Chains entanglements and flow within the mobile surface layers of glassy polymers

The existence of a mobile surface layer of several nanometers thick has been established as critical to dynamical deviations in nanostructured polymers. Several questions arise, such as i) How are segments in this region entangled? and ii) How do polymers in this region flow especially when polymer size is larger than thickness of the surface layer? Herein, we design a creep experiment to investigate rheology in this layer. In this experiment, a micro-droplet was placed atop polymer surfaces to induce a nanometer deformation, called a wetting ridge, by the vertical component of surface tension of the droplet. The time evolution of the height of the wetting ridge, reflecting polymer relaxation at different length scales, was monitored. Results show that segments in the very thin surface layer are readily entangled. The terminal relaxation time was scaled by molecular weight (M_w) with a power law of 1.5. We proposed a 2D entanglement and limited flow to explain the phenomena. Within the surface layer, segments were entangled in direction parallel to the surface, but free of entanglements in film thickness direction. Flow of polymer in surface layer was constrained by the glassy underlying bulk, resulting in lower M_w dependence.