Compressibility Effects in Neutron Scattering by Polymer Blends

Abstract

In this paper, we consider the effects of finite compressibility on the thermodynamics of polymer blends as deduced by neutron scattering. We show that, within the framework of the lattice fluid model and contrary to past conclusions by us and others, the compressibility effects are insignificant for weakly interacting blends. An explanation for the commonly observed parabolic concentration dependence of the Flory-Huggins parameter in isotopic and model polyolefin blends must therefore be sought elsewhere. A compressible interpretation is advantageous for strongly interacting blends, such as polystyrene/poly(vinyl methyl ether), because it reduces significantly the composition dependence of the interaction parameter. We note however, that, even in this case, compressibility need not be introduced into the analysis of the scattering data. The entire analysis can be conducted in terms of the Flory-Huggins model, and the results can then be reexpressed in terms of a compressible model, if that is desired.