Development of asymmetric BTDA-TDI/MDI (P84) co-polyimide hollow fiber membranes for ultrafiltration: the influence of shear rate and approaching ratio on membrane morphology and performance

In this paper, the effects of dope composition (especially approaching ratio α), rheology and spinning conditions on the morphology and performance of asymmetric BTDA-TDI/MDI co-polyimide (P84, CAS#: 58698-66-1) hollow fiber membranes spun from P84/N-methyl-2-pyrrolidone (NMP, CAS#: 872-50-4)/H2O dope solutions were studied. The velocity distribution and rheology of the dope solution within the spinneret were analyzed. The experimental results indicated that the defects on the outer surface of hollow fiber membranes were suppressed by increasing the shear rate of the dope solution within the spinneret. Therefore, membranes with a low flux and a high rejection could be obtained. By changing in shear rate, the hollow fiber membranes ranging from ∼20,000 to >200,000 MWCO can be obtained within a single formulation (such as 21 wt.% P84; H2O/NMP: 3.6/96.4 (w/w). An increase of shear rate also increased the outer diameter and thickness of the hollow fiber membranes. The approaching ratio α of the dope solution played an important role in determining the membrane structure. The hollow fiber membranes spun at α = 0.45 (21 wt.% P84; H2O/NMP: 3.6/96.4 (w/w) had a finger-like structure, but sponge-like structure was obtained for α = 0.82 (21 wt.% P84; H2O/NMP: 6.4/93.6 (w/w). An increase of approaching ratio α resulted in a poor rejection and a high flux. With a decrease in the volume ratio of dope solution to bore fluid, the membrane became thinner and the flux increased. Compared to the dry-jet wet spinning process, the hollow fiber membranes from a wet spinning process had a high flux, a large outer diameter and a poor rejection. For a dry-jet wet spinning process, the increase in the air gap decreased the rejection of the hollow fiber membranes.