TOWARD AN INTEGRATED ANALYTIC DESCRIPTION OF DEMIXING IN TERNARY SOLUTIONS OF NONIDEAL UNCHARGED LATTICE POLYMERS, HARD PARTICLES, AND SOLVENT

We present a new analytic scheme for the description of the phase beha vior of solutions of lattice polymers and hard particles. The theory c overs mixtures containing short polymers (i.e., the radius of gyration smaller than the hard particle radius) as well as mixtures containing very long polymers. The lattice polymers have excluded volume interac tions, and the hard particles can have a spherical, convex, or dumbbel l shape. The theory combines the Flory theory for polymer solutions, t he Boublik-Nezbeda equation of state for isotropic fluids of hard part icles, and a simple correction term for the reduction of polymer confi gurations near a wall. In order to be able to integrate these differen t approaches, typical properties of the hard particles, such as the av erage number of surface-fluid contacts of a particle, are expressed in terms of lattice sites using a simple numerical simulation. These sim ulation results are fitted for general use by simple second-order poly nomials. The resulting equation of state predicts entropy-driven as we ll as energy-driven demixing and restabilization. As examples, the ter nary mixtures lattice polymer + solvent + (hard spheres or hard dumbbe lls) are discussed, as well as the comparison with the Flory theory fo r two different polymers and a solvent.