A series of poly(urethane-urea) (PUU) membranes based on contributing hard segments in gas separation were prepared using two novel bulky chain extenders, polytetramethylene-glycol, isophorone and hexamethylene-diisocyanate. The chain extenders were synthesized by cyanuric chloride, aniline/naphthylamine and hydrazine. According to FTIR, AFM and DSC, bulkier PUU membranes showed more phase separation and lower glass transition temperature consistent with higher fractional free volume, which were correlated with single (CO2, CH4, O-2 and N-2) and mixed (CO2:N-2 and CO2:CH4) gas permeation results. The main findings indicate these polyurethanes serve efficiently designed molecular structures favoring bulky hard segments contribution in gas separation. The bulky PUU membranes afford simultaneous increased permeability and selectivity, while maintaining high tensile properties. Enhanced O-2/N-2 diffusivity selectivity by bulkier PUU and CO2 sorption/desorption study by Ellipsometry confirmed hard segments' association, too. The bulkiest membrane provided 126% and 54% improvement for single CO2 permeability and CO2/N-2 selectivity compared to linear one.