Broadband dielectric spectroscopy (BDS) is conventionally exploited to convey information on molecular dynamics of dipolar systems. In such a way, BDS is especially suitable to explore the molecular spontaneous fluctuations associated to glassy dynamics, that is, the so-called $\alpha$ relaxation and secondary relaxations. These measurements provide an exhaustive overview about equilibrium dynamics in the unperturbed glass former, but lack any information regarding the transition from the melt to the non-equilibrium glassy state, the so-called vitrification or glass transition, and the subsequent recovery of equilibrium in the glassy state, generally addressed as physical aging. Here, I will show how several observables delivered by BDS can be successfully used to determine the glass transition temperature (Tg) and to monitor physical aging in the glassy state. Special attention will be paid to polymer glasses confined at the nanoscale. It will be shown how, when these systems are subjected to some specific conditions of confinement, they exhibit suppressed Tg and accelerated physical aging with respect to the corresponding bulk polymer. Importantly, this takes place at invariant and bulk-like molecular dynamics. This results hints towards a decoupling between vitrification and physical aging on the one hand, and molecular dynamics on the other.

Vitrification and physical aging in polymer glasses by broadband dielectric spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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