Fast scanning differential calorimetry. All fast scanning differential calorimetry (FDSC) measurements were done using a Mettler Toledo Flash DSC 2+ device. Samples were initially crushed under a scalpel blade. For each sample, a single grain (microgram range) of the homogenous sample was loaded onto the active area of a Mettler Toledo pre-conditioned and pre-corrected UFS1 sensor using a hair (see example of a loaded sample in Supplementary Fig. ). The sample was then prepared at temperatures ranging from 200-300 °C i.e., well above Tg of the modified ZIF glasses, to establish a good thermal contact between the sensor and the sample. All sample scans were performed in a nitrogen atmosphere (flow of 20 mL min -1 ). The with an inner diameter of 1.03 mm and a 0.051 mm wall thickness to achieve good transmission of X-rays. The in situ measurements were performed using an exposure time of 60 s at each 10 K temperature step. Heating and cooling between steps were performed at a ramp of 25 K min - 1 using a hot air blower . Real and setpoint temperatures were checked using the (2 0 0) Bragg reflection of a copper sample. We found a difference between real and setpoints temperatures of <10 K (Supplementary Figs. 37-38) for all measurements and we thus used the setpoint temperature for all reported temperatures of the total scattering data. The ex situ measurements were performed at room temperature using an exposure time of 600 s.