Dielectric study of the segmental relaxation of low and high molecular weight polystyrenes under hydrostatic pressure

In this work we analyze by means of dielectric spectroscopy the dynamics of the alpha-relaxation process of low and high molecular weight polystyrene over a wide range of pressures and temperatures. The results are interpreted in terms of a recently proposed equation which describes the behavior of the structural relaxation time, tau(T, P), as a function of both pressure and temperature. This equation has been derived from the Adam-Gibbs (AG) theory by writing the configurational entropy, S-c, in terms of the excess thermal heat capacity and of the excess thermal expansion. Consequently, the molecular dynamic of glass-forming liquids can be linked to its thermodynamic properties. The pressure dependence of the segmental dynamics for both polymers is here measured and analyzed in the AG framework for the first time. tau(T, P) was found to be very well described using the extended AG equation. Additionally, the pressure dependence of the fragility and glass transition temperature (T-g) is analyzed and discussed in terms of the role of chain length and end groups. (c) 2007 Elsevier B.V. All rights reserved.