EFFECT OF MONOMER STRUCTURE AND COMPRESSIBILITY ON THE PROPERTIES OF MULTICOMPONENT POLYMER BLENDS AND SOLUTIONS .2. APPLICATION TO BINARY BLENDS

The general lattice cluster theory of corrections to classic Flory-Huggins theory from paper 1 is applied to compressible binary polymer blends. The properties considered are the small-angle neutron scattering intensity and excess thermodynamic quantities, with more emphasis placed on the former because its pressure dependence has not previously been studied. We first illustrate the theory with the idealized athermal limit blend, which is of interest because it yields a pure entropic contribution to the interaction parameter from extrapolated zero-angle neutron scattering. This pure athermal limit takes all three independent van der Waals interaction energies to vanish, and our calculations show that the small-angle neutron scattering intensity displays several interesting variations with composition, pressure, molecular weights, and monomer structures. However, this idealized athermal system only yields stable one-phase, dense blends at quite elevated pressures, so it is inappropriate for real polymer blends. We therefore introduce a more realistic quasiathermal blend model in which the small-angle neutron scattering intensity is insensitive to temperature but in which van der Waals interactions are present to make the blend stable at normal conditions. We study the influence of pressure, variations of the three independent van der Waals interaction energies, molecular weights, and monomer structures on the neutron scattering and excess thermodynamic properties of nonathermal blends. Other measures of the Flory interaction parameter are obtained from the entropy of mixing (in the athermal limit), the heat of mixing, or the free energy of mixing and are shown often to depart from the behavior of the interaction parameter deduced from small-angle neutron scattering. For instance, there are examples in which the latter interaction parameter is positive, suggesting interactions that are unfavorable for blend formation, whereas the former measures are negative and correctly predict blend stability. The neutron scattering interaction parameter displays the most interesting variations with pressure, monomer structure, composition, and van der Waals interactions, while the behavior of the volume change on mixing is most in line with expectations.