The effect of preparation temperature on the swelling behavior of poly(N-isopropylacrylamide) gels

The role of the preparation temperature of poly(N-isopropylacrylamide) (PNIPA) gels on their swelling behavior in water and in aqueous solutions of sodium dodecylbenzenesulfonate was investigated. PNIPA gels were prepared by free-radical crosslinking copolymerization of N-isopropylacrylamide and N,N'-methylene(bis) acrylamide in water at fixed monomer and crosslinker concentrations. The equilibrium swelling ratio of the gels increases first slightly up to about 20 degrees C then rapidly with increasing gel preparation temperature. Magnitude of the collapse transition in water at 34 degrees C becomes larger as the gel preparation temperature increases. Calculations indicate a decrease in the effective crosslink density of PNIPA gels with increasing preparation temperature. The gels prepared at temperatures higher then 20 degrees C were heterogeneous consisting of highly crosslinked regions interconnected by the PNIPA chains.