SCATTERING PROPERTIES OF POLY(VINYL ACETATE) GELS IN DIFFERENT SOLVENTS

The thermodynamic properties of lightly cross-linked PVAc gels swollen in different solvents (acetone at 25-degrees-C, isopropyl alcohol at 70-degrees-C, isopropyl alcohol at 52-degrees-C) are investigated by osmotic and scattering techniques. The first two diluents are good solvents, and the last is a theta solvent. The relaxation rates of the concentration fluctuations, measured by dynamic light scattering, are greater in the gels than in the corresponding solutions. In all cases, however, the longitudinal osmotic modulus M(os) is significantly smaller than the osmotic modulus K(os) of the corresponding solution. This change is related to the presence of large-scale static nonuniformities in the gels, which are revealed by small-angle neutron scattering. A scattering function of the form I(Q) = I(d)(0)/(1 + Q2xi2) + I(s)(0) exp{-(Qxi)p} is fitted to the neutron data, where Q is the transfer wave vector, xi and XI are correlation lengths, and p = 0.7. Evaluation of I(d)(0) and I(s)(0), the intensities of the scattering due to dynamic and static concentration fluctuations, respectively, provides an independent estimate of M(os) that is in satisfactory agreement with the value obtained by dynamic light scattering.