Figure 4. Scanning electron micrographs of the catalytically-grown
multi-walled carbon nanotube-reinforced epoxy composite.
The effect of volumetric carbon loading on the electrical conductivity
is illustrated in Figure 5 for the catalytically-grown multi-walled
carbon nanotube-reinforced epoxy composite. The results are suggestive
of a typical percolation system wherein the electrical properties of the
composite would not be changed from those of the bulk polymer until the
average distance between the carbon nanotubes is reduced such that
either electron tunneling through the polymer or physical contacts may
be formed. Polymer compositions are being increasingly used in a wide
range of areas that have traditionally employed the use of other
materials, such as metals [59, 60]. Polymers possess a number of
desirable physical properties, are light weight, and inexpensive [61,
62]. In addition, many polymer materials may be formed into a number
of various shapes and forms and exhibit significant flexibility in the
forms that they assume, and may be used as coatings, dispersions,
extrusion and molding resins, pastes, powders, and the like [63,
64]. There are various applications for which it would be desirable to
use polymer compositions, which require materials with electrical
conductivity [65, 66]. However, a significant number of polymeric
materials fail to be intrinsically electrically or thermally conductive
enough for many of these applications [67, 68]. Most composites are
made with the understanding that there will be only weak secondary bonds
that exist between the fibers and polymer [69, 70]. This makes it
necessary for very high aspect ratios of fibers to be used in order to
get reasonable stress transfer, or else the fibers will slip upon load.
Some commercial applications of carbon fiber-reinforced polymer matrix
composites include aircraft and aerospace systems, automotive systems
and vehicles, electronics, government defense and security, pressure
vessels, and reactor chambers, among others [71, 72]. Progress in
the development of low-cost methods to effectively produce carbon
fiber-reinforced polymer matrix composites remains very slow [73,
74]. Currently, some of the challenges that exist affecting the
development of carbon fiber-reinforced polymer matrix composites viable
for use in real world applications include the expense of the materials
and the impracticality of the presently used chemical and mechanical
manipulations for large-scale commercial production [75, 76]. It
would thus be desirable for a low-cost method to produce a carbon
fiber-reinforced polymer matrix composite suitable for large-scale
commercial production that offers many property advantages, including
increased specific stiffness and strength, enhanced electrical and
thermal conductivity, and retention of optical transparency [77,
78]. Among the challenges introduced in the fabrication of carbon
nanotube-filled polymer composites is the necessity to creatively
control and make use of surface interactions between carbon nanotubes
and polymeric chains in order to obtain an adequate dispersion
throughout the matrix without destroying the integrity of the carbon
nanotubes.