Effect of pressure on the cooperative relaxation properties and glass transition temperature of amorphous polymers

The Adam and Gibbs (AG) theory which describes the cooperative relaxation phenomena in the main transition region in terms of the configurational entropy of the system has been extended to describe also the dependence of both the relaxation times and the glass transition temperature Tg on the external pressure P. The resulting relations contain quantities accessible both by thermo-dynamic (discontinuities in the isobaric heat capacity and in the thermal expansion coefficient) and by viscoelastic (parameters of the temperature dependence of relaxation times) measurements. Theoretical relations adequately describe pressure experimental data on polystyrene, poly(vinyl acetate), and poly(vinyl chloride). The expression dTg /dP derived from the AG theory is at variance with Ehrenfest's relation; the connection between the AG theory and the order parameter description of the glass transition is discussed.