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 α-process and the space-time factorization of the scattering function on the intermediate time scale of the MCT β-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 ≤ T g ≤ T c , these results suggest that also the glass transition temperature should decrease for stronger confinement.