VFT to Arrhenius crossover at the dynamic glass transition of an epoxy network as revealed by dielectric experiments in continuous immersion

The influence of water sorption on the α-relaxation of an epoxy network was studied by performing impedance measurements in continuous immersion (in aqueous solution) at various temperatures in the range [20; 80] °C. Relaxation times associated with the α-relaxation were found to shift towards lower temperatures as a result of plasticization, accompanied by a decrease in the relaxation strength. The most striking feature of the impedance measurements was a crossover from a Vogel-Fulcher-Tammann (VFT) to an Arrhenius dependence of the relaxation times upon cooling, in the vicinity of the glass transition of the epoxy network. Complementary experimental techniques (namely differential scanning calorimetry, dielectric spectroscopy, thermally stimulated currents) were used on dry and wet epoxy samples to better understand the physical origin of this crossover. Various scenarios were discussed; the most convincing involved confinement-like effects. The deviation from VFT behaviour below Tg was possibly due to long-living H-bonds between the epoxy network and water molecules, which limited the cooperativity of the α-relaxation. The water molecules H-bonding to the network localized the mobility of the main chain, as confirmed by thermally stimulated currents performed on a wet sample.