Investigating the Effects of Nonsolvent Additive and Spinning Conditions on Morphology and Permeation of PAN Hollow Fiber Membranes

In this study, polyacrylonitrile hollow fiber (PAN HF) membranes were designed and fabricated with desired morphology and permeability for low pressure-driven applications via the dry-wet phase-inversion spinning process. The effects of multiple design and fabrication parameters on permeation, morphology, thickness, and pore size of membranes were investigated using various techniques. Moreover, the effects of water as the nonsolvent additive, chemical composition of the bore solution, dope solution flow rate, air gap, coagulation bath temperature, and tensile ratio (take-up speed) were investigated. The addition of 3% wt. water as the nonsolvent additive into the dope solution resulted in a seventeen-times increment in the water flux up to 495 L m-2 h-1 bar-1. It showed a significant improvement in membrane porosity compared with those prepared without water nonsolvent additive. Utilization of 90% wt. of the solvent in water solution as the bore solution instead of pure water showed the most significant effect on the water flux and membrane structure among the fabrication parameters. It resulted in a reduction in threshold permeation pressure by more than ten times. The results revealed that whichever variation, including increased dope solution flow rate or coagulation bath temperature, or reduced air gap or traction, leads to promotion of membrane water flux. Still, different effects on the structure and morphology of the membranes were observed. Based on the outcomes of this study and according to the SEM images, one can conclude that outer surface pore size reduction from about 300 nm resulted in decreased water flux from 448 to 226 L m-2 h-1 bar-1, so that no pores were observed in the outer surface until 50 K magnification of the SEM image. The findings in this study provide instructive guidelines for the design and fabrication of high-performance hydrophilic PAN-based hollow fiber membranes with the desired morphology and water flux. Best ranges of investigated parameters for relatively high permeate water flux and desired membrane morphology were reported.