  The presence of a viscoelastic mechanism distinctly different from the
segmental a-relaxation and the Rouse modes within the glass-rubber transition
zone of polymers had been justified by theoretical considerations, and
subsequently experimentally verified in different bulk polymers by various
techniques, and in several laboratories. Referred to in the literature as the
sub-Rouse modes, they were also found in polymer thin films by the creep
compliance measurements of McKenna and co-workers Established by experiment and
theoretical considerations is the enhanced mobility of sub-Rouse modes in thin
PS films by the combination of effect from the free surface, finite size, and
induced chain orientations, concomitant with the segmental a-relaxation.
Induced chain orientations effect is present only when h is less than the
end-to-end distance of the high molecular weight polymer chains. In this paper,
the proven enhanced mobility of sub-Rouse modes at the surface of polymers is
used to explain recent experimental investigations of viscous flow at the
surface of low molecular weight PS by Chai et al. [Science, 343, 994 (2014)],
and by Yang et al. [Science, 328, 1676 (2010).]. Viscous flow of polymers is by
global chain motion, therefore the observed large reduction of viscosity at the
surface of low molecular weight PS originates from the sub-Rouse modes, and not
the segmental a-relaxation. This distinction is not commonly recognized in the
current literature. The accerleration of the sub-Rouse modes at the surface
explains the experimental findings.
