Calorimetric Study of the Glassy State. XIV. Calorimetric Study on Unusual Glass Transition Phenomena in CFCl2–CFCl2

Abstract

Abstract The heat capacities of the glassy crystalline, plastic crystalline, and liquid phases of CFCl2–CFCl2 were measured by an adiabatic calorimeter over a temperature range from 13 to 310 K, with a sample of 99.93% purity. The triple point was determined to be 297.91 K, the enthalpy and entropy of fusion being 3.666 kJ mol−1 and 12.30 J K−1 mol−1, respectively. Three kinds of relaxation phenomena were found during the heat capacity measurement centered at around 60, 90, and 130 K, respectively. The first was attributable to the β relaxation. The second was for primary glass transition, where overall rotation or reorientation of the molecules might be frozen. The third was analyzed by ascribing it to the freezing of conversion between the trans and the gauche conformers of the molecule. Activation enthalpy for the internal rotation and energy difference between the two conformers were determined to be 41 kJ mol−1 and (790±60) J mol−1, respectively. This experiment provides the first example in which three different freedoms of molecular motion are frozen successively at different temperatures as the crystal is cooled down. An effort to get a possible low-temperature ordered crystalline phase was partially successful by long annealing of the crystal. The extremely sluggish nature of the phase transition into the ordered phase will be related with the slow rate of the internal rotation of the molecule below the transition temperature.