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

Several sulfonic acid copolymer gels were prepared by radical copolyme rization of N,N-dimethylacrylamide (DMAA) and 2-acrylamido-2-methylpro panesulfonic acid (AMPS) using N,N'-methylenebisacrylamide (BIS) as th e 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 %, respecti vely, while the mole fraction f of AMPS in dried gels determined by el emental analysis varied from 0 to 0.608. Swelling ratios for our sampl e 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 rep resent the charge density in the swollen gels. The swelling capacity o bserved in pure water at 25 degrees C increases monotonically with inc reasing f, showing a higher increasing rate when f > 0.3 without any s ign of counterion condensation. The experimental swelling curve was in terpreted with the Flory-Huggins theory and with the Hasa-Ilavsky-Duse k (HID) theory, but both theories failed to provide a reasonable appro ach to the swelling equilibria in our gels with a large amount of stro ngly dissociating electrolytes. The reciprocal of polymer volume fract ion in a swollen gel, 1/phi, predicted with the Flory-Huggins theory w as much higher than that observed when f > 0.2, while the calculated r esults given by the HID theory using the experimental nominal value of chain density were considerably lower than measured data in the entir e 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 th e weakening of electrostatic repulsion during increasing V-a in the mi xture.