Poly(acrylamide-co-acrylic acid) [P(AAm-co-AAc)] hydrogels were synthesized by the free-radical cross-linking copolymerization of acrylamide (AAm) monomer at fixed amount, but at changing amount of acrylic acid (AAc) as the ionic comonomer in water. The swelling behavior of the hydrogels thus obtained was analyzed in buffer solutions at various pHs. The polymer-solvent interaction paramet(chi) (chi) and the average molecular mass between crosslinks (M,) of the hydrogels were calculated from swelling studies in buffer solutions at various pHs and were related to the AAc content. The results indicated that the swelling behavior of the hydrogels at different pHs agreed with the modified Flory-Rehner equation based on the affine network model and the ideal Dorman theory. In addition, from differential curves of dO(upsilon)/dpH versus pH, the volume phase transition pH (VPT-pH) was found to be 4.33 +/- 0.05, independent of the AAc content in the hydrogel. Scanning electron microscope (SEM) was used to identify the morphological changes in the hydrogel as the concentration of AAc increased. In water at 22 degrees C, an increase of the AAc content further increased the response rate of the hydrogels because of simultaneous increase of both the porosity and hydrophilicity of the network. In buffer solution with a pH increase from 9.0 to 2.0 at 22 degrees C, the hydrogels with more AAc also exhibited a more rapid deswelling rate than that of the hydrogels with less AAc. (c) 2007 Wiley Periodicals, Inc.