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Poly(ethylene oxide) (PEO) chains are introduced as graft chains maintaining freely mobile ends in thermo-responsive cross-linked poly(N-isopropylacrylamide) (PIPAAm) hydrogels by copolymerization of IPAAm with α-acryloyl-ω-methoxy-PEO. The deswelling response on raising the temperature of this gel above the gel phase transition temperature (T P) takes place within 10 min, whereas a conventionally cross-linked PIPAAm gel of the same dimensions requires 1 month for deswelling. This difference is due to the formation of water release channels within the skin layer by the hydrophilic PEO graft chains. The rapid deswelling of the grafted gel is compared with the deswelling changes of random copolymer gels composed of IPAAm and hydrophilic acrylic acid (AAc), which also accelerates gel deswelling. Deswelling is fastest in copolymers containing 1.3 wt % AAc and in grafted gels containing 13 wt % PEO. These results were interpreted as reflecting the gel structure.

Amino semitelechelic poly(Ar-isopropylacrylamide)s (PIPAAm) with three different molecular weights were synthesized by telomerization of IPAAm monomer with 2-aminoethanethiol as a chain transfer agent, changing the molar ratio of monomer to chain transfer agent. Macromonomers of thermosensitive PIPAAm were synthesized by condensation reaction of amino semitelechelic PIPAAm with A'-acryloxysuccinimide. The molecular weights of macromonomers determined by titration of the terminal amino groups were 2900, 4000, and 9000, respectively. The comb-type grafted PIPAAm hydrogels having different lengths of graft chains were synthesized by radical copolymerization of IPAAm monomer with PIPAAm macromonomer in the presence of AT,AT'-methylenebisacrylamide as a cross-linker. An important aspect of the graft-type gels is the construction of a molecular architecture different from PIPAAm normal type of gel even though the composition is same. Higher equilibrium swellings at lower temperatures were observed in graft-type gels in contrast to the normal-type gel, and longer graft chains resulted in higher equilibrium swelling due to the freely mobile grafted chains. Both the normal-type and the graft-type gels exhibited reversible swelling-deswelling changes in aqueous milieu in response to an alteration of temperature. The deswelling kinetics at 40 °C changed from equilibrium swelling states at 10 °C, however, exhibited remarkable differences, and rapid responses were observed for grafttype gels. The rapid dehydration of graft chains during gel shrinking was confirmed by analysis of DSC measurements. These dehydrated graft chains strongly aggregated with hydrophobic intermolecular forces, inducing the rapid deswelling of the gels. The attractive forces operating between dehydrated chains were larger in the gel having longer grafted chains, resulting in faster deswelling. A deswelling mechanism distinct from polymer network collective diffusion was demonstrated with these comb-type grafted hydrogels having freely mobile grafted chains in the network.

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