The second‐generation polysulfone gas‐separation membrane. I. The use of lewis acid: Base complexes as transient templates to increase free volume

Abstract

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.

Functionalized polysulfones from 4,4‐bis(4‐hydroxyphenyl)pentanoic acid, 2,2‐isopropylidenediphenol and bis(4‐chlorophenyl) sulfone: synthesis, behaviour and polymer analogous amidation of the carboxylic groups

“…After dissolving the polysulfone in THF, it was reprecipitated in 250 mL EtzO and finally dried for 3 days at 90 "C under reduced pressure. The analytical data are summarized in Tab = 6,96 (d,7,8 Hz);2: 7,80 (d,7,5 Hz);3: 6,89 (d,6,5 Hz);4: 7,19 (d,7,4 Hz);5: 1,60;6 1,96;7: 2,43;8: 5,79;9: 3,15;1 0 1,39;11: 1,27;1 2 0,84. I3C NMR (62,9 MHz, 100 mg 5a in 0,5 mL CDCI, at 25 "C, assignment of the carbon-atoms: seeFig.2b);1:6= 161,61;2117,54;3:129,44,4:135,11;5:152,69;6119,61;7:128,74;8:148,18; 9: 45,07; 1 0 27,47; 11: 31,37; 12: 32,04; 13: 172,49; 1 4 39,07; 15: 31,37; 16: 19,81; 17: 13,53 Preparation of membranes 0,l g of the polycondensate 4a, 4b or 4c was dissolved in 10 mL DMF, cast on a glass plate (diameter: 9 cm) and dried for 2 h at 80°C.…”

Membrane formation with presence of Lewis acid–base complexes in polymer solution

“…When the propionic acid (PA)/N-methyl-2-pyrrolidone (NMP) molar ratio increased, the O 2 permeability did also whereas the O 2 /N 2 selectivity stayed rather constant. 7 However, in the case of He/N 2 , when increasing the PA/NMP molar ratio, the selectivity increased and the He permeability diminished. 5 Besides the solvent molecule size, these results are a clear indication that other factors may also affect the free volume and permeability of the membrane.…”

Fabrication of Hollow‐Fiber Membranes by Phase Inversion

“…Under these circumstances, the humidity effects on membrane separation performance may be negligible. The molecular sizes and solubility parameters of solvents and internal and external coagulants play important roles on membrane morphology (Kesting et al, 1990;Kesting and Fritzsche, 1993). Large-size solvents may have difficulties to leach out during the precipitation.…”

“…Other mechanisms such as Marangoni effects (Levich and Krylov, 1969;Shojaie et al, 1994), osmosis pressure (McKelvey and Koros, 1996), and moisture effects (Menut et al, 2002;Tsai et al, 2005;Khare et al, 2005) have also been proposed. Ways to reduce macrovoids or modify the phase-inversion conditions to yield a spongelike structure have been studied; they may consist of (1) using high polymer concentration solutions (Kesting et al, 1990;Kesting and Fritzsche, 1993), (2) using high viscosity spinning solutions (Chung et al, 1994), (3) spinning at high shear rates (Ren et al, 2002), (4) the induction of delayed demixing (Kim et al, 2001) or gelation (Lin et al, 2002), (5) the addition of surfactants (Tsai et al, 2001), (6) the addition of high-viscosity components , or (7) the reduction of air gap distance and flow channel thickness in the spinneret (Widjojo and Chung, 2006).…”