New class of plastic crystal shows its fragility
New class of plastic crystal shows its fragility lead image
The molecules of plastic crystals form a 3-D lattice, like a solid. But unlike in solids, the molecules in plastic crystals can rotate. Because the plastic crystal phase acts like both a liquid and a solid, these systems are often used to better understand the still-mysterious glass transition, which is also a phase between liquid and solid.
In The Journal of Chemical Physics, authors report using various experimental techniques to thoroughly characterize a new class of plastic crystals, called anhydrosaccharides, for the first time. They performed calorimetric, X-ray diffraction, infrared, and dielectric studies on 1,6-anhydro-β-D-mannopyranose (anhMAN), 1,6-anhydro-β-D-galactopyranose (anhGAL), and 1,6-anhydro-β-D-glucopyranose (anhGLU), three plastic crystals, as well as on 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-glucopyranose, an acetyl derivative of anhGLU.
The dynamic, thermodynamic, and structural measurements of the anhydrosaccharides showed that they’re not like ordinary plastic crystals. Anhydrosaccharides are unexpectedly fragile: They’re the most fragile plastic crystals besides Freon 113. Co-author Ewa Kamińska said that their fragility is probably due to multidirectional hydrogen bonds that affect the number of available conformations, density states, and potential barriers in the energy landscape of these compounds. In this context, fragility does not mean that the samples easily shatter, but rather indicates that the dynamics in the plastic crystal are a very strong function of temperature.
The researchers also revealed that the thermodynamic stability of these carbohydrates, or their tendency to undergo the transition from plastic crystal to fully ordered crystal, is closely related to hydrogen bond strength as well as space symmetry of the lattice. Additionally, in the examined anhydrosaccharides, there are enormously long-range static correlations between molecules reaching even 120 angstroms.
The authors will next work to study the molecular dynamics of anhydrosaccharides at high pressure.
Source: “Anhydrosaccharides—A new class of the fragile plastic crystals,” by Ewa Kamińska, Olga Madejczyk, Magdalena Tarnacka, Karolina Jurkiewicz, Kamila Wolnica, Wioleta Edyta Śmiszek-Lindert, Kamil Kamiński, and Marian Paluch, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5011672 .
