Water transport in epoxy–aliphatic amine networks – Influence of curing cycles

In this work, polyepoxide network based on an aliphatic amine hardener were cured under different curing temperatures and their water sorption characteristics was systematically investigated as function of cure conditions. Attention was focused on the analysis of equilibrium properties, swelling behaviour and thermomechanical properties. For the undercured systems (epoxide conversion around 0.90), additional crosslinking reactions take part during water immersion, even at 20 °C. This effect was found to be mainly related to plasticizing of the network by water molecules. Local dynamics and plasticization effects were also monitored by dynamic mechanical analysis (DMA). The changes in the glass transition for the two well-crosslinked epoxies were greater than those predicted by considering the free volume changes only. The β process was significantly affected by the absorbed water. A decrease of the average relaxation time with moisture uptake was observed. The disruptions of the α and β relaxations were found to be reversible after re-drying of the wet material for the nearly fully cured epoxies. The fully crosslinked network was found to absorb slightly more water and at a greater rate than a slightly less crosslinked network (conversion around 0.98). This anomalous behaviour was analysed and a tentative explanation was presented in this paper.