Development of a novel RAFT-UV grafting technique to modify polypropylene membrane used for NOM removal

Surface modification of polypropylene microporous membrane was conducted by reversible addition-fragmentation chain transfer (RAFT) polymerization under UV irradiation. Dibenzyltrithiocarbonate was used as a RAFT agent. Attenuated total reflection-Fourier transform infrared spectroscopy and filed emission scanning electron microscopy were used to characterize the structural and morphological changes on the membrane surface. The degree of grafting of poly(acrylic acid) (PAAc) on the membrane surface increased with the increase of UV irradiation time, and decreased with the increase of the concentration of chain transfer agent. Surface-initiated block graft copolymerization with acrylamide was carried out on the PAAc modified membrane surface in the presence of free radical initiator, 2,2′-azobisisobutyronitrile. The degree of grafting of polyacrylamide (PAAm) increased with the reaction time. These results demonstrated that block graft copolymer of PAAc and PAAm on the membrane surface was accomplished by the combination of UV irradiation and the surface-initiated RAFT method.The pure water flux through the PAAc grafted membranes increased with the degree of grafting up to 0.67 wt.%, and then it decreased. The membrane filtration of natural organic matter from the Yangtze River showed that the degree of grafting played a very important role. Reduction from pure water flux was lower, the flux recoveries after water and caustic cleaning and flux ratios after fouling were higher for the modified membrane than those of the unmodified membrane. The PAAc grafted PPMM with a degree of grafting of 2.51 wt.% possessed the best antifouling characteristics. The high hydrophilicity, high permeability, and low fouling character of the modified membranes make microporous polypropylene membranes desirable for water treatment.