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Free surfaces in glassy polymer films are known to induce surface mobilelayers with enhanced dynamics. Using molecular dynamics simulations of abead-spring model, we study a wide variety of layer-resolved structural anddynamical properties of polymer films equilibrated at a low temperature.Surface enhancement on thermally induced particle hopping rate is found toterminate abruptly only about 5 particle diameters from the free surface. Incontrast, enhancement on the net motions of particles measured at longer timescales penetrates at least 2 particle diameters deeper. The diverse penetrationdepths show the existence of a peculiar sublayer, referred to as theinner-surface layer, in which surface enhanced mobility is not caused by morefrequent particle hops but instead by a reduced dynamic heterogeneityassociated with diminished hopping anti-correlations. Confinement effects ofthe free surface thus provide a unique mechanism for varying the dynamicheterogeneity and hopping correlations while keeping the hopping rate constant.Our results highlight the importance of correlations among elementary motionsto glassy slowdown and suggest that dynamic facilitation is mediated viaperturbations to the correlations rather than the rate of elementary motions.

Deeper penetration of surface effects on particle mobility than on hopping rate in glassy polymer films