Calorimetric Study of the Glassy State. VI. Phase Changes in Crystalline and Glassy-Crystalline 2,3-Dimethylbutane

Abstract

Abstract 2,3-Dimethylbutane has two crystalline phases, high temperature form (crystal-I) and low temperature form (crystal-II). On account of the supercooling effect of crystal-I, its glassy crystalline state (non-equilibrium frozen-in state) is established below Tg (76 K) by rapid cooling. The heat capacities for various phases were determined from 13 to 300 K from principal interest in the thermodynamic properties of the glassy crystal. The transition temperature from crystal-II to crystal-I and melting point were determined to be 136.02 K and 145.05 K, respectively. The heats of transition and of fusion were determined to be 6427 J mol−1 and 788 J mol−1, respectively. From these data, the residual entropy and T2 temperature were determined to be 7.4 JK−1 mol−1 and 65 K, respectively. We also calculated the ratio of Tg to T2 for several glassy crystals known hitherto, which amounts to 1.15–1.20. The stabilization effect was studied by the measurement of the change of the configurational enthalpy in the glassy crystalline state. Also, we discovered a new meta-stable intermediate phase (crystal-III) in the course of irreversible transition from the supercooled crystal-I to crystal-II. The thermal properties of this phase were also studied and its residual entropy was determined to be 2.7 JK−1 mol−1.