Hollow fiber membranes spun from lewis acid : Base complexes. I. Structure determination by oxygen plasma ablation

Abstract

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.

Relationship between substructure resistance and gas separation properties of defect-free integrally skinned asymmetric membranes

“…A direct measurement of the individual resistances is, therefore, very difficult. Removal of the skin layer by plasma etching has been used extensively to characterize the resistance of the substructure of asymmetric hollow fiber membranes (Fritzsche, 1989;Fritzsche et al, 1990;Bauer et al, 1990).…”

The art of surface modification of synthetic polymeric membranes

“…Inorganic gas plasma is known to promote the implantation of atoms, radical generation, and etching reactions, and is called a nonpolymer forming plasma. It is reported that highly reactive particles from gas plasma can etch a surface very gradually 115, 116. Van't Hoff et al115, Fritzsche et al116‐118 and Weigel et al119 performed etching experiments with oxygen plasma on PES GS HFMs and on asymmetric PSf hollow fibers, respectively.…”

“…It is reported that highly reactive particles from gas plasma can etch a surface very gradually 115, 116. Van't Hoff et al115, Fritzsche et al116‐118 and Weigel et al119 performed etching experiments with oxygen plasma on PES GS HFMs and on asymmetric PSf hollow fibers, respectively. They noticed that it was possible to determine the sublayer resistance after etching the fiber.…”

Morphology of integral‐skin layers in hollow‐fiber gas‐separation membranes

“…These posttreated membranes can exhibit effectively dense film gas selectivities and ultrathin skin thicknesses. Without posttreatment, the membranes generally display unattractive gas selectivities between Knudsen values and 50% of dense film values 18, 22–35…”