Impact of polar groups concentration and free volume on water sorption in model epoxy free films and coatings

Two epoxy networks based on DGEBA resin and an amine hardener (TETA or DAMP) are tested. Thanks to a controlled curing protocol, two controlled macromolecular architectures are obtained. The network microstructure differs only in the crosslink density and the polar group number. Then, the free films are hygrothermally aged. Network microstructure impacts the solubility and the Arrhenius parameters associated to the water diffusion process. It appears that is principally governed by water–polar group interaction mechanisms. •Two model epoxy systems with two controlled macromolecular architectures.•Both systems with different polar group density and free volume fraction.•Free films and coatings ageing followed by gravimetric and EIS measurements.•Water diffusion and solubility governed by a water polar group interaction mechanism. The water sorption characteristics have been determined for two model epoxy systems based on the DGEBA resin and two amine hardeners (TETA and DAMP). A particular attention was paid to the curing protocol to control the macromolecular architectures. Using FTIR, DSC and DMTA measurements, it was demonstrated that both network microstructures differ mainly in the polar groups density, the potential crosslinking nodes and the free volume. DGEBA/DAMP system presents a higher free volume fraction and a lower polar groups concentration compared to the DGEBA/TETA system. Gravimetric measurements on free films and EIS experiments onto coated steel Q-panels were carried out to follow the water sorption kinetics at different temperatures (30, 40, 50 and 60°C). For both epoxy systems, the diffusion processes follow a pseudo-fickian behaviour with two diffusive stages. Solubility and diffusion processes are discussed in relation with the microstructural specificities of the polymer networks using a thermodynamic framework. The main result is that in DGEBA/DAMP and DGEBA/TETA systems, the water diffusion and solubility are mainly governed by a water polar group interaction mechanism.