Thickness and air gap dependence of macrovoid evolution in phase-inversion asymmetric hollow fiber membranes

We have studied, for the first time, the effects of spinneret dimension and its coupling effect with air gap distance on macrovoid evolution. Spinnerets with different annulus gaps, i.e., from 0.05 to 0.50 mm, were fabricated. Hollow fiber membranes spun from thinner spinneret annulus gaps show only inward-pointed long, teardrop, and elliptical shape macrovoids, while those spun from thicker annulus gaps have both inward-and outward-pointed macrovoids. On the basis of SEM examination, it is concluded that nonsolvent intrusion due to local surface instability and skin rupture accounts for most inward- and outward-pointed long macrovoid formation, while the diffusion mechanism with the aid of solutocapillary convection possibly accounts for the formation of small teardrop and elliptical shape macrovoids. In addition, the number of inward-pointed macrovoids increases, while the number of outward-pointed macrovoids decreases with an increase in air gap distance. Clearly, many forces affecting macrovoid formation compete with one another starting from within the spinneret through the air gap region. It is found that (1) the shear stresses developed within the thinner spinneret annulus gaps, (2) the gravity-induced elongational drawing, and (3) the moisture-induced partial phase separation in the air gap region all hinder the nonsolvent intrusion and suppress outward-pointed macrovoid formation. The solutocapillary convection may prevail in thicker annulus gaps and longer air gap distances, and most likely accounts for the formation of small inward-pointed teardrop and elliptical shape macrovoids.