Polyaryl ether sulfone semipermeable membrane and process for producing same

A polyaryl ether sulfone semipermeable membrane made of a polyaryl ether sulfone comprised of recurring units represented by the general formula (I):

• ##STR6## wherein X, X', X" and X'", which may be identical to or different from each other, are non-dissociative substituents, and l, m, n and o, which may be identical to or different from each other, are integers of from 0 to 4, which membrane has pores of a diameter increasing progressively and continuously from first and second major opposite surface layers of the membrane to the center portion thereof, which center portion is located approximately equidistant from the first and second major opposite surface layers and the membrane is comprised of a single continuous polymer phase;
• the diameter of the pores present on or in the immediate proximity of the first and second major surfaces being such that the percentage rejection of dextran molecules having an average molecular weight of 10,000 is not greater than 95%, the percentage rejection of dextran molecules having an average molecular weight of 70,000 is in the range of from 5% to 95% and the percentage rejection of dextran molecules having an average molecular weight of 500,000 is at least 5%, and;
• the average diameter of pores present in said center portion being in the range of from 0.05 to 10 microns.