The Effects of pH and Ionic Strength of Swelling of Cationic Gels

A model is reported for the mechanical response of a cationic polyelectrolyte gel subjected to swelling in a water bath with an arbitrary pH and an arbitrary concentration of a monovalent salt. A gel is treated as a three-phase medium consisting of a polymer network, solvent (water), and solute (mobile ions). Transport of solvent and solute is thought of as their diffusion through the network accelerated by the electric field formed by mobile ions and bound charges and accompanied by chemical reactions (self-ionization of water molecules, protonation of functional groups, and formation of ion pairs between fixed cations and mobile anions). Material constants in the governing equations are determined by fitting equilibrium swelling diagrams on several copolymer gels with vinylimidazole component. Simulation demonstrates that the anti-polyelectrolyte effect (nonmonotonic changes in degree of swelling with molar fraction of salt) can be observed in cationic gels and core-shell microgels.