Reorganization of the surface geometry of hollow-fiber membranes using dip-coating and vapor-induced phase separation

Phase separation is one of the major methods to prepare a hollow-fiber membrane in industry. Despite the strong demands for the control of the inner membrane structure and surface geometry, it is still difficult to obtain the desired membrane structure and surface geometry simply by phase separation. In this work, we employed thermally induced phase separation, dip-coating and vapor-induced phase separation to prepare a hollow-fiber membrane with the controlled surface geometry and homogeneous inner structure, which led to high water permeability and high mechanical strength. First a poly(vinylidene fluoride) (PVDF) hollow-fiber membrane was prepared via thermally induced phase separation (TIPS) and then the outer surface of a membrane was dip-coated with another PVDF solution, followed by vapor induced phase separation (VIPS) to reorganize the outer surface of the membrane. The dip-coating and the VIPS treatment produced a highly porous mesh-like layer on the membrane surface. Filtration experiments using nanospheres and protein solutions revealed that the newly-formed layer served as a “separation layer” to determine the separation properties. The membrane pore-size was controlled to some extent by the conditions for the dip-coating and the VIPS treatment. The surface-reorganized membrane kept its intrinsic high water permeability and mechanical strength. In addition, the membrane exhibited improved low-fouling properties in the filtration of humic acid and protein solutions.