Electromechanical and electrothermal behaviours of carbon whisker reinforced elastomer composites

Abstract

The electromechanical and electrothermal properties of conducting carbon whisker reinforced thermoplastic elastomer (TPE) composites were investigated. The carbon whiskers were derived by a catalytic chemical vapour Deposition (CCVD) process and the TPE was a styrene-ethylene-butylene-styrene (S-EB-S) block copolymer. The electrical resistivity (ϱ) of the composites can be varied either by uniaxial deformation (101–108 Ω cm) or by temperature (101–105 Ω cm). The temperature-resistivity studies indicated that the resistivity of these composites was influenced by the glass transition temperature (Tg) of the TPE. The ϱ versus 1/T curves exhibited two distinct regimes each with a different negative slope which intersected at the Tg of the elastomer. This was correlated to the Tgof the EB segments in the S-EB-S block copolymer (∼ -50°C) by the dynamic mechanical thermal analysis. Further, uniaxial deformation studies at room temperature (20 °C) demonstrated that the resistivity increased exponentially with the deformation. Processing technique considerations and electron micrographs of the morphology of the composites indicated the formation of polymeric film on the carbon whiskers. Thus, the electrical conduction between carbon whiskers in these highly loaded (33 and 52 vol % fraction) composites occurred through the elastomeric film by electron tunnelling. This is explained on the basis of Mott's electron hopping theory, for conduction through several carbon-polymer-carbon (C-P-C) junctions. Further studies by scanning electron microscopy, dielectric thermal analysis and voltage-current characteristics confirmed this observation. Mechanical and electrical properties of the composites indicated that CCVD carbon whiskers can be used to improve the strength and electrical conductivity of TPEs. The change in resistivity (up to five orders of magnitude) of the composites with respect to the deformation or temperature can find use in electromechanical and electrothermal device applications.