Figure 3. Effect of the weight fraction of the graphene-carbon nanotube
hybrid material on the modulus of elasticity for the fiber-reinforced
polymer composite.
The effect of the weight fraction of the graphene-carbon nanotube hybrid
material on the hardness is illustrated in Figure 4 for the
fiber-reinforced polymer composite. Regarding the enhancement of
mechanical properties, superior dispersion relates to the maximization
of Young’s modulus, which may be expected if the graphene-carbon
nanotubes are dispersed homogeneously at the level of individual
graphene and carbon nanotubes. However, as can relate to other
considerations, when enhanced electrical conductivity is the goal, the
development of a contiguous, cellular graphene-carbon nanotube structure
yielding electrical percolation can result in greater property
enhancement than a relatively homogeneous graphene-carbon nanotube
dispersion. The hybrid material exhibits great improvements in hardness
and yield strength and major deteriorations in strain at break. A modest
degree of chemical attachment between the derivatized graphene-carbon
nanotubes and the polymer matrix could be tolerated, while retaining the
thermoplastic properties. Physical blending of the graphene-carbon
nanotubes with the polymer can be enhanced by the derivatization
process. For instance, a polymer composite material containing pure
graphene-carbon nanotubes may be desired so that the polymer would have
certain enhanced conductive properties; however, the pure and
underivatized graphene-carbon nanotubes may not sufficiently disperse in
the polymer. By derivatizing the graphene-carbon nanotubes with a
particular moiety, the derivatized graphene-carbon nanotubes could then
be dispersed adequately. In this manner, the conductivity of the
material can be recovered. Polymer properties are enhanced by
incorporating therein a combination of graphene or carbon nanotubes.
Additionally, graphene-carbon nanotubes prevent delamination and provide
structural stability in polymer composites. Because graphene-carbon
nanotubes have uniquely high strength to mass ratio, intrinsic light
weight, thermal conductivity, electrical conductivity, and chemical
functionality, and prevent delamination and provide structural stability
in polymer composites, they can impart these properties to polymers when
effectively combined therewith. Incremental additions of graphene-carbon
nanotubes to the polymer matrix are necessary to produce a composite
that contains a high fraction of graphene-carbon nanotubes. It is
important 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 graphene-carbon nanotubes and the
polymer matrix. After completion of the mixing process, the composite
material, now having a rubber-like consistency, is extracted from the
barrel at the mixing temperature. Larger samples of the fiber-reinforced
polymer composite can be prepared using an integrated high shear mixing
and injection molding apparatus. ASTM standard test bars can be
fabricated and evaluated for mechanical properties. Preliminary tests
performed on small samples indicate significant improvements in
stiffness and strength. Typically, fiber-reinforced thermoplastic
composites suffer from lower impact resistance than the polymer alone.
Additionally, the high-shear mixing process can efficiently disperse the
graphene-carbon nanotube agglomerates, forming a uniform distribution of
graphene-carbon nanotubes in the polymer matrix.