  To understand the role of collective motion in the often large changes in
interfacial molecular mobility observed in polymer films, we investigate the
extent of collective motion in the interfacial regions of a thin supported
polymer film and within the film interior by molecular dynamics simulation.
Contrary to commonly stated expectations, we find that the extent of collective
motion, as quantified by string-like molecular exchange motion, is similar in
magnitude in polymer-air interfacial layer to that in the film interior, and
both are generally distinct from the bulk material. This finding is consistent
with Adam-Gibbs description of the segmental dynamics within mesoscopic film
regions where the extent of collective motion is related to the configurational
entropy of the film as whole rather than a locally defined extent of collective
motion or configurational entropy.
