The distribution of glass-transition temperatures in nanoscopically confined glass formers

Despite the decade-long study of the effect of nanoconfinement on the glass-transition temperature (T-g) of amorphous materials, the quest to probe the distribution of T(g)s in nanoconfined glass formers has remained unfulfilled. Here the distribution of T(g)s across polystyrene films has been obtained by a fluorescence/multilayer method, revealing that the enhancement of dynamics at a surface affects T-g several tens of nanometres into the film. The extent to which dynamics smoothly transition from enhanced to bulk states depends strongly on nanoconfinement. When polymer films are sufficiently thin that a reduction in thickness leads to a reduction in overall T-g, the surface-layer T-g actually increases with a reduction in overall thickness, whereas the substrate-layer T-g decreases. These results indicate that the gradient in T-g dynamics is not abrupt, and that the size of a cooperatively rearranging region is much smaller than the distance over which interfacial effects propagate.