Research paper of the month
December 2015 - Hybrid glasses from strong and fragile metal-organic framework liquids
Hybrid glasses connect the emerging field of metal-organic frameworks (MOFs) with the glass formation, amorphization and melting processes of these chemically versatile systems. Though inorganic zeolites collapse around the glass transition and melt at higher temperatures, the relationship between amorphization and melting has so far not been investigated. Here we show how heating MOFs of zeolitic topology first results in a low density ‘perfect’ glass, similar to those formed in ice, silicon and disaccharides. This order–order transition leads to a super-strong liquid of low fragility that dynamically controls collapse, before a subsequent order–disorder transition, which creates a more fragile high-density liquid. After crystallization to a dense phase, which can be remelted, subsequent quenching results in a bulk glass, virtually identical to the high-density phase. We provide evidence that the wide-ranging melting temperatures of zeolitic MOFs are related to their network topologies and open up (i) the possibility of ‘melt-casting’ MOF glasses and (ii) the ability to design hybrid glasses incorporating a wide array of desirable functionalities.
Figure: (Top) Highlighting the rings and imidazolate linkages in zeolitic topologies, in the ordered structure of crystalline ZIF-4 (left) and ZIF-8 (centre), and the disordered amorphous phase (right) obtained by Molecular Dynamics modeling. Zn, orange; N, blue; C, green; and H, grey. (Bottom) X-ray pair distribution function data D(r) measured for the melt quenched glass (green), ZIF-4 (broken black) and the amorphous phase (broken pink). Inset: optical images of (left) ZIF-4 and (right) the melt quenched glass, showing the typical fracture pattern of a non-metallic bulk glass. Scale bars, 100μm.
Thomas D. Bennett, Jin-Chong Tan, Yuanzheng Z. Yue, Emma F. Baxter, Caterina Ducati, Nick J. Terril, Hamish H. M. Yeung, Zhongfu Zhou, Wenlin Chen, Sebastian Henke, Anthony K. Cheetham and Neville G. Greaves, “Hybrid glasses from strong and fragile metal-organic frameworks”, Nat. Commun., 2015, 6, 8079.