Short-Time Dynamics Reveals T_g Suppression in Simulated Polystyrene Thin Films

Suppression of the glass transition temperature, T_g, in polymer thin films is of great practical importance and theoretical significance. It is widely believed that such T_g suppression results from enhanced segmental mobility at the free surface. To investigate these effects, we carry out united-atom molecular dynamics simulations on free-standing polystyrene thin films. Care has been taken to ensure consistent behavior between thin films and the bulk. The dilatometric T_g inferred from the density versus temperature shows substantial reduction in thin films compared to the bulk even at high cooling rates. Furthermore, we find that dynamical T_g shifts, obtained by collapsing temperature-dependent short-time dynamical properties onto a master curve, vary with film thickness just like the dilatometric T_g. We apply the same data collapse procedure to dynamics of segments within a given distance from the free surface to obtain the local T_g(z), which reveals a mobile surface layer of about 4 nm.