Raman Spectroscopy Studies of Carbon Nanotube-Polymer Composites
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Natural carbon exhibits its distinct properties in two forms: diamond, an insulator, gives the strongest mechanical strength; graphite, a brittle material, conducts electricity at room temperature. A recently synthesized carbon form, the carbon nanotube (CNT), combines the mechanical strength of diamonds with the electric conductivity of graphite. However, the as-grown form of this almost perfect new material is actually a mixture of semiconducting and metallic tubes, in the approximate 2:1 ratio estimated by the tight binding model. Furthermore, as-grown single-wall nanotubes (SWNTs) generally possess a very high aspect ratio of length to diameter (on the order of 1000:1). They tend to bundle together as ropelike hydrophobic aggregates—not an individual tube of either semiconductor or metallic tube. However, controlling growth and follow-up processing and treatment conditions can change the metallic and semiconducting ratio in the mixture (see “Carbon Nanotubes”).
To take advantage of their unique electronic properties, mixed SWNT bundles can be dispersed and separated to tailor to various device applications. One ideal matrix for separation and self-assembly is a scheme using polymers consisting of a carbon backbone because of their structural compatibility. The electric conductivity of the filler-based polymer composite is substantially increased with CNT loading. Raman frequency shifts have demonstrated their noncovalent interfacial interactions, through tensile and strain as well as compression transferred by the outer and inner layers of the multiwalled nanotube (MWNT), respectively. These interfacial interactions, along with the high aspect ratio (D/L in the order of 1:1000), are believed to enhance conduction path through hopping and tunneling in the composite.
The study and design of the CNT/polymer composite emerged with increased knowledge of the interactions in the composites. Most recently, CNT-based composites appear promising for applications in telecommunication, actuators, photovoltaic cells, light-emitting diodes (LEDs), field emission and radiation shielding. The performance and lifetime of devices are enhanced through CNT composites. This also opens a venue for many nonconducting polymer applications with good structural stability and flexible processability. Because mobility can be increased to avoid electostatic effect, the composites can be used for passive electronic components as well as electromagnetic shielding. By forming the CNT composites, we overcome the disadvantages pertinent to nonconducting polymers to improve their performance as a result of the superior electric, thermal, and mechanical properties of CNT.