Effect of the rheology of poly(vinylidene fluoride- co-hexafluropropylene) (PVDF–HFP) dope solutions on the formation of microporous hollow fibers used as membrane contactors

Abstract

The rheological properties of polymer dope solutions influence the membrane structure and performance. In the present work, the effect of shear rate on the fabrication of poly(vinylidene fluoride- co-hexafluropropylene) (PVDF–HFP) asymmetric microporous hollow fiber membranes has been studied in terms of membrane morphology, pure water flux, filtration rejection, mechanic strength and the performance of the membrane contactors made by as-spun hollow fibers for CO 2 capture. The PVDF–HFP copolymer possesses strong viscoelasticity, which is believed to be associated with the occurrence of an irregular morphology in the PVDF–HFP hollow fibers made at a high dope extrusion rate. For the first time, the viscoelastic Mach number was applied to elaborate the deformation of the inner contour of the membranes by means of characterizing the delayed die swell in the process of hollow fiber fabrication. Based on the Mach number, several batches of PVDF–HFP dope solutions with an appropriate viscosity were adopted for hollow fiber fabrication at given dope extrusion rates. The effects of the shear rate on the permeation flux and molecular weight cutoff (MWCO) of the membranes made by Batch III-2L.5T dope solution (2 wt.% LiCl and 0.5% Tween 80 as the additive) presented a V shape curve; while for the polymer dope of Batch V-2P9G (2 wt.% PVP and 9 wt.% glycerol as the additive), the effect of the shear rate on the resultant membranes showed a different pattern: increasing the shear rate resulted in smaller MWCOs, enhanced water permeation flux and higher elongation strength. These behaviors have been discussed thoroughly based on the shear-induced macromolecular chain orientation and close packing, variation of the porosity with the shear rate and shear-thinning rheological property of the dope solution under a high shear stress.