Fine structure of polymer networks as revealed by solvent swelling

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