Thermo-Mechanical Properties of Stretchable Nanocomposites Based on Honeycomb Networks of Carbon Nanotubes.

Abstract

Waterborne polymer colloidal particles (i.e. latex) have been used as a template to fabricate carbon nanotube (CNT) composites. Plasticized colloidal crystals are able to assemble CNTs into ordered hexagonal, or honeycomb-like, networks with periodicity defined by the size and deformability of the polymer latex spheres used to form the nanocomposite. In this work, two-dimensional hexagonal networks have been formed by spin-coating the latex nanocomposite dispersion onto a substrate. The resulting monolayer composites have very interesting thermal expansion behaviour. Due to the significantly lower thermal expansion (CTE) of single-walled CNTs (SWNTs) compared to polymers, nanotube networks act as static belts surrounding latex particles in the thin film plane. The matrix with considerably higher CTE is, therefore, restricted to expand in the film plane. The polymer particles, consequently, expand in the out-of-plane direction, thus displaying a higher CTE in that direction. Ultimately the monolayer films may find application as nanoscale thermal actuators. By using latex matrices with low and high glass transition temperature values, random and ordered honeycomb-like CNT networks can be formed in bulk composite samples, respectively. The ability to control CNT network formation results in a variation of thermal conductivity (K) of the nanocomposites. Compared to the composite with random CNT networks, a K enhancement has been found when ordered segregated nanotube networks are created, especially in the case of SWNT composites that offer higher K than the case of composites consisting of multi-walled CNTs. In addition, the thermal percolation threshold has been found to be markedly low due to large excluded volume of the polymer matrix in ordered networks of SWNTs. After spin-coating particle monolayers of CNT-latex blends onto a polymer substrate, the mechanics of the unusual monolayer elasticity has been investigated. Polarized Raman spectroscopy reveals a uniform alignment of the nanotube networks in the strained monolayers. Systematic changes in the resulting Raman spectra of the monolayer under strain indicate that stress is transferred from the colloidal matrix to SWNT inclusions as observed from the Raman G’-band shift. These are explained by strain and slippage of individual SWNTs in the bundles. Additionally, elastic recovery of the monolayer has been found after being strained beyond failure, which may be related to the inter-tube van der Waals forces pulling individual tubes back to their bundles and, therefore, latex particles back to original morphology