Hypersensitivity of the Glass Transition to Pressure History in a Metal- Organic Framework Glass

A Qiao and SS Sorensen and M Stepniewska and CAN Biscio and L Fajstrup and Z Wang and XH Zhang and L Calvez and I Hung and ZH Gan and MM Smedskjaer and YZ Yue, CHEMISTRY OF MATERIALS, 34, 5030-5038 (2022).

DOI: 10.1021/acs.chemmater.2c00325

Metal-organic framework (MOF) glasses feature numerous unique properties and potential applications, but many fundamental questions remain unanswered, especially concerning their glass transition. Here, we report a discovery about ZIF-62 glass (a typical MOF glass), namely, the hypersensitivity of its glass transition to pressure history. Specifically, upon quenching the ZIF-62 melt under a modest pressure of 60 MPa, the derived glass exhibited a significantly lower glass transition temperature (T-g) than the glass formed under ambient pressure. The sensitivity parameter of T-g to pressure for ZIF-62 glass is much higher than that of other glass systems, including polymer glasses that typically feature a huge increase in T-g upon compression. The origin of the hypersensitivity of T-g to pressure history was revealed by structural analyses, positron annihilation lifetime spectroscopy, and molecular dynamics simulations. We found that the medium-range structure change induced the drop in T-g.

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