Polyether and especially poly(ethylene oxide) (PEO) based segmented block copolymers are very well known for their high CO2 permeability combined with a high CO2/light gas selectivity, but most (commercially) available block copolymers have incomplete phase separation between the soft and hard blocks in the polymer leading to reduced performance. Here we present a polyether based segmented block copolymer system with improved phase separation behavior and gas separation performance using poly(ethylene oxide) (PEO) and/or poly(propylene oxide) (PPO) as a soft segment and short monodisperse di-amide (TΦT) as a hard segment. In this work we tune the mass transport properties of such multi-block copolymers for CO2 capture by systematically investigating the effect of the type and length of soft segment in the block copolymer at constant short hard segment. The effect of (1) the length of the PEO soft segment, (2) the type of soft segment (PPO vs. PEO) and (3) the use of a mixture of these two different types of soft segment as a method to tune the gas separation performance and its relation with the thermal-mechanical properties is investigated. The use of such a polyether based segmented block copolymer system as presented here offers a very versatile tool to tailor mass transfer and separation properties of membranes for gas and vapor separation.