A Simulation Method for the Phase Diagrams of Polymer Blends

Computer simulations are a powerful tool for establishing the relationship between molecular interactions and the phase behavior of polymer blends. Phase diagrams are usually obtained from molecular simulations via the Gibbs Ensemble method with the absense of interfaces between phases. The method has challenges for polymers because chain insertion has a very low acceptance probability. Another approach is to simulate a system with an interface; the challenge here is that the interface moves during the course of the simulation making it difficult to estimate the coexisting concentrations (or densities). In this work we propose a method where the concentration profiles at different time slots are aligned by identifying the interface in each time window using a spatial density autocorrelation function. We validate the method by calculating the phase diagrams of the Widom-Rowlinson model and symmetric blends of freely-jointed polymers, and comparing to previous Gibbs ensemble simulations. We use the method to calculate the phase diagram of poly(ethylene oxide) in the ionic liquid BMIM BF₄.