Glassy dynamics in thin polymer films: recent MD results

The influence of a film geometry on the glass transition is investigated via molecular dynamics (MD) simulations of a (non-entangled) polymer melt. The confinement is realized by two identical potential barriers of the form U(wall) = z(-9), where z denotes the distance of a particle from the wall. Despite the geometric confinement, basic qualitative features of the system dynamics can be well described in the framework of the mode-coupling theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time-temperature superposition property of the late time alpha-process and the space-time factorization of the scattering function on the intermediate time scale of the MCT beta-process. A comparison of the dynamics in the film and in the bulk shows an acceleration of the relaxation processes due to the presence of the walls. This leads to a reduction of the critical temperature, T(c), of MCT with decreasing film thickness. A comparison of the quantities like the static structure factor and the mean-square displacements for the bulk and for the film suggests that T - T(c)(D) is a relevant temperature scale for the dynamics at intermediate times. Furthermore, we also analyze the sharp rise of the relaxation times at low temperatures by the Vogel-Fulcher-Tammann (VFT) equation and thus estimate the VFT-temperature T(0)(D). We observe that, similar to T(c)(D), also T(0)(D) decreases for smaller D. As T(0) less than or equal to T(g) less than or equal to T(c) these results suggest that also the glass transition temperature should decrease for stronger confinement. (C) 2002 Elsevier Science B.V. All rights reserved.