| The traditional hydrophobic PVDF were chosen to fabricate the PVDF hollow fiber membranes. The improved performances of modified membranes by MWNTs, HAP were systematically investigated. Moreover, the structure parameter of MWNTs and electric charge effect of resultant membranes were also comprehensively verified.Firstly, the MWNTs were selected to modify the PVDF membranes prior to the incorporation into the polymer matrix using a simple oxidation method. The PVDF/PFSA/O-MWNTs hollow fiber memrbanes with 90.0wt% NMP aqueous solution as bore liquid are prepared by wet spinning method. The membranes with the singly introduced PFSA or O-MWNTs additive are also investigated for comparison. The synergism effects of the two additives of PFSA and O-MWNTs are experimentally verified. The results indicate that the PVDF/PFSA/O-MWNTs hollow fiber membranes show asymmetrical structure. The fingerlike voids in the membranes run through the whole cross-sections. However, the PVDF/O-MWNTs fiber(MC-75) has an evidently different cross-section morphology due to the absence of PFSA. The surface hydrophilicity and permeation flux of PVDF/PFSA/O-MWNTs membranes are evidently improved with the addition of O-MWNTs. The hydraulic permeability Jw increases from 100.4 L·M-2·H-1·bar-1 to 229.4 L·M-2·H-1·bar-1 as O-MWNTs loading increases from 0.25 wt% to 0.75 wt% for M-25~M-75, and then slightly decreases to 205.3 L·M-2·H-1·bar-1 for M-100. The fibers of M-0 and MC-75 containing a single additive, PFSA or O-MWNTs, only have 80.7L·M-2·H-1·bar-1 and 32.3L·M-2·H-1·bar-1, respectively. The mechanical properties of PVDF/PFSA/O-MWNTs fibers are also improved by adding O-MWNTs up to 0.75 wt%, whereas the fibers only containing a singleadditive(PFSA or O-MWNTs) present a lower mechanical strength. The improved PVDF membrane is more suitable for practical use.Secondly, On the basis of earlier work, the structure parameters of O-MWNTs were further verified by introducing MWNTs with different diameters(10-20 nm, 20-40 nm and 40-60nm). The morpholpgies of resultant membranes have no obvious difference, whereas the water permeation rises in the first stage and then decreases with the increasing diameter of O-MWNTs. With the O-MWNTs content increases up to 0.75 wt.%, the water permeation increases. It demonstrates that the increasing diameter of O-MWNTs enlarge the area of mass channel, so that the water permeation increases. With the further expansion of the diameter of O-MWNTs, the PVDF is much easier to immerse in the channel of O-MWNTs during the severe mechanical agitation in the preparation of dope solution process. This phenomenon contributes to the decline of water permeation. Furthermore, the mechanical properties of membranes decrease with the increasing diameter of O-MWNTs.Thirdly, the charged PVDF hollow fiber membranes were prepared by introducing NH2-MWNTs, which showed improved rejection performance by aperture sieve effect as well as charged repulsion. The charged PVDF membranes can be practically used in the separation of proteins and dyes. The results indicates that the polyoxyethylene surfactant(denoted as O-25) effectively improve the dispersion of NH2-MWNTs. The finger-like pores gradually shortened and replaced by sponge-like structure with the increasing content of NH2-MWNTs. The water permeation of membranes was 275.6 L·M-2·H-1·bar-1 with the addition of 0.5wt.% NH2-MWNTs. The rejection of LYM and BSA regularlly changed at different p H. The mechanical properties improved with the addition of NH2-MWNTs.Finally, the PVDF/PVP/HAP hollow fiber membranes were fabricated with the wet spinning method. The effects of two additives and the synergism on the morphologies, surface properties, permeation performances, antifouling ability and mechanical properties of the PVDF/PVP/HAP membranes were characterized by numerous state-of-the-art analytical techniques, and reasonably elucidated accompanying with precipitation kinetics and shear viscosity of dopes. The results show that with the addition of HAP, the finger-like structure of PVDF/PVP/HAP membranes is gradually suppressed and replaced by sponge-like structure, and the hydrophilicity is evidently improved. The hydraulic permeability Jw firstly increases from 224.2 to 549.1 L·M-2·H-1·bar-1 by adding 1.0 wt% HAP nano whiskers and then decreases to 425.9 and 316.3 L·M-2·H-1·bar-1 with increasing content of HAP. The rejections of three proteins and humic acid range from 31.4% to 98.4%, and slightly decrease as HAP content increases in the membranes. The mechanical properties of the membranes are markedly improved with the addition of HAP nano whiskers. The membranes containing dual additives have higher permeability and mechanical strength than those of the fibers containing either single additive, implying the synergism of them in improving membrane properties. Moreover, the modified membranes is much more suitable for the practical application of separation process with large capacity and stength. |
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