  Free surfaces in glassy polymer films are known to induce surface mobile
layers with enhanced dynamics. Using molecular dynamics simulations of a
bead-spring model, we study a wide variety of layer-resolved structural and
dynamical properties of polymer films equilibrated at a low temperature.
Surface enhancement on thermally induced particle hopping rate is found to
terminate abruptly only about 5 particle diameters from the free surface. In
contrast, enhancement on the net motions of particles measured at longer time
scales penetrates at least 2 particle diameters deeper. The diverse penetration
depths show the existence of a peculiar sublayer, referred to as the
inner-surface layer, in which surface enhanced mobility is not caused by more
frequent particle hops but instead by a reduced dynamic heterogeneity
associated with diminished hopping anti-correlations. Confinement effects of
the free surface thus provide a unique mechanism for varying the dynamic
heterogeneity and hopping correlations while keeping the hopping rate constant.
Our results highlight the importance of correlations among elementary motions
to glassy slowdown and suggest that dynamic facilitation is mediated via
perturbations to the correlations rather than the rate of elementary motions.
