MECHANICAL AND STRESS-OPTICAL PROPERTIES OF STRONGLY SWOLLEN HYDROGELS

Crosslinked polyelectrolytes exhibit extreme swelling in aqueous syste ms; fluid uptake usually amounts to 100 to 1000 times of their dry vol ume. The mechanical and stress-optical properties of such highly swoll en gels show some peculiarities because the network chains are stretch ed far beyond the Gaussian limit. The shear modulus increases markedly at high degrees of swelling, and the stress-optical coefficient decre ases. A quantitative treatment has to be based on non-Gaussian chain s tatistics. In order to provide experimental data, stress-optical and m odulus measurements were carried out on hydrogels made from partially hydrolyzed poly-(acrylamide) and other polyelectrolytes. The degree of swelling was adjusted by using aqueous NaCl-solutions of different co ncentration as the swelling medium. Using a description based on the i nverse Langevin function, the course of the shear modulus on the degre e of swelling can be fitted to the experimental data. The optical modu lus, DELTAn/(lambda2-lambda-1), is predicted with somewhat less agreem ent. However, a different set of parameters has to be used for the mec hanical and optical properties. This shows that the treatment is not f ully consistent.