SWELLING EQUILIBRIA AND VOLUME PHASE-TRANSITION IN HYDROGELS WITH STRONGLY DISSOCIATING ELECTROLYTES

Several sulfonic acid copolymer gels were prepared by radical copolymerization of N,N-dimethylacrylamide (DMAA) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) using N,N'-methylenebisacrylamide (BIS) as the cross-linker. The concentration of pregel solutions and BIS content in dried gels were held constant at 0.75 mol/L and 1.3 mol %, respectively, while the mole fraction f of AMPS in dried gels determined by elemental analysis varied from 0 to 0.608. Swelling ratios for our sample of f = 0.468 were compared with those for an acrylic acid copolymer gel of f = 0.494 and 25 degrees C in aqueous buffers from pH = 2 to pH = 10 at 0.01 M ionic strength, and the results indicated that even in the gel state the sulfonic acid group could dissociate completely in the whole pH range. Therefore, f in this work can be designated to represent the charge density in the swollen gels. The swelling capacity observed in pure water at 25 degrees C increases monotonically with increasing f, showing a higher increasing rate when f > 0.3 without any sign of counterion condensation. The experimental swelling curve was interpreted with the Flory-Huggins theory and with the Hasa-Ilavsky-Dusek (HID) theory, but both theories failed to provide a reasonable approach to the swelling equilibria in our gels with a large amount of strongly dissociating electrolytes. The reciprocal of polymer volume fraction in a swollen gel, 1/phi, predicted with the Flory-Huggins theory was much higher than that observed when f > 0.2, while the calculated results given by the HID theory using the experimental nominal value of chain density were considerably lower than measured data in the entire f range. A volume phase transition occurred in these gels when f > 0 when the acetone volume fraction V-a in the acetone/water mixture was beyond a certain value, and this transition could be attributed to the weakening of electrostatic repulsion during increasing V-a in the mixture.