Aqueous quenched asymmetric polysulfone membranes prepared by dry/wet phase separation

Abstract

A process for producing ultrathin and defect-free selecting layers on asymmetric membranes using dry/wet phase separation has been reported earlier for organic coagulation media. The present study seeks to generalize this process by demonstrating the ability to form equally thin defect-free polysulfone membrane using an aqueous coagulation medium. In addition, this study presents the fundamental issues involved in altering flat sheet casting solutions to produce solutions, potentially useful for preparing hollow fibers within, defect free selective layers. Transport properties of integrally-skinned asymmetric polysulfone membranes coagulated in water are reported for the separation of various gases of 24°C and upstream driving pressures of 50 to 100 psig. These asymmetric membranes have essentially the intrinsic permselectivities of dense polysulfone films formed by simple solvent casting techniques. The novel aspect of both the organic and the aqueous quenched structures in their ultrathin selective layers, which can be made as thin as 200 to 800 Å under optimum formation conditions without introducing defects that reduce permselectivity. Unlike other thin-skinned structures reported recently, the present membranes do not require any post-formation solvent treatments or silicone coatings to achieve essentially the intrinsic separation factors of the material. High resolution scanning electron photomicrographs of the membrane structures are presented to clarify the morphologies responsible for their desirable properties.