  We investigate the near-surface relaxation of freestanding atactic
\glsdesc{ps} films with molecular dynamics simulations. As in previous
coarse-grained simulations, relaxation times for backbone segments and phenyl
rings are linked to their bulk relaxation times via a power law coupling
relation. Variation of the coupling exponent with distance from the surface is
consistent with depth-dependent activation barriers. We also quantify a
reduction of dynamical heterogeneity at the interface which can be interpreted
in the framework of cooperative models for glassy dynamics.
