Manifestation of sub-Rouse modes in flow at the surface of low molecular   weight polystyrene
  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. 
K. L. Ngai, S. Capaccioli, Daniele Prevosto, Luigi Grassia
cond-mat.soft