Permselective composite membrane having improved gas permeability and selectivity

A permselective composite membrane which comprises:

• a porous support made of a polymer selected from the group consisting of an aromatic polysulfone having recurring units of the formula (1) and an organic polyester sulfone having recurring units of the formula (2) ##STR9## and a ultrathin film of a polymer selected from the group consisting of polyacetylenes having recurring units of the general formula (3) and (4), respectively, ##STR10## wherein R.sub.1 represents a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms, R.sub.2 represents --(CH.sub.2).sub.m --CH.sub.3 wherein m represents an integer of from 0 to 3, R.sub.3 represents a halogen atom, a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms, and R.sub.4 represents a phenyl group or an alkyl group having from 3 to 6 carbon atoms;
• and
• an interface layer formed between said porous support and said ultrathin film whereby gas separation is facilitated by the presence of the interface layer, said ultrathin film being deposited on said porous support by a method wherein said polymer is dissolved in a solvent, spread over the surface of water, allowed to stand to evaporate the solvent thereby forming a thin film, and the thin film is taken up by the porous support whereby said interface layer is formed between said porous support and said ultrathin film.