SWELLING AND PHOTOELASTIC BEHAVIOR OF IONIZED HYDROGELS OF POLY(ACRYLIC ACID)

The photoelastic and swelling behaviour of poly(acrylic acid) gels swo llen in 1 M NaC1 aqueous solution was studied as a function of crossli nking degree (1-5 wt.% ethylene glycol dimethacrylate) and degree of i onization alpha(g) = 0-1. The degree of swelling passes through a mini mum at alpha(g) approximately-equal-to 0.15 regardless of the degree o f crosslinking, probably due to the formation of hydrogen bonds betwee n ionized and nonionized carboxyl groups. In the same neutralization r egion, the equilibrium modulus reaches a maximum; on the other hand, t he modulus related to the dry state is independent of alpha(g) for all crosslinker concentrations. Thus the mechanical behaviour of poly(acr ylic acid) gels is in accord with the Gaussian theory and no contribut ion of permanent physical interactions to the modulus is observed. The optical behaviour is more complex - the sign of the stress-optical co efficient C-e changes three times with increasing alpha(g) due to the orientation of the side groups. The analysis of the optical data shows that the C-e value is controlled by the degree of swelling and by the ionization.