Conductive Probe Microscopy Investigation of Electrical and Charge Transport in Advanced Carbon Nanotubes and Nanofibers-Polymer Nanocomposites

1. Introduction

Since their identification, (Oberlin et al., 1976; Iijima, 1991) carbon nanotubes (CNTs) have captured the imagination of society as much as engineers and scientists. At small scales, and for pristine CNTs, CNT properties are unrivaled by any other material, especially when the properties are normalized to density, for example, specific stiffness, modulus over density (E/ρ).CNTs are envisioned as a constituent in numerous applications, particularly as reinforcement in advanced structural and functional composites, owing to their numerous and attractive multifunctional properties. The unique CNTs properties includes their high aspect ratio length/diameter (∼1000), high strength-to-weight ratio, and extraordinary mechanical properties: their axial elastic modulus and tensile strength were theoretically predicted to be as high as 1–2 TPa and 200 GPa(Lu, 1997; Wong et al., 1997), respectively. Furthermore, CNTs have superior thermal and electrical properties: thermally stable up to 2800°C in a vacuum or inert atmosphere, thermal conductivity about twice as high as diamond (Kim et al., 2001), and electric-current-carrying capacity 1000 times higher than copper wires (Collins et al., 2000).

Polymers can be easily processed and fabricated into intricately shaped components without damaging CNTs during processing using conventional methods, and hence the manufacturing cost can be certainly reduced. Therefore, CNT based polymer nanocomposites (CNT/polymer nanocomposites) stimulate great interests and have been extensively investigated. Several recent reviews summarizing different aspects of the composite investigations display a broad spectrum of properties useful for production of electrodes for batteries and fuel cells, sensors for chemical vapour, electromagnetic radiation shielding materials, electrical heaters, as well as pressure, deformation and temperature sensors and photovoltaic cells (Thostenson et al., 2001; Popov, 2004; Breuer & Sundararaj, 2004; Ramirez, 2005; Moniruzzaman & Winey, 2006; Baibarac & Gomez-Romero, 2006; Rajesh et al., 2009; Spitalsky et al., 2010). Specials interest is represented by electrical properties of the composites because of the variety of potential applications.

A lot of the research work aimed at understanding the properties and associated mechanisms of these materials has focused largely on bulk properties. An important conclusion of these works is that because of the complexity of the system during the formation of aggregates (from a uniform dispersion of nano-fillers to formation of the mesostructure), the analysis of the overall electric and dielectric response has often been done on a phenomenological basis. It is now clear that the dispersion of the individual tubes inside the polymeric matrix is of a major concern, and the processing conditions must carefully realized to avoid the tendency of CNTs to agglomerates. Furthermore, understanding the interaction of nanoparticles and polymer matrices, structural and transport property characteristics at the nano- and micro- meter range is crucial to exploring their full potential in various applications. While it is critical to characterize the composite structure/properties at the sub-micron or nanoscale, only a few techniques are suitable for this purpose. Although electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have high spatial resolution, high energy electrons can damage polymer films and induces undesired structural changes to CNTs (Banhart, 1999; Li & Banhart, 2004; Yuzvinsky et al., 2005). Scanning probe microscopy (SPM), however, has proven itself to be a valuable tool for nanocomposite characterization and often used to quantify the bulk/surface properties.