a composite membrane for separating a gas from a mixed gas stream includes a fibrous non - woven substrate including consolidated synthetic thermoplastic fibers , and coextensively disposed on a surface of the fibrous non - woven substrate a continuous polysulfide rubber film adhered thereto . a method of separating a gas component from a mixed gas stream includes 1 ) contacting a surface of the above - described composite membrane with the mixed gas stream under conditions such that a product gas enriched in the gas component diffuses through the composite membrane ; and 2 ) collecting the product gas .