Preparation And Separation Performances Of Poly（Vinylidene Fluoride） Composite Membrane

With the acceleration of the process of industrialization,the emergence of various water pollution continues to endanger the living environment of human beings,which has led to increasing demands for green and sustainable development.Membrane separation technology is a green,eco-friendly,powerful and effective method for treating water pollution systems.Poly（vinylidene fluoride）（PVDF）is a kind of the outstanding polymeric membrane materials due to its superior mechanical strength as well as thermal stability and chemical resistance.However,the high hydrophobic nature and low surface energy of PVDF membrane lead to poor antifouling ability and permeability,which limits its wide applications.To this end,the composite membrane with hydrophilicity,high permeation,outstanding antifouling property and excellent separation performance based on PVDF substrate membrane was fabricated by a variety of modification methods.In this paper,Polydopamine（PDA）was coated on poly（vinylidene fluoride）（PVDF）membranes with different structures to select the most suitable structure for achieving outstanding hydrophilicity,permeability,stability and antifouling properties.PVDF membranes were prepared with different solvents（N-methyl-2-pyrrolidone（NMP）,N,N-dimethylacetamide（DMAc）,N,N-dimethylformamide（DMF）,NMP/DMAc,NMP/DMF,DMAc/DMF,and NMP/DMAc/DMF）,and the factors which influenced the phase inversion were discussed.Fourier transform infrared（FTIR）spectroscopy,scanning electron microscopy（SEM）and atomic force microscopy（AFM）were employed to characterize various membranes.Results indicated that PDA was coated on PVDF membrane surface and formed a smoother PDA layer.Water contact angle（WCA）measurement confirmed the increased hydrophilicity of PDA modified membranes,but water flux decreased due to the adhesion of PDA on pore walls.The decrease of static adsorption of bovine serum albumin（BSA）,the increase of flux recovery ratio（FRR）and the decline of fouling resistance for PDA modified membranes demonstrated that their antifouling properties have been improved significantly.The effects of oxidation and acidic/alkaline solutions on the performance of PVDF and PDA/PVDF membranes were examined to study the structural stability,the result shown that PDA/PVDF membranes exhibited better structural stability.The membrane prepared with NMP/DMAc displayed the highest decay rate of contact angle（16.6%）,the higher water flux（1358.5L/（m~2·h））,the highest decrease of BSA adsorption（129.3μg/cm~2）,the highest increase rate of BSA rejection（13.6%）,and the highest FRR（92.8%）.This indicated that NMP/DMAc membrane possessed the most suitable structure to improve membrane performances by PDA coating.A composite membrane constructed by intercalating zeolitic imidazolate framework-67（ZIF-67）into the graphene oxide（GO）nanosheets and PVDF substrate was synthesized under the modification of polydopamine（PDA）,and GO@ZIF-67/PDA/PVDF composite membrane were prepared by in-situ growth and self-assembly method.XRD,FTIR,SEM,AFM and water contact angle goniometer were employed to characterize chemical composition,structure and surface properties of membranes.The composite membrane was used for dye separation to evaluate membrane permeability,dye separation performances and antifouling abilities.Compared with PVDF membrane,the performances of GO@ZIF-67/PDA/PVDF membrane were shown as below:the composite membrane possesses“layer by layer”structure,and the roughness decreased from 48.9 nm to 25.7 nm;The static water contact angle decreased from 55.5°to 33.4°,and the rate of dynamic water contact angel increased from 9.76%to51.58%;Although the water flux decreased from 1090.4 L/（m~2·h）to 803.1 L/（m~2·h）,its dye rejection enhanced significantly;Especially,the membrane’s interception rate of Congo Red increased to 97.64%;The total resistance ratio decreased from 52.45%to 42.66%;The retention of dyes could be attributed to the size exclusion,electrostatic interaction and adsorption.Novel high-flux and antifouling membranes were prepared by introducing two monomers（GO and ZIF-67）into polyvinylidene fluoride（PVDF）casting solution.In addition,GO nanosheets were cross-linked by ethylenediamine（EDA）.The porous PVDF/GO/ZIF-67membrane is expected to combine the advantages of the two monomers for excellent permeability and outstanding antifouling property.The chemical composition,structure and surface properties of PVDF/GO/ZIF-67 membrane were characterized and analyzed by FTIR,SEM,AFM and WCA.The permeability,protein/dye separation performances and antifouling abilities of PVDF/GO/ZIF-67 membrane was evaluated.The results showed that the layered scaffold formed by the hydrophilic GO nanosheets embedded with ZIF-67 nanoparticles can greatly promote the rapid penetration of water molecules;the water contact angle reduced to39.8°;the value of pure water flux increased to 1369.8 L/（m~2·h）.In addition,the abundant oxygen-containing functional groups on the GO surface made the PVDF/GO/ZIF-67 membrane possess a high dye rejection.Thus,the static adsorption of BSA decreased to 61.8μg/cm~2;BSA and MB rejection increased to 89.54%and 86.47%,resepectively.Additionally,anti-fouling performances of PVDF/GO/ZIF-67 membrane was tested.The results showed that the membrane possessed the highest FRR（89.05%）and the smallest total resistance ratio（57.21%）,which demonstrated the anti-fouling ability of PVDF/GO/ZIF-67 membrane was enhanced.