Microstructural reorganization in model epoxy network during cyclic hygrothermal ageing

The water sorption characteristics and physico-chemical properties have been determined for a fully cured model epoxy systems based on the DGEBA resin and DAMP amine hardener, during cyclic hygrothermal ageing (absorption-desorption-reabsorption-redesorption). Gravimetric measurements on epoxy free films were carried out to follow the water sorption kinetics at different temperatures (30, 40, 50 and 60 degrees C). The water diffusion processes follow a pseudo-Fickian behaviour with two diffusive stages during the first sorption and a Fickian behaviour during the first desorption. During the first sorption, a plasticization phenomenon is observed leading to a decrease of rigidity and Tg but this phenomenon is reversible, showing that water reacts only with the polymeric network via low energy links. The water diffusion processes evolve during the second absorption-desorption cycle. They follow a Fickian behaviour which implies a disappearance of diffusion heterogeneity of the first sorption. This behavior was explained by a microstructure reorganization during cyclic ageing highlighted through the FTIR and DMA techniques: chain fractions became accessible to the water after being relaxed in the first absorption-desorption cycle. A thermodynamic approach applied to solubility and diffusion processes corroborates these results. This study allows to a better understanding of diffusion phenomena in epoxy model systems. (C) 2017 Elsevier Ltd. All rights reserved.