PVDF hollow fiber membrane formation: Fundamental study on the roles of phase inversion, crystallization and rheology on morphology and mechanical properties

Abstract

This study explores the fundamental science and engineering of fabricating poly(vinyl fluoride) (PVDF) hollow fiber membranes as well as elucidates the relationship among membrane morphology, crystallinity and mechanical properties as functions of non-solvent additives and dope rheology in the phase-inversion process. The phase inversion of semi-crystalline PVDF membranes is dominated by liquid-liquid demixing or solid-liquid demixing accompanying crystallization. These precipitation mechanisms drastically influence the resultant morphology and mechanical integrity of the membranes. Interestingly, the membrane morphology transforms from an interconnected-cellular type to an interconnected-globule type when adding water, methanol, ethanol, or isopropanol into the spinning dopes or into the external coagulation bath. In other words, the use of a weak non-solvent (i.e. methanol, ethanol or isopropanol) as an additive or a component in the external coagulant induces delayed demixing and crystallization and the resultant membranes are macrovoid-free with globule structure comprising of spherulitic crystallites. The size of spherulitic globules and crystallinity in the morphology are controlled by the amounts of non-solvents presented in the systems. Another interesting finding is the conventional perspective of macrovoid-free membranes having better mechanical properties may not be applicable for semi-crystalline polymers like PVDF. Despite the macrovoid-free structure, the membrane with spherulitic globule packing exhibits a lower mechanical strength than those with interconnected-cellular type morphology. The rheological behavior of dope solutions is also explored and the relationship between elongation viscosity and mechanical properties has been elaborated. The mechanical strength of as-spun fibers tends to increase as the elongation viscosity of the polymer dope increases. Analytical methods and molecular dynamics simulations are employed to provide insights mechanisms from the views of thermodynamic and kinetics as well as the state of polymer chains involved in the phase inversion process. It is believed that this pioneering work will significantly enhance the science of understanding PVDF hollow fiber formation and improve the PVDF membrane production in the industry.