Equilibrium and Out-of-Equilibrium Dynamics in Confined Polymers and Other Glass Forming Systems by Dielectric Spectroscopy and Calorimetric Techniques (original) (raw)

Glassy dynamics under nanoscale confinement is currently a topic under intense debate in soft matter physics. The reason is that this kind of studies may deliver important insight on the glassy dynamics in general. Furthermore, from a technological point of view, there exists a rising interest in the understanding of how properties are modified at the nanoscale in comparison to the corresponding bulk system. Within this context, this chapter critically discusses the experimental findings in the field. The vast majority of results concerns thin polymer films. However, other geometries of confinement, such as polymer nanocomposites and nanospheres, are considered as well. Special attention is devoted to the kind of information achieved by a specific technique. Within this context, the ability of dielectric and calorimet-ric techniques is highlighted. Particular attention is devoted to the determination of the different aspects of glassy dynamics in confinement, that is, the equilibrium dynamics in terms of the rate of spontaneous fluctuations as probed by experiments where a perturbation in the linear regime is applied, on the one hand, and the out-of-equilibrium dynamics in terms of thermal glass transition temperature (T g) and the physical aging on the other. In the latter case, the application of a temperature ramp for T g measurements and the recovery of equilibrium in physical aging imply the application of large perturbations, in particular with amplitude well beyond that of spontaneous fluctuations. It is demonstrated how, in view of numerous experimental results, the two aspects are not one-to-one related in confinement. Specifically, the reduction in T g and the acceleration of equilibrium recovery in the aging regime does not imply a concomitant speed-up of the rate of spontaneous fluctuations, which is in several cases found to be unaltered in comparison to the bulk. Finally, a description of suitable frameworks to describe such phenomenology is presented with special attention to the free volume hole diffusion (FVHD) model. This is shown to quantitatively catch the acceleration of physical aging and the T g depression with no need to assume any acceleration on the intrinsic molecular mobility of the glass former.