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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.