Thickness-dependent reduction of the glass-transition temperature in thin polymer films with a free surface

We present results of molecular dynamics simulations for free-standing and supported thin films of a nonentangled polymer melt using a coarse-grained (headspring) model. Our discussion is mainly concerned with the equilibrium properties of the films above the critical temperature (T-c) of mode-coupling theory, although we also determine the glass-transition temperature (T-g) by measurements of the film thickness h upon cooling. We explore the influence of confinement on the structure and dynamics of the polymer films. We find that the dynamics in the films is accelerated compared to the bulk, that this enhanced mobility originates from the surfaces, and that the effect is larger at the free than at the supported surface. Thus, the films have lower T-c values relative to the bulk. T-c depends on film thickness h; this dependence can be well parametrized by T-c(h) = T-c(bulk)/(l + h(o)/h), a function proposed by experiments on supported polystyrene films. (c) 2006 Wiley Periodicals, Inc.