provide reactive sites on the fiber ends, which represent
sites for strong bonding, such as covalent bonding, by the polymers
having the attributes mentioned above. The resulting solid composites
have improved mechanical properties upon cooling, and the optimal fiber
length, and, subsequently, cost will be greatly reduced by this bonding.
In one aspect, the present study provides a high efficiency mixing
method to transform a polymer composite that contains carbon fibers into
broken carbon fibers having reactive ends or edges, by compounding in a
batch mixer or extruder that imparts repetitive, high shear strain
rates. The method is low cost to produce a carbon fiber-reinforced
polymer matrix composites that offers numerous property advantages,
including increased specific stiffness and strength, enhanced electrical
and thermal conductivity, and retention of optical transparency.
Furthermore, these properties are tunable by modification of the
process, vide infra. In some cases, an inert gas or vacuum may be used
during processing. Other advantages of in situ carbon fiber breaking are
that it avoids handling size reduced carbon fibers, and also avoids the
need to disperse them uniformly in the polymer matrix phase. Superior
mixing produces finer composite structures and very good particle
distribution.
It should be understood that essentially any polymer inert to carbon
fibers or nanotubes and capable of imparting sufficient shear strain to
achieve the desired carbon fiber breakage may be used in the method of
the present study. Mechanical functionalization of carbon fibers within
a polymer matrix may be accomplished by a polymer processing technique
that imparts repetitive high shear strain events to mechanically break
the carbon fibers within the polymer matrix. A succession of shear
strain events is defined as subjecting the molten polymer to an
alternating series of higher and lower shear strain rates over
essentially the same time intervals so that a pulsating series of higher
and lower shear forces associated with the shear strain rate are applied
to the carbon fibers in the molten polymer. Higher and lower shear
strain rates are defined as a first higher, shear strain rate that is at
least twice the magnitude of a second lower shear strain rate. After
high-shear mixing, the mechanically size reduced carbon fibers are
uniformly dispersed in the molten polymer, are randomly oriented, and
have high aspect ratio. Graphite microparticles are also added to the
molten polymer and are mechanically exfoliated into graphene via the
succession of shear strain events. The amount of graphite added to the
molten polymer can be an amount up to and including the amount of carbon
fibers and nanotubes added. The shear strain rate within the polymer is
controlled by the type of polymer and the processing parameters,
including the geometry of the mixer, processing temperature, and speed
in revolutions per minute. The required processing temperature and speed
for a particular polymer is determinable from polymer rheology data
given that, at a constant temperature, the shear strain rate is linearly
dependent upon speed in revolutions per minute. Polymer rheology data
collected for a particular polymer at three different temperatures
provides a log shear stress versus log shear strain rate graph.
A polymer-carbon nanotube composite differs from a conventional
carbon-fiber composite in that there is a much higher interface area
between reinforcing carbon and polymer matrix phases. It has been
proposed that introducing a uniform distribution of carbon nanotubes
into a polymer matrix should yield property enhancements that go beyond
that of a simple rule of mixtures [39, 40]. The challenge is to take
full advantage of the exceptional properties of carbon nanotubes in the
composite material. Carbon nanotubes are considered to be ideal
reinforcing material for polymer matrices because of their high aspect
ratio, low density, remarkable mechanical properties, and good
electrical and thermal conductivity [41, 42]. One of the matrices
that has been studied is commercially important epoxy [43, 44].
However, property improvements have not been significant to date,
apparently due to poor interfacial carbon nanotube-polymer bonding and
severe carbon nanotube agglomeration [45, 46]. These obstacles have
now been overcome by utilizing a new processing route that involves
high-shear mixing in a molten polymer to induce de-agglomeration and
dispersal of carbon nanotubes, while enhancing adhesive bonding and
covalent bonding by creating new sites on the carbon nanotubes to which
the polymer chains can bond. An attempt is also being made to increase
impact energy absorption by forming a biphasic composite, comprising a
high fraction of strong carbon nanotube-reinforced epoxy particles
uniformly dispersed in a tough epoxy matrix. A carbon nanotube consists
of a sheet of hexagonal-bonded carbon atoms rolled up to form a tube. A
single-walled carbon nanotube comprises a single layer of this tubular
structure of carbon atoms. However, the structure of a multi-walled
carbon nanotube is still open to some debate. In one model, a
multi-walled carbon nanotube is imagined to be a single graphene sheet
rolled up into a scroll. In another model, a multi-walled carbon
nanotube is considered to be made of co-axial layers of
helically-aligned carbon hexagons, with matching at the joint lines,
leading to a nested-shell structure. In yet another model, a combination
of scroll-like and nested-shell structures has been proposed.
