Effect of PVP, lithium chloride, and glycerol additives on PVDF dual-layer hollow fiber membranes fabricated using simultaneous spinning of TIPS and NIPS

Poly(vinylidene fluoride) (PVDF) dual-layer hollow fiber membranes with porous layers were successfully prepared by simultaneous spinning of thermally induced phase separation (TIPS) and non-solvent induced phase separation (NIPS) through a triple orifice spinneret (TOS). The support layer was produced using a TIPS system from PVDF, with gamma-butyrolactone (GBL) as the diluent. The prepared membranes were evaluated by analysis of their morphology, water flux, and tensile strength. The NIPS dope solution plays an important role in suppressing the formation of the dense top layer and forms a porous coating layer on the TIPS support layer. In addition, the effect of various non-solvent additives (polyvinylpyrrolidone (PVP), lithium chloride (LiCl), ethylene glycol (EG), and glycerol (Gly)) employed in the TIPS process was investigated. A specific morphology, namely, micro-sized holes in the spherulite structure, was observed on addition of specific mixtures of the non-solvent additives PVP, LiCl, and glycerol. Controlling the amount of added glycerol can help tune the dimensions of the holes. Because of this structure, the water flux of the membrane significantly increased, while a slight decrease in the tensile strength was observed. The specific morphology (hole structure) was very effective in controlling the porosity of the support layer on PVDF dual-layer hollow fiber membranes.