The structure of cellulose acetate membranes 1. The development of porous structures in anisotropic membranes

Large cellular cavities have often been noted in cellulose acetate and other membranes prepared from solution by methods similar to those described by Loeb and his colleagues. These were heretofore thought to be gas or aqueous occlusions entrained during membrane formation, which resulted in pinholes during the pressurization of the membrane. These "bubbles" have now been shown to be aqueous intrusions which grow actively during the gelling stage of membrane formation, and are unrelated to any entrainment process. Their formation can be modified and sometimes inhibited by control of the conditions of gelation and by the use of additives in the casting solutions or gelling medium. These fingerlike formations called intrusion cells appear to be related to several effects noted in the literature for analogous systems, and which derive from hydrodynamic instabilities connected with surface tension or viscosity changes. In this preliminary note, the effect is described qualitatively and a tentative explanation given for its mechanism. It is suggested that on a sub-microscopic scale, a similar mechanism could explain the porosity of various membranes produced in a similar way.