Simultaneous enhancement of mechanical properties and CO2 selectivity of ZIF-8 mixed matrix membranes: Interfacial toughening effect of ionic liquid

Abstract

Fine-tuning of interfacial adhesion between fillers and polymer matrices is crucial to the separation performance of mixed matrix membranes (MMMs) and their mechanical properties. Direct incorporation of metal-organic framework (MOF) particles in polymers often leads to insufficient adhesion and non-selective interfacial voids. Herein, we present a novel strategy to develop toughened MOF-polymer interface by confining a room temperature ionic liquid (IL) [bmim][Tf2N] into ZIF-8 cages. The diffusion of the IL out of the IL-incorporated ZIF-8 (IL@ZIF-8) was restrained by a water-washing process, confirmed by energy-dispersive X-ray spectra. Scanning electron microscopy images and differential scanning calorimetry measurements verified the interfacial toughening between the polymer and the IL@ZIF-8 filler. As a result, the mechanical properties and gas separation performance of IL@ZIF-8/Pebax membranes were significantly improved at the same time. The maximum tensile strength and elongation at break of IL@ZIF-8/Pebax membranes, obtained at a loading of 15wt%, were 20% and 280% higher than those of pure Pebax membrane, respectively. Moreover, IL@ZIF-8/Pebax membranes showed improved molecular sieving properties compared to ZIF-8/Pebax membranes, due to a stiffer interphase between the filler and the polymer and a reduction in the effective aperture size of ZIF-8. The MMM with 15wt% IL@ZIF-8 has a substantial improvement in both CO2 permeability (increased by 45%) and selectivity (increased by 74% for CO2/N2 and 92% for CO2/CH4) over pure Pebax membrane. The presented strategy of fabricating IL@MOF/polymer membranes with desired interfacial structures can be conveniently expanded to a wide range of material combinations.