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Milling-induced Phase Transformations, Underlying Mechanisms and Resulting Physical States in an Enantiotropic System: the Case of Bezafibrate

Abstract : The mechanisms driving the milling-induced transformations of an enantiotropic polymorph system, Bezafibrate, are depicted. The phase transformations promoted by milling at two different temperatures and the resulting physical states were carefully investigated by the cross-use of structural (X-Rays powder diffraction), thermodynamic (differential scanning calorimetry) and dynamic (dielectric relaxation spectroscopy) techniques. Our results highlight that milling of the commercial alpha phase at Tg - 50 °C (-10 °C) leads to a complete amorphisation (Tg = 40 °C), whereas at Tg - 15 °C (25 °C) it leads to the stable β phase. We establish that, as for monotropic situations, the solid-solid conversion is mediated by a transient amorphous state resulting from a milling-induced disordering of the crystalline structure partly counterbalanced by a slower re-crystallisation. However, the monitoring of the transformation kinetic (phases ratio, crystallites' size) reveals that at least 10% of amorphous phase is required to trigger the re-crystallisation toward the stable β form instead of the metastable alpha form. For the first time, the molecular mobility of the physical states resulting from milling is finely investigated by dielectric relaxation spectroscopy. Strikingly, it evidenced, for the crystalline phases produced by milling, a residual mobility (detection of localised intra-molecular motions but absence of wide amplitude motions characterising the amorphous state) which originating through the mobility of part of molecules at the surface of crystallites. This outstanding result emphasizes and unravels the highly defective nature of the crystalline phases generated by milling.
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https://hal.univ-lille.fr/hal-03537652
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Soumis le : jeudi 20 janvier 2022 - 15:08:42
Dernière modification le : mercredi 23 mars 2022 - 15:51:12

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Emeline Dudognon, Florence Danede, Mathieu Guerain. Milling-induced Phase Transformations, Underlying Mechanisms and Resulting Physical States in an Enantiotropic System: the Case of Bezafibrate. Crystal Growth & Design, American Chemical Society, 2022, Crystal Growth & Design, 22 (1), pp.363-378. ⟨10.1021/acs.cgd.1c00997⟩. ⟨hal-03537652⟩

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