Analytical and Computational Micromechanics Analysis of the Effects of Interphase Regions and Orientation on the Effective Coefficient of Thermal Expansion of Carbon Nanotube-Polymer Nanocomposites

Abstract

The interest in developing multifunctional materials for use in advanced structures in the aerospace industry has been one of the contributing factors encouraging the development of nanocomposites. In particular, nanocomposites consisting of single-wall carbon nanotubes (SWCNTs) dispersed in a polymer matrix have been proposed by many as a material capable of providing enhanced elastic, thermal and electrical properties relative to the neat polymer matrix materials typically used in traditional structural carbon fiber composites. The intent is to allow for the development of carbon fiber composites which can serve not only as a key structural element, but which are capable of providing improved thermal management, electrostatic static discharge, and structural health monitoring abilities with negligible increases in weight. As a result of the orders of magnitude difference in properties between SWCNTs and most polymers , it is believed that only a small amount of SWCNTs would be needed to impart large increases in the elastic, thermal and electrical properties. Recent characterization efforts have shown this to certainly be the case for the electrical properties of nanocomposites where fractions of a weight percent of SWCNTs have been shown to lead to percolation and a corresponding six to eight orders of magnitude increases in electrical conductivity relative to that of the neat polymer. Relatively large increases of 20 30% and 30-100% have also been observed in elastic properties and thermal conductivities, respectively, of nanocomposites containing on the order of 1% SWCNTs, thereby confirming the potential of nanocomposites as multifunctional matrix materials for use in structural carbon fiber composites. In an effort to explore the design space for multifunctional nanocomposite materials, there has been a significant amount of research devoted to developing multiscale models for carbon nanotube-polymer nanocomposites. Some of our recent work in this area has focused on the development of both analytic and computational micromechanics models for assessing the effects of interphase regions, clustering, orientation, distribution, and nanoscale effects such as interfacial thermal resistance and electron hopping on the effective elastic properties and thermal and electrical conductivities of carbon nanotube-polymer nanocomposites; making use of input from lower length scale molecular dynamics simulations when possible. In particular, our efforts to model the thermal conductivity of nanocomposites have made use of molecular dynamics simulations for the measurement of the nanoscale effect associated with an interfacial thermal resistance between the nanotube and the surrounding polymer. This effect has been incorporated into both analytic and computational micromechanics approaches as a zero-thickness interface layer in the calculation