pH-Sensitive hydrogels based on non-ionic acrylic copolymers

Hydrogels based on ethoxytriethyleneglycol monomethacrylate/methyl methacrylate copolymers were prepared by free radical polymerization at 70 °C in bulk with 2,2′-azobisisobutyronitrile as initiator. The swelling behaviour of the hydrogels was studied by immersion of the films in water and in buffered solutions at various pHs. The hydration process was followed gravimetrically measuring the water uptake of the films with the time of treatment. The kinetics fit adequately a Fickian law for reduced sorption coefficients, Mt/M∞, lower than 0.6. The values of the diffusion coefficient were lower at neutral pH than at acidic or alkaline pHs, and they seemed to be independent of the copolymer composition. The results obtained were consistent with the stress relaxation model of polymer chains predicted for a relatively fast hydration process. The swelling process is reversible and the polymeric films are stable after repeating cycles of hydration and desorption promoted by the change of pH in appropriate buffered solutions.