Osmotic and SANS Observations on Sodium Polyacrylate Hydrogels in Physiological Salt Solutions

The volume transition induced by the addition of calcium into lightly cross-linked fully neutralized sodium polyacrylate hydrogels, swollen in 40 mM NaCl solutions, is investigated by osmotic swelling pressure and small-angle neutron scattering (SANS) measurements. In this reversible transition, the polymer volume fraction æ changes by more than an order of magnitude. The longitudinal osmotic modulus Mos deduced from macroscopic measurements shows an abrupt increase at the transition. The intensity of the scattered radiation can be decomposed into a static and a dynamic component. The former exhibits Porod scattering behavior, while the latter obeys an Ornstein-Zernike relation. The observed intensity of the dynamic part has a maximum at the transition and exhibits a functional dependence on the calcium concentration, which is similar to that of the osmotic susceptibility æ/Mos, determined from direct osmotic measurements.