Bioinspired Fabrication Of Ultrafiltration Membranes And Tentative Exploration Of Antifouling Mechanism At Molecular Level

Membrane fouling constitutes a bottleneck limiting the efficient and wide application of polymeric membranes. Investigating the antifouling mechanism and developing methods to inhibit membrane fouling have become one of the most important research issues in membrane separation area. Learn from nature, investigating