Thermodynamic Approach to Glass Transitions of Plastically Crystalline Cyanoadamantane and Isocyanocyclohexane

The heat capacity of plastically crystalline cyanoadamantane was measured with an adiabatic calorimeter in the temperature range 5–310 K. A glass transition appeared at 163 K in the supercooled plastic phase. The ordered phase obtained by annealing underwent a first-order transition to the plastic phase at 273 K. Enthalpy relaxation was also measured around T g to investigate the molecular reorientation with long relaxation times (10 3 < τ <10 5 s). Dielectric permittivity of plastically crystalline isocyanocyclohexane was measured in the temperature range 120–230 K and frequency range 20–10 6 Hz to determine the relaxation times of the molecular reorientation. The fragility of cyanoadamantane was small while those of isocyanocyclohexane and cyclohexanol were intermediate between the fragile and the strong systems. The configurational entropies of these plastic crystals were calculated as functions of temperature. Their relaxation times were reproduced well by an extended version of the Adam-Gibbs theory which is modified by considering the configurational entropy inside the cooperatively rearranging region.