Phase transition in swollen gels

Summary

The swelling and mechanical behaviour of networks of copolymers of acrylamide (AAm) with the ionic crosslinker (I) — N,N'-(1,4-phenylenedimethylene)bis(2-acrylamidoethyldimethylammonium chloride) — in water-ethanol mixtures was investigated. Two series of networks were prepared: a) AAm and ionic crosslinker (I) were copolymerized in series 1 (mole fraction x_I=0.005-0.1); b) AAm, ionic crosslinker (I) and non-ionic crosslinker — methylenebisacrylamide (MBAAm) — were copolymerized in series 2 (x_I+x_MBAAm=0.1, x_I was used in the same region as in series 1). In the range x_I≥0.01 the phase transition was observed for both series. While for networks of series 2 (constant total amount of crosslinkers) the extent of transition (jumpwise change in the gel volume) at the collapse increases with x_I, for samples of series 1 this extent is practically independent of x_I due to the compensation of the effect of the increasing crosslinking density and increasing charge concentration on the swelling. The critical concentration of ethanol in the mixture, at which collapse takes place, increases with x_I for both series. The jumpwise change in the gel volume is accompanied by a similar change in the shear modulus of the gel.