For example, membranes described in U.S. Pat. No. 4,073,733 are prepared from a casting solution of PVA polymers belonging to the group of vinyl alcohol homopolymers having an average degree of polymerization in the range of 500 to 3,500 and a degree of saponification in the range of 85 to 100 mole percent; PVA copolymers containing less than 30 (10) mole percent of such monomers as ethylene, viny