Method for casting ultrathin methylpentene polymer membranes

An improved method is described for solvent casting ultrathin non-porous methylpentene polymer films. The casting solution comprises methylpentene polymer or a mixture thereof with organopolysiloxane-polycarbonate copolymer dissolved in a solvent system and is characterized by being able to spontaneously spread over the surface of the liquid casting substrate. In a preferred embodiment, large-area films less than 200 Angstroms in thickness may be repeatedly formed. The solidified films remain relatively free of tensile stress during formation. The casting solution is deposited in a narrow, longitudinally-extending defined region at the surface of the casting substrate. The surface area and perimeter of the defined region are enlarged over the surface of the casting substrate in a manner whereby the casting solution is moved relative to the casting substrate and, after desolvation, the polymer film formed remains stationary relative to the casting substrate and relatively free of tensile stress. The films are useful as semipermeable membranes for selectively removing oxygen gas from mixtures thereof with nitrogen gas.