| The main research of this dissertation focuses on the hydrophilic and anti-foulingmodification of polyvinylidene fluoride (PVDF) membrane. PVDF is currently widelyapplied in MBR membrane market. However, PVDF membrane ususlly has somelimitations such as protein fouling and low permeation flux due to the hydrophobicproperty. The main modification methods to improve the hydrophilicity are surfacegraft and material blending. Atom transfer radical polymerization (ATRP) is the“Controlled/Living” new technology developed in the past two decades. ATRPreactions, where metal catalysts are utilized in combination with ligands yieldingwell-controlled structures and narrow molecular weight distributions, have beencarried out since1995.The modification of PVDF membrane by coating3,4-dihydroxyphenylalanine(DOPA) for the purpose of reducing protein fouling is first studied.Both the oxidizedand deoxidized DOPA are self-polymerized and adhered to the membrane surfaces inmild base aqueous environments. The chemical compositions of modified membranesurfaces are determined by Fourier transform infrared spectrometer (FTIR-ATR)analysis and X-ray photoelectron spectroscopy (XPS). Morphological changes ofmembrane surfaces are detected by using scanning electron microscopy (SEM) andDOPA has been coated on the membrane surface and sub-layers as well. Thedeoxidized polyDOPA coating increases the hydrophobicity of PVDF membranebecause phenolic hydroxyl groups are prone to be oxidized into quinone carbonylgroups in strong alkali and dry conditions. However, the anti-fouling performance ofthe deoxidized polyDOPA coated membrane is still better than the oxidizedpolyDOPA coated membrane due to existence of unoxidized phenolic hydroxylgroups and dipolar ions(carboxylic and amine groups) in wet environment. Thestability of the oxidized polyDOPA coated membrane is more excellent due to thecoexistence of phenolic hydroxyl and quinone carbonyl groups. However, the apparent black color illustrates the formation of melanin.The covalently binding of2-hydroxyethyl methacrylate (HEMA) and2-(dimethylamino)ethyl methacrylate(DMAEMA) brushes onto the PVDF membranesurfaces via surface-initiated atom transfer radical polymerization (ATRP) to preparethe fouling resistant and antibacterial membrane is studied. Prior to ATRP, PVDF iscoated with polyDOPA. The hydroxyl groups on the polyDOPA-coated PVDFmembrane surface and pore surface are used for the immobilization of alkyl halideATRP initiator. By controlling the reaction temperature at40℃, the identicalmonomer/initiator concentration, the effect of different reaction time on the graftingrate is studied. It is found the grafting rate linearly increases with the polymerizationtime. The surface and the cross-sectional morphologies of PVDF modifiedmembranes are monitored by SEM. It is found that the grafting chains partially orcompletely fill the pores on the coated membrane surface, the uneven pedals-likelayers remain on the surface of PHEMA-grafted membrane reacted for5h, whilePHEMA-grafted membrane for10min exhibits porous but smooth surface. Thevarying pore architecture from the cross-sectional images after surface modificationindicates that the graft polymerization occurs in the sub-layers as well. The image ofatomic force microscope(AFM) indicates that the roughness of PHEMA-graftedmembrane is increased with grafting time and excess PHEMA grafted on modifiedmembrane damages the mechanical property of the membrane. Water contact anglesare employed to evaluate the effect of the membrane surface hydrophilicity andwetting characteristic. The water contact angle of non-treated hydrophobic PVDFmembrane is100.6o. The contact angle of PVDF-g-PHEMA membrane reaches alowest value of35o after1h polymerization. While the static angles ofDMAEMA-grafted PVDF membranes are even higher than90o. With increase ofHEMA grafting degree (graft polymerization time), the fluxes of pure water and BSAsolution decrease because of the effect of higher grafting degree on pore blocking.The water flux recoveries for pristine and polyDOPA-coated membranes are44.75and63.60%in the successive three ultrafiltration cycles，respectively, whereas itincreases to81.77%for the PVDF-g-PHEMA with the grafting time of10min, indicates anti-fouling and cleaning efficiency of PVDF membrane are improved bygrafting PHEMA chains on the membrane surface and the pore surface. Thequaternized PVDF-g-PDMAEMA membrane (with an ATRP time of1h) hasantibacterial rate of90.0%after being contacted with S. aureus for about1h.Besides the membrane surface grafting, blending is also a convenient andpromising method to improve the PVDF membrane hydrophilicity. PVDF-g-PHEMAand PVDF-g-PDMAEMA are synthesized in both methanol and water by ATRP, andused as additives in the fabrication of novel PVDF ultrafiltration(UF) membranes byimmersion precipitation phase conversion. The hydroxyl groups on the PVDF-OHpolymer obtained by Fenton reaction are used for the immobilization of alkyl halideinitiator of ATRP. The chemical compositions on blend membrane are investigated byFTIR-ATR, TGA and XPS. The typical curves showed that PHEMA and PDMAEMAare quantitatively grafted on PVDF polymers. The monomer contents in copolymersare increased with the polymerization time and the reaction monomer content. Thehydrophilic chains also increase on PVDF blend membrane surface through theirmigration to the membrane surface. The flux, hydrophilic and anti-fouling ability ofPVDF/PVDF-g-PHEMA blend membrane is improved greatly. The anti-bacterial(E.coli) efficiencies of the PVDF/PVDF-g-PDMAEMA andPVDF/PVDF-g-PDMAEMA-b-PHEMA blend membrane (PDMAEMA brush graftedfor20min) reach80.0%.By fixing the reaction temperature at40℃and the identical initiator density,changing the polymerization time and reaction monomer contents, respectively, the“Controlled/Live” reaction characteristic of ATRP is evaluated. The grafting yieldof PHEMA is determined by Mn which is increased with the polymerization time andreaction monomer contents. The polydispersity index of PVDF-g-PHEMA remainsnarrow Mw/Mn at around1.21-1.45throughout the reaction. The water intake,contact angle and BSA adsorption properties are increased with the increase ofPHEMA content in copolymer (polymerization time and reaction monomer contentincrease). Due to the addition of the copolymer PVDF-g-PHEMA in blend membrane,the membranes exhibit macrovoid formation. Water permeability of blend membrane using PEG20000as pore-forming agent is more excellent than that using PEG400dueto formation of larger pore sizes on the PVDF membrane surface and somePEG20000tangling up with amphiphilic copolymer in PVDF matrix. The cyclicfiltration tests using BSA solution prove the excellent fouling resistance of themembrane, and the initial flux can be recovered only by water cleaning instead ofstrong chemical cleaning. Such membranes might be expected to exhibit substantiallylonger operational lifetimes and reduced membrane process costs.Because ATRP can linearly control the graft or block chain length with reactiontime and monomer concentration, which makes the hydrophilic monomers stablybond on the hydrophobic PVDF membrane surface and material, also effectivelycontrols the membrane pore size and distribution, so that the separation performanceof modified membrane is improved. The ATRP technology has become the essentialtool for the design and synthesis of advanced, noble and novel materials andmembranes. Modified PVDF blend membrane is of the potential application to reducethe protein adsorption and bacteria fouling in membrane treatment process. Inaddition, the free radical synthesis technology used in commercial application canobtain more chemical structure with matrix and side chain. The increased cost is justthe price difference between the ratio of copolymer added in the membrane castingsolution and the matrix polymer instead. |
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