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.
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