Swelling-deswelling kinetics of poly(N-isopropylacrylamide) hydrogels formed in PEG solutions

A series of poly(N-isopropylacrylamide) (PNIPA) hydrogels was prepared by free-radical crosslinking copolymerization of N-isopropylacrylamide (NIPA) and N,N'-methylenebisacrylamide (BAAm) in aqueous solutions of poly(ethylene glycol) of molecular weight 300 g/mol (PEG). The amount of PEG in the polymerization solvent, the crosslinker (BAAm) content, and the gel preparation temperature (T-prep) were varied in the gelation experiments. The hydrogels were characterized by the equilibrium swelling and elasticity tests as well as by the measurements of the deswelling-reswelling kinetics of the hydrogels in response to a temperature change between 25 and 48 degrees C. The rate deswelling of the swollen gel increases while the rate of reswelling of the collapsed gel decreases as the amount of PEG in the polymerization solvent is increased or as the crosslinker content is decreased. The Tprep effect on the swelling kinetics of the hydrogels was only observed if the PEG content of the polymerization solvent is less than 20%, which is explained with the screening of H-bonding interactions in concentrated PEG solution. (c) 2005 Wiley Periodicals, Inc.