Localization of antifouling surface additives in the pore structure of hollow fiber PVDF membranes

Industrial hollow fiber membranes for the purification of wastewater were manufactured by the phase inversion method in a dry-wet spinning process, adding an amphiphilic diblock copolymer to a standard PVDF/Polyvinylpyrrolidone (PVP) formulation. The copolymer designed to provide antifouling properties is composed of a PMMA block and a hydrophilic block bearing pendent hydroxy groups. As PMMA is miscible with PVDF, the copolymer is expected to anchor in the matrix more efficiently than the hydrophilic but water soluble PVP which is washed away overtime during repetitive cleaning procedures. Infrared experiments attest that both antifouling additives are well included in the fibers after the inversion process. Performances of the fibers were checked by permeability, burst pressure and tensile tests. The pore distribution observed by scanning electron microscopy goes from 10 to 30 nm at the outer surface to 0.1-10 inside the fiber. Observation of the pore structure by transmission electron microscopy, made possible by embedding the fibers into a resin that filled in the voids, and specific staining of the antifouling additives, shows that both PVP and the amphiphilic copolymer are located all around the pores. This study is the first direct proof at the microscopic scale of pore surface enrichment by antifouling agents.