Fast heating inhibits endothermic solid-solid polymorphic transition giving a melting of low temperature polymorph with the next cold crystallization

Abstract

Heating at the rate of several hundred Kelvin per second in fast scanning calorimetry (FSC) experiments was found to change the route of polymorphic transformation. Upon the increase of the heating rate first the endothermic solid-state polymorphic transition is hindered to give a consecutive melting of low-temperature form and crystallization of high temperature polymorph. At higher heating rates, only melting of low-temperature polymorph is observed much below an equilibrium melting point. This thermal behavior with depression of final melting point by 30 K was observed for tert‑butylthiacalix[4]arene tetrasubstituted with (ethoxycarbonyl)methoxy groups (1), which does not sublimate at heating. A similar shift of the endothermic solid-state polymorphic transition by temperature at fast heating was found for caffeine (2), however the melting of low-temperature polymorph is not observed due to sublimation of the compound at elevated temperatures. The heating rates, at which a significant depression of the melting points of 1 and 2 may be expected in absence of sublimation, were calculated using equations of thermal kinetics derived from the data of conventional differential scanning calorimetry. The results give a practical guide for the preparation of glassy forms of the substances capable of endothermic solid-solid transition below melting point of high-temperature form.