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 φ changes by more than an order of magnitude. The longitudinal osmotic modulus M os 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 φ 2 / M os, determined from direct osmotic measurements.