NOVELTY - Preparation of a composite membrane (I) comprises: forming a blend of poly(aryl ether ketone) polymer with a polyimide; shaping the blend; optionally annealing obtained shaped substrate; contacting the substrate with a primary amine to decompose the polyimide into low molecular weight fragments while functionalizing the poly(aryl ether ketone) with the primary amine; removing the low molecular weight fragments; washing the obtained porous poly(aryl ether ketone) substrate; drying; and grafting the substrate with a perfluorohydrocarbon, for forming (I).    USE - (I) is useful for separating a fluid mixture into a fraction enriched in a first component and a fraction depleted in the first component, where the method is a micro filtration, an ultrafiltration, a nanofiltration, a membrane distillation or a gas separation process; and the fluid mixture is a hydraulic fluid, a jet fuel, a bio-diesel fluid or transformer oil (all claimed). (I) is useful as membrane contactors for membrane distillation and for degassing and dewatering of fluids. (I) is useful: for filtration and purification processes, such as filtration of waste water, preparation of ultra pure water and in medical, pharmaceutical or food applications, including removal of microorganisms, dialyses and protein filtration; for separating components of liquid mixtures; and in gas separations such as natural gas treatment, gas dehydration, and hydrogen recovery from petrochemical and refinery streams. (I) is useful for removal of dissolved gases from liquids and for dehydration of liquids.    ADVANTAGE - (I) is a chemically resistant substrate and has a uniform narrow pore size distribution with pore diameter below 100 nm. The process is simple, cost-effective and industrially feasible for the preparation of chemically and thermally durable composite membranes. (I) can be operated at high temperatures.    DETAILED DESCRIPTION - Preparation of a composite membrane (I) comprises: (a) forming a blend of poly(aryl ether ketone) polymer with a polyimide; (b) shaping the blend to form a shaped substrate; (c) optionally annealing the shaped substrate; (d) bringing the shaped substrate into contact with a primary amine to decompose the polyimide into low molecular weight fragments while functionalizing the poly(aryl ether ketone) with the primary amine; (e) removing the low molecular weight fragments from the substrate to form a porous poly(aryl ether ketone); (f) washing the porous poly(aryl ether ketone) substrate; (g) drying the porous poly(aryl ether ketone) substrate; and (h) grafting the porous poly(aryl ether ketone) substrate with a perfluorohydrocarbon, for forming (I). INDEPENDENT CLAIMS are included for:    (1) preparation of (I) comprising steps (a)-(c), bringing the shaped substrate into contact with a primary amine to decompose the polyimide into low molecular weight fragments without functionalizing of the poly (aryl ether ketone) with the primary amine, steps (e)-(g), functionalizing the surface of the porous poly(aryl ether ketone) substrate with reactive functional groups to form a modified poly(aryl ether ketone) substrate, and grafting the modified poly(aryl ether ketone)substrate with a perfluorohydrocarbon, for forming (I); and    (2) separating a fluid mixture into a fraction enriched in a first component and a fraction depleted in the first component, comprising contacting the fluid mixture with a fluid separation membrane while maintaining partial pressure differential across the membrane, the membrane having been prepared by a process which includes forming a perfluorohydrocarbon layer on a surface of a porous poly(aryl ether ketone) substrate, where the fraction enriched in the first component and the fraction depleted in the first component are generated by preferentially permeating a portion of the fluid mixture through the fluid separation membrane.