Molecular dynamics simulations of concentrated polymer solutions in thin film geometry. II. Solvent evaporation near the glass transition.

We perform molecular dynamics simulations of a coarse-grained model of a polymer-solvent mixture to study solvent evaporation from supported and freestanding polymer films near the bulk glass transition temperature Tg. We find that the evaporation process is characterized by three time (t) regimes: An early regime where the initially large surplus of solvent at the film-vapor interface evaporates and the film thickness h varies little with t, an intermediate regime where h decreases strongly, and a final regime where h slowly converges toward the asymptotic value of the dry film. In the intermediate regime the decrease of h goes along with an increase of the monomer density at the retracting interface. This polymer-rich “crust” is a nonequilibrium effect caused by the fast evaporation rate in our simulation. The interfacial excess of polymer gradually vanishes as the film approaches the dry state. In the intermediate and final time regimes it is possible to describe the simulation data for h(t) and the so...