STRUCTURE AND THERMODYNAMICS OF FLEXIBLE POLYMER GELS

Unlike the situation for polymer solutions, for swollen polymer networks, there is no generally accepted relation between the macroscopic osmotic properties and the scattering behavior. Complementary measurements involving osmotic and elastic observations, small angle neutron and x-ray scattering, as well as quasielastic light scattering, can, however, yield detailed information on the relationship between the structure and the thermodynamics in polymer gels. Here, we apply such a complementary investigation to two types of networks, differing in the mechanism of cross linking. The neutron scattering spectra of the gels contain two components, dynamic and static. The former is governed by the macroscopic osmotic and elastic moduli of the swollen network, and is associated with the thermodynamic concentration fluctuations. The static component of the scattered intensity is generated by local variations of the cross-link density. The amplitude of the static concentration fluctuations <delta phi(2)>phi(2) is determined by the detailed mechanism of the cross-linking procedure. Its concentration dependence can be expressed in terms of the same macroscopic osmotic and elastic variables as govern the thermodynamic properties of the gel.