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Abstract

Asymmetric gas separation membranes having graded density skins and macrovoid-free morphology comprised of glassy, hydrophobic polymers which when contacted on the graded density skin surface with an effective amount of permeation modifier and a coating in occluding contact, exhibit permeation selectivities greater than the intrinsic permeation selectivities for at least one gas of a gaseous mixture than a bulk sample of the polymers. The permeation modified membranes have increased free volume as evidenced by the membrane first heat T.sub.g which is greater than the T.sub.g of the bulk sample of the glassy, hydrophobic polymers; however the permeation modified membranes having an effective amount of permeation modifier added in non-uniform mode are achieved without significant loss in the physical properties of the membrane, including T.sub.g.

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