Glassy dynamics and glass transition in nanometric layers and films: A silver lining on the horizon

The long lasting highly controversial discussion on glassy dynamics and the glass transition temperature of polymers in nanometric layers and films seems to be converging based on a multitude of recent experiments: (i) Linear response spectroscopies (e.g. alternating current calorimetry (ACC), broadband dielectric spectroscopy (BDS)) measuring in the liquid state do not observe shifts of the mean relaxation rate in dependence on the 1-dimensional confinement down to layer thicknesses of 8 nm (ACC) and 4 nm (BDS); (ii) Frequency dependent photobleaching techniques working essentially below the bulk glass transition temperature find as well a glassy dynamics in thin (freestanding or supported) films of polystyrene (PS) which is primarily bulk-like and does not depend on the thickness of the layer as demonstrated down to 14 nm. (iii) Evidence exists, that close to the bulk Tg, a layer of strongly enhanced mobility-and having a temperature dependent thickness-is formed on a free polymer surface. This enormously complicates the interpretation of ellipsometric and fluorescent based experiments, and might be the reason for the widely diverging results. In summary the dynamic glass transition does not show a confinement effect above and below the glass transition temperature in pronounced contrast to the non-equilibrium dynamics in the glassy state, which depend strongly on a variety of parameters including the layer thickness. This decoupling is well described by the free volume hole diffusion (FVHD) model as developed by Cangialosi et al.