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.