The formation mechanism of asymmetric membranes

Aromatic polyamide asymmetric “skin type” membranes have been prepared by the Loeb-Sourirajan technique. Two different structures were obtained, depending on the rate of precipitation. Low precipitation rates produced membranes with sponge-like structures. These membranes usually had high salt rejections and low water fluxes. High precipitation rates produced membranes with large finger-like pores. These membranes had low salt rejections and high water fluxes. Possible mechanisms producing these two structures are discussed. It is proposed that high concentrations of polymer in the casting solution, and hence high viscosities at the point of precipitation, or a thick viscous sublayer in advance of the precipitating polymer front both favour sponge-structured membrane formation. The suggested mechanisms are supported by data showing the effect of various preparation parameters such as polymer concentration and casting solution and precipitation bath composition on membrane structure.