Effect of Preparation Parameters on the Structure of PVDF Hollow Fiber Membranes

Hollow fibers (HFs) provide a number of advantages in comparison with traditional flat sheet (FS) membranes. The ability to pack them closely enables the creation of modules with a large specific surface area. Furthermore, this geometry does not require any support material and offers excellent mechanical properties. Nonsolvent induced phase separation (NIPS) is the most used method for HF spinning. However, the parameters commonly used for FS manufacturing cannot be simply translated to HF production since there is a far greater number of factors that may be adjusted to produce membranes with the desired properties. In this study, HFs are prepared by means of the NIPS technology using polyvinylidene fluoride (PVDF) as polymer and dimethyl sulfoxide as solvent. The surface and cross-section characteristics are tailored by tuning the nonsolvent strength (i.e., by using pure water or water/ethanol solutions). Nearly dense selective skins, suitable for gas separation processes, can be obtained when harsh nonsolvents are used, whereas porous and more symmetric structures, suitable for membrane filtration systems, are formed with weaker nonsolvents. Finally, the effect of the dope solution concentration is evaluated, registering a decrease of the membrane porosity as the polymer amount is increased.