Crystalline thiophene - Calorimetric evidence for a glassy crystalline state in both phase sequences

Abstract
Both phase sequences of crystalline thiophene have been investigated during a new calorimetric study from 14 to 300 K. The phase transition temperatures are reported Around 40 K, each phase sequence exhibits a glass transition which seems to arise from the freezing of the reorientational motion of the molecules at the low temperature stage of the ordering process. Kinetic data that characterize the enthalpy relaxation in both glassy crystalline states have been determined : this leads to activation enthalpies which are close to those associated with the in-plane reorientational motions of thiophene molecules in the corresponding phase sequences. This supports the hypothesis that both processes are related. Some speculations are made about the influence of the molecular quasi-five-fold symmetry

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