A lattice model for dense polymer solutions and polymer mixtures in three dimensions is presented, aiming to develop a model suitable for efficient computer simulation on vector processors, with a qualitatively realistic local dynamics. It is shown that the bond fluctuation algorithm for a suitable set of allowed bond vectors has the property that due to the excluded volume constraint no crossing of bonds by local motions can occur, and entanglement restrictions thus are fully taken into account. For athermal binary (AB) symmetrical polymer mixtures, the dependence of both self-diffusion coefficient and interdiffusion coefficient on polymer density is obtained, simulating a thin film geometry where a film of polymer A is coated with a film of polymer B. For one density, the dependence of the interdiffusion coefficient on an attractive energy between unlike monomers is also studied. For weak attraction an enhancement of interdiffusion proportional to this energy occurs. For strong attraction, however, a rather immobile tightly bound AB layer forms in the interface which hampers further unmixing.

No abstract

No abstract

Carlo methods, using the bond fluctuation model on simple cubic lattices, for TV = 16 to TV = 256 at a volume fraction of occupied lattice sites <#> = 0.5 (which corresponds to a dense melt). Applying recently developed efficient simulation techniques (grand-canonical sampling of the mixture thermodynamics is combined with multiple histogram data evaluation and finite size scaling techniques), very precise estimates of critical temperatures, phase diagrams, composition-dependent effective Flory-Huggins parameters, and, last but not least, critical exponents and amplitudes are obtained. The data provide clear evidence for a linear dependence of the critical temperature on chain length, k^TJt ~2.157V + 1.35, and thus disagree with the integral equation theory prediction (Tc <* y/N) of Schweizer and Curro. Consistent with the work of Sariban and Binder, however, it is concluded that the naive application of Flory-Huggins theory would overestimate strongly the proportionality constant relating Tc with TV. For the first time, clear evidence for a crossover from Ising-like critical behavior (dominating at small TV) to mean-field critical behavior (which emerges in the limit TV -®) is seen in simulations, consistent with the Ginzburg criterion for polymer mixtures.

No abstract