New Graphene Oxide/Polyvinylidene Fluoride Composite Ultrafiltration Membrane

Poly(vinylidene fluoride) (PVDF) is a typical polymer materials for ultrafiltration(UF) membranes due to its outstanding properties such as high mechanical strength,thermal stability, chemical and radiation resistance. However, the highhydrophobicity of PVDF has a negative effect on anti-fouling properties of UFmembranes. A lot of works have been carried out to improve the performance of UFmembranes made by PVDF. The Graphene Oxide (GO) as the promising and novelcarbon nanomaterials with its excellent hydrophilic property has paved a new way tothe PVDF membrane modification. How to controll the size of as-prepared GOsheets which is significantly important for UF membrane fabrication is a criticallyunsolved problem.Two strategies were deliberately designed to synthesize GO, marked as GO-1(sythesized by original Hummer method) and GO-2 (sythesized by modifiedHummer method by varying the used oxidants and reacting time) on the basis ofHummer method, and the as-prepared GO-1 was further modificated by mixed acidsunder ultrasonic (GO-Ac) or physical ball milling (GO-m). The results of FrontierTransform Infrared Spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS)and X-ray Diffraction (XRD) characterizations of GO samples demonstrated that ahigher amount of oxygen function groups like hydroxyl, epoxy and carboxyl hadshown in the carbon skeleton of GO, and the layered structrue was clearly observedby Transmission Electron Microscopy (TEM). The C/O ratios of GO-1 and GO-2were 2.18 and 1.88 respectively. The size of GO-1 and GO-2 were ~400nm and~40nm respectively. After modification, the size of GO-1 could be reduced to~100nm. Besides, the GO-Ac whose carbon skeleton was heavily destroyed bymixed acid treatment had much higher oxidation degree.GO, modified GO and Polyvinylpyrrolidone (PVP) were used as additives to prepareGO/PVDF and GO-PVP/PVDF composite ultrafiltration (UF) membranes byimmersion phase inversion method. The prue water flux (F0), rejection (R) of bloodserum albumen (BSA) and flux reduction coefficient (m) of PVDF UF membraneswere tested in details. Scanning Electron Microscopy (SEM) and Atomic ForceMicroscopy (AFM) were applied to characterize the morphologies of cross-sectionand surface structures of membranes. Contact angle of composite membranesdecreased ~20°compare with pristine PVDF membranes. When GO and PVP wereadded into the casting solution together, the water flux increased dramaticlly with PVP concentration; however, the BSA rejection reduced greatly. The resultsillustrated that the optimized additive concentration sof GO and PVP were both 0.5wt%, under which the value of F0, R and m were 348.56L/(m2?h), 84.37%, 56.52%respectively. In summary, the addition of nano particle of GO improved thehydrophilicity of PVDF UF membranes and the high water flux, good anti-foulingperformance and appropriate BSA rejection were achieved compared to prestinePVDF UF membranes.