The slow molecular mobility in the amorphous solid and supercooled liquid phases of brucine, an anti-cancer drug. A study by differential scanning calorimetry (DSC), thermostimulated currents (TSC) and dielectric relaxation spectroscopy (DRS)

Abstract

In this work, we explore in detail the molecular mobility in amorphous brucine, i.e. in the glassy state and in the supercooled liquid immediately above the glass transition, where viscosity is high and where a spatially heterogeneous dynamic is detected. Brucine is an alkaloid used in ancient times by Chinese medicine in the treatment of cancer, it can be obtained in the amorphous form by cooling from the melt and the high stability exhibited makes it a good candidate for being use in novel formulations.Combining thermostimulated depolarization currents (TSDC), and dielectric relaxation spectroscopy (DRS) different types of motions have been probed. In the glassy state, two secondary relaxations γ and β were characterized, the latter with features compatible with a Johari-Goldstein mobility. The cooperative α relaxation was clearly identified, and by combining also with results taken from differential scanning calorimetry (DSC), it was possible to determine parameters such as the glass transition temperature, and fragility index which indicate a moderately fragile glass former. In addition, the determination of the βJG relaxation times and activation energy, both above and below Tg, were a new result of the current investigation.Additionally, TSDC measurements, although with some uncertainty, reveal the existence of a mobility above Tg. On the other hand, in the supercooled liquid region, the analysis of the conductivity obtained by DRS, shows a breakdown of the Debye-Stokes-Einstein relation pointing to a heterogeneous dynamic that could be in the origin of the observed mobility by TSDC.