Hydrophilic Modification Of PVDF,PVC Microporous Membranes

The main research of this dissertation focused on the hydrophilic modification of PVDF and PVC membranes.PVDF and PVC are widely applied in current membrane market. However,these membranes usually have some limitations such as low permeation flux and protein fouling due to the hydrophobic property.Different modification methods were tried to improve the hydrophilicity like surface grafting,material grafting and blending.Poly(vinylidene fluoride)(PVDF)membranes were pre-irradiated by electron beam,and then the hydrophilic sulfonate groups were introduced by the single-step grafting method with binary monomer solution of acrylic acid(AA)and sodium 4-Styrenesulfonate(SSS).The effect of irradiation dose,total monomer concentration,reaction temperature,reaction time, binary monomer ratio and pH of reaction solution on the degree of grafting was investigated. The surface chemical change was characterized by Fourier transform infrared attenuated total reflection spectroscopy(FTIR-ATR)and X-ray photoelectron spectroscopy(XPS). Morphological changes on the membrane surface were characterized by Scanning Electron Microscopy(SEM)and atomic force microscopy(AFM).The modified membranes were characterized through water contact angle,water flux measurement and fouling-resistant experiment.The water droplet can be completely adsorbed by the grafting PVDF membrane with 25.2%in 25s.The adsorption amount of bovine serum albumin(BSA)on the modified membrane decreased.The BSA solution permeation of modified membrane increased.The permeation flux could reach 0.90 after water or chemical cleaning.The results indicate the PVDF membranes have the better fouling resistance and filtration performance through the surface grafting of AA/SSS.To study the effect of the unique architecture of grafted chains on the membrane hydrophilicity,poly(ethylene glycol)methyl ether methacrylate(PEGMA)was grafted onto the membrane surface through the electron beam to form the comb-like polymer brushes.The degree of grafting was significantly influenced by irradiation dose,molecular weight of the monomer,reaction temperature,monomer concentration and the pH value of the reaction solution.The surface chemical changes were characterized by the Fourier transform infrared attenuated total reflection spectroscopy(FTIR-ATR)and X-ray photoelectron spectroscopy (XPS).Combining with the analysis of the nuclear magnetic resonance proton and carbon spectra(¹H NMR and ¹³C NMR),the PEGMA was mainly grafted onto the membrane surface. The surface mole content of PEGMA is 0.15 compared with the bulk mole content 0.01. Morphological changes were characterized by scanning electron microscopy(SEM)and atomic force microscopy(AFM).The porosity and bulk mean pore size changes were determined by a mercury porosimeter.The hydrophilicity was evaluated on the basis of static water contact angle,dynamic water contact angle and the dynamic adsorption process.The initial pure water flux of modified membrane increased from 180L/m²h to 229L/m²h.The BSA solution flux decreased slowly with increasing the degree of grafting.The flux of fouled membrane can be recovered more than 90%of the original one.All these results demonstrate that both hydrophilicity and fouling resistance of the PVDF membrane can be improved by the immobilization of hydrophilic comb-like polymer brushes on the membrane surface.Besides the membrane surface grafting,blending is also a convenient and promising method to improve the PVDF membrane hydrophilicity.Amphiphilic PVDF-g-PEGMA(P^a and P^b)was synthesized through ATRP in both DMAc and SCCO₂/DMAc,and then the blending PVDF membrane was prepared by the phase inversion method with PVDF-g-PEGMA as additives.The chemical structures of the copolymers were characterized by 1H NMR,and the PEGMA content was also calculated(W₁(P^a)=19.6wt%, W₂(P^b)=6.8wt%).Differential scanning calorimetry(DSC)showed that the melting temperature of PVDF-g-PEGMA decreased slightly,and the crystallinity of PVDF-g-PEGMA synthesized in SCCO₂/DMAc increased to 30.2%due to the induced nucleation of CO₂.The surface composition of membrane was determined by XPS.Significant surface segregation was observed to occur during the coagulation process.Adding solvent into the water bath would promote the surface segregation of PEGMA.The kinetics process for precipitation of PVDF blending membrane was studied through the light transmission experiments.The amphiphilic copolymer delayed dynamically the phase separation and made the hydrophilic chains have enough time to segregate to the membrane surface.The hydrophilicity of the membrane was characterized by the dynamic contact angle and dynamic adsorption process. Fouling resistance was characterized by the static BSA adsorption experiment.It was found that the segregation of amphiphilic PVDF-g-PEGMA improved the hydrophilicity and fouling resistance of PVDF membrane.Poly(vinyl chloride)(PVC)is promising as membrane materials for the economic and comprehensive properties.PVC powders were irradiated by electron beam in vacuum at 20KGy to produce living free radicals,and then reacted with acrylic acid(AA)in solution to obtain the PVC-g-AA copolymers.The copolymers were characterized by Fourier transform infrared spectroscopy(FTIR).Porous membranes were prepared from copolymers by the phase inversion technique.The morphology of membrane was studied by field emission scanning electron microscopy(FESEM).The mean pore size and pore size distribution were determined by a mercury porosimeter.The mean pore size was 0.19μm,and the bulk porosity was 56.0%.The apparent static water contact angle was 89.0°.The water drop penetration rate was 2.35 times to the original membrane.The maximum stress was 4.10MPa.Filtration experiments were carried out to evaluate the fouling resistance of the PVC-g-AA membrane. The results show that the modified membrane has higher permeation flux and better fouling-resistance.Amphiphilic copolymers including poly(vinyl chloride-vinyl acetate)(LCA),poly(vinyl chloride-vinyl acetate-maleic anhydride)(LCM),poly(vinyl chloride-vinyl acetate-vinyl alcohol)(LCAA),poly(vinylidene chloride-acrylonitrile-methyl methacrylate)(LAM)would segregate and self-organize on the membrane surface during the PVC blending membrane fabrication process by the phase inversion method.The thermodynamic compatibility of copolymers with PVC was evaluated by three-dimensional solubility parameters.The phase inversion behavior of PVC/LCA/DMAC/H₂O system was studied through the thermodynamic ternary phase diagram.Light transmission experiment showed that increasing the content of amphiphilic LCA would make the PVC/LCA/DMAc system change from instantaneous phase demixing to delayed phase demixing.The morphology of PVC blending was characterized by scanning electron microscopy(SEM).The surface composition was analyzed by X-ray photoelectron spectroscopy(XPS),the mole ratio(O/Cl)of PVC/LCM system is 1.13,the content of hydrophilic vinyl acetate and maleic anhydride chains on the membrane surface is 13.2%.The hydrophilicity of PVC blending membrane was evaluated by the contact angles. The contact angle of PVC/LCM membrane decreased from 85.3°to 35.3°in 200s.