Memory Effect and Crystallization of (R, S)-2-Chloromandelic Acid Glass

Abstract

Chloromandelic acid, which can crystallize in racemic crystals (forms α and β) or a conglomerate (form γ), has been studied for its glass-forming behavior. Below the glass transition temperature, samples of the title compound crack into pieces. Correlation plots of DSC results have been used to investigate what determines the cracking and its occurrence temperature. We found that the latter is influenced by the polymorph from which the melt state has been obtained, showing that a certain memory of the previous crystalline phase persists in the undercooled melt. Moreover, this residual structure could be eliminated by elongating the annealing period or increasing the annealing temperature. Investigation using broadband dielectric spectroscopy confirmed such a memory effect. Finally, we studied the role of cracking in the control of the crystallization. In contrast with previous literature on other glass-forming molecular systems, we verified that the crystallization upon reheating is not impacted by the occurrence of cracks in the glassy state. This observation challenges the current views on polymorphic crystallization from organic glasses.

Accessing Selective Crystallization of ROY Polymorphs Using Directional Crystallization from the Melt

“…The procedure of sample preparation is presented in Figure S4. First, ROY was completely melted at the hot side T h = 130 °C (for both ∇ T 1 and ∇ T 2 ) with an annealing time of 5 min, which allow for erasing any memory of the form from which the melt was prepared . Then, sandwiched thin films of molten ROY were withdrawn from the cold side at various speeds.…”

“…First, ROY was completely melted at the hot side T h = 130 °C (for both ∇T 1 and ∇T 2 ) with an annealing time of 5 min, which allow for erasing any memory of the form from which the melt was prepared. 27 Then, sandwiched thin films of molten ROY were withdrawn from the cold side at various speeds. In these conditions, several nucleation spots were present on the coldest side of the sample and crystal growth occurred (Supporting Information, Videos V1, V2, V3, and V4).…”

Memory Effect by Melt Crystallization Observed in Polymorphs of a Benzothieno-Benzothiophene Derivative

“…Figure shows the XRD patterns recorded before and after the heat treatment for the different polymorphic thin films. For other systems, a similar memory effect was already observed for crystallization from melt, although it is also known that the heat treatment at higher temperatures or elongation of the annealing time cancels this effect …”

“…For other systems, a similar memory effect was already observed for crystallization from melt, although it is also known that the heat treatment at higher temperatures or elongation of the annealing time cancels this effect. 24 A series of experiments were performed on phase-pure Form IV samples after annealing above the melting temperature and then cooling them to room temperature. The annealing temperature was varied between 373 and 393 K for a period of 30 min for equilibration.…”

“…This ability suggests that even after heating well above the melting point, temperature-induced nucleation into the initial crystal structure is observed upon cooling below the melting temperature . This intriguing phenomenon is frequently observed during the crystallization of slowly crystallizing materials like polymers and glasses. , In the case of polymer crystallization, a specific polymorph could appear at temperatures far above the melting point due to heterogeneous nucleation at impurities, by homogeneous nucleation from the internal structure in the melt (self-nucleation), or slightly above the melting temperature due to crystal fragments in the melt (self-seeding). − However, the melt memory effect is hardly known for molecular crystals, despite the fact that it can be of technological importance. ,− Here, the melt memory effect is evidenced for three different solvates of the molecule OEG-BTBT; recrystallization back to their initial structure is observed even after annealing far above their melting temperatures.…”