Thermoresponsive properties of porous poly(N-isopropylacrylamide) hydrogels prepared in the presence of nanosized silica particles and subsequent acid treatment

Porous poly(N-isopropylacrylamide) hydrogels were prepared by the free-radical polymerization of its monomer and a suitable crosslinker in the presence of spherical silica particles of different sizes (74 and 1600 nm) and by the subsequent acid extraction of silica. The yields were 81-83%, and the yields were not affected by the silica content. Scanning electron microscopy observations revealed the porous structure of the hydrogels. Porous and nonporous hydrogels showed volume phase transitions from swelling states to deswelling states at approximately 30 degreesC, as analyzed by the ratio of the diameter of cylinder-shaped hydrogels to that of the glass tube used for the hydrogel preparation at the corresponding temperature. Deswelling, which was analyzed by rapid changes in the temperature of the aqueous media from 20 to 40 degreesC, was facilitated by decreased silica particle size and increased silica content. The deswelling rate constant of the hydrogel prepared with 74-nm silica at 10 v/v % (silica/solvent) was more than 1500 times greater than that of conventional hydrogels. Swelling was similarly analyzed through changes in the temperature from 40 to 20 degreesC and was independent of the pore structure. The deswelling-swelling cycle was repeatable with reasonable reproducibility. Moreover, the mechanical strength of the porous hydrogels was significantly maintained compared with that of conventional nonporous hydrogels. This method produced thermoresponsive hydrogels of suitable mechanical strength and remarkable deswelling properties. (C) 2002 Wiley Periodicals, Inc.