Polymer films become glassy near their glass transition temperature — molecular motion slows dramatically, and the material behaves like a solid on experimental timescales. But the surface of a polymer film is not the bulk. At a free surface, molecules have fewer neighbors, more freedom to rearrange, and faster dynamics. The surface layer is more mobile than the interior. This has been known for decades.
Starr and Douglas (arXiv 2602.23106, February 2026) discover that this enhanced surface mobility has a counterintuitive effect on chain dynamics. Polymer chains are long, and their relaxation follows Rouse scaling — a power-law relationship between the mean-squared displacement and time, with exponent γ. In the bulk, dynamic heterogeneity (the spatial variation of local relaxation rates) enhances the Rouse exponent, compressing the Rouse regime and accelerating chain-level motion. The bulk heterogeneity helps chains relax.
At the surface, the opposite happens. The accelerated segmental mobility at the free interface suppresses the Rouse exponent to values as low as γ = 0.4 — well below the bulk value. The surface speeds up the segments but slows down the chain. The enhanced local mobility disrupts the cooperative motion that chains need for Rouse-regime relaxation.
A linear gradient separates the two regimes. Moving from the surface into the film interior, the Rouse exponent crosses over from surface-suppressed to bulk-enhanced, passing through a minimum at intermediate depth. The Rouse scaling exponent acts as a spatially resolved probe of glassy dynamics — it maps the depth-dependent interplay of interfacial effects and bulk heterogeneity in a single number.
Faster parts, slower whole. The surface is mobile but uncoordinated. The bulk is slow but coherent. The chain relaxes fastest where the segments compromise between speed and cooperation.