Evidence of Organic Polymeric Behavior in the Glass Transition of Metal-Organic Frameworks

The origin of the glass transition is still an open debate, especially for the new class of glasses, formed from metal-organic compounds. High-temperature in situ ²H Nuclear Magnetic Resonance (NMR) experiments are performed on deuterated samples of ZIF-62 (Zn(C₃H₄N₂)_2-x(C₇H₆N₂)_x, with x = 0.25 and x = 0.05), the prototypical metal-organic framework glass former. Using lineshape analysis, frequencies and angular amplitudes of oscillations of the imidazolate ring during heating up to the melt progressively increasing from ≈10 to 150 MHz, and from ≈5° to 25° are found. This behavior is compositionally dependent and points to the origin of the glass transition lying in organic linker movement, in a similar vein to that witnessed in some organics and contrary to the purely inorganic-based view of Metal-Organic Framework (MOF) glasses taken to date. This experimental approach shows the potential to elucidate the melting and/or decomposition process for a wide range of MOFs.