Study On Fabrication Of Charged Polyethersulphone Ultrafiltration Membrane And Its Surface Electrokinetic Phenomena

In this work, a series of Physical and Chemical methods were employed to modify the Polyethersulphone ultrafiltration membrane. That is to say, the optimization of membrane structure and the endowment of special functions to membrane were tried to achieve by the modification of membrane materials and manufacturing methods of membrane, respectively.Firstly, in order to improve the structure and properties of PES UF membrane, several kinds of membrane fabricating technologies, including the addition of non-solvent (CH₃COOH) into the casting solution and salt (Na₂CO₃) into the coagulant, were tested. It was worth noting that the following chemical reaction takes place when the CH₃COOH and Na₂CO₃ were employed as additives in the casting solution and the coagulant, respectively, at the same time:2 CH₃COOH + Na₂CO₃ → 2 CH₃COONa + CO₂ ↑ + H₂OTherefore, the effects of the chemical reaction on the structure and properties of membrane were studied in detail besides the influence of non-solvent additives and salt additives. The experiments showed that the addition of additives into the casting solution and the coagulant and the chemical reaction between the additives of casting solution and coagulation medium played significant roles in the PES membrane morphology and performance. For an example, the chemical reaction can facilitate the mass transfer between the casting solution and coagulant. Furthermore, the aggregate and regular release of the gas product produced by the reaction may contribute to improve the interconnectivity, enlarge the mean pore size regularly, and even increase porosity. Hence, an increase of permeability and relatively narrow pore size distribution were observed at the same time. In a word, the PES membrane structure and performance could be ameliorated to a certain extent by means of the phase inversion method modified by the chemical reaction.Secondly, the charged PES UF membranes were prepared by blending with the synthesized charged polymer, PAN-c-AMPS. In the experiments, some important parameters and properties of membranes, such as flux, pore size, surface and cross-sectional morphology, chemical composition of membrane surface, charged state of membrane surface and their antifouling abilities, were investigated in detail. The ATR-FTIR, ¹H NMR, Elements Analysis and Viscosity measurement showed that the charged polymer, PAN-c-AMPS, could be synthesized through the water phase precipitation copolymerization process in which the K₂S₂O₈/Na₂SO₃ were used as initiator. FESEM images of membrane surface and cross-section displayed that pore size decreased, the porosity increased, the top layer became thicker and the finger-like macrovoids disappeared gradually as the PAN-co-AMPS was added. From the surface analysis by XPS experiment, ATR-FTIR experiment, tangential streaming potential measurement and static water contact angle measurement, it was found that the charged groups tend to accumulate onto the membrane surface. This result indicatedthat membrane surface modification for imparting surface electrical properties could be carried out by blending charged polymer. The solute rejection experiments in which the PEG and KC1 was used as solutes, respectively, showed that the retention of the charged membranes could be achieved by means of the steric effects of membrane pores and the electrochemical interaction between the membrane and charged solutes. In addition, based on a relatively elaborate study on the electrostatic interaction between the membrane surface and protein, it was found that these charged membranes could meet different demands of membrane applications, such as resisting protein fouling or protein separation, through adjusting solution pH value.Thirdly, the charged PES UF membranes were manufactured by surface graft polymerization of methylacrylic acid initiated by redox system. Surface chemical changes and membrane morphology changes before and after graft polymerization were investigated by the ATR-FTIR and FESEM, respectively, to ascertain the formation and location of graft. In the hydraulic permeability experiments and diffusional permeability experiments using VB₁₂ and KC1 as solutes, the grafted membranes prepared using the reported method exhibited marked, rapid and reversible pH-response. In addition, the experiments, such as the observation of the graft degree under different reactional conditions, the charged state of membrane surface and the antifouling abilities of membranes were carried out systematically.At last, in order to obtain the information on the charged properties of the membrane and even the change of pore size, the electrokinetic phenomena occurring through and across membrane were studied systematically. Firstly, the perpendicular streaming potential measurement was carried out to evaluate the effect of membrane fabricating technologies on pore sizes based on the understanding of the relation between the pore size and streaming potential occurring through membrane pore by means of the irreversible process thermodynamics and the space charge model. In addition, a tangential streaming potential measurement system was successfully fabricated on the basis of our previous work. Furthermore, a relatively novel way that using constant current DC to measure the system total conductance was incorporated into the measurement system. The experimental results showed that both the preparing method of Ag/AgCl electrode and the four-electrode mode used to measure total conductance were simple and effective. The experimental results also displayed the prominent contribution of surface conductance and body conductance to the determination of Zeta potential values. And, it was noteworthy that, from the profiles of Zeta potential versus pH curves and the magnitude of Zeta potentials, the determination of Zeta potential was dependent not only on the electrical properties of membrane surface but also on its hydrophilicity.