Molecular thermodynamics of polymer solutions

Abstract

Abstract The previously revised Freed lattice model is used to replace the Flory-Huggins mean-field theory for binary polymer/solvent mixtures. Liquid-liquid binodals calculated for model systems are in good agreement with those from computer simulation and those from Freed's rigorous lattice cluster theory. A double lattice model is used to account for oriented interactions such as hydrogen bonding. To account for free-volume effects, an essentially empirical, two-step process is adopted. First, pure components are mixed to form a close-packed polymer solution. Then, holes are Introduced to mix with the close-packed solution which is considered to be a pseudo-pure substance. The revised Freed model is applied for both steps. A size parameter C r accounts for the composition dependence of effective chain lengths. Two binary energy parameters are used: ϵ is the conventional interchange energy of a nearest-neighbor i-J segment-segment pair, and ξ corrects the quadratic rule for mixing the two components to form a pseudo-pure substance. Pure-component parameters, Γ i o and ϵ ii , are obtained by fitting experimental pure-component vapor pressure and pVT data. A few examples indicate that the semi -empirical model can describe satisfactorily a variety of liquid-liquid equilibria Including UCST, LCST, XXXiscibility-loop and hour-glass-shaped phase diagrams, as well as the pressure dependence and the molar-mass dependence of liquid-liquid binodals.