Effect of solidification rate of polymer solution on the die-swell during hollow fiber spinning by non-solvent induced phase separation

• Viscoelasticity and solidification rate of polymer solution were measured.
• We examined the effects of solidification and rheological properties on die swell.
• Solidification by inner solution in air gap significantly affected die-swell.

The diameter of the hollow fiber membrane is of great importance in relation to space efficiency in a membrane module. The die-swell effect is one of the major contributors to the membrane diameter during nonsolvent induced phase separation (NIPS) by wet spinning. In this study, we investigate the effects of viscoelasticity and solidification rate of the polymer solution on the die-swell effect and diameter of hollow fiber membrane during NIPS in the spinning process. We characterized the rheological properties and solidification rates of the polymer solutions during phase separation. The die swell in hollow fiber spinning and the cross-sections of the prepared hollow fiber membranes were then determined. From the results, the outer and inner diameters of the hollow fiber membranes increased and decreased due to die swell, respectively, with increasing first normal stress difference. Furthermore, decreasing solidification rate of the polymer solution enhanced die swell and significantly affected the outer and inner diameters of the prepared hollow fiber membrane. Surprisingly, the solidification rate of the inner solution influenced die swell and significantly affected not only the inner diameter but also the outer diameter.