Figure 7. Stress-strain responses of the tensile deformation of the
graphene-carbon nanotube fiber-reinforced polymer composite system.
The effect of strain on the bond order parameters of the tensile
deformation is illustrated in Figure 8 for the graphene-carbon nanotube
fiber-reinforced polymer composite system. 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,
for example, a polyaromatic hydrocarbon [79, 80]. This type of
functionalization results in higher degrees of functionalization as the
entire length of the carbon nanotubes can be functionalized rather than
just the ends and specific active sites [81, 82]. Like
end-functionalization, non-covalent functionalization also opens up the
possibility for tailoring the functionalization via the choice of
molecule [83, 84]. The methods may be used to create light weight,
high strength structures [85, 86]. This results in improving the
mechanical properties of the interface between the carbon nanotubes and
the polymer thereby imparting many of the valuable properties of carbon
nanotubes into the polymer matrix resulting in a significantly improved
polymer-carbon nanotube composite [87, 88]. Carbon nanotubes are
ideal reinforcing material for polymer matrices because of their high
aspect ratio, low density, remarkable mechanical properties, and good
electrical and thermal conductivity. The graphene-carbon nanotube
fiber-reinforced polymer composite differs from a conventional
carbon-fiber composite in that there is a much higher interface area
between reinforcing carbon and polymer matrix phases. Introducing a
uniform distribution of graphene-carbon nanotubes into a polymer matrix
should yield property enhancements that go beyond that of a simple rule
of mixtures. The challenge is to take full advantage of the exceptional
properties of graphene-carbon nanotubes in the composite material.
However, property improvements are not significant to date, apparently
due to poor interfacial graphene-carbon nanotube-polymer bonding and
severe graphene-carbon nanotube agglomeration. These obstacles can be
overcome by utilizing a new processing route that involves high-shear
mixing in a molten polymer to induce de-agglomeration and dispersal of
graphene-carbon nanotubes, while enhancing adhesive bonding and covalent
bonding by creating new sites on the graphene-carbon nanotubes to which
the polymer chains can bond. 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 graphene-carbon nanotube having
the same magnitude as the radius of gyration of the polymer chain.