Phase separation kinetics in compressible polymer solutions: computer simulation of the early stages

A coarse-grained model for solutions of polymers in supercritical fluids is introduced and applied to the system of hexadecane and carbon dioxide as a representative example. Fitting parameters of the model to the gas–liquid critical point properties of the pure systems, and allowing for a suitably chosen parameter that describes the deviation from the Lorentz–Berthelot mixing rule, we model the liquid–gas and fluid–fluid unmixing transitions of this system over a wide range of temperatures and pressures in reasonable agreement with experiment. Interfaces between the polymer-rich phase and the gas can be studied at temperatures both above and below the end point of the triple line where liquid and vapour carbon dioxide and a polymer-rich phase coexist. In the first case interfacial adsorption of fluid carbon dioxide can be demonstrated. Our model can also be used to simulate quenches from the one-phase to the two-phase region. A short animation and a series of snapshots help to visualize the early stages of bubble nucleation and spinodal decomposition. Furthermore we discuss deviations from classical nucleation theory for small nuclei.