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.