Nonequilibrium behavior of thin polymer films.

The rheological behavior of 100-nm-thick polystyrene films cast from various solvents was examined using an electric field to weakly perturb the free surface of the polymer melt. The effective viscosity and residual stresses of the as-spun films are seen to strongly depend on the properties of the casting solvent and the solvent quality. Both effects are explained in terms of the coil dimension at the solvent-polymer composition at which the film vitrifies. The more compact chains in a near-Θ-solvent are less entangled and less deformed when quenched to the dry melt compared to the more swollen chains in an athermal solution. Despite chain conformations that are further from equilibrium for the Θ-solvent cast chains, these films have reduced stored stresses compared to the chains cast in films from athermal solvents. A more detailed analysis of the data suggests that the formation of a surface-near region with more strongly deformed chains during spin coating. Since thermal equilibration of spin-cast high-molecular-weight films is unpractical, solvent vapor annealing was used to equilibrate films on timescale of a few hours.