Novel poly(ethylene glycol) /poly(2-acrylamido2-methylpropanesulfonic acid-co-N-isopropylacrylamide) semi-interpenetrating network gels were synthesized with free-radical initiation at 70 degrees C and characterized with differential scanning calorimetry, Fourier transform infrared and X-ray photoelectron spectroscopy, X-ray diffraction, and scanning electron microscopy. The first thermal decomposition peaks for semi-interpenetrating network gels with various proportions of 2-acrylamido-2-methylpropanesulfonic acid and N-isopropylacrylamide were in the range of 278-289 degrees C, whereas for poly (2-acrylamido-2-methylpropanesulfonic acid-co-N-isopropylacrylamide) copolymer gets with corresponding proportions of the comonomers, decomposition began at about 203 degrees C. A significant proportion of the amide groups in the semi-interpenetrating network gels with a high proportion of 2-acrylamido-2-methylpropanesulfonic acid were found to be protonated as a result of the relatively strong acid strength of 2-acrylamido-2-methylpropanesulfonic acid. The surface morphology of the dried gels varied with the relative proportions of the comonomers and was significantly different for the semi-interpenetrating network gels and corresponding copolymer gels. Strong hydrophobic and hydrogen-bonding interactions between poly (ethylene glycol) with an average molecular weight of approximately 6000 g/mol and the copolymer network were responsible for different thermal behaviors of the poly(2-acrylamido-2-methylpropanesulfonic acid-co-N-isopropylacrylamide) copolymer gels and the semi-interpenetrating network gels. (c) 2007 Wiley Periodicals, Inc.