Thermodynamic study of 2-chlorothiophene: Observation of two subsequent glass transitions in the stable crystalline state

Abstract

Heat capacities of 2-chlorothiophene were measured between 13 and 300 K using an adiabatic calorimeter and the standard thermodynamic functions were evaluated. The triple point of fusion was found to be 201.30 K and the enthalpy and entropy of fusion were determined to be 8.966 kJ mol −1 and 44.54 J K −1 mol −1, respectively. Two glass transitions were discovered in the stable crystalline phase; one at 164 K with a heat-capacity jump of 0.204 J K −1mol −1 and the other at 186 K with that of 4.22 J K −1 mol −1. The endothermic enthalpy-relaxation at 183.2 K, which was high enough for the relaxation of the low-temperature mode not to be detected with respect to its characteristic time, showed a nonexponential behavior with a β value of 0.60 in the stretched exponential function. With this result, the two relevant reorientational modes were suggested to couple with each other in their static properties, and the situation in 2-bromothiophene was discussed in comparison with that of the chlorine compound.