TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The gases are separated from natural gas and comprise at least one of carbon dioxide, hydrogen, oxygen, nitrogen, water vapor, hydrogen sulfide or helium gas. Preferred Components: The gases are volatile organic compounds comprising toluene, xylene or acetone. The gases comprise a mixture of carbon dioxide and at least one of hydrogen, flue gas or natural gas. The gases are a mixture of olefins and paraffins or iso and normal paraffins. The gases comprise a mixture of nitrogen and oxygen, carbon dioxide and methane, hydrogen and methane or carbon monoxide, helium and methane.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The fluorinated ethylene-propylene copolymer comprises many first repeating units of substituted hydrofluorocarbon compounds of formula ((-C(F)(CF3)-C(H)(H))n-(-C(F)(F)-C(H)(H)-)m) (I).n, m = 100-20000.The fluorinated ethylene-propylene copolymer further comprises structural units derived from other monomers. The other monomers comprise hexafluoropropene. The second polymer is polyethersulfone, sulfonated polyethersulfone, cellulosic polymers, polyamide, polyimide, poly(arylene oxide), PVC, poly(vinyl fluoride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinyl alcohol), and/or polymers of intrinsic microporosity. The second polymer is a cellulosic polymer comprising cellulose acetate or cellulose triacetate. The second polymer is poly(arylene oxide) comprising poly(phenylene oxide) or poly(xylene oxide). The fluorinated ethylene-propylene copolymer comprises 20-99 mol.% 2,3,3,3-tetrafluoropropene-based structural units and 1-80 mol.% vinylidene fluoride-based structural units. The weight ratio of the fluorinated ethylene-propylene copolymer to the second polymer in the polymeric blend membrane is 1:20-20:1, preferably 1:10-10:1. The second polymer is cellulose acetate. The polymeric blend membrane comprises fluorinated ethylene-propylene copolymer comprising 70-90 mol.% 2,3,3,3-tetrafluoro-propene-based structural units and 10-30 mol.% vinylidene fluoride-based structural units. EXAMPLE - 6.86 g Cellulose acetate polymer and 1.72 g fluorinated ethylene-propylene polymer (comprised of 100 ml degassed deionized water, 2.112 g sodium monohydrogen phosphate heptahydrate, 0.574 g monosodium phosphate, and 2.014 g perfluorooctanoate-amine, 0.3068 g ammonia, 111.3 g 2,3,3,3-tetrafluoropropene, 11.8 g vinylidene fluoride, 0.2942 g sodium metabisulfite, 91.1 mol.% 2,3,3,3-tetrafluoropropene and 8.9 mol.% vinylidene fluoride) comprising 90 mol.% 2,3,3,3-tetrafluoropropene-based structural units and 10 mol.% vinylidene fluoride-based structural units were dissolved in 28.7 g acetone. The resulting mixture was stirred for 2 hours to form homogeneous casting dope. The resulting homogeneous casting dope was filtered and allowed to degas overnight. The polymeric blend dense film membrane was prepared from the bubble free casting dope on a clean glass plate using a doctor knife with a 22-mil gap. The membrane together with the glass plate was dried at room temperature for 12 hours and was then dried at 40 degrees C under vacuum for at least 48 hours to completely remove the residual acetone solvent to form polymeric blend dense film membrane.