Deswelling mechanism for comb-type grafted poly(N-isopropylacrylamide) hydrogels with rapid temperature responses

Deswelling mechanism for comb-type grafted poly(N-isopropylacrylamide) (PIPAAm) hydrogel was investigated to make clear the effect of gel architecture on the gel deswelling kinetics. Deswelling rate and mechanism were compared with those for conventional crosslinked PIPAAm gel and poly(ethylene oxide) (PEO)-grafted PIPAAm gel in response to temperature increase from below to above the gel-phase transition temperatures. The deswelling rate for the conventional PIPAAm gel are reciprocal to the gel size square, indicative of collective diffusion mediated manner for crosslinked network into water. Although PEO graft chains formed water release channels within the network to produce fast gel deswelling changes, deswelling change of PEO-grafted gel also demonstrated collective diffusion mediated manner of crosslinked backbone chains. On the other hand, one order faster rate of deswelling change of PIPAAm-grafted gel was observed than that of PEO-grafted gel with identical gel dimension. The deswelling change of PIPAAm grafted gel did not obey collective diffusion mediated manner, demonstrating reciprocal to the gel size to 1.58th power. This deswelling mechanism was explained in terms of rapid hydrophobic aggregation of freely mobile PIPAAm graft chains and an operation of intrinsic elastic forces of polymer network. The operation of both elastic force and strong hydrophobic aggregation force between the dehydrated PIPAAm graft chains released water inside the gel, causing more rapid deswelling changes of the PIPAAm-grafted gels.