Three styrene-butadiene copolymer (SBC)-derived membranes were prepared using different polystyrene/polybutadiene (PS/PB) ratios, including a graft copolymer with 4 wt% PB (PS-4BR) and block copolymers with 55 and 70 wt% PB (PS-55BR and PS-70BR). The SBC-derived membranes were confirmed by Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA); the CO₂/N₂ separation performances of the membranes were studied under long-term operation. The SBC-derived membranes adopted different microphase-separated structures, including spherical, lamellar and cylindrical structures for PS-4BR, PS-55BR and PS-70BR, respectively. This result indicated that the CO₂ separation performance of the SBC-derived membranes was affected by the CO₂ solubility in the PB domain and the crystalline PS domain in the microphase-separated structure. The cylindrical PS-70PB membrane that contained a γ crystalline PS domain showed a high CO₂ permeability of 50–60 Barrer, with a CO₂/N₂ selectivity of 15. Moreover, after 28 days, the cylindrical PS-70BR membrane showed greater stability in CO₂ separation performance compared to the pure PS and PB membranes due to the high stability of the microphase-separated structure.