Combination of pyrrolidone based solvents for the production of porous membranes

A method for producing a porous membrane, the method comprising the following steps (a) to (d):

• (a) providing a casting solution, wherein the casting solution comprises polyethersulfone as a membrane-forming polymer dissolved in a solvent system comprising 2-pyrrolidone and N-n-butyl-2-pyrrolidone but not comprising dimethyl sulfoxide, wherein the casting solution further comprises at least one additive selected from the group consisting of polyvinylpyrrolidone, polyvinylpyrrolidone-co-polyvinylacetate, and polyethylene glycol;
• (b) forming a polymer film from the casting solution provided in step (a);
• (c) contacting the polymer film formed in step (b) with a liquid precipitation bath, with a gaseous phase comprising a precipitation-inducing agent or with a combination thereof to induce membrane formation, thereby obtaining a porous membrane;
• and
• (d) drying the porous membrane obtained in step (c),
• wherein the content ratio of 2-pyrrolidone to N-n-butyl-2-pyrrolidone in the solvent system is in the range of from 80%:20% to 50%:50% based on mass %.