Membranes are used to separate fluids for the production of oxygen-enriched air, nitrogen-enriched-air, for the separation of carbon dioxide from hydrocarbons, and the separation of hydrogen from various petrochemical and oil refining streams. Membranes are discussed that provide a resistance to interaction with process components or contaminants, which can lead to plasticizing of the membrane, while providing the mechanical strength required to withstand high membrane differential pressures and high process temperatures. Membranes of blended polymers are used to improve the mechanical strength of the polymers used to make separation membranes. Specifically, polyimide polymers are combined with a blend polymer that is a polyamide and/or a polyamide-imde polymer. The resulting polymer mix is used to produce various forms of high strength, chemically resistant membranes, including hollow-fiber membranes that are suitable for high pressure, high temperature applications.