Research On The Modification Of Polyvinylidene Fluoride Membrane’s Preparation And Properties

Poly（vinylidene fluoride）（PVDF）is a kind of semi-crystalline polymer,which has excellent performance such as corrosion resistance,heat resistance,ageing resistance,radiation resistance and chemical stability.Moreover,it also has good mechanical properties and membrane-forming property.Above properties make PVDF used in water treatment materials widely.However,an extremely low surface energy of PVDF make it has a strong hydrophobicity,which leads to strong driving force and contaminate in water treatment process.This decreased the membrane working time and increased costing.Therefore,it’s necessary to use effective method to improve hydrophilic performance and other special function of PVDF membrane.In This paper,membrane-forming process of PVDF was research.Besides,methods of in situ cross-linking and surface modification were taken to improve hydrophilic performance of PVDF membrane and discussed the influence of modification process to PVDF’s properties.Besides research the adsorption capacity of modified membrane to Cu²⁺ which was common heavy metal in waster water.Firstly,PVDF porous membrane was successfully prepared and DMAc used as a solvent,PVP used as pore-forming agent by a solution phase inversion method.The effects of casting solution concentration,ratio of pore-forming agent,coagulation temperature were analyzed.The results show that,with the increasing of casting solution concentration from 11% to 20%,the porosity of PVDF membrane was decreased,the pore number in membrane surface was decline,bovine serum albumin（BSA）rejection rate was increased in slow speed.With the increasing of pore-forming agent（PVP）concentration from 0.5 to 5%,PVP as water-soluble polymer promote the process of phase separation from liquid-solid phase to liquid-liquid phase,the porosity and water flux were increased,however,when pore-forming agent（PVP）concentration reach 1%,the effect of improve porosity and water flux was not obvious.With the increasing of coagulation temperature form 15℃ to 55℃,the double diffusion rate speed up in the process of phase separation,which make liquid-solid phase convert to liquid-liquid phase,the porosity and water flux of PVDF membrane were increased,bovine serum albumin（BSA）rejection rate was decreased.PVDF membrane have balanced performance when casting solution concentration is 15%,pore-forming agent（PVP）concentration is 1%,coagulation temperature is 25℃,water flux is 172 L/（h·m²）,porosity is 77.2%,bovine serum albumin（BSA）rejection rate is 13.5%.Secondly,the method of in situ cross-linking was used to make acrylic acid（AA）and vinyltriethoxysilane（VTES）to modified PVDF membrane.PVDF/AA（wt/wt）ratios were 10/0,10/2,10/3 and 10/4.The change of modified and unmodified membrane’s surface and section morphology were analyzed by field emission scanning electron microscopy（FESEM）and atomic force microscope（AFM）.Besides,research the modified PVDF membrane’s hydrophilic performance,thermal performance,mechanical property,water flux,BSA rejection ratio and adsorptive property of Cu²⁺.The results show that,the pore in the membrane surface reduced with the introduction of AA and VTES to PVDF membrane.Meanwhile,finger-like pore decreased in section,which looks like sponge.When PVDF/AA（wt/wt）ratio is 10/2,finger-like pore content was little in section,water flux greatly decreased,BSA rejection ratio increased.When PVDF/AA ratio reached to 10/4,the initial contact angle of PVDF membrane decreased from 82.6° to 45.4°after modified,the hydrophilic performance of PVDF membrane was improved.When membrane treated by ethanol in 48 h,the initial contact angle of modified membrane is 50°,which dosen’t have significant change.AA has good stability in membrane by in situ cross-linking method.When PVDF/AA ratio was 10/2,the breaking strength and breaking elongation of modified PVDF membrane was significantly improve.With the increase of AA content,the breaking strength and breaking elongation were decreased,but still better than unmodified PVDF membrane.Research on adsorption ability of Cu²⁺,with the increase of AA content,the adsorption ability of Cu²⁺ is better.With the increase of solution p H,modified membrane’s adsorption ability of Cu²⁺ was reduced after growth.When p H was 6,PVDF/AA ratio was 10/4,modified membrane’s adsorption ability of Cu²⁺ was 13.3μg/cm².Repeated adsorption regeneration research,after four cycle,modified membrane’s adsorption efficiency was upon to 94%,which means modified membrane had good repeat adsorption capacity.Finally,dopamine-modified poly（vinylidene fluoride）membrane（PDA-PVDF）was synthesized by using dopamine modifying in the PVDF membrane surface,which was subsequently grafted with polyethylene imine（PEI）to obtain PEI/PDA-PVDF membrane.Fourier Transform Infrared Spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）were used to characterize the surface element content and functional groups after modification,while the contact angle,BSA rejection rate,as well as surface morphology of the membrane were also obtained,respectively.Last but not least,the adsorption performance was studied between the membrane and Cu²⁺.Specifically,in contact with PVDF membrane,the roughness of PDA-PVDF membrane is larger.With the increasing of reaction time,there are several granular substances in the surface,blocking some pores.However,the substances almost disappeared when the reaction time increased to 9h,on account of the formation of PDA.After grating with PEI for 6h,the ratio of element N and O was increased from 0.38 to 1.06,indicating that the PEI was successfully grafted to the PDA-PVDF membrane.The surface permeability was visibly increased and the contact angle increased from 5° to 25° within 2min.After grafting,the study of adsorption performance with Cu²⁺ showed that when solution p H higher,the adsorption capacity was better.When p H is 7,modified membrane’s adsorption ability of Cu²⁺ was 32.43μg/cm².Isothermal adsorption of PEI/PDA-PVDF membrane and copper ions conforms to the Freundlich model,while dynamic adsorption accounts with the pseudo-second order equation.Repeated adsorption regeneration research,after four cycle,modified membrane’s adsorption efficiency was upon to 90%,which means modified membrane had good repeat adsorption capacity.