Study On The Surface Zwitterionic Modification Of PVDF Membrane And The Properties Of The Modified Membrane

Polyvinylidene fluoride（PVDF）has been widely used in the preparation of ultrafiltration membrane for its advantages of chemical stability,UV degradation resistance,thermal stability,and good film forming properties.However,the PVDF material is highly hydrophobic due to its C-F bonds.Thus,PVDF membrane is easy to be fouled when dealing with aqueous solutions containing organic foulants and usually bears serious flux decline during filtration.Studies have demonstrated that the improvement of membrane hydrophilicity can promote the formation of hydration layer on membrane surface through hydrogen bonding,which can hinder the deposition of foulants on membrane surface and thus enhance membrane anti-fouling property.Of the hydrophilic modification methods,zwitterionic modification can endow the membrane with better hydrophilicity.Therefore,it is of important research significance to improve the anti-fouling performance of the PVDF membrane by zwitterionic modification.In this thesis,alkali treatment and acrylic acid grafting were firstly performed with the base of a commercial PVDF porous membrane,then the modified membrane was aminated through grafting polyethylene imine（PEI）using a peptide condensing agent,the tertiary amine groups of grafted PEI were finally employed to introduce zwitterionic units through ring-opening reaction with 1,3-propanesultone（1,3-PS）.Fourier transform infrared spectroscopy（ATR-FTIR）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscope（SEM）,Zeta potential measuring instrument and water contact angle tester were used to characterize membrane property including chemical composition,morphology,charge and hydrophilicity.Cross-flow permeation tests were conducted to evaluate separation performance,anti-protein-fouling property,and oily wastewater treatment performance.The conclusions obtained from experiments are as follows:（1）Alkali treatment could generate carbon-carbon double bonds on membrane surface through defluorination,the followed acrylic acid grafting initiated by free radical and PEI grafting activated by EDC/s-NHS could achieve the amination of the membrane surface,and the final ring-opening reaction between tertiary amine group and 1,3-PS could construct zwitterionic surface.The ATR-FTIR spectrum of the zwitterionic membrane showed characteristic absorption peaks of C=O,-NH-and SO₃^-,XPS spectrum showed the signal peaks of O,N and S elements.Zwitterionic modification altered the wettability of membrane surface from hydrophobic to hydrophilic,weakened membrane surface negative charge under neutral p H,and had nearly no effect on membrane morphological structure.（2）The optimal concentrations of PEI and 1,3-PS for zwitterionic modification of PVDF membrane were 10 g/L and 20 g/L respectively.The pure water flux increased from 397.0 L/（m²·h）of the original membrane to 557.4 L/（m²·h）of the modified membrane.To BSA aqueous solution,he permeation flux increased f zwitterionic rom70.6 L/（m²·h）to 264.9 L/（m²·h）,the BSA rejection enhanced from 27.05%to 55.56%,the flux recovery ratio increased from 31.79%to 80.22%,the total membrane resistance declined from 5.70×10¹² m^-1 to 1.52×10¹² m^-1,and the membrane fouling resistance decreased from 2.21×10¹² m^-1 to 0.18×10¹² m^-1,showing that the zwitterionic modification of current work could effectively improve the separation and anti-fouling performances of PVDF membrane.（3）The flux recovery ratio increased from 34.6%of the original membrane to 79.9%of the desired zwitterionic membrane in filtration oily wastewater.The analyses of resistances in filtration of oily wastewater demonstrated that the total permeation resistance decreased from 6.65×10¹² m^-1 of the original membrane to 3.72×10¹² m^-1of the desired zwitterionic membrane,and the fouling resistance declined from 2.94×10¹² m^-1 of the original membrane to 1.02×10¹² m^-1 of the desired zwitterionic membrane,showing that zwitterionic modification of current could effectively improve the anti-oil-fouling property of PVDF membrane.However,the retention performance to oily wastewater decreased slightly from 94.4%of the original membrane to 91.8%of the zwitterionic modified membrane.