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

Hydrogels of N-vinylimidazole (VI) and sodium styrenesulfonate (SSS) were s ynthesized by radical crosslinking copolymerization. Contrary to expectatio n, for some SSS compositions, the equilibrium swelling (S-eq) in deionized water shows a minimum with values close to those of collapsed states. In aq ueous solutions of NaCl, S-eq either remains constant, increases. or decrea ses with changes of the ionic strength, depending on the gel composition. T hese peculiarities are interpreted in terms of a sodium-proton exchange fol lowed by the interaction of sulfonate groups with neighboring protonated im idazole rings, which behave as ionic crosslinks. The crosslink density due to ionic interactions was determined, as a function of composition, in deio nized water. in methanol, and in 0.2 N NaCl aqueous solution. (C) 2001 Else vier Science Ltd. All rights reserved.