Study of the nature of glass transitions in the plastic crystalline phasesof cyclo-octanol, cycloheptanol, cyanoadamantane and cis-1,2-dimethylcyclohexane

Cycloheptanol, cyclo-octanol, cyanoadamantane and cis-1,2-dimethylcyclohexa ne are known to form plastic crystals which can be supercooled to show a gl ass transition at a temperature T-g. The molecular dynamics in different pl astic phases is studied in their supercooled states using dielectric spectr oscopy (frequency range: 10(6) Hz-10(-3) Hz) and differential scanning calo rimetry (DSC) over a wide temperature range. The kinetic freezing of the va rious dielectric processes have been critically examined in relation to the T-g found in the DSC experiments. The plastic phase I of cyclo-octanol sho ws two T-g's: one at 148.5 K and the other at 164 K, the former of which is not found in well annealed phase I. The dielectric alpha -modes correspond to the latter. Cycloheptanol exhibits many T-g's for the different plastic phases. Unlike the cyclic alcohols, the dielectric spectra of cyanoadamant ane and cis-1,2-dimethylcyclohexane is clearly found to follow the Havrilia k-Negami equation, both of which interestingly show very little molecular m obility, often referred to as the secondary relaxation in their glassy stat es. In addition, the temperature dependence of the relaxation rates and the dielectric strengths are critically examined for various supercooled phase s to gain an insight into the nature of the molecular mobility in those pha ses. (C) 2001 American Institute of Physics.