Acceleration of Deswelling of Poly(N-isopropylacrylamide) Hydrogel by the Treatment of a Freeze-Dry and Hydration Process

The deswelling rate of a thermally-sensitive gel consisting of poly(N-isopropylacrylamide) crosslinked by 4% N,N′-methylenebisacrylamide (MBA) was accelerated at above the lower critical solution temperature (LCST) by the treatment of a freeze-dry and hydration process (freeze-dry treatment); it was roughly 102 times faster than the gel without a freeze-dry treatment. The values of the apparent activation energy (Ea’s) for the conventional gel crosslinked by 2 and 4% MBA was roughly 140 kJ mol−1 each, and those for freeze-dry treated gel crosslinked by 2, 4, and 10% MBA were 24, 25, and 12 kJ mol−1, respectively. An SEM photomicrograph showed that the freeze-dry treatment made the gel porous. The freeze-dry treated gel exhibited a weaker shrinking force, and showed easy expansion compared with the conventional gel. An expansion of the diffusion area of water and a decrease of the thickness of the surface layer for a macroporous gel could be the reason for the increase in the deswelling rate.