Preparation And Performance Study Of Poly(arylene Ether) Antifouling Ultrafiltration Membrane

Membrane technology, especially ultrafiltration (UF), as an effective and powerfultechnique has in recent years superseded other traditional separation processes inindustry for removal of dispersed or dissolved contaminants from water and as itinvolves a number of attractive features mainly low energy consumption, mildoperating conditions, no additive requirements, no phase change andenvironmentally friendly. However, the use of ultrafiltration in most applications isoften limited by membrane fouling caused by inorganic and organic substanceswhich can adsorb on the surfaces or adhere to the pores, resulting in progressivedeterioration of membrane performance, alteration of membrane selectively andincrease in costs of energy and membrane replacement. Therefore, it is critical todevelop advanced antifouling ultrafiltration membranes that have high chemical andbiological stability, high resistance to fouling, and tailored separation ability to meetvarious demands. The excellent chemical and physical characteristics ofpoly(arylene ether) material make it an ideal candidate for the preparation ofultrafiltration membranes. However, the poly(arylene ether) membrane applicationin aqueous phase separation is limited by its inherent hydrophobic property. On theone hand, the hydrophobicity restricts the permeate flux that is a key parameter indefining the membrane performance; on the other hand, when feed solutionscontaining substances like proteins are filtered, the hydrophobic interaction betweenthe poly(arylene ether) membrane and protein molecules often causes nonspecificadsorption and deposition of proteins on the membrane surface or in pores, andresults in serious membrane fouling. In this dissertation, in order to improve themembrane resistance toward fouling, two different modification strategies, including(1) direct membrane material modification before membrane preparation, and (2) advanced surface modification or functionalization after membrane preparation hadbeen used. The more detailed studies are summarized as follows:First, ultrafiltration membranes were prepared with polyphenylsulfone (PPSU) asthe membrane-forming material and polyvinylpyrrolidone (PVPK30) as the additivethrough phase inversion technique, and dimethyl formamide (DMF) was chosen asthe solvent. The effects of the compositions of casting solutions (e.g. theconcentrations of PPSU and PVP) and the preparing conditions (e.g. the evaporationtime, coagulation temperature and humidity) on the membrane performances werethoroughly studied. It was found that with the increase of PPSU concentration in thecasting solution, the water flux decreased, and the rejection to BSA increased. Withthe increase of PVP concentration, the water flux firstly increased and thendecreased, but the rejection kept constant. The water flux significantly increased andthe rejection constantly decreased when the coagulation temperature increased. Withthe elongation of evaporation time, the water flux firstly increased and thendecreased, but the rejection slightly increased. With the increase of ambient humidity,the water flux firstly increased then decreased, but the rejection didn't havesignificant change. However, the flux recovery ratio was only55.7%for PPSUmembrane, meaning the existence of serious membrane fouling, especiallyirreversible membrane fouling.The second section focused on the synthesis of a series of sulfonatedpolyphenylsulfone (SPPSU) random copolymers with various controlled sulfonationlevels by using sulfonated monomer in direct copolymerization method and thepreparation of antifouling SPPSU ultrafiltration membranes via the conventionalimmersion precipitation phase inversion method. The chemical structures of theSPPSU copolymers were confirmed by using Fourier transform infrared spectrometer(FTIR) and their thermal properties were thoroughly characterized by differentialscanning calorimetry (DSC) and thermogravimetric analysis (TGA), indicating theirgood thermal stability. The morphologies of the SPPSU membranes were investigatedby scanning electron microscopy (SEM), and the morphology changes of these resultant membranes had been detailedly explained and verified by Hansen solubilityparameters (HSP) from thermodynamic theory. In addition, the surface hydrophilicityand charged property of the SPPSU membranes were studied by water contact angleand membrane potential measurements, and the results indicated that the sulfonationof membrane material is really an effective way to enhance the hydrophilicity andnegatively charge the membrane. The pure water flux and protein solution permeationthrough the prepared membranes were increased with the increase of the degree ofsulfonation. The cycle untrafiltration experiments for protein solution revealed thatnonspecific protein adsorption, especially irreversible protein adsorption, for theSPPSU membranes was significantly reduced, giving a good antifouling performance.However, the excess of sulfonation degree lead to the membrane excessive swollen,which is negative to the membrane mechanical performance.The third section was aimed at optimizing polymer design by employing especialmonomer in direct copolymerization method that provides a good opportunity toimprove the resultant membrane performance without sacrificing mechanical strength.A series of poly(arylene ether ketone) containing carboxylic acid groups(PAEK-COOH) random copolymers with various well controlled carboxylic acidgroup contents were synthesized by precise adjusting the ratios of different monomers.The morphologies of the PAEK-COOH membranes were investigated by SEM, andthe remarkably change of morphology had been explained. In addition, both watercontact angle and membrane potential experiments confirmed the highly hydrophilicand negatively charged characteristics of the PAEK-COOH membranes. The cycleultrafiltration experiments showed that the pure water flux and protein solutionpermeation through the PAEK-COOH membranes were increased with the increase ofthe content of carboxylic acid group. Meanwhile, the PAEK-COOH membranes hadhigher flux recovery ratio and lower extent of membrane fouling, especiallyirreversible membrane fouling, as compared to those of the SPPSU membranes,which displayed a better antifouling performance. This method can effectivelyenhance the hydrophilicity, negatively charge the membrane and significantlyimprove ultrafiltration membrane performance. The last section described the polymerization of a zwitterionic polymer,poly(2-methacryloyloxyethyl phosphorylcholine)[poly(MPC)] onto the poly(aryleneether sulfone) containing hydroxyl groups (PES-OH) membrane via redox graftpolymerization using ceric (IV) ammonium nitrate (CAN) as initiator. Due to thepresence of the activated hydroxyl groups which were used for the immobilization ofMPC monomer, the grafting process was facile and effective which avoid the complexand unfavorable pretreatment process and/or hydroxylated treatment. Attenuated totalreflectance Fourier transform infrared spectrometer (FTIR-ATR), X-ray photoelectronspectroscopy (XPS) and SEM were used to characterize the chemical compositionsand surface morphologies of the unmodified (PES-OH) and modified membrane(PES-g-MPC), respectively. Static water contact angle measurement indicated that theintroduction of poly(MPC) promoted remarkably the surface hydrophilicity of thePES-OH membrane. The cycle untrafiltration experiments for protein solutionrevealed that nonspecific protein adsoption, especially irreversible protein adsorption,for the zwitterionic membrane was significantly reduced, suggesting superiorantifouling performance. This work not only introduced a modification approach toobtain a PES-OH membrane grafting hydrophilic poly(MPC) chains, but also gave thezwitterionic membrane a long time life and excellent ultrafiltration performance.