Preparation and characterization of flexible composite membranes with MOFs grown inside hollow nanofiber

The flexible composite membranes materials based on metal organic frameworks (MOFs) play a significant role in many fields such as gas separation, pollutant removal and controlled drug release system. Utilizing coaxial electrospinning technology, the composite nanofibers of core-shell structure was prepared by mixing organic ligand 2-methylimidazole (2-mI) in polyvinylidene fluoride (PVDF) polymer solution as shell spinning solution, and adding zinc ion to glycerol as core spinning solution. With hydrothermal synthesis, the glycerol in the core layer dissolves to form a hollow structure, while the zinc ion diffuses outward and contacts the organic ligands on the inner surface of the polymer shell layer at the same time. The in-situ growth of ZIF-8 MOF crystals inside PVDF hollow nanofibers (HNFs) were obtained as the ZIF-8@HNFs flexible composite membrane material. The effect of different metal salt and ligand proportions, different hydrothermal growth time and temperature on the formation of ZIF-8 in HNFs were studied. The molar ratio of zinc ion and 2-mI ligand was 1:40, and the hydrothermal growth condition was at 65℃ for 4 h with water as solvent, which proved to be the optimal preparation circumstance. The structure and performance of ZIF-8@HNFs was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA) and the nitrogen adsorption-desorption test, etc. The results indicated that the ZIF-8 crystals were in-situ synthesized and loaded inside the PVDF HNFs with 3.351% content. The Brunner-Emmet-Teller measurements (BET) surface area was 38.189 m²/g, the pore volume was 0.204 cm³/g and the pore size distribution were at 4.678 nm and 7.573 nm. Furthermore, the ZIF-8@HNFs still maintains its structural stability after being treated with 200℃ high temperature, multiple bending and soaking with water or alkali solution for 4 h. The characteristic of ZIF-8 that itself can dissociate under acidic conditions, which makes the ZIF-8@HNFs flexible composite membrane material potentially valuable in areas such as contaminant adsorption-desorption and controlled drug release system.