Molecular weight effects on the phase-change-enhanced temperature coefficient of resistance in carbon nanotube/poly(N-isopropylacrylamide) composites

Abstract

We investigate the temperature coefficient of resistance (TCR), of carbon nanotube composites with the phase-change polymer poly(N-isopropylacrylamide). Our results shed light on the underlying mechanism of electron transport through networks of conductive paths in the composites and its dependence on the molecular weights (MWs) of the phase-changing polymer. Measurements of the humidity dependence of the TCR reveal the dominant conduction mechanism to be variable range hopping with an exponent of ¼, corresponding to transport in a 3D system. In a humid atmosphere, a twenty-fold change in the hopping activation energy is observed as the temperature is swept across the polymer’s hydrophilic-to-hydrophobic phase-transition, while no change is observed in low humidity atmospheres or in vacuum. The TCR was found to depend on MW but to also be affected by details of the dynamics of hydration/dehydration of the composites. For operation in dry atmospheres and in vacuum the maximum TCR observed was only .002%, while in a humid atmosphere the TCR as large as 60% were observed—an order of magnitude higher than those found in common high-TCR materials used in thermal sensing applications.