Probing interfacial mobility profiles via the impact of nanoscopic confinement on the strength of the dynamic glass transition

Abstract

Dielectric measurements of ultrathin polymer layers capped between metallic electrodes revealed, besides controversial deviations in the dielectric glass transition temperature, a universal and continuous decrease of the dielectric strength which scales with the surface/volume ratio. In an attempt to describe the thickness dependence of this last quantity, proportional to the number of segments relaxing on the time and length scale of the dynamic glass transition and being a unique probe of the deviations from bulk behavior, we propose a model based on the impact of adsorption on the segmental dynamics and on the assumption of a smooth distribution of mobility inside the film. Differently to simplified bilayer and trilayer-models already described in the literature, our approach is able to reproduce the behavior of all the polymer systems so far investigated. Moreover, our calculations allow determining the penetration depth of the surface induced perturbations along with the thickness of the dead (or reduced) mobility layer where motions of polymer segments are highly inhibited (or reduced) on the time scale of the glassy dynamics. Analysis of the temperature dependence of this quantity confirmed that the gradient of mobility is limited by the gyration radius at temperatures much larger than T_g while in deeply supercooled melts the penetration of the interfacial interactions exceeds the dimension of the single polymer chain.