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

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 (Tg) than the glass formed under ambient pressure. The sensitivity parameter of Tg 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 Tg upon compression. The origin of the hypersensitivity of Tg 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 Tg.