Study On The Hydrophilic Modification And Separation Properties Of PVDF Membranes

As an emerging and environmental-friendly separation technology,membrane separation has been attracting much more attention in the field of water treatment.Polyvinylidene fluoride?PVDF?could be utilized to fabricate porous separation membranes owing to the excellent stability and mechanical properties.However,the inherent hydrophobicity of PVDF membrane material results in a decrease in water permeability as well as serious fouling by proteins or microorganisms in aqueous environment.Therefore,it was urgent to improve the fouling-resistance of PVDF membrane with effective strategies.In this thesis,four kinds of modified PVDF membranes were fabricated based on in-situ blending?"inside-out"?and force-applied surface coating?"outside-in"?methods.The surface morphology,structure,wettability,separation performance,antifouling performance and stability of modified membranes were thoroughly investigated.Firstly,a hydrophilic polymer?Cate-PEG?with catechol and ether oxygen groups was synthesized by the covalent combination of Dopamine and Poly?ethylene glycol?diglycidyl ether.Afterwards,the synthesized Cate-PEG polymer was introduced into PVDF ultrafiltration membrane by in-situ blending.Under the free surface segregation,PVDF/Cate-PEG modified membrane was constructed.At the optimized conditions,the water flux and BSA rejection of PVDF/Cate-PEG membrane was as high as 194.0 Lm^-2 h^-1and 94.8%,respectively,with a decreased contact angle and improvement of hydrophilicity.The cycling filtration test results showed that the water flux recovery ratio?FRR?of the modified membrane was up to 95.5%,while the irreversible fouling ratio?Rir?was only 0.8%.In addition,the PVDF/Cate-PEG membrane exhibited good long-term stability based on the result of hot-water treatment.To enhance the water flux and antifouling performance of modified membrane,a zwitterionic hydrophilic polymer?SZ-PEG?with catechol groups was synthesized by the substitution reaction of Cate-PEG and sodium 2-bromoethanesulfonate.SZPEG polymer was introduced into the PVDF membrane matrix by in-situ blending to construct the PVDF/SZ-PEG ultrafiltration membrane.SEM images exhibited that pore size of the supporting layer on the modified membrane was increased significantly,meanwhile the continuity of holes was improved on section of the supporting and selective layers.The decrease of contact angle and BSA adsorption implied the upgrade of hydrophilicity of membrane.What's more,the mechanical strength of modified membrane was enhanced.At the optimized condition,the PVDF/SZ-PEG membrane showed excellent antifouling performance and stability with high water-flux(337.1 Lm^-2 h^-1),BSA rejection?93.8%?,FRR?96.9%?and low Rir?<10%?.To further enrich the content of hydrophilic polymer on the surface of modified membranes,the zwitterionic nanoparticle with core-shell structure?Fe₃O₄@PSBMA?was synthesized.Subsequently,it was developed to form a homogeneous casting solution with PVDF by in-situ blending.In pre-conversion process,Fe₃O₄@PSBMA would be forced segregation towards the membrane surface with the help of magnetic field,and gradually formed the PVDF/Fe₃O₄@PSBMA ultrafiltration membrane.The SEM images and EDX mapping implied that Fe₃O₄@PSBMA nanoparticles were enriched on the surface of membrane with significantly improved hydrophilicity.The water flux and BSA rejection of PVDF/Fe₃O₄@PSBMA membrane were increased simultaneously,while the trade-off between them was broken.In addition,the FRR and Rir were 93.8% and 6.2% after the cycling experiment.Notably,Fe₃O₄@PSBMA nanoparticles could retain into the PVDF/Fe₃O₄@PSBMA membrane with a longterm stability.Considering the modification efficiency,a composite coating was constructed on the porous PVDF membrane with the Dopamine and Dimethyloctadecyl[3-?trimethoxysilyl?propyl]ammonium chloride?QAC?under the assisted of oxygen?"outside-in" method?.The self-polymerization of dopamine and hydrolytic condensation of QAC occurred simultaneity on both the surface and inner pore walls of modified membrane.At the optimized condition,the water flux and BSA rejection were improved simultaneously towards 519.7 Lm^-2 h^-1 and 95.1%,respectively.The trade-off restrictions of water flux and rejection was broken,while the water flux recovery ratio was upto 94.2%.In addition,the modified membrane showed highly effective antibacterial?on both of E.coli and S.aureus was over 90%?and adhesion resistance perormances.Meanwhile,the coating layer on PVDF membrane was stable in neutral aqueous solution.