Relaxation processes in polystyrene melts and ultra-thin films

By means of large-scale computer simulations we investigate relaxation processes in polystyrene melts and ultra-thin films. The local orientational mobility of the phenyl bonds is studied with the help of Legendre polynamials of the second-order P{\_}2(t). The spectral density of P{\_}2 (t) shows several distinctive peaks. They are caused by the large- scale motions of cooperative segments ($\alpha $ relaxation), smaller-scale structural dynamics ($\beta $ relaxation), and transient processes. Our simulations reveal that interfaces affect $\alpha -$ and $\beta $-relaxation processes differently. The most puzzling observation is a slight decrease in the structural relaxation time in the middle of the film, compared to that near the free surface. As expected, the $\alpha $-relaxation time is shorter near the free surface. The glass transition temperature, obtained from a plot of thickness versus temperature, decreases with decreasing film thickness, which is in agreement with an observed decrease in the $\alpha -$relaxation time. Surprisingly, the structural relaxation time is roughly the same for the bulk and for films. Our results will be compared with published experimental data.