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.