Capture and immobilisation of iodine (I2) utilising polymer-based ZIF-8 nanocomposite membranes

Abstract

Polymer nanocomposites made up of nanoporous metal–organic frameworks (MOFs) are fast becoming a staple of next generation hybrid composites, and are currently being intensely developed for gas capture and separation. This work reports the first attempt to capture and retain iodine (I₂) using polymer-MOF (ZIF-8) nanocomposites. Membranes of ZIF-8-based nanocomposites (comprising either a glassy Matrimid or a rubbery polyurethane (PU) matrix) were prepared via a colloidal-mixing approach and their viability for I₂ capture and retention effects was determined through absorption experiments, nanoindentation mechanical measurements, and thermogravimetric (TGA) analysis. The absorption experiments demonstrated that I₂ capture and retention is possible in all of the nanocomposite membranes, although the PU/ZIF-8 30 wt% nanocomposite exhibited higher affinity for I₂ absorption (>32 wt%). It is reasoned that the molecular affinity and attraction between I₂, (2-methylimidazolate organic ligands of) ZIF-8 nanoparticles, and polymer matrices (Matrimid and PU) will catalyse the formation of weak secondary bonds, resulting in the ‘capture’ and ‘retention’ of I₂ within molecular segments of the polymers and inside the pores of ZIF-8. The enhancement of the Young's modulus (E) of the PU/ZIF-8 30 wt% nanocomposite (E increased by ∼6%) is postulated to be due to I₂ rigidification, while TGA analysis proved that I₂ retention within both free volume of the polymer and ZIF-8 sodalite cages remained intact up till the points of structural degradation, at ∼200 °C for the PU-based nanocomposites, and at ∼300 °C for the Matrimid-based nanocomposites. We propose that the affinity of the organic ligands in ZIF-8 and the formation of free volume in the nanocomposites from the presence of ZIF-8 attracted I₂, and the formation of secondary bonds between these constituents (H-bonds) strengthened not only the nanocomposite, but also kept I₂ from being released despite the larger pore size and gate-opening dynamics of ZIF-8. It was therefore concluded that a combination of nanoparticles of porous MOFs and a rubbery polymer is promising for further development to enable I₂ capture and retention applications.