NOVELTY - Preparing a polymeric material comprises: providing a polymeric matrix comprising at least one polymer and at least one porogen; and degrading the porogen at a temperature (T) of less than or equal to 1.1 Tg, where Tg is a glass transition temperature of the polymeric matrix, where (a) the degrading step comprising exposing the polymeric matrix to at least one treatment is thermal degradation, chemical degradation, electrical degradation or radiation degradation.    USE - The method is useful for preparing polymeric material, which is used in gas separation membranes for separating components of fluids (all claimed).    ADVANTAGE - The method: provides membranes having increased free volume and decreased pore size distribution, thus exhibiting improved permeability and selectivity; provides membranes having enhanced temperature resistance, and high resistance to liquid exposure; and prevents fouling while improving membrane durability.    DETAILED DESCRIPTION - Preparing a polymeric material comprises: providing a polymeric matrix comprising at least one polymer and at least one porogen; and degrading the porogen at a temperature (T) of less than or equal to 1.1 Tg, where Tg is a glass transition temperature of the polymeric matrix, where (a) the degrading step comprising exposing the polymeric matrix to at least one treatment is thermal degradation, chemical degradation, electrical degradation or radiation degradation, (b) the polymeric material has a permeability at least 1.2 times a permeability of the polymeric matrix for a gas, and a selectivity of the polymeric material is at least 0.35 times a selectivity of the polymeric matrix for a gas pair, and (c) the polymeric material is a gas separation membrane, which exceeds Robeson's upper bound relationship for at least one gas separation pair. INDEPENDENT CLAIMS are also included for:    (1) a polymeric material prepared by the method, and adapted for use as a gas separation membrane;    (2) a gas separation membrane comprising the polymeric material; and    (3) separating components of a fluid comprising (i) providing a separation device comprising the polymeric material as a separation membrane, (ii) feeding a feed fluid to the separation device, where the feed fluid comprises a mixture of a first fluid and at least one second fluid, and (iii) collecting a product from the separation device, where the product contains the first fluid at a higher purity than the feed fluid.