Thermal conductivity enhancement of electrically insulating syndiotactic poly(styrene) matrix for diphasic conductive polymer composites

In co-continuous conductive polymer composites (CPCs) designed for heat generation by the joule effect, thermal conductivity km of the electrically insulating matrix is the limiting factor to obtain good heat dissipation. To prevent the establishment of non-desirable high temperature gradient in heating elements, it is necessary to enhance the thermal conductivity of the composite, k(c), independently from its electrical conductivity, sigma (already optimized). Several adequate fillers for this purpose have been selected, boron nitride (BN), talc Mg3Si4O10 (OH)(2), aluminum nitride (AIN) and aluminum oxide (Al2O3) particles, and their impact on thermal properties and syndiotactic polystyrene phase transition temperatures have been studies. Lewis and Nielson, Cheng and Vachon, Agari and Uno models were used to predict the evolution of thermal conductivity with filler content and were found to describe correctly thermal conductivity of CPCs after the determination of phi(m) the maximum packing fraction, A = f(D/l) the shape factor, C-1 and C-2 parameters function of respectively modifications of crystals and ability of the filler particles to associate into chains. These models show that whatever the filler and unlike electrical conductivity, thermal conductivity does not go through a sharp insulator/conductor (percolation threshold). Only BN shows a real exponential increase of conductivity over 20% v/v filler. Consequently, in best conditions introducing 30% v/v of BN allows the thermal conductivity to be multiplied by six. BN particles were also found to shift the CPCs glass transition temperature (T-g), the non-isothermal crystallization temperature (T-c,T-n) and the melting temperature T-m. Copyright (c) 2006 John Wiley & Sons, Ltd.