Deswelling Kinetics of Macroporous Poly(N-isopropylacrylamide) Gel in Salt Solution

Abstract

Abstract The deswelling kinetics of a macroporous poly(N-isopropylacrylamide) gel in a NaCl solution was investigated. NaCl was used as the probe in order to trace out any hydration from the hydrated polymer chains. In the case of macroporous gel immersed in a NaCl solution, the minimum of the initial deswelling rate (v = 0.06 s−1) appeared at around 0.2—0.3 M (1 M = 1 mol dm−3) when the equilibrated gels (at 22 °C) were heated to 40 °C. The maxima of the apparent activation energy (Ea = 110 kJ mol−1) and the change in the activation entropy (ΔS313 = 80 J K−1 mol−1) were at 0.2—0.3 M NaCl. These results indicate that deswelling of the gel in a NaCl solution advanced on the condition of the following two factors. One is an electrostatic repulsion between Cl− ions below 0.2—0.3 M. Another is a salting-out effect due to NaCl above 0.2—0.3 M. The deswelling kinetics and the lower critical solution temperatures of the gels in other alkalimetal halide solutions were also measured in order to clarify the effect of the species of the cations and the anions on the interaction between the water and polymer chains.