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

## TL;DR

This paper explores how soft polymorphic materials like MOFs and COFs respond to anisotropic stresses and provides best practices for modeling their phase transitions.

## Contribution

The study introduces best practices for simulating phase transitions in soft porous crystals under nonhydrostatic stresses using the Cauchystat.

## Key findings

- Normal stresses on MIL-53(Al) induce phase transitions at stresses below critical hydrostatic pressure.
- COF-5 delaminates when critical shear stress is applied, causing layer instability.
- Proper selection of Cauchystat control parameters is crucial for accurate predictions.

## Abstract

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.