The photoelastic behaviour and small-angle x-ray scattering of ionized gels of copolymers of 2-hydroxyethyl methacrylate with methacrylic acid

Abstract

The photoelastic behaviour and small-angle X-ray scattering of ionized, loosely crosslinked gels of copolymers of 2-hydroxyethyl methacrylate (HEMA) with methacrylic acid (MA) swollen in 0.5 M NaCl are characterized by the following specific features. Increasing neutralization of gels causes a steep decrease in the volume fraction of the polymer in the gel, v 2, and in the modulus G. The time dependences of photoelastic functions, which are the most pronounced for PMA, become weaker with swelling. The slopes of the dependence of the extrapolated equilibrium modulus log G c on log v 2 is large (φ 0.6) 1.5 for PMA. The stress-optical coefficiént C passes at about v 2 φ 0.25 through a pronounced maximum, probably due to a decisive effect of the shape contribution, caused by possible correlation structures in the copolymers. Increasing ionization leads to a rise in swelling, first increasing the contrast of the correlation structures but then obviously causing their dissociation. The small-angle X-ray data indicate a considerable heterogeneity of the order 10 nm in size of gels in the ionized state: the structural characteristics calculated using a two-phase model are in agreement with the assumed shape contribution to birefringence.