Equilibrium and non-equilibrium phase transitions in copolymer polyelectrolyte hydrogels

A continuum model of polyelectrolyte hydrogel phase transitions has been developed and used to analyze the equilibrium and non-equilibrium swelling transitions observed in copolymer N-isopropylacrylamide (NIPA), acrylic acid (AAc), and acrylamide (AAm) hydrogels. The contribution of the dominant forces to the overall swelling response has been illustrated experimentally by systematically varying both the hydrogel and bath composition. We have interpreted the equilibrium swelling pattern theoretically by separately examining the role of charge, acid dissociation, and solvent mediated effects. The non-equilibrium swelling response to changes in bath ionic strength has been examined in terms of competing coupled ion, diffusion reaction, polymer network mechanisms, and the potential role of film boundary effects. We have obtained qualitative agreement between our theory and measurements and have critically examined both the strengths and limitations of our model. (C) 1997 American Institute of Physics.