SPINODAL DECOMPOSITION OF POLYMER MIXTURES - A MONTE-CARLO SIMULATION

The initial stages of phase separation are studied for a symmetrical lattice model of a polymer mixture, where both polymers A and B are modeled by self-avoiding walks of N(A) = N(B) = 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), choosing a volume fraction phi-v = 0.6. We study two cases of energy parameters, (i) epsilon = epsilon-AA = -epsilon-BB (if two neighboring sites are taken by monomers of the same kind) and epsilon-AB = 0 (if two neighboring sites are taken by monomers of different kind) and (ii) epsilon = epsilon-AB and epsilon-BB = 0. For chain lengths N = 8 and 32 and volume fractions phi-A/(1 - phi-v) = 0.5 and 0.2, the system is quenched from the randomly mixed initial state (epsilon/kappa-B T = 0) to various unstable inside the spinodal, and the time evolution of various structure factors after the quench is monitored, as well as the time evolution of the chain radii and various types of nearest-neighbor contacts. It is shown that in case (ii) also some segregation of solvent (vacancies) occurs, accompanied by a contraction of the chains, while in case (ii) the vacancies remain ramdonly distributed in the system, and the radii of the chains do not change significantly. In neither case can the data be accounted for by the linearized (Cahn-type) theory of spinodal decomposition. We suggest that this theory should hold only for much longer chains and rather shallow quenches. Performing runs also in the grandcanonical ensemble of the mixture, long-lived metastable states are identified but only in the close neighborhood of the binodal curve. Consequences of our results for the interpretation of experiments are briefly discussed.