Quantitative Comparison of Structural and Mechanical Properties of 6-Chloro-2,4-dinitroaniline Polymorphs

This work is focused on three polymorphic forms of 6-chloro-2,4-dinitroaniline crystals that are drastically different in their mechanical behavior. We analyze (1) the anisotropy of the mechanical properties in equilibrium state, (2) the changes of the crystal structures during the virtual tensile test, and (3) the subsequent full relaxation of stretched structures. In case of shearing Form I, the structural changes during stretching are mainly caused by the sliding of the molecular layers relative to each other, which is accompanied by a reconfiguration of the weak interlayer bonds. The plastic behavior of Form II during stretching, with no shape memory, is due to the mobility of the strong noncovalent bonds that form the two crystal motifs. Form III has the least anisotropy of elastic moduli among three considered polymorphs. Its brittleness is confirmed by the combination of rigidity in one spatial direction with the possibility of crack formation in other directions found during the virtual tensile test. Full relaxation of the stretched Form III crystals demonstrates the possibility of self-healing. We argue that it is necessary to use several analysis methods to correctly predict the mechanical properties of molecular crystals.

Automated summary

In this study, we focus on the mechanical behavior of three polymorphic forms of 6-chloro-2,4-dinitroaniline crystals. Through analyzing the anisotropy of mechanical properties, crystal structure changes during stretching, and relaxation of stretched structures, we find distinct differences in mechanical behavior among the three forms. The sliding of molecular layers and reconfiguration of interlayer bonds contribute to the structural changes in Form I, while the plastic behavior of Form II is attributed to the mobility of strong noncovalent bonds. Form III exhibits the least anisotropy in elastic moduli and demonstrates the possibility of self-healing. Multiple analysis methods are necessary for accurately predicting the mechanical properties of molecular crystals.