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The presence of a viscoelastic mechanism distinctly different from thesegmental a-relaxation and the Rouse modes within the glass-rubber transitionzone of polymers had been justified by theoretical considerations, andsubsequently experimentally verified in different bulk polymers by varioustechniques, and in several laboratories. Referred to in the literature as thesub-Rouse modes, they were also found in polymer thin films by the creepcompliance measurements of McKenna and co-workers Established by experiment andtheoretical considerations is the enhanced mobility of sub-Rouse modes in thinPS films by the combination of effect from the free surface, finite size, andinduced chain orientations, concomitant with the segmental a-relaxation.Induced chain orientations effect is present only when h is less than theend-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 isused to explain recent experimental investigations of viscous flow at thesurface 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 byglobal chain motion, therefore the observed large reduction of viscosity at thesurface of low molecular weight PS originates from the sub-Rouse modes, and notthe segmental a-relaxation. This distinction is not commonly recognized in thecurrent literature. The accerleration of the sub-Rouse modes at the surfaceexplains the experimental findings.

Manifestation of sub-Rouse modes in flow at the surface of low molecular weight polystyrene