Molecular dynamics simulations of concentrated polymer solutions in thin film geometry. I. Equilibrium properties near the glass transition

We report on results of molecular dynamics simulations for supported polymer films with explicit solvent. The simulation represents the polymers by bead-spring chains and the solvent particles by monomers. The interaction between polymer and solvent favors mixing. We find that the solvent acts as a plasticizer. The glass transition temperature $Tg$ is reduced relative to the pure polymer film. Near $Tg$ we explore equilibrium properties as a function of temperature and solvent concentration. We find that the structure and dynamics of the films are spatially heterogeneous. The solvent density is enriched at the supporting wall and at the free surface where the film is in equilibrium with solvent vapor. At both interfaces the solvent dynamics is fast, but smoothly crosses over to bulk dynamics when moving from the interfaces toward the center of the film. A smooth gradient from enhanced dynamics at the interfaces to bulk behavior in the film center is also found for the monomers. We show that the same formula used to parametrize the spatial gradient of the dynamics in the pure polymer film may also be applied here. Furthermore, we determine the concentration dependence of the relaxation time of the solvent in the center of film and compare this dependence to models proposed in literature.