PHASE-SEPARATION OF SYMMETRICAL POLYMER MIXTURES IN A COMMON GOOD SOLVENT IN THE SEMIDILUTE CONCENTRATION REGIME

Monte Carlo simulations of lattice models of binary (AB) symmetric polymer mixtures (chain lengths N-A = N-B = N) in a common good solvent are carried out and the phase diagrams and critical properties of the unmixing transitions are estimated and interpreted in terms of recent theories. Polymers are modeled by self-avoiding walks of length N = 16, 32 and 64 on the simple cubic lattice. Data for vacancy concentrations of phi(V) = 0.6, 0.8 and 0.85 are analyzed. It is shown that for N = 16, phi(V) = 0.85 no phase separation occurs, down to the lowest temperature, while for N = 32, phi(V) = 0.85 still phase separation occurs but no longer is complete. Our results are compatible with a scaling theory based on a ''renormalization'' of the Flory-Huggins chi-parameter due to ''blob'' effects.