SynopsisThe 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 permeability 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 emergence of the trilayer morphology wm the discovery of a hitherto unsuspected relationship between the size of solvent mokcules 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)'". 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.

SynopsisAll integrally skinned asymmetric membranes contain some defects which are attributable to the incomplete coalescence of the nodule aggregates of which the skin layer is composed. When such defects are small in size and few in number, they can be effectively sealed by coating with a highly permeable polymer. The resulting composite then exhibits the selectivity to gas permeation which is characteristic of the base polymer. Prior to their sealing, therefore, such membranes can be said to exhibit the potential for intrinsic selectivity. However, not all gas separation membranes can be effectively sealed. In the present study the relationship between sol properties, the presence of macrovoids in the substructure of the gel, and the subsequent failure of the fibers to achieve the potential for intrinsic selectivity are considered. Macrovoid-free fibers with the potential for intrinsic selectivity can be prepared by the utilization of high viscosity, high total solids sols with low nonsolvent tolerance whose solvent vehicles consist of appropriate Lewis acid base complexes.

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SynopsisOxygen plasma ablation has been used to define the structure of polysulfone hollow fiber membranes spun from Lewis acid:base complex solvents in which the molar ratio of propionic acid to N-methylpyrrolidone was varied. I t was found that the helium/nitrogen separation factor of the unetched samples increases with increasing PA/NMP molar ratios. This increase implies a decrease in surface porosity of the resultant hollow fiber as larger fractions of the total NMP in the solvent are complexed with propionic acid. The results also suggest that the differences between the outer separating layer and the supporting matrix increase with increases in the PA/NMP molar ratios. Therefore, a more rapid transition from the separating layer to the porous supporting matrix exists in hollow fiber membranes spun from the higher PA/NMP ratios.

SynopsisThe structures of polysulfone hollow fiber membranes, spun from the propionic acid : N-methylpyrrolidone complex and from a formylpiperidine/formamide mixture were investigated as a function of progressive surface removal with an oxygen plasma. Oxygen plasma ablation experiments were performed on both unexposed and isopentane-treated hollow fiber membranes. Pure gas permeation rates were obtained on these samples as well as oxygen plasma etched samples which were then subsequently coated with polydimethyl-siloxane from an isopentane solution. The results show that the hollow fiber membrane spun from the propionic acid : N-methylpyrrolidone complex has both a thinner active separating layer and a thinner skin than the polysulfone hollow fiber membrane spun from the formylpiperidine/formamide mixture. Also, the resistance to flow of the porous substrate of the complex spun hollow fiber membrane is significantly less than that of the polysulfone hollow fiber membrane spun from the mixture. Therefore, the substrate of the PA : NMP complex spun hollow fiber membrane has greater porosity and less tortuosity than its FP/FA congener. The oxygen plasma ablation results and the scanning electron micrographs demonstrate a nonequivalence between the active separating layer and the microscopically observable skin of the hollow fiber membrane. It is believed that membranes prepared from Lewis acid : base complex solvents possess a porous substructure and a nonuniform (graded-density) skin which consists of a very thin active separating layer whose effective thickness varies depending upon the gases to be separated and a thin less dense transition layer, which may contain pores whose sizes are below the limits of resolution by SEM. Both are components of the microscopically observable skin. Membranes possessing this structure belong to the trilayer class of integrally skinned membranes. If membranes are so fabricated that the density gradient in the active separating layer approaches zero, a bilayer membrane with a porous substructure and a thin skin of uniform density results. Membranes prepared from conventional solvent/nonsolvent mixtures, i.e., formylpiperidine/formamide, exhibit a diminished density gradient in the skin approximating the bilayer model.