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 shor t, 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 simulati ons, the wall separation is varied from about one to about 15 times the bul k radius of gyration R-g. The confinement influences both static and dynami c 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 co nfinement effects are therefore observed for film thicknesses D less than o r similar to 4R(g). The preferential alignment of the chains with respect t o the walls suppresses reorientations in perpendicular direction, whereas p arallel reorientations take place in an environment of high monomer density . Therefore, they have a relaxation time larger than that of the bulk. On t he other hand, the influence of confinement on the translation motion of th e chains parallel to the walls is very weak. It almost coincides with the b ulk behavior even if D approximate to 1.5R(g). Despite these differences be tween translational and reorientational dynamics, their behavior can be wel l reproduced by a variant of Rouse theory which only assumes orthogonality of the Rouse modes and determines the necessary input from the simulation. (C) 2001 Elsevier Science Ltd. All rights reserved.