Non-Debye dielectric relaxation around the liquid-glass transition of a glass-forming polymer

Abstract

The dynamics of glass-forming liquids shows universal features like non-Debye behavior, scaling, and non-Arrhenius temperature dependence of the characteristic rates. These features have been mainly established in the time range between ${10}^{\mathrm{-}9}$ and 10 s. It is well known that when the temperature of a glass-forming liquid is lowered and the characteristic relaxation time becomes larger than hundreds of seconds, the system falls out of equilibrium and a glass is obtained. However, whether or not the typical features of the glass-forming-liquid dynamics extend to the glass phase through the experimental liquid-glass transition, ${\mathit{T}}_{\mathit{g}}$ is still unknown. Trying to address this question better we have performed a study of the dielectric relaxation of a glass-forming polymer, poly(bisphenol-A, 2-hydroxypropylether), around ${\mathit{T}}_{\mathit{g}}$. By using both frequency-domain and time-domain dielectric techniques, we have studied the dynamical processes in the range ${10}^{\mathrm{-}5}$–${10}^5$ s. We have also carried out calorimetric measurements for comparison. In the supercooled-liquid state (T>${\mathit{T}}_{\mathit{g}}$) we have observed all the universal features expected. However, although in the glassy state (T≲${\mathit{T}}_{\mathit{g}}$) the non-Debye character is preserved the scaling is broken, i.e., we observed a relaxation shape depending on the temperature and the state of the glass. Moreover, the temperature dependence of the characteristic time scale of the dielectric relaxation below ${\mathit{T}}_{\mathit{g}}$ is Arrhenius-like albeit the apparent activation energy is too high to be a truly activated process. All the features observed have been discussed in the framework of several theoretical approaches.