Suitable polymeric materials include cellulose esters such as mono-, di- and triacylates including mixed esters, cellulose ethers such as ethyl cellulose, poly(dialkylsiloxanes), poly(methacrylic acid) esters, poly(acrylic acid) esters, poly(phenylene oxides), poly(vinyl alcohols), aromatic nitrogen-containing polymers, regenerated cellulose and other membrane-forming materials suitable for use in