The second‐generation polysulfone (PSU) gas‐separation membrane is seen as a trilayer that is considerably more permeable and at least as selective as the first‐generation bilayer that it has replaced. In air separation, a fourfold increase in oxygen permeabiliy has been obtained with no loss in oxygen/nitrogen selectivity. The enhanced performance is the result of a membrane skin that is not only thinner, but also exhibits increased free volume and a graded density. The key to the emergency of the trilayer morphology was the discovery of a hitherto unsuspected relationship between the size of solvent molecules within a sol and the free volume and permeability in the resultant gel! Solvent molecules with a molar volume V > ˜ 147 cc/mol function as transient templates (spacers) that decrease macromolecular packing density. As a practical matter, the low diffusivity (difficult extractibility) of large solvent molecules is circumvented by the use of 1:1 Lewis acid: base (A:B) complexes such as propionic acid: N‐methyl pyrrolidone instead of neat solvents. Complexes whose acid and base strengths, respectively, lie between (Gutmann 47 < AN < 53 and 27 < DN < 28) are sufficiently stable to function as templates, while at the same time exhibiting the hydrolytic instability that leads to their ready disassociation and extraction by water. Selectivity is maintained by the use of A:B complexes whose Hildebrand solubility parameters differ from that of PSU by less than ˜ 1.3 (cal/cc)½. The emergence of the trilayer membrane is considered to be the second decoupling of permeability from selectivity. By the formation of an anisotropic (graded density) skin, permeability has been increased and selectivity maintained. This is analogous to the first decoupling by Loeb and Sourirajan who essentially replaced a thick dense monolayer film with a bilayer consisting of a thin skin of uniform density in series with a thick porous substructure. Copyright © 1990 John Wiley & Sons, Inc.
