Method for the casting of ultrathin polymer membranes

Claim 1

In the process of casting a polymer film wherein a quantity of casting solution comprising polymer dissolved in a solvent system is deposited in a narrow, longitudinally-extending defined region on the surface of a liquid casting substrate, both the surface area and perimeter of said defined region are enlarged over the surface of said casting substrate in a direction transverse to the longitudinal direction of the original defined region and said quantity of casting solution is converted to a thin desolvated, solid polymer film covering an area much larger than the area of said defined region, the improvement comprising the steps of: (a) employing a casting solution able to spontaneously spread over the liquid casting substrate, the polymer therein being insoluble in and not subject to swelling by the liquid of said casting substrate, (b) controllably enlarging both the surface area and perimeter of said defined region over the surface of said casting substrate at such a rate relative to the volatility of the solvent system that only casting solution moves relative to said casting substrate during such enlargement, the solid polymer film being formed from the casting solution upon desolvation thereof and remaining stationary relative to said casting substrate and (c) removing at least a portion of the solidified film from the surface of said casting substrate, said portion of solidified film being substantially free of applied tensile stress during the formation thereof and having a surface area of at least about one square foot.