Swelling equilibria for ionized temperature-sensitive gels in water and in aqueous salt solutions

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Abstract
Swelling equilibrium data in water and in aqueous NaCl solutions are presented for thermally sensitive N‐isopropylacrylamide (NIPA) hydrogels containing 0–4 mol % quaternized amine (positively ionizable) comonomer. We report the effect of gel charge and solution ionic strength on the temperature‐induced collapse of NIPA gels. Experimental swelling equilibria are compared with predictions based on a recently proposed oriented‐quasichemical model. This model has been shown previously to describe lower critical solution behavior in uncharged aqueous polymer solutions and gels (i.e., aqueous NIPA gel). We apply the model here to ionized NIPA gel. Semiquantative predictions are obtained for the effects of gel charge and solution ionic strength on temperature‐dependent swelling behavior.