PHASE-TRANSITION IN SWOLLEN GELS .19. EFFECT OF THE CROSS-LINKER WITHA NEGATIVE CHARGE ON THE COLLAPSE OF POLYACRYLAMIDE NETWORKS

The swelling and equilibrium mechanical behaviour of networks of copol ymers of acrylamide (AAm) with a ionic crosslinker - sodium salt of 2, 2-bisacrylamidopropionic acid (I) in water-ethanol mixtures was invest igated. Two series of networks were prepared. While in series 1 AAm an d ionic crosslinker were copolymerized (mole fraction x(I) = 0.005 to 0.1), in series 2, AAm, ionic crosslinker and non-ionic crosslinker - methylenebisacrylamide (MBAAm) - were copolymerized in such way that X (I)+x(MBAAm) = 0.05 (x(I) varied from 0 to 0.05). In the region x(I) g reater-than-or-equal-to 0.01 the phase transition was observed for bot h series. As expected, for networks of series 2 with constant crosslin ker concentration the extent of transition (jumpwise change of the vol ume of the gel) increases with increasing charge concentration, x(I). On the other hand, for samples of series 1 this extent slightly decrea ses with x(I) which means that the effect of increasing crosslinking d ensity on the swelling is greater than that of increasing charge conce ntration. The critical ethanol concentration at which collapse takes p lace, e(c), increases with x(I) in both series. The jumpwise change in the gel volume is accompanied by a similar change in the equilibrium modulus of the gel.