Preparation of porous membranes

A new method for preparing plastic membranes with very small pores is described. The method involves dissolving two or more film‐forming polymers in a common solvent. A thin layer of the resulting solution is spread on a smooth surface and the solvent is allowed to evaporate. One of the polymers is then extracted from the resulting film with a selective solvent. The selective permeability of ethyl cellulose was altered considerably using polyethylene glycol as the second polymer. The apparatus for determining the permeability of these films is described. Pictures of the membranes at different stages in their preparation are shown. Electron micrographs demonstrating the effect of this treatment on ethyl cellulose are presented.