Structure, thermal properties and molecular mobility in cholesteryl hydrogen phthalate: Different approaches to the crystal, the glassy crystal and the mesophase

Abstract

Cholesteryl hydrogen phthalate (CHP) is a good glass-former: cooling the isotropic liquid forms the cholesteric mesophase, and subsequent cooling leads to the formation of an orientationally ordered glass. The thermal behavior of CHP was studied by DSC and it was found that, once melted, the commercial crystalline form cannot recover through heat treatments: cold crystallization gives rise to different polymorphic forms; in contrast, recrystallization of the commercial crystal from an ethanol solution leads to the same crystalline form.The structure of the stable ordered polymorph (commercial) was resolved by single-crystal X-ray diffraction (SXRD) at room and low temperature (167 ± 2 K); it belongs to the monoclinic space group P21.The glass transformation of CHP was studied by DSC and thermally stimulated depolarization currents (TSDC). The values obtained for the glass transition temperature and the dynamic fragility indicate that CHP is a moderately fragile glass former.Concerning secondary relaxations, the results suggest that the mobility detected by TSDC has a Johari-Goldstein nature. On the other hand, the interpretation of the results above Tg raises difficulties eventually due to the non-isothermal nature of the TSDC technique together with the temperature dependent structure of the cholesteric mesophase.