INFLUENCE OF FREELY MOBILE GRAFTED CHAIN-LENGTH ON DYNAMIC PROPERTIES OF COMB-TYPE GRAFTED POLY(N-ISOPROPYLACRYLAMIDE) HYDROGELS

Amino semitelechelic poly(N-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 PIPA;Am were synthesized by condensation reaction of amino semitelechelic PIPAAm with N-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 N,N'-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 degrees C changed from equilibrium swelling states at 10 degrees C, however, exhibited remarkable differences, and rapid responses were observed for graft-type 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.