Effect of additives on the fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes

The effects of two typical additives, lithium chloride (LiCl) and glycerol, on the fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes were investigated in terms of membrane morphology, structure, permeation performance, hydrophobicity and mechanical property.The addition of the additives into the dope solution altered the morphology and structure of the resultant membranes, which was believed to be associated with the change of the thermodynamic and kinetic properties of the system in the phase inversion process. The membranes with improved pure water permeability (PWP) from 6.908 × 10−5 (7) to 4.835 × 10−4 (49), 3.256 × 10−4 (33) and 5.003 × 10−4 L/h m2 Pa (51 L/h m2 atm) were obtained when 2 wt.% LiCl, 4 wt.% LiCl and 10 wt.% glycerol was used as the additive, respectively. Among these, the membrane made from the dope containing 4 wt.% LiCl possessed the highest retention capability of 40 kDa molecular weight cut-off (MWCO). The LiCl or glycerol addition into the polymer dope also made the membranes exhibit a narrow pore size distribution. Moreover, the membrane hydrophobicity was affected less by LiCl and glycerol than by poly(vinyl pyrrolidone) (PVP). The addition of 4 wt.% LiCl into the dope could form the membrane with four times stronger strain strength than the poly(vinylidene fluoride) (PVDF) commercial membrane while keeping similar rigidity.