Construction of an adiabatic calorimeter at low temperatures and glass transition of crystalline 2-bromothiophene

Abstract

A low-temperature adiabatic calorimeter was constructed for measuring heat capacities accurately and following enthalpy relaxations stably for a long time. The calorimetry is feasible in the temperature range between 2 and 400 K and with quite high precision in the thermometry of 10 −5K. The imprecision in the heat capacity measurement was found to be within ±0.06% and the inaccuracy to be within ±0.3% and ±0.2% below and above 35 K, respectively. Heat capacities of 2-bromothiophene were measured between 4 and 300 K with this calorimeter and the standard thermodynamic functions evaluated. The temperature of fusion was found to be 205.30 K and the enthalpy and entropy of fusion were determined to be 7.903 kJ mol −1 and 38.43 J K −1 mol −1, respectively. A glass transition in the stable crystalline phase was discovered around 120 K. The enthalpy relaxation in the transition region showed a nonexponential behavior with β value of 0.93 in the stretched exponential function, and its majority molecular process was estimated to have an activation energy of (38.2 ±0.7) kJ mol −1. A small anomaly of heat capacity was observed around 55 K with an entropy change of 0.251 J K −1 mol −1 and interpreted as being due to a structural phase transition of the displacive type.