Shear-induced demixing in polymeric membrane dopes

One of the critical challenges in producing hollow fiber membranes is controlling fiber microstructure. The dope solutions involved possess (by design) an inherent thermodynamic sensitivity to phase separation, and are therefore susceptible to shear-induced microstructural changes when they are subjected to the complicated flow conditions that accompany spinning. This study demonstrates that shear-induced demixing is relevant in polymeric membrane dopes, and that it can affect macrovoid formation. Furthermore, we find that surprisingly subtle changes in shear history can affect demixing. Our investigation consisted of a two-tier approach. First, we used shear-small-angle light scattering (shear-SALS) to observe the microstructural evolution in liquid dope solutions before, during, and after shear flow. By varying the speed at which samples were syringe-loaded into the rheometer, we found that the pattern (and hence, the microstructure) observed after demixing varied based on shear history. In the second set of experiments, dopes with varying shear histories were extruded into a non-solvent bath to form solid fibers. Despite the fact that the fibers were all extruded under identical conditions, scanning electron microscopy (SEM) showed that the number of macrovoids observed varied widely between fibers. A comparison of the shear-SALS and extrusion results showed that shear-demixing is strongly linked to macrovoid formation. Our findings suggest that low-shear, upstream flow conditions (e.g., in supply tubing) could be just as important for fiber microstructure as the high-shear flow within the actual spinneret.

► Shear can change the phase behavior of polymeric dopes, even at low shear rates.
► The microstructure created by shear-induced demixing varies based on shear history.
► SALS and extrusion results show that shear-demixing enhances defect formation.