Interfacial wetting-induced nanorheology of thin polymer films

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.