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The rheological response and chain dynamics of thin polymer films underpin nanoscale polymer processing, yet molecular confinement alters such behavior. Using the interfacial wetting force of an immiscible liquid droplet to deform the films and linear elastic theory to describe the time evolution of the deformation profile, we demonstrated that the linear viscoelastic spectra, i.e., the frequency-dependent storage and loss moduli, of nanoscale polymer films are experimentally accessible over a wide frequency range. Our measurements on polystyrene nanofilms evidence an acceleration of polymer diffusion at a large confining length scale, i.e., at film thicknesses of hundreds of nanometers. This long-range perturbation in chain dynamics was interpreted as the fast relaxation of surface chains with reduced entanglements provoking loosening of entanglement constraints of the underlying chains, allowing the accelerated reptation mobility at the surface to extend deeply into the film interior. This suggests a surface-induced constraint release effect dominating the dynamics and rheology of polymers confined at a large length scale. 

Abstract Over the past three decades, studies have indicated a mobile surface layer with steep gradients on glass surfaces. Among various glasses, polymers are unique because intramolecular interactions — combined with chain connectivity — can alter surface dynamics, but their fundamental role has remained elusive. By devising polymer surfaces occupied by chain loops of various penetration depths, combined with surface dissipation experiments and Monte Carlo simulations, we demonstrate that the intramolecular dynamic coupling along surface chains causes the sluggish bulk polymers to suppress the fast surface dynamics. Such effect leads to that accelerated segmental relaxation on polymer glass surfaces markedly slows when the surface polymers extend chain loops deeper into the film interior. The surface mobility suppression due to the intramolecular coupling reduces the magnitude of the reduction in glass transition temperature commonly observed in thin films, enabling new opportunities for tailoring polymer properties at interfaces and under confinement and producing glasses with enhanced thermal stability.