Influence of temperature and pressure on dielectric relaxation in a supercooled epoxy resin

Isothermal and isobaric dielectric measurements of a supercooled epoxy resin have been compared. A simple scaling relates isobaric and isothermal spectra corresponding to the same frequency of the main loss peak. Thus, the main and secondary processes retain a relative weight that is the same under isothermal and isobaric conditions. It is inferred that both pressure and temperature, equivalently, are able to take effect on the relaxation processes, without changing the relaxation mechanism itself. Careful analysis of the structural relaxation time behavior revealed that the traditional free volume equation, where only the macroscopic volume controls the pressure evolution of free volume, is not a suitable description of the data, as well as a Vogel-Fulcher (VF) type pressure dependent function. Based on a derivative method, a different function for describing the bidimensional surface tau(T,P) has been proposed, which accounts for the observed behavior through a nonlinear correction of the critical temperature T0 in the VF law. The function we propose predicts pressure dependencies of the glass transition temperature and fragility which are appealing in view of a comparison with experimental results in this and many other systems. Interesting hints for interpreting the phenomenological results can be obtained within the Adam-Gibbs theory.