Glass transition and molecular mobility in polymers under nanoscale confinement

In the last couple of decades, a plethora of studies have shown that confined polymer glasses with free interface exhibit significantly modified vitrification kinetics, resulting in substantial T_g suppression. Beside this observation, a number of studies, where the molecular mobility is measured by techniques probing the linear response, show the presence of a dominant component with bulk-like dynamics. To explain these results, two main hypotheses have been recently formulated. On one side, it has been argued that in nanoscale confinement T_g and molecular dynamics are decoupled. Beside, the role of free surface to speed dynamics, which has been shown in several studies, has been invoked to explain T_g suppression in confinement.
This contribution aims to provide compelling experimental evidence on the decoupling between glass transition and molecular mobility in polymeric glasses under geometrical confinement. Special attention is devoted to calorimetric techniques where the two aspects can be unequivocally separated by applying temperature perturbations either in the linear and non-linear regime. A number of examples are provided where, despite the ubiquitous presence of a prominent component with bulk-like linear dynamics, the T_g is suppressed. This outcome is discussed in view of the conceptual difference between the two aspects of glass dynamics.