Water and polymer dynamics in poly(hydroxyl ethyl acrylate-co-ethyl acrylate) copolymer hydrogels

Water and polymer dynamics in hydrogels based on random copolymers of hydrophilic poly(hydroxyl ethyl acrylate) (PHEA) and hydrophobic poly(ethyl acrylate) (PEA), in wide ranges of composition, were investigated by means of two dielectric techniques, thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) at several levels of relative humidity/water content. Water sorption of the hydrogels was studied by equilibrium sorption isotherms (ESI). Two secondary relaxations (γ and β_sw) and the primary (segmental) α relaxation associated with the glass transition of the copolymer matrix were followed and analyzed against copolymer composition and water content. The results show that the copolymers are homogeneous at nm scale, except at very high PEA content. Correlations were observed between results on the organization of water in the hydrogels and on water effects on polymer dynamics. Distinct changes in the dielectric response, in particular in the time scale and the dielectric strength of the β_sw relaxation, at the water content of the completion of the first hydration layer indicate that water molecules themselves contribute to the dielectric response at higher water contents. Proton conductivity of the hydrogels at various levels of water content was also studied and correlation to segmental dynamics (decoupling) was analyzed.