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Mechanical Properties and Processing Techniques of Bulk Metal-Organic Framework Glasses
preprintsubmitted on 22.10.2018, 08:31 and posted on 22.10.2018, 14:38 by Thomas Bennett, Shichun Li, Rene Limbach, Louis Longley, Amir Shirzadi, John C. Walmsley, Duncan Johnstone, Paul A. Midgley, Lothar Wondraczek
Glasses formed by melt quenching metal–organic frameworks (MOFs) are attracting growing attention because they exhibit an improved processability compared with their crystalline counterparts. Melt quenched MOF glasses also define a new category of glass, distinct from metallic, organic and inorganic glasses, owning to the role that metal-ligand coordination bonding plays in their three-dimensional structures. The mechanical properties of glasses in general are of importance given their application in protective coatings, display technologies and screens. Little, however, is known about the mechanical behavior of MOF-glasses, and experimental elucidation of key properties such as their scratch resistance has been limited by the lack of processing methodologies capable of producing bulk glass samples. Here, nanoindentation was used to investigate the Young’s modulus and hardness of four melt-quenched glasses formed from zeolitic imidazolate frameworks (ZIF): agZIF-4, agZIF-62, agZIF-76 and agZIF-76-mbIm. The creep resistance of the melt-quenched glasses was studied via strain-rate jump (SRJ) tests, and through constant load and hold (CLH) indentation creep experiments. Values for the strain-rate sensitivity were found to be close to those for other glassy polymers and Se-rich GeSe chalcogenide glasses. One glass, agZIF-62, was used to explore two strategies for the preparation of bulk glass samples, i.e. (i) vacuum hot-pressing and (ii) remelting and annealing. Vacuum hot-pressing resulted in an inhomogeneous bulk sample containing the glass and amorphous, non-melt quenched aZIF-62. Remelting and annealing however, resulted in the fabrication of a transparent, bubble-free, bulk specimen, which allowed the first scratch testing experiments to be performed on a MOF-glass. The results are of high significance for potential applications of MOF-glasses.