PHASE-TRANSITION IN SWOLLEN GELS 24 - EFFECT OF THE CONCENTRATION ANDSTRUCTURE OF IONIC COMONOMERS ON THE COLLAPSE OF POLY(ACRYLAMIDE) HYDROGELS

The swelling and mechanical behaviour of ionized networks of copolymer s of acrylamide (AAm), methylenebisacrylamide and sodium 2-[(3-carboxy pyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, molar fraction of sa lt x(S) = 0-0.20) was investigated in water-acetone (w/a) mixtures and in aqueous NaCl solutions. In the range x(s) greater than or equal to 0.01, a first-order phase transition (collapse) was found in w/a mixt ures; with increasing x(S), both the critical acetone concentration a( c) in the mixture at which collapse takes place and the extent of coll apse (jump in the swelling ratio X), Delta log X, increases. A compari son of these results with the results obtained for charged PAAm networ ks with sodium 2-[(2-carboxybenzoyl)oxy]ethyl methacrylate (BeNa) as i onic comonomer led to a conclusion that substituting benzoyl for pyrid yl groups in the side chain slightly decreases Delta log X and the swe lling degree in water, and does not change critical a(c) values. On th e other hand, a comparison with charged PAAm networks with sodium meth acrylate (MNa) as ionic comonomer showed that at constant x(S), the sw elling degree in water, the extent of collapse Delta log X and the con centration of elastically active network chains strongly decreased wit h the increased distance of the bound charge from the main chain (leng th of the side chain of the ionic comonomer). The expected decrease in the swelling degree of ionized networks with increasing NaCl concentr ation was found. The experimental swelling data in w/a mixtures and in aqueous NaCl solutions could be described by a theory proposed previo usly if an effective degree of ionization i is introduced (i < x(S)). The mechanical behaviour of gels in w/a mixtures is predominantly dete rmined by the degree of swelling. On the other hand, different depende ncies of the reduced modulus on the swelling ratio X found in w/a mixt ures and in aqueous NaCl solutions in the high swelling region suggest s that contracted gels with different microstructure were formed durin g gel deswelling in bad solvents or salt in water. (C) 1998 Elsevier S cience Ltd. All rights reserved.