MOLECULAR THERMODYNAMICS OF POLYMER-SOLUTIONS

The previously revised Freed lattice model is used to replace the Flor y-Huggins mean-field theory for binary polymer/solvent mixtures. Liqui d-liquid binodals calculated for model systems are in good agreement w ith those from computer simulation and those from Freed's rigorous lat tice cluster theory. A double lattice model is used to account for ori ented interactions such as hydrogen bonding. To account for free-volum e effects, an essentially empirical, two-step process is adopted. Firs t, 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 comp osition dependence of effective chain lengths. Two binary energy param eters are used: epsilon is the conventional interchange energy of a ne arest-neighbor i-j segment-segment pair, and r corrects the quadratic rule for mixing the two components to form a pseudo-pure substance. Pu re-component parameters, r(i)o and epsilon(ii), are obtained by fittin g experimental pure-component vapor pressure and pvT data. A few examp les indicate that the semi-empirical model can describe satisfactorily a variety of liquid-liquid equilibria including UCST, LCST, miscibili ty-loop and hour-glass-shaped phase diagrams, as well as the pressure dependence and the solar-mass dependence of liquid-liquid binodals.