Calorimetric Study of the Glassy State. VII. Phase Changes between the Crystalline Phases of Cycloheptanol with Various Degrees of Stability

Abstract

Abstract The heat capacities of 8 kinds of stable and meta-stable crystalline phases including the glassy crystal of cycloheptanol were measured at temperatures from 13 to 300 K. The crystalline phases have been named crystals I, II, II′, III, III′, III″, Iβ, and glassy crystal. Transition temperatures between crystals II and I, III and II, II′ and II, III′ and III, III″ and III were determined to be 258.45, 227.26, 172.23, 152.30, and 128.2 K, respectively. The melting point and glass transition point of the glassy crystal were determined to be 280.30 and 140 K, respectively. The heats of transition between crystals II and I, III and II, III′ and III and the heat of melting were determined to be 876, 554, 58.1, and 1604Jmol−1, respectively. The residual entropy of the glassy crystal was found to be 0.7 JK−1mol−1 assuming that of crystal III′ to be zero. The stabilization phenomenon in the glassy crystalline state was also studied from the measurements of enthalpy relaxation. The stability of the various phases was determined by calculating their free energies. The degree of freedom which is frozen in the glassy crystalline state was discussed from molecular viewpoints. Classification of various phase transitions found in this material was also made from the viewpoint of thermodynamical stability.