A small percentage of carbon nanotubes can improve the robustness of the
material without significantly compromising the elastomeric properties.
Increases in mechanical strength properties reduce blade edge tears and
substantially extend blade life due to edge wear. Low percentage
additions of carbon nanotubes can also significantly increase electrical
and thermal conductivity. Enhanced electrical conductivity can dissipate
charge accumulation at the blade edge due to rubbing against the
photoreceptor and air breakdown from the accumulation of charged toner
at the blade edge. Enhanced thermal conductivity can aid heat
dissipation due to friction at the blade-photoreceptor interface.
Polymer properties such as electrical conductivity can be enhanced by
incorporating therein a combination of carbon fibers or carbon
nanotubes. Additionally, carbon nanotubes can prevent delamination and
provide structural stability in polymer composites. Because carbon
nanotubes have uniquely high strength to mass ratio, intrinsic light
weight, thermal conductivity, electrical conductivity, and chemical
functionality, and can prevent delamination and provide structural
stability in polymer composites, they can impart these properties to
polymers when effectively combined therewith. Though carbon nanotubes
have extraordinary mechanical properties, their ability to strengthen
polymers and epoxies is limited by the strength of interfacial bonding.
As a result, when incorporated into polymeric resin without
cross-linking or functionalization, they lack the ability to transfer
loads across the structure. Carbon nanotubes can be functionalized via
covalent or non-covalent bonding, to either the ends of the nanotubes or
to the sidewalls. Covalent functionalization often requires beginning
with modified tubes, such as fluorinated nanotubes, or with purified
tubes where defect sites in the carbon nanotubes are produced by
oxidation. Because these modifications often result in the disruption of
the bonds along the tubes themselves, covalent functionalization can
degrade the mechanical and electrical properties of the nanotubes and,
thus, is not ideal for all applications. Non-covalent functionalization
to the sidewalls of carbon nanotubes can be attained by exploiting the
van der Waals and pi-pi bonding between the pi electrons of the carbon
nanotubes and that of a polyaromatic molecule. This type of
functionalization results in higher degrees of functionalization as the
entire length of the carbon nanotube can be functionalized rather than
just the ends and specific active sites. Like end-functionalization,
non-covalent functionalization also opens up the possibility for
tailoring the functionalization via the choice of molecule. For the
purpose of polymerizing the carbon nanotube to a polymer resin or epoxy,
in one embodiment, a polymerizable ligand comprising a polyaromatic
molecule with an appropriate polymerizable group such as a vinyl,
styryl, or amino group can be non-covalently bonded to the carbon
nanotubes.
It is known that increases in elastic modulus and strength of
epoxy-carbon nanotube composite resulted from making small additions of
carbon nanotubes to polymer matrices. While Van der Waals bonding
dominates interactions between carbon nanotubes and polymers, adhesion
in some carbon nanotube composites also occurs via covalent bonds, which
plays a role in reinforcement of carbon nanotube composites.
Measurements by atomic force microscopy of the pull-out force necessary
to remove a given length of an individual multi-walled carbon nanotube
embedded in copolymers demonstrate covalent bonding between the outer
layer of a multi-walled carbon nanotube and the polymer matrix. The
polymer matrix in the near vicinity to the interface behaves differently
than the polymer in the bulk, which is attributed to the outer diameter
of a carbon nanotube having the same magnitude as the radius of gyration
of the polymer chain. Because of the tendency of carbon nanotubes to
agglomerate, difficulty of aligning them in the matrix and often poor
load transfer, attempts are made to produce composites using different
polymer matrix phases. The present study provides remarkable
improvements in stiffness and strength of a carbon nanotube-reinforced
epoxy composite. The composites are characterized by an increase in
impact energy absorption. Processing parameters which achieve superior
mechanical properties and performance are provided herein.
Incremental additions of carbon nanotubes to the molten epoxy are
necessary to produce a composite that contains a high fraction of carbon
nanotubes. It takes about one hour to ensure that mixing parameters
remain as stable as possible. The rapid increase in melt viscosity
during mixing is attributed to chemical bonding between dispersed carbon
nanotubes and epoxy polymer matrix. After completion of the mixing
process, the composite material is extracted from the barrel at the
mixing temperature. Upon cooling to ambient temperature, the material
became hard and brittle. This is further evidence for chemical bonding
between dispersed carbon nanotubes and epoxy matrix. Larger samples of
carbon nanotube-reinforced epoxy can be prepared using an integrated
high shear mixing and injection molding apparatus. American Society for
Testing and Materials standard test bars can be fabricated and evaluated
for mechanical properties. Preliminary tests performed on small samples
indicate significant improvements in stiffness and strength.