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 ɛ(ω,T) of the orientationally disordered phase of cyclo-octanol. The frequency dependence (${10}^{\mathrm{-}3}$–${10}^4$ Hz) of the real and imaginary parts of ɛ 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.