Figure 6. High-resolution scanning electron micrographs of the graphene-carbon nanotube hybrid material for the production of fiber-reinforced polymer composites.
The stress-strain responses of tensile deformation are illustrated in Figure 7 for the graphene-carbon nanotube fiber-reinforced polymer composite system. Carbon nanotubes can be functionalized via covalent or non-covalent bonding, to either the ends of the carbon nanotubes or to the sidewalls [69, 70]. Covalent functionalization often requires beginning with modified carbon nanotubes, such as fluorinated carbon nanotubes [71, 72], or with purified carbon nanotubes where defect sites in the carbon nanotubes are produced by oxidation [73, 74]. Because these modifications often result in the disruption of the bonds along the carbon nanotubes themselves [75, 76], covalent functionalization can degrade the mechanical and electrical properties of the carbon nanotubes [77, 78] and, thus, is not ideal for all applications. Though graphene and 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. Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes for use in these applications because they have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes of similar diameter. Defects are less likely to occur in single-walled carbon nanotubes than in multi-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Single-walled carbon nanotubes exhibit exceptional chemical and physical properties that have opened a vast number of potential applications. However, the availability of these new single-walled carbon nanotubes in quantities and forms necessary for practical technology is still problematic. Large scale processes for the production of high-quality single-walled carbon nanotubes are still needed, and suitable forms of the single-walled carbon nanotubes for application to various technologies are still needed. The fibers are broken in the presence of molten polymers during melt processing. Fiber breakage can be accomplished either by having a specially designed cutting tool in the melt processing equipment, or through high shear during melt processing, or by a combination of the two. The opening up of new fiber ends by breaking the fibers while surrounded by liquid polymers introduces dangling bonds, or reactive free radicals, on the fiber ends that represent sites for strong bonding by the polymers with the graphene-carbon nanotube hybrid material. The resulting solid composites have improved mechanical properties.