Dielectric measurement of the model glass transition in orientationally disordered cyclo-octanol

Abstract

Orientationally disordered crystals possess translational order while retaining rotational and conformational degrees of freedom. Because these materials exhibit a glasslike transition, they are of great theoretical and experimental interest as model systems for structural glasses. We present detailed measurements of the complex dielectric susceptibility \ensuremath{\varepsilon}(\ensuremath{\omega},T) of the orientationally disordered phase of cyclo-octanol. The frequency dependence (${10}^{\mathrm{\ensuremath{-}}3}$--${10}^{4}$ Hz) of the real and imaginary parts of \ensuremath{\varepsilon} are fit to Cole-Davidson and Kohlrausch-Williams-Watts shape functions to extract values for the peak frequency and width exponents as functions of temperature. The temperature dependence of the peak frequency is described by a Vogel-Tamman-Fulcher form with ${\mathit{T}}_{0}$=92.4 K for the Cole-Davidson fit and 94.7 K for the Kohlrausch-Williams-Watts fit. The correlation between the width exponent and fragility is consistent with the behavior of other glasses and glasslike materials. The data also obey a scaling form proven successful for a variety of organic glass formers. Both analyses demonstrate the close similarity of the transition in orientationally disordered cyclo-octanol to the structural glass transition and confirm that cyclo-octanol is an ideal candidate for further theoretical and experimental treatment as a model glass.