Abstract

We review the present state of polymer nanocomposites research in which the fillers are singlewall or multiwall carbon nanotubes. By way of background we provide a brief synopsis about carbon nanotube materials and their suspensions. We summarize and critique various nanotube/polymer composite fabrication methods including solution mixing, melt mixing, and in situ polymerization with a particular emphasis on evaluating the dispersion state of the nanotubes. We discuss mechanical, electrical, rheological, thermal, and flammability properties separately and how these physical properties depend on the size, aspect ratio, loading, dispersion state, and alignment of nanotubes within polymer nanocomposites. Finally, we summarize the current challenges to and opportunities for efficiently translating the extraordinary properties of carbon nanotubes to polymer matrices in hopes of facilitating progress in this emerging area.

Introduction

Carbon nanotubes (CNT) were first reported by Iijima1 in 1991, and the first polymer nanocomposites using carbon nanotubes as a filler were reported in 1994 by Ajayan et al.2 Earlier nanocomposites used nanoscale fillers such as carbon blacks, silicas, clays, and carbon nanofibers (CNF) to improve the mechanical, electrical, and thermal properties of polymers. Carbon nanotubes possess high flexibility,3 low mass density,4 and large aspect ratio (typically ca. 300-1000). CNT have a unique combination of mechanical, electrical, and thermal properties that make nanotubes excellent candidates to substitute or complement the conventional nanofillers in the fabrication of multifunctional polymer nanocomposites. Some nanotubes are stronger than steel, lighter than aluminum, and more conductive than copper. For example, theoretical and experimental results on individual single-wall carbon nanotubes (SWNT) show extremely high tensile modulus5 (640 GPa to 1 TPa) and tensile strength6 (150-180 GPa). Depending on their structural parameters, SWNT can be metallic or semiconducting, which further expands their range of applications. Because of the nearly one-dimensional electronic structures, metallic nanotubes can transport electrons over long tube lengths without significant scattering (electronic mean free path for metallic SWNT is on the order of several microns7). Similarly, SWNT exhibit large phonon mean free path lengths that result in high thermal conductivity (theoretically8 >6000 W/(m K)). Because of these extraordinary properties of isolated carbon nanotubes, great enthusiasm exists among researchers around the world as they explore the immense potential of these nanofillers. The level of activity is illustrated by the number of journal articles and patents published within a short period of time (Figure 1).