INFLUENCE OF BLEND COMPRESSIBILITY ON EXTRAPOLATED ZERO-ANGLE COHERENT SCATTERING AND SPINODAL - LIMITATIONS OF RPA ANALYSIS

Three different statistical thermodynamic models of compressible binary polymer blends are used to compute the extrapolated zero-angle coherent scattering S11(0) by one of its components. The models are taken from Flory-Huggins theory, equation of state theories, and Dickman-Hall continuum modifications of lattice theories. The computed zero-angle scattering is analyzed using the conventional incompressible RPA model to produce the effective Flory interaction parameter, χeff. When input polymer-polymer interactions that are composition and molecular weight independent are used, the computed χeff values exhibit strong composition and molecular weight dependences, solely as a result of the inappropriateness of the incompressible blend model used to define χeff. The vanishing of the reciprocal of S11(0) is shown to describe the spinodal line for a compressible binary blend at constant volume, but this is found to differ from the constant-pressure spinodal line. While our computed χeff values display some composition-dependent trends observed experimentally, several others cannot be generated. Hence, future studies will investigate the influence of composition and molecular dependences of the polymer-polymer interactions on χeff. Nevertheless, the simplest models employed here focus on the need for analyzing experimental data with thermodynamically more faithful models than the incompressible RPA if the desire is to obtain fundamental molecular information concerning basic polymer-polymer interactions in a blend. © 1990, American Chemical Society. All rights reserved.