Equilibrium swelling properties of poly(N-vinylimidazole-co-sodium styrenesulfonate) hydrogels

Hydrogels of N-vinylimidazole (VI) and sodium styrenesulfonate (SSS) were synthesized by radical crosslinking copolymerization. Contrary to expectation, for some SSS compositions, the equilibrium swelling (Seq) in deionized water shows a minimum with values close to those of collapsed states. In aqueous solutions of NaCl, Seq either remains constant, increases, or decreases with changes of the ionic strength, depending on the gel composition. These peculiarities are interpreted in terms of a sodium–proton exchange followed by the interaction of sulfonate groups with neighboring protonated imidazole rings, which behave as ionic crosslinks. The crosslink density due to ionic interactions was determined, as a function of composition, in deionized water, in methanol, and in 0.2 N NaCl aqueous solution.