Ionic poly(N-t-butylacrylamide-co-acrylamide) [P(TBA-co-AAm)] hydrogels were synthesized by the free-radical crosslinking copolymerization of N-t-butylacrylamide and acrylamide monomers in fixed amounts and the maleic acid (MA) comonomer in methanol in different amounts with N,N-methylene bis(acrylamide) as the crosslinker, ammonium persulfate as the initiator, and N,N,N',N'-tetramethylenediamine as the activator. The swelling behavior of these hydrogels was analyzed in buffer solutions at various pHs. The polymer-solvent interaction parameter (chi) and the average molecular weight between crosslinks of the ionic P(TBA-co-AAm) hydrogels were calculated from swelling studies in buffer solutions at various pHs and were related to the MA content. The results indicated that the swelling behavior of the ionic P(TBA-co-AAm) hydrogels at different pHs agreed with the modified Flory-Rehner equation based on the affine network model and the ideal Donnan theory. The enthalpy (Delta H) and entropy (Delta S) changes appearing in the chi parameter for the hydrogels were also determined with the modified Flory-Rehner equation. The negative values for Delta H and Delta S indicated that the hydrogels had a negative temperature-sensitive property in water, that is, swelling at a lower temperature and shrinking at a higher temperature. The experimental swelling data of the hydrogels at different temperatures agreed with the modified Flory-Rehner approach based on the affine network model, which ensured that the sensitive dependence of the chi parameter on both the temperature and polymer concentration was taken into account. (c) 2006 Wiley Periodicals, Inc.