Development and positron annihilation spectroscopy (PAS) characterization of polyamide imide (PAI)-polyethersulfone (PES) based defect-free dual-layer hollow fiber membranes with an ultrathin dense-selective layer for gas separation

► Defect-free dual-layer hollow fiber membranes with ultrathin dense-selective layer. ► O2/N2 selectivity of 7.73 with a dense-selective layer thickness of 63 nm. ► PAS to explore the morphology and predict the gas performance. ► PAS estimate the outer-layer thickness and demonstrate the layered structure. ► PAS fitted data agrees well with the experimental result.