We describe high-performance, dual-phase, all-polymeric membranes that simultaneously exhibit improved CO2 permeability and CO2/N-2 selectivity. The membranes are fabricated by incorporating the graft copolymer poly (2-[ 3-(2H-benzotriazol-2-yl)-4-hydroxyphenyl] ethyl methacrylate)-graft-poly(oxyethylene methacrylate) (PBE) in a poly(amide-b-ether) block copolymer (Pebax) matrix. The amphiphilic PBE filler selectively interacts with Pebax, resulting in the formation of interconnected CO2-philic network channels within the matrix. The membranes exhibit a dual-phase, microphase-separated morphology, and good thermal/mechanical properties, as confirmed using transmission electron microscopy, differential scanning calorimetry, X-ray diffraction analysis, thermogravimetry analysis, and measurements performed on a universal testing machine. The CO2 and N-2 permeabilities increase with the PBE content, with the maximum selectivity being observed at a PBE content of 5 wt%. The improved performance is attributable to the increased CO2 solubility owing to the efficient clustering of CO2-philic groups such as ether oxygens and triazole groups resulting from the dual-phase, interconnected morphology. The best performance corresponded to a CO2 permeability of 175.3 Barrer and a CO2/N-2 selectivity of 48.2, which are much higher than those of neat Pebax (CO2 permeability of 103.3 Barrer and selectivity of 31.2) and approach the 2008 Robeson upper limit. Thus, these all-polymeric membranes are suitable for the commercialization of gas separation owing to their simple fabrication method and low cost.