DIELECTRIC MEASUREMENT OF THE MODEL GLASS-TRANSITION IN ORIENTATIONALLY DISORDERED CYCLO-OCTANOL

Orientationally disordered crystals possess translational order while retaining rotational and conformational degrees of freedom. Because th ese materials exhibit a glasslike transition, they are of great theore tical and experimental interest as model systems for structural glasse s. We present detailed measurements of the complex dielectric suscepti bility epsilon(omega,T) of the orientationally disordered phase of cyc lo-octanol. The frequency dependence (10(-3)-10(4) Hz) of the real and imaginary parts of epsilon are fit to Cole-Davidson and Kohlrausch-Wi lliams-Watts shape functions to extract values for the peak frequency and width exponents as functions of temperature. The temperature depen dence of the peak frequency is described by a Vogel-Tamman-Fulcher for m with T-0=92.4 K for the Cole-Davidson fit and 94.7 K for the Kohlrau sch-Williams-Watts fit. The correlation between the width exponent and fragility is consistent with the behavior of other glasses and glassl ike materials. The data also obey a scaling form proven successful for a variety of organic glass formers. Both analyses demonstrate the clo se similarity of the transition in orientationally disordered cyclo-oc tanol to the structural glass transition and confirm that cyclo-octano l is an ideal candidate for further theoretical and experimental treat ment as a model glass.