Effects of the thermodynamics and rheology of BTDA-TDI/MDI co-polyimide (P84) dope solutions on the performance and morphology of hollow fiber UF membranes

In this work, hollow fiber UF membranes were prepared using BTDA-TDI/MDI co-polyimide (P84, synthesized by the condensation of 2,4-diisocyanato-1-methylbenzene and 1-1′-methylenebis[4-isocyanatobenzene] with 5-5′-carbonylbis[1,3-isobenzofurandione])/N-methyl-2-pyrrolidone (NMP)/acetone/H2O/LiCl/polyoxyethylenesorbitan monooleate (Tween80) dope solutions with two approaching ratios (α = 0, 0.2). The effects of the thermodynamic characteristic (described with the approaching ratio α) of the dope solution, the rheological property (described with shear rate) of the dope solution within the spinneret and other spinning conditions such as air gap, the volume ratio of the dope solution to the bore fluid (ω) on the structure, morphology and performance of P84 hollow fiber membranes were studied.The hollow fiber membranes spun with the approaching ratio α = 0.2 was found to have a high permeation flux and a large molecular weight cut-off (MWCO) due to the existence of water in the dope solution compared with those spun with the approaching ratio α = 0. The performance of the hollow fiber membranes was dominated by the shear rate of the dope solution within the spinneret according to shear induced molecular orientation. With an increase in shear rate, a low permeation flux and a small MWCO were achieved, and the pore size distribution curve of the membrane shifted to the left, indicating the membrane exhibited a good selectivity. The transition of the spinning process from the wet spinning to the dry-jet wet spinning strongly influenced the performance of the hollow fiber membranes, and a relative low permeation flux and a small MWCO could be obtained when the air gap was not less than 1 cm. With a decrease in the volume ratio of the dope solution to the bore fluid (ω), the membrane thickness became thinner, resulting in the increase of permeation flux and the MWCO.