Determination of pore sizes and surface porosity and the effect of shear stress within a spinneret on asymmetric hollow fiber membranes

Abstract

We have developed a simple approach to determine surface porosity and mean pore size of asymmetric hollow fiber membranes based on the resistance model and Poiseuille and Knudsen gas transport mechanisms. We specifically decouple Poiseuille and Knudsen flows in the calculation, because they should take place at different pore sizes according to their definitions. Elucidation was made to calculate the effect of shear stress within a spinneret on hollow fiber membrane structures in terms of the thickness of apparent dense skin layer, surface porosity, and mean pore size. Calculation results suggest that solution-diffusion, Poiseuille and Knudsen flows coexist, but pores for Knudsen flow occupy a large portion of total surface porosity. A high shear tends to eliminate surface pores of hollow fiber membranes, especially for big pores of Poiseuille flow. Hollow fiber membranes spun with high shear rates apparently have a thicker skin layer because of shear-induced chain orientation and packing. Shear reduces pores for Poiseuille and Knudsen flows through two mechanisms, namely, shear-induced chain packing and orientation, and shear-induced pore deformation and transformation. The former tightens chains and reduces pore sizes to be suitable for Knudsen flow and solution-diffusion, while the latter transforms or deforms big pores originally for Poiseuille flow to small pores suitable for Knudsen flow. We also report, for the first time, the mean pore size of the dense selective layer of hollow fibers for Knudsen flow as a function of shear rate.