Free Surface Relaxations of Star-Shaped Polymer Films

The surface relaxation dynamics of supported star-shaped polymer thin films are shown to be slower than the bulk, persisting up to temperatures at least 50 K above the bulk glass transition temperature Tgbulk. This behavior, exhibited by star-shaped polystyrenes with functionality f=8 arms and molecular weights per arm Marm<Me (Me is the entanglement molecular weight), is shown by molecular dynamics simulations to be associated with a preferential localization of these macromolecules at the free surface. This new phenomenon is in notable contrast to that of linear-chain polymer thin film systems, where the surface relaxations are enhanced in relation to the bulk; this enhancement persists only for a limited temperature range above the bulk Tgbulk. Evidence of the slow surface dynamics, compared to the bulk, for temperatures well above Tg and at length and time scales not associated with the glass transition has not previously been reported for polymers.