Suppression of the glass transition temperature, Tg, in polymer thin films is of great practical importance and theoretical significance. It is widely believed that such Tg 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 Tg 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 Tg shifts, obtained by collapsing temperature-dependent short-time dynamical properties onto a master curve, vary with film thickness just like the dilatometric Tg. We apply the same data collapse procedure to dynamics of segments within a given distance from the free surface to obtain the local Tg(z), which reveals a mobile surface layer of about 4 nm.
All Science Journal Classification (ASJC) codes
- Organic Chemistry
- Polymers and Plastics
- Inorganic Chemistry
- Materials Chemistry