The preparation of membrances

A membrane is defined as a solid, porous sheet which, when placed between two phases, may allow the passage of small particles but hinder or prevent the passage of large particles. With this definition as a basis a review is given of methods employed to prepare membranes, with special emphasis on the preparation of organic membranes. Directions are then given for the preparation of thin, uniform membranes 100 to 1500 A. or thicker. Membranes of 500 or 600 A., in thickness with a variation of less than 25 A. across a section as large as 3.5 cm. sq. are described, together with evidence for physical and chemical stability. These are prepared from pyroxylin. Thin membranes were also prepared from Formvar 15/95 E (polyvinylformal from Shawinigan Resins Corp.) and VYDR, blend 7 (a high molecular weight vinylchloride‐acetate resin containing approximately 96% vinylchloride and 4% vinylacetate, from Bakelite). No success was achieved with DYNH (a polythylene from Bakelite); and polyisobutylmethacrylate (Monomer‐Polymer).