Characteristics and performance of PVDF membrane prepared by using NaCl coagulation bath: Relationship between membrane polymorphous structure and organic fouling

In ultrafiltration (UF) applications, the antifouling modification of semi-crystalline poly (vinylidene fluoride) (PVDF) membrane aroused extensive attention. However, efforts have paid less attention to the roles of PVDF polymorphs in fouling alleviation. In this study, we report a new strategy to tailor the polymorphs and pore structure of the PVDF membranes, simultaneously, via incorporating the in-situ blending of Pluronic F127 (F127) with a sodium chloride (NaCl) coagulation bath (CB). The NaCl CBs were utilized to create ion-dipole interactions between Na+ and PVDF molecule chains, and to promote the formation of polar beta-PVDF phase during phase inversion process. When cooperated with the NaCl CBs, due to the intensified copolymer-polymer interactions, blending amphipathic F127 was demonstrated more effective in manipulating the membrane morphology compared with blending hydrophilic additives. The prepared membrane possessed a highly porous surface with narrowed size distribution, resulting in a great enhancement for both the permeability and selectivity. Furthermore, isopropanol (IPA) post-treatment was purposely performed to wash out the F127 embedded in the membrane matrix. The dominating beta-PVDF phase almost doubled the surface energy of the pure PVDF membrane compared with the alpha-PVDF phase, especially in electron donor functionality, which endowed the membrane enhanced fouling resistance for natural organic matter. As the fouling resistance introduced by polymer itself is more attractive than by exotic additives due to the better stability, we anticipate this new strategy will have numerous potential applications in the future.