| Membrane separation as a kind of green fluid separation technology,with the good advantages of low energy consumption,high efficiency,simple process,and less investment,etc,has been more and more widely used in wastewater treatment,drinking water purification,pharmaceutical industry and environmental separation fields.Poly(vinyl chloride)(PVC)and poly(vinylidene fluoride)(PVDF)were often prepared into microfiltration,ultrafiltration or nanofiltration membranes and used in water treatment due to their good mechanical and chemical properties,corrosion resistance and other advantages.Although PVC and PVDF membranes were different in the structure and chemicalproperties,they were all easily fouled in aqueous solutions due to their low surface energy,which could shorten their working life and increase the postoperating cost.Therefore,surface modification to the membrane is becoming necessary.It is one of the most effective methods to improve the membrane surface performance by grafting functional groups on the membrane surface.In this study,themembranes were modified by atom transfer radical polymerization.Hydrophilic charged groups were grafted onto the membrane surface to improve the hydrophilic,charged and anti-fouling properties of the membrane.The detail research contents and major conclusions are listed as follows:1.Preparation and separation performance of poly(vinyl chloride)membrane modified by poly(ionic liquid)brushesPoly(ionic liquid)brushes were grafted onto the poly(vinyl chloride)(PVC)membrane surface via a two-step atom transfer radical polymerization(ATRP)reaction.The analysis of the FTIR,XPS and Zeta potential confirmed that the successful grafting of PHEMA and PBVIm-Br onto the PVC membrane surface,and the grafting density(GD)of PBVIm-Br increased gradually with the increasing grafting time.The modified membrane exhibited a positive charged and enhanced surface hydrophilicity.The static water contact angle of the membrane surface decreased from 92.3° to 51.6° as the GD of the PBVIm-Br brushes increased.Filtration experiments indicated that the water flux of the modified membrane increased with increasing GD,and their recovered fluxes were more than twice than that of the original.In addition,the total fouling ratio of the membranes decreased from 89%in MO to 67%in M5,and among them,the reversible foulingrate of the membrane M5 is 62%and the irreversible fouling was only 5%,which indicates that most of the foulingin membrane M5 was reversible.These results indicated that the positive charged poly(ionic liquid)brushes featuring hydrophilic properties would have potential applications in membrane separation.2.Preparation of poly(ionic liquid)brushes modified PVDF membranes and the reversible switching between hydrophilicity and hydrophobicity of the membraneFirstly,a polyDOPA-coated PVDF membrane was prepared by self-polymerization of dopamine,then the hydroxyl groups on the polyDOPA-coated PVDF membrane surface were used for the immobilization of alkyl halide,and then PBIVm-Br was grafted onto the membrane surface by the ATRP reaction,thus poly(ionic liquid)brushes modified PVDF membranes were prepared.Chemical and morphology changes of the membrane surface were characterized by FTIR,XPS,SEM,AFM and Zeta potential instrument.The changes of hydrophilicity/hydrophobicity of membrane surface were investigated by contact angle instrument.The results revealed that the grafted poly(ionic liquid)brushes could improve the hydrophilicity of the membrane,and make the zeta potential of the membrane change from negative to positive.The contact angles decreased from 92 ° of the membrane MO to 55.5 ° of the membrane M5.Anion exchange experiment showed when Br-anion was exchanged with hydrophobic anion,the hydrophobicity of the modified membrane would increase,and at this time the contact angle would increase and the water flux would decrease in the following order:PF6->CF3SO3->BF4-.After Br-anion was exchanged with PF6-anion,the static contact angle of the membrane M5 increased from 55.2 ° to 104.2 °and the water flux decreased from 30.47L/m2 h to 4.89L/m2 h.The water flux of membrane M4 decreased obviously from 37.73L/m2·h to 4.72L/m2 h,which had only the 12.54%of the original flux of the membrane,and the corresponding static contact angle increased from 57.9 ° to 102.2 0.After BF4-anion was exchanged with Br-anion,the contact angle of the membrane M4 was increased to 85.1 ° and at the same time the water flux remained 52.95%as compared with the original flux of the membrane.Further study found that the flux recovery rate was ordered as the follows:BF4->CF3SO3->PF6-when the hydrophilic anoins was further exchanged with Br-anions.Among that,the water flux of membrane M4 was recovered to 79.48%of the original when PF6-anion was exchanged with Br-anion.When CF3SO23-anion was exchanged with Br-anion the flux recovery rate of the membrane M4 was 92.81%,and the flux recovery rate reached highest value as 99.53%when Br-anion was exchanged with BF4-anion.These phenomena indicated that the the hydrophilicity and hydrophobicity of the modified PVDF membrane could be reversible controlled by anion exchange.3.Separation performance of poly(ionic liquid)brushes modified PVDF membraneThe filtration and anti-fouling performance of poly(ionic liquid)brush modified membrane were investigated by the separation of dye,protein and the drug clopidogrel sulfate.Dye separation and adsorption experiment indicated that the separation flux of orange G and Luo Danming 6G by membrane M4 reached 30.21L/m2 h and 24.05L/m2 h respectively.And the rejection rate of the membrane to Luo Danming 6G reached 91.18%,which was 30.78%higher than that of the membrane to orange G.The reason might be that the membrane M4 had electrostatic repulsion to Luo Danming 6G,thus,the membrane M4 had an electrostatic adsorption effect on the negatively charged dye,the adsorption capacity and rejection to orange G were 39.87ug/cm2 and 97.8%,respectively.Protein separation experiment showed that the water and BSA separation flux of membrane M4 were the largest,and the water flux was 1.65 times higher than that of the membrane MO,reached 37.82L/m2 h,and the BSA flux was 13.92L/m2 h.The pH value of the solution had a great influence on the separation performance of the membrane.The rejection and flux recovery of M4 reached 94.65%and 90.07%at pH=3.6,respectively.The irreversible fouling rate was only 9.93%.Clopidogrel sulfate separation experiments found that the drugs flux and rejection of the modified membrane were greater than that of the original membrane MO when the concentration of clopidogrel sulfate was 10mg/L,the flux and rejection of membrane M4 reached 27.62L/m2·h and 89.03%,respectively.The separation performance was better when the drug concentration was lower.When the drug was 5mg/L,the rejection of membrane M4 and M5 reached 90.79%and 91.58%,respectively.After the separation of cloridine sulfate solution for 60 h,the membrane flux was still kept at 18.05L/m2·h,which was only decreased 9.57L/m2·h as compared with the initial flux,and the rejection of the membranes increased from 89.03%to 91.38%.The above results indicated that the surface charged properties of the membranes were improved by poly(ionic liquid)brushes which also improved the hydrophilicity and anti-fouling performance of the modified membranes.The reversible switching between hydrophilicity and hydrophobicity of poly(ionic liquid)brushes modified PVDF membrane surface could be achieved by simple ion exchange.At the same time,the modified membrane exhibited good antifouling and filtration performance in separation of dyes,BSA and drugs.These properties would make the membranes have good application in the fields of electrochemical,intelligent materials and membrane separation. |
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