Porosity and pore size determination in polysulfone hollow fibers

Average pore sizes and effective porosity of microporous polysulfone hollow fibers were determined by the gas permeability method. Surface structure and porosity were determined by scanning electron microscopy. The values of effective porosity ε/q² (porosity/tortuosity factor) are approximately one order of magnitude lower than those reported previously for flat sheet porous membrane. These lower values are a direct outcome of a higher polymer concentration in the spinning dope. Correlations between the wall void volume, equivalent pore size, and hydraulic permeabilities of the hollow fibers have been determined. Rather low values of ε/q² have been calculated compared to those of the void volume; these effective porosity values indicate either a very high tortuosity factor or a large number of “dead end” pores. Exposure of the fibers to elevated temperature (110°C) for a short period of time drastically reduces the surface pore size and narrows the pore size distributions, whereas overall fiber dimensions are reduced only by 1%, and 85% of the fiber's hydraulic permeability is retained. The scanning electron microscopy study reveals the formation of a relatively dense skin in some spun fibers. For such “skinned” fibers, kinetic (permeability) evaluation of static structure such as mean pore size is not realistic and is further generalized to the term “equivalent pore size.”