Thermo‑mechanical performance and novel applications of nanocomposites incorporating metal‑organic frameworks

This work investigated the thermo-mechanical responses of two classes of prototypical polymer/Metal-Organic Framework (MOF) composites: Matrimid/Zeolitic Imidazolate Framework (ZIF)-8 and Polyurethane (PU)/ZIF‑8, and explored their potential application in the uptake/retention of iodine and the reversible uptake of ethylene. The aim is to develop a structure-property relationship and elucidate the influence of ZIF-8 nanoparticles on the performance of the nanocomposite, while establishing a contrast between glassy (Matrimid) and rubbery (PU) matrices vis-à-vis its interactions with ZIF‑8 nanoparticles (MOF fillers). Experimental techniques such as dynamic mechanical analysis, uniaxial tensile tests, and nanoindentation were employed to study the variation to thermo-mechanical properties as a function of the concentration(s) of MOF nanoparticles. With 30 wt.% ZIF-8 content, it was demonstrated that the addition of filler nanoparticles decreased the quasi-static mechanical properties of the Matrimid/ZIF-8 nanocomposite (17.5% decrease in the Young's modulus and ~90% drop in ductility), but benefit the PU/ZIF-8 nanocomposite (~700% increase in the Young's modulus while retaining ~70% ductility of the neat PU matrix). The dynamic mechanical response of Matrimid/ZIF-8 nanocomposite remained almost comparable with its matrix, but PU/ZIF-8 nanocomposite exhibited tunability in its dynamic response. Higher concentrations of ZIF-8 nanoparticles resulted in a significant rise in storage moduli and decrease in loss moduli. The PU/ZIF-8 (30 wt.% of ZIF-8) nanocomposite exhibited significant uptake/retention of iodine (~32 wt.%) and reversible uptake of ethylene (~0.6 mmol g-1 at 5 °C). This is attributed to the molecular accessibility afforded by the PU matrix in conjunction with the porosity and chemical affinity of the 2-methylimidazolate ligands with iodine and ethylene guest species. In contrast, the glassy Matrimid matrix demonstrated poorer uptake performance due to its limited accessibility and robust microstructural features, which greatly limited the interactions between the ZIF‑8 nanoparticles with iodine and ethylene. It was also confirmed that the response of the matrix dictates the overall performance of the nanocomposites because they constitute the structural majority of the composite system, and determine crucial factors such as accessibility to the functional MOF nanoparticles incorporated within.