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 N aCl solutions, is investigated by osmotic swelling pressure and small-angle neutron scattering (SANS) measurements. In this reversible transition, the polymer volume fraction psi changes by more than an order of magnitude. Th e longitudinal osmotic modulus M-os deduced from macroscopic measurements s hows an abrupt increase at the transition. The intensity of the scattered r adiation can be decomposed into a static and a dynamic component. The forme r exhibits Pored scattering behavior, while the latter obeys an Orstein-Zer nike relation. The observed intensity of the dynamic part has a maximum at the transition and exhibits a functional dependence on the calcium concentr ation, which is similar to that of the osmotic susceptibility psi (2)/M-os, determined from direct osmotic measurements.