Mechanical properties of zeolitic imidazolate framework crystal-glass composites: A molecular dynamics study

Zeolitic imidazolate framework (ZIF) glasses have potential applications as battery materials or gas separation membranes, but this requires an improvement in their fracture resistance. In this study, we investigate the mechanical properties and fracture mechanism of ZIF crystal-glass composites using molecular dynamics simulations based on a recently developed machine learning force field. The composites are made from ZIF-4 or ZIF-zni crystals embedded in a melt-quenched ZIF-4 glass matrix. ZIF-4 and ZIF-zni share the same chemical composition (ZnIm₂, where Im is imidazole), but ZIF-zni has a denser crystal structure. By examining ZIF-4 glass based composites with different sizes and shapes of ZIF-4 and ZIF-zni crystals, we study the relationship between atomic-scale structure and mechanical properties. Our findings reveal that the structural differences between ZIF-4 and ZIF-zni crystals lead to distinct mechanical behaviors. The composites based on the stiffer ZIF-zni crystals exhibit greater resistance to irreversible atomic rearrangements compared to those based on ZIF-4 crystals, allowing for crack deflection around the crystals, thereby slightly increasing fracture toughness. Furthermore, the morphology of the crystals plays a crucial role in determining the crack path, influencing both crack deflection and the structural arrangement ability in the elastic state. Overall, the study identifies the key atomic-scale factors, such as the zinc bond switching propensity, for optimizing the mechanical properties of ZIF crystal-glass composites.