Thermally stimulated depolarisation currents in model epoxy networks: experimental evidence for local and non-local relaxation processes

We have shown in a recent publication, (Henn et al, J. Appl. Phys. vol.85, p.2821, 1999), that it is possible to distinguish between local and non-local, i.e. collective processes of dielectric relaxation measured in various classes of solids. This discrimination is made feasible by varying the heating rate of the thermally stimulated depolarisation currents (TSDC) and by measuring the evolution of the shape of the glow curve. If the thermally stimulated dielectric response arises from a distribution of relaxation times, then the width of the glow curve increases with increasing heating rate, On the other hand, in the case of a polymeric material, the TSDC signals obtained in the Tg region exhibit an opposite heating rate dependence: the width of the glow curve is a decreasing function of the heating rate. This behaviour has been explained as being the result of non-local relaxation processes. The aim of the present work is to show that these findings can be generalised to other types of polymeric materials, in which a progressive change from local to non-local relaxation modes is observed. For this purpose, we selected a series of epoxy networks, based on the diepoxide DGEBA and aliphatic amines, and whose sub-Tg relaxational behaviour has already been identified as involving both isolated and short-scale cooperative molecular motions.