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Top1. Introduction
There has been intense interest in carbon nanotubes (CNTs) since their discovery by Ijima in (1991), because they possess exceptional stiffness, extraordinary resilience, remarkable thermal and electrical properties which conceivably, could have a far reaching impact on the next generation of advanced products (Lau and Hui, 2002). Due to their excellent structural, mechanical and electrical properties, CNTs are considered as potential candidate for reinforcement of polymeric materials (Thostenson, Ren, and Chou, 2001). Carbon nanotubes occur as single walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) (Ash, Siegel, and Schadler, 2004) . A SWCNT can be viewed as a single sheet of graphite (i.e. graphene), which has been rolled into the shape of a tube (Saito, Dresselhaus, and Dresselhaus, 1998). SWCNTs have radii on the order of nanometers and lengths on the order of micrometers resulting in large aspect ratios beneficial to their use in composites (Roche, 2000; Saito, Dresselhaus, and Dresselhaus, 1998). Many believe that CNTs may provide the ultimate reinforcing materials for the development of a new class of nanocomposites. However, improvements in properties are by no means guaranteed, and the results are often sensitive to the particular polymer chosen, in addition to the quantity and quality of CNTs used in the composite. If we attempt to trace what controls the performance of materials, we can identify that properties of materials are largely dictated by the atomic structure, composition, microstructure, defects and interfaces in materials (Schaffer et al., 1995). As a result, it is very important to quantitatively understand the mechanics and physics of nano-structured materials, and only then we can effectively employ them in multi-scale material design.
Experimental measurement of the effective properties of CNT reinforced polymer matrix composites have indicated substantial increases in the composite modulus over the matrix modulus. Schadler et al. (1998) found a 40% increase in the effective stiffness of CNT reinforced epoxy as compared to the matrix value with 5% weight CNTs. Qian et al. (2000) also found an increase in the effective modulus of CNT reinforced polystyrene to be of the order of 40% for just 1% weight CNTs. Similar results were also reported by Xu et al. (2002) for MWNT/epoxy resin composites. A wide variety of composites containing CNTs have been manufactured. Peigney et al. (2000) have fabricated composites of CNTs embedded in ceramic powders while Milo, Shaffer, and Windle (1999) have embedded CNTs in poly vinyl alcohol.