Hollow fiber ultrafiltration membranes with microstructured inner skin

Hollow fiber membranes with microstructured inner surfaces were fabricated from a PES/PVP blend using a spinneret with a microstructured needle. The effect of spinning parameters such as polymer dope flow rate, bore liquid flowrate, air gap and take-up speed on the microstructure and shape of the bore and its deformation was investigated. It was found that when a high bore liquid flowrate was used, the microstructure in the bore surface was destroyed. The bores were deformed to an oval shape when the fiber walls were thick. This was attributed to buckling of the fiber shell as a result of the coagulation and shrinkage of the outer surface. Fibers were also fabricated with a round-needled spinneret for comparison. The intrinsic pure water permeabilities (based on the actual bore surface areas) of fibers with structured and round bores were found to be similar. On the other hand, the structured fibers have larger pores in the skin layer. Smaller pores on the round fibers are considered to form when the inner surface coagulates and the skin layer is pulled inwards due to the shrinkage caused by phase separation. When the bore is structured, the wavy shape can damp this contraction effect resulting in larger pores. The skin layer thickness of the fibers was investigated using a colloidal filtration method. It was shown that fibers with microstructured bores which have mostly uniform skin layer thickness and reasonably narrow pore size distribution can be fabricated.