Temperature-responsive characteristics of poly (N-isopropylacrylamide) hydrogels with macroporous structure

Macroporous poly(N-isopropylacrylamide) (PNIPA) hydrogels were synthesized by free-radical crosslinking polymerization in aqueous solution from N-isopropylacrylamide monomer and N,N-methylenebis (acrylamide) crosslinker using poly(ethylene glycol) (PEG) with three different number-average molecular weights of 300, 600 and 1000 g mol(-1) as the pore-forming agent. The influence of the molecular weight and amount of PEG pore-forming agent on the swelling ratio and network parameters such as polymer-solvent interaction parameter (chi) and crosslinking density (v(E)) of the hydrogels is reported and discussed. Scanning electron micrographs reveal that the macroporous network structure of the hydrogels can be adjusted by applying different molecular weights and compositions of PEG during polymerization. At a temperature below the volume phase transition temperature, the macroporous hydrogels absorbed larger amounts of water compared to that of conventional PNIPA hydrogels, and showed higher equilibrated swelling ratios in aqueous medium. Particularly, the unique macroporous structure provides numerous water channels for water diffusion in or out of the matrix and, therefore, an improved response rate to external temperature changes during the swelling and deswelling process. These macroporous PNIPA hydrogels may be useful for potential applications in controlled release of macromolecular active agents. (c) 2006 Society of Chemical Industry.