Unveiling the structure-mechanical strength relationship of single crystal mixed-linker ZIF-8 and ZIF-67 via...

Zeolitic imidazolate frameworks (ZIFs) have garnered considerable attention in recent years due to their facile synthesis, versatile chemical properties, large pore volumes, and chemical stability. Researchers have developed organic glasses/membranes based on ZIFs for various applications, including gas storage and molecular separation. Probing the structural-mechanical property relationship of ZIFs can provide the foundation for designing high-performance ZIF-based materials with controllable physical structures and mechanical responses. Therefore, we aim to define the factors governing mechanical performance through systematic structural variations and comprehensive mechanical characterization. We synthesized micron-sized single crystals of ZIF-8 and ZIF-67, which have identical crystal structures but different metal-linker connectivity, and studied their mechanical properties using single-particle nanoindentation. We found that ZIF-67 exhibited a higher Young's modulus and hardness when compared with ZIF-8, which was attributed to the stronger Co-N bond in ZIF-67. Moreover, we controlled the linker content (20% imidazole and 80% 2-methylimidazole), as confirmed by NMR for both ZIF-8 and ZIF-67 and by single-crystal X-ray diffraction (SCXRD) for ZIF-67. The resulting mixed-linker ZIFs showed decreased crystallinity compared with the parent ZIFs and exhibited missing-linker defects, as determined by thermogravimetric analysis (TGA). As a result, the mixed-linker ZIFs displayed lower Young's modulus and hardness. Our approach offers significant insight into the design and fabrication of mechanically robust ZIFs/MOFs, paving the way for future research.