Swelling and photoelastic behaviour of ionized hydrogels of poly(acrylic acid)

The photoelastic and swelling behaviour of poly(acrylic acid) gels swollen in 1 M NaC1 aqueous solution was studied as a function of crosslinking degree (1-5 wt.% ethylene glycol dimethacrylate) and degree of ionization alpha(g) = 0-1. The degree of swelling passes through a minimum at alpha(g) approximately-equal-to 0.15 regardless of the degree of crosslinking, probably due to the formation of hydrogen bonds between ionized and nonionized carboxyl groups. In the same neutralization region, the equilibrium modulus reaches a maximum; on the other hand, the modulus related to the dry state is independent of alpha(g) for all crosslinker concentrations. Thus the mechanical behaviour of poly(acrylic acid) gels is in accord with the Gaussian theory and no contribution of permanent physical interactions to the modulus is observed. The optical behaviour is more complex - the sign of the stress-optical coefficient 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.