Rate-Determining Step of Deswelling for Conventional and Macroporous Poly(N-isopropylacrylamide) Gels

Abstract

Abstract The kinetic parameters of deswelling for conventional and macroporous poly(N-isopropylacrylamide) gels were determined in order to point out the rate-determining step on the deswelling process in a NaCl or KCl solution. The apparent activation energy for the deswelling of the conventional gel in water (130 kJ mol−1) was larger than that of the macroporous gel in water (25 kJ mol−1). The split process of the skin layer, which was formed on the surface of the gel by heating from outside of gel, was considered concerning the rate-determining step in the case of the conventional gel. The activation entropy changes at 313 K (ΔS313’s) of the macroporous gel were determined to be −65 J K−1 mol−1 in 0.1 M NaCl (1 M = 1 mol dm−3), of which the cation exhibits a structure-forming property, and 180 J K−1 mol−1 in 0.1 M KCl, of which the cation exhibits a structure-breaking property. The sign of ΔS313 for the macroporous gel was negative at higher than 0.3 M NaCl or KCl. In order to point out the rate-determining step, the process of the salting-out effect, that of bond formation between the polymer chains, and that of forming water clusters in the presence of the cation inside of the gel are discussed.