Study by Thermostimulated Currents of Dielectric Relaxations through the Glass Transition in an Amorphous Polymer: Poly(n-butyl methacrylate)

The study of poly( n -butyl methacrylate) (PnBMA) by thermostimulated currents has been performed in order to give a better definition of the molecular mobility when crossing the glass transition. It reveals the existence of two dipolar relaxation modes: α, ascribed to the glass−rubber relaxation, and α‘, which might be the dielectric manifestation of the liquid−liquid transition. The distribution of relaxation times and the evolution of the activation enthalpies when increasing the temperature have been studied by the fractional polarizations technique. It appears that the α and α‘ modes behave differently, showing the crossover from an Arrhenian to a Vogelian behavior. Furthermore, up to T α, the cooperativity of molecular mobility is highlighted by the existence of a compensation law in agreement with Starkweather's criterion, and above T α, results might be explained by the existence of an intermediate state neither glassy nor completely liquid.