Application of the Tool--Narayanaswamy--Moynihan model to the study of the alpha relaxation by thermally stimulated depolarization currents

Abstract

The dielectric alpha relaxation is the dielectric manifestation of the glass transition. In spite of this fact, the more commonly used models, the Arrhenius model and the Williams--Landel--Ferry model, do not take into account the structural state of the system to modelize this relaxation. In thermally stimulated discharge current (TSDC) experiments, the sample is out of equilibrium during most of the discharge ramp. Not surprisingly, the capability of these models to represent the data points at temperatures well below the glass transition temperature is very poor. To overcome this limitation, we have used the Tool--Narayanaswamy-Moynihan (TNM) model, that takes into account the structural state of the system, to modelize TSDC data. Although it is mostly applied to calorimetric and volumetric experiments we show how it can be employed on dielectric data. The numerical results support the applicability of the model and suggest how the dielectric and the structural relaxation times may be related. The TNM model turns out to be physically sound since the modelization of dielectric data gives also a reasonable structural kinetics of the system. The qualitative differences between the TNM model and the equilibrium models for different TSDC experiments is discussed. Experimental data coming from TSDC experiments where the thermal history of the sample is changed do not provide additional evidence but nevertheless are compatible with the parameter values that were obtained in the fits to the TNM model.