Preparation Of Zwitterionic Polymer Modified Polyvinylidene Fluoride Ultrafiltration Membrane

Polyvinylidene fluoride(PVDF)is a kind of semi-crystalline polymer,which have excellent properties of high mechanical strength,thermal stability and chemical resistance,and have become a widely used separation membrane material.However,due to its strong hydrophobicity,poor wettability,and high permeation resistance,PVDF have low water flux.At the same time,organic pollutants and microorganisms of metabolites are easily adsorbed on membrane surface and block pores during the separation process,reducing the permeability and the final separation performance,thereby affecting the service life of the membrane.In this article,non-solvent induced phase separation method is used to achieve hydrophilic modification of PVDF membrane by doping with zwitterionic polymer in casting solution.Different conditions are invested to explore the membrane properties;the membrane structure is observed by electronic scanning electron microscopy,and the hydrophilicity of the membrane is measured by the contact angle.The membrane performance is evaluated by measuring the membrane flux,water flux,retention,and fracture strength of the membrane.Firstly,zwitterionic polymers are prepared by free radical polymerization and quaternary amination reactions.They are blended into casting solutions to improve the hydrophilic antifouling properties of PVDF membranes.The chemical structure and elements of the membrane surface are analyzed by infrared spectroscopy and X-ray photoelectron spectroscopy,and the results show that the zwitterionic polymer is successfully introduced into the membrane structure.To explore the effects of casting fluid composition PVDF,polyvinylpyrrolidone(K30),methanol and zwitterionic polymers on the structure and properties of flat membranes.The results show that with the increase of the PVDF mass fraction,the mechanical properties of the membranes are continuously improved,and the fluxes raise to the maximum and then begin to reduce,with the addition of the porogen K30,the porosity of the membrane surface increases,the flux increases,and the mechanicalproperties increase.After the K30 mass fraction exceeds 9%,the flux and mechanical properties decrease.With the increase of the mass fraction of methanol in the non-solvent,the membrane structure is changed from the finger pore structure to the sponge-finger structure,both the flux and mechanical properties show a trend of increasing first and then decreasing.With the increase of the mass fraction of the zwitterionic polymer in the casting solution,the contact angle of the membrane is decreased continuously,the pure water flux raises to the maximum and then begins to reduce,and the fracture strength decreases first and then increases.In addition,anti-pollution test is carried out on the membrane with bovine serum albumin as the pollutant source.The flux recovery rate of the blend membrane is significantly higher compares with the original membrane,indicating that the hydrophilic modification of the zwitterionic polymer can improve the membrane’s anti-pollution ability.Secondly,the effect of spinning process conditions on the structure and properties of hollow fiber membranes is investigated.The results show that with the increase of coagulation bath,the pure water flux,fracture strength,and flux recovery rate of hollow fiber membranes raise to the maximum and then begin to reduce,while the rejection rate decreases to a minimum and then increases.Increasing the temperature of the coagulation bath accelerates the rate of phase separation and transition of the zwitterionic polymer toward the membrane surface,thereby increasing the porosity of the membrane surface,the number of hydrophilic segments,and enhancing the antifouling performance.However,when the temperature of the coagulation bath continues to increase,some secondary relaxation of the polymer chains causes shrinkage of the membrane pores,thereby deteriorating the membrane performance.As the height of the dry spinning process increases,the membrane have larger flux,strength,and retention properties at the same time.The increase in the height of the dry spinning process results in a longer diffusion time for the solvent to diffuse into the core fluid,a thinner wall thickness,a less solvent diffusion resistance,a better hole penetration,.However,when the height of the dry spinning process exceeds 12 cm,the large amount of volatile solvent in the outer layer will be large,which will lead to a reduction in the number of pores in the outer skin layer and a significant decrease in membrane performance.As the draw ratio increases,the mechanical properties of the membrane are improved.The increase of draw ratio can provide a strong fiber axial tensile stress,which makes the macromolecules of primary fibril membranes easier to arrange in the axial direction.But when the draw ratio is too large,it will cause defects in the hollow fiber membrane structure.,thereby reducingthe strength and elongation at break.By optimizing the preparation conditions,the water flux of the hollow fiber membrane reaches 190L?m-2?h-1,the retention rate is 91%,the recovery rate of flux is90.4%,the breaking strength is 3.35 Mpa,and the elongation at break is 164%.It will have a good development prospect in the fields of water treatment and medical treatment.