Fine Structure of Polymer Networks As Revealed by Solvent Swelling

Measurements are reported of small-angle neutron scattering (SANS) from lightly cross-linked poly(vinyl acetate) gels swollen in good and ϑ solvents. In all the solvent conditions investigated the scattering response can be described satisfactorily by two characteristic length scales: a thermodynamic correlation length ξ that is related to the osmotic swelling pressure and a larger distance Ξ that is governed by the range of the random elastic forces exerted by the cross-links. ξ decreases significantly as the overall polymer concentration increases; the static correlation length Ξ, however, decreases only weakly as the gels shrink. For samples in a good solvent undergoing uniaxial stretching, a butterfly pattern is observed which is also well described in terms of two length scales. In ϑ conditions, however, this effect is masked by incipient phase separation in the gel in which the precipitated regions are elongated along the stretching direction.