example alternatively this invention is useful for removing carbon dioxide and other acid gases such as hydrogen sulfide from raw natural gas to produce natural gas of pipeline quality in this method the membrane may comprise a composite hollow fiber membrane that comprises a supporting core layer and a fluid separating sheath layer the polymer blends of the current invention are uniform and can be formed from the component polymers in conventional ways for example the component polymers can be synthesized separately and mixed in solution by dissolving each copolyimide in one or more suitable solvents if the blend is solvent mixed the solution can be stored or used directly in subsequent membrane fabrication steps or the solvent can be removed to provide a solid blend for later use homogeneity of the dry i e solvent free blend either before or after membrane formation can be checked by detecting only a single compositional dependent glass transition temperature lying between the glass transition temperatures of the constituent components differential scanning calorimetry and dynamic mechanical analysis can be used to measure glass transition temperature one preferred embodiment of the blend is formed by dissolving the component polymers in separate solutions combining the solutions and agitating the combined solutions to obtain a dissolved blend mild heating to temperatures in the range of about 50 to 100 u00b0 c can optionally be used to accelerate.