Can Thickness and Interfacial Interactions Univocally Determine the Behavior of Polymers Confined at the Nanoscale?

The behavior of polymers confined in ultrathin films (thickness < 200 nm) can sensitively differ from that observed in macroscopic samples. Based on the simple arguments of finite size and interfacial effects, film thickness, and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. However, recent models suggest that a third key parameter, namely, the interfacial free volume, should also be considered. We describe a novel methodology that quantifies the volume available for structural relaxation at the interface between a thin polymer layer and its supporting substrate. Experiments performed at different annealing conditions verified that the shift in glass transition temperature, measured in thin films upon confinement, is proportional to the degree of adsorption and, thus, to the interfacial free volume.