Orientational glass, orientationally disordered crystal and crystalline polymorphism: A further study on the thermal behavior and molecular mobility in levoglucosan

Abstract

Levoglucosan has very particular features in its phase diagram. On the one hand it has two crystalline phases, one orientationally ordered (ORC), stable at room temperature, which is transformed by heating in an orientationally disordered crystal (ODIC), a plastic crystal. On the other hand, it is difficult to form the structural glass by cooling down from the isotropic liquid, but it is easy to form the orientational glass (O-G) of levoglucosan by cooling from the ODIC phase. The stability of the O-G and ODIC phases allowed the study of the orientational glass transition, O-G → ODIC, by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) with the determination of the respective dynamic fragility. In this context a critical analysis of the procedures using the effect of the DSC scanning rate on the temperature location of the DSC glass transition signal was performed.Two polymorphs of levoglucosan, not yet described in the literature, were discovered. These crystalline ORC forms are converted into an ODIC phase by heating, as happens with the most stable polymorph (as received). The three ORCs show ORC → ODIC transitions that differ greatly in temperature, enthalpy, and entropy of transition. We must therefore consider four levoglucosan polymorphs, three ORCs and one ODIC.TSDC was also used to study the secondary relaxations in the levoglucosan orientational glass, and it was possible to identify a β-type mobility (Johari-Goldstein) in addition to fast relaxations. Finally, the two dielectric techniques, dielectric relaxation spectroscopy (DRS) and TSDC, were compared in their ability to provide relevant information on the secondary mobility.