Structure and dynamics of thin polymer films: a case study with the bond-fluctuation model

This paper reports Monte Carlo simulation results of a polymer melt of short, non-entangled chains which are embedded between two impenetrable walls. The melt is simulated by the bond-fluctuation lattice model under athermal conditions, i.e. only excluded volume interactions between the monomers and between the monomers and the walls are taken into account. In the simulations, the wall separation is varied from about one to about 15 times the bulk radius of gyration R_g. The confinement influences both static and dynamic properties of the films: Chains close to the walls preferentially orient parallel to it. This parallel orientation decays with increasing distances from the wall and vanishes for distances larger than about 2R_g. Strong confinement effects are therefore observed for film thicknesses D?4R_g. The preferential alignment of the chains with respect to the walls suppresses reorientations in perpendicular direction, whereas parallel reorientations take place in an environment of high monomer density. Therefore, they have a relaxation time larger than that of the bulk. On the other hand, the influence of confinement on the translation motion of the chains parallel to the walls is very weak. It almost coincides with the bulk behavior even if D≈1.5R_g. Despite these differences between translational and reorientational dynamics, their behavior can be well reproduced by a variant of Rouse theory which only assumes orthogonality of the Rouse modes and determines the necessary input from the simulation.