provided is a composition for use in fabricating a carbon molecular sieve membrane , including a fluorine - containing polymer matrix and polysilsesquioxane . the composition shows high selectivity to the gas to be separated and high separation quality by controlling the mixing ratio of the fluorine - containing polymer matrix with polysilsesquioxane as well as the type of fluorine - containing polymer matrix and polysilsesquioxane . ancillary selective pore formation is enhanced by a so - called “ autogenous fluorinated gas induced siloxane etching ” mechanism of fluorine - containing polymer / polysilsesquioxane blend precursors during carbonization . therefore , it is possible to effectively separate gases having a small difference in particle size , which , otherwise , are difficult to be separated with the conventional polymer membranes .