Semipermeable membranes of copolyamides

A porous semipermeable membrane for inverse osmosis and ultra filtration of highly-concentrated salt solutions consisting essentially of a fully aromatic copolyamide of

• (A) 10 to 90% of units corresponding to the formula (I) ##STR17## in which R, R' and R" which may be the same or different represent hydrogen, C.sub.1-4 -alkyl, or halogen, and
• Ar represents an alkyl- or halogen-substituted by-valent aromatic radical or an unsubstituted aromatic radical and of
• (B) 10 to 90 mole % of units corresponding to formulae II or IV below ##STR18## in which R, R', R", R'", R.sup.IV, R.sup.V, R.sup.VI and R.sup.VII which may be the same or different represent hydrogen, C.sub.1-4 -alkyl or halogen,
• Ar is as just defined, and
• X, Y and Z which may be the same or different represent a direct bond or a bridge member corresponding to a member selected from the group consisting of ##STR19## the physical properties of said membrane being a water absorption capacity of from 4.5 to 10% by weight, measured on approximately 40.mu. thick films at room temperature and at 65% relative air humidity;
• a through-flow of at least 80 l/m.sup.2 d for a desalination level of at least 85% as measured in a pressure osmosis apparatus in which a 3.5% NaCl-solution is pumped in a circuit past the surface of the membrane at room temperature under a pressure of 110 bars at a rate of 15 l/h and consisting of a fully aromatic copolyamide with a relative viscosity of .gtoreq.1.4, as measured on a 0.5% N-methyl pyrrolidone solution at 20.degree. C.