Reverse osmosis membrane quenching

A process for preparing permselective membranes comprising the steps of casting a film or hollow fiber from a solution including:

• (a) synthetic organic nitrogen-linked aromatic polymer represented by the formula:
• --(LR).sub.2 --
• where
• (1) each L independently is a divalent linking group of the formula --(D.sub.i E.sub.j D.sub.k E.sub.l D.sub.m)-- wherein
• (i) D is ##STR9## and E is ##STR10## or vice versa:
• each X independently is O or S;
• each Z independently is H, lower alkyl, or phenyl, provided that at least about 1/4 of the Z's in the polymer are H;
• and all non-terminal ##STR11## occur in pairs;
• (ii) i and j each represent the numerals 1 or 2, k, l, and m each represent the numerals 0, 1, or 2;
• provided that if l=0;
• then m=0;
• and if k=0, then l=0;
• and further that i+j+k+l+m.ltoreq.8;
• (2) each R independently is a divalent organic radical, both of whose terminal atoms are carbon atoms, at least about 1/2 of all such terminal atoms bonded to ##STR12## and at least about 2/3 of all such terminal atoms bonded to ##STR13## and the L links being members of aromatic nuclei;
• and such that (i) (N.sub.R)/(s) is less than about 10,
• (ii) f.sub.M is less than about 1/5, and
• (iii) P.I. is less than about 1/500,
• where:
• N.sub.R =N.sub.R .degree.-10N.sub.1 --N.sub.H
• N.sub.R .degree.=(number of atoms in R, exclusive of H-atoms) 50
• N.sub.1 =number of ionic groups in R
• N.sub.H =number of H-bonding units contributed by polar groups in R
• N.sub.R =average value of N.sub.R for the polymer,
• s=1/2[(number of ##STR14## groups in L)+1] ##EQU2## (3) n is an integer sufficiently large to provide film-forming molecular weight;
• and
• (4) the polymer has a solubility of at least about 10% by weight in a medium consisting of 0-3% by weight of lithium chloride in a solvent selected from the group consisting of dimethylacetamide, dimethyl sulfoxide, N-methyl pyrrolidone, hexamethyl phosphoramide, and mixtures thereof at 25.degree. C.;
• and (b) organic solvent for the polymer;
• contacting the cast film or hollow fiber with an aqueous quenching liquid for a period of about from 5 seconds to 15 minutes, the quenching liquid comprising about from 5 to 50 weight percent of at least one polar liquid selected from the group consisting of carbonyl compounds of the general formula ##STR15## wherein A is H, NH.sub.2, CH.sub.3, or CH.sub.2 CH.sub.3 and B is OH, NH.sub.2, or CH.sub.3 ;
• and then immersing the quenched film or hollow fiber in water for a time necessary to extract substantially all of the polar liquid from the film or hollow fiber.