Phase behavior of monomeric mixtures and polymer solutions with soft interaction potentials

We present Gibbs ensemble Monte Carlo simulations of monomer–solvent and polymer–solvent mixtures with soft interaction potentials, that are used in dissipative particle dynamics simulations. From the simulated phase behavior of the monomer–solvent mixtures one can derive an effective Flory–Huggins χ-parameter as a function of the particle interaction potential. We show that this χ-parameter agrees very well with the free energy difference between a monomer surrounded by solvent particles, and a solvent particle surrounded by solvent particles. We develop a new “identity change” Monte Carlo move to equilibrate the polymer–solvent mixtures. In this move a polymer chain from one box is exchanged with an equal number of solvent particles from the other box. At realistic densities this new move offers a large computational advantage over the convential insertion method for a polymer chain using a configurational bias Monte Carlo algorithm. The new algorithm is demonstrated for polymer–solvent mixtures with a chain length of up to 150 segments. Significant differences are found between the simulated polymer–solvent phase behavior and results predicted by mean-field theory. Finally, we fit a master–equation to the simulated binodal curves at different chain lengths. This function is used to make a quantitative comparison between the simulations and experimental data for the phase equilibrium of the polystyrene–methylcyclohexane system.