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.