Permeation modified asymmetric gas separation membranes having graded density skins

A permeation modified asymmetric gas separation membrane having a graded density skin and macrovoid-free morphology comprised of glassy, hydrophobic polymers, the membrane having increased free volume as evidenced by the membrane first heat T.sub.g which is greater than the T.sub.g of a bulk sample of the glassy, hydrophobic polymers, the membrane having been contacted on the graded density skin with an effective amount of a permeation modifier and when combined with a coating in contact with the graded density skin exhibits a separation factor greater than the intrinsic separation factor of a dense membrane of the bulk sample of the polymers for one gas of a gaseous mixture.

A permeation modified asymmetric gas separation membrane comprised of a coating in occluding contact with a graded density skin of an asymmetric membrane which has macrovoid-free morphology and is comprised of glassy, hydrophobic polymers, the graded density skin being characterized as having an active layer and a transition layer which communicates with the active layer and a porous substrate of the membrane, wherein all three provide increased free volume to the membrane as evidenced by the membrane first heat T.sub.g which is greater than the T.sub.g of a bulk sample of the glassy, hydrophobic polymers, the membrane having been contacted on the graded density skin with an effective amount of a permeation modifier, the permeation modified membrane exhibits a separation factor greater than the determined intrinsic separation factor of a dense membrane of the bulk sample of the polymers for one gas of a gaseous mixture and an increase in permeability for the one gas of the gaseous mixture when compared to unmodified asymmetric membranes of the same polymers having uniform skins and coated with the same coating materials.