Challenges and Best Practices in Modeling Anisotropic Stresses in Soft Polymorphic Materials

Soft polymorphic materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), often display distinct anisotropy. Yet, their phase transition behavior has been predominantly characterized under isotropic stimuli, such as temperature or pressure variations, up to now. In this work, we employed the Cauchystat to investigate how MIL-53-(Al) and COF-5, two prototypical soft porous crystals, respond to anisotropic stresses instead. For MIL-53-(Al), we showed that normal stresses induce a phase transition already at stresses below the critical hydrostatic pressure, depending on the directionality of the applied stress. For COF-5, we determined the critical shear stress needed to induce a layer instability, leading to delamination. In both cases, we highlighted the importance of selecting adequate values of the Cauchystat control parameters to obtain accurate predictions. Based on these insights, we formulated best practices to simulate phase transitions in soft porous crystals under nonhydrostatic loadings, which is required for, e.g., nanosensors and -dampers.