PHASE-SEPARATION OF POLYMER MIXTURES IN THE PRESENCE OF SOLVENT

A model of a symmetrical polymer mixture, where both polymers A and B are modeled by self-avoiding walks of N//A equals N//B equals N steps on the simple cubic lattice and a lattice site is taken by either an A monomer, a B monomer, or a solvent molecule (or a vacancy V, respectively), is analyzed by Monte Carlo simulations, in order to analyze the validity of the predictions of the Flory-Huggins theory. We attribute huge discrepancies found between the 'exact' critical temperature of phase separation for this model and those predicted by the Flory-Huggins approximation to the crude treatment of chain configurational statistics in this approximation. We also obtain binodal curves and a scattering function and discuss the crossover from Ising-like critical behavior to mean-field critical behavior as N approaches infinity . Consequences for experimental work are briefly discussed.