Cooperative Motion as a Unifying Principle to Understand Confinement Effects on Glass Formation

We examine how confinement scale and interfacial interactions affect polymer glass formation, studied via molecular dynamics simulations. We consider both thin supported polymer films and polymer-nanoparticle composites. By varying the film thickness, nanoparticle loading fraction, or polymer-interfacial interactions, we can significantly alter both $T_g$ and the fragility of glass formation, leading to a seemingly intractable degree of complexity. However, we find that all our observations can be described in unified way by using the scale of collective motion as a measure of ``cooperatively rearranging regions'' in the Adam-Gibbs (AG) description of glass formation. For thin films, we show how the scale of cooperative motion relates to the scale of enhanced interfacial dynamics at the free surface, offering a promising route to experimentally determine the scale of cooperative motion. These string-like motions can further be described as a living polymerization. Combining polymerization theory with the AG approach, we theoretically predict the relaxation time at much lower $T$, which suggests a return to Arrhenius behavior that avoids a Kauzmann ``entropy crisis.'' Finally, we consider the applications of these ideas to ultra-stable polymer films formed by vapor deposition.