Polymer-based hollow fibre composite membranes are composed of a porous support coated with a highly permeable gutter layer, and a thin selective top layer. They are promising candidates for industrial gas separation, but there are challenges in maintaining their performance under high pressure or during long-term operation. To tackle this, we incorporated ZIF-8 into Pebax-1657 (polyamide-polyether copolymer) for gas separation membrane fabrication. To fully understand the behaviour of hollow fibre composite membranes, special attention was paid to the effect of ZIF-8 on the Pebax chain structure using dense film flat sheet membranes. The presence of porous ZIF-8 disrupted the original well-packed polymer chain structure, leading to higher fractional free volume and gas permeability. However, the introduced microvoids were non continuous and the gas selectivity was only slightly reduced. On the other hand, the stable structure of ZIF8, together with the hydrogen bonds between ZIF-8 organic ligands and polyamide chains, significantly improved the linear glassy polymer chain stiffness, ensuring good operational stability under elevated pressure for both flat sheet and hollow fibre membranes. Finally, good long-term operational stability was observed with the hollow fibre membrane, indicating the ZIF-8/Pebax coating overcame the aging previously observed for the poly[1-(trimethylsilyl)-1-propyne] (PTMSP) gutter layer.