Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization, and rheology

This study explores the fundamental science of fabricating poly(vinylidene fluoride) (PVDF) hollow fiber membranes as well as elucidates the correlation among membrane morphology, crystallinity, and mechanical properties as functions of nonsolvent additives and dope rheology in the phase inversion process. A series of nonsolvents (i.e., water, methanol, ethanol, isopropanol) are used either as nonsolvent additives in the dope or as a component in the external coagulant. Depending on the strength of the nonsolvent, the phase inversion of semicrystalline PVDF membranes is dominated by liquid-liquid demixing or solid-liquid demixing accompanying crystallization. As a result, the membrane morphology transforms from an interconnected cellular type to an interconnected globule transition type with lower mechanical strengths when adding water, methanol, ethanol, or isopropanol into the spinning dopes or into the coagulation bath. The crystallinity and size of spherulitic globules in the morphology are controlled by the amounts of nonsolvents present in the systems. The rheological behavior of dope solutions is explored and the relationship between elongation viscosity and mechanical properties has been elaborated. Analytical methods and molecular dynamics simulations are employed to provide insight mechanisms from the views of thermodynamic and kinetic aspects as well as the state of polymer chains involved in the phase inversion process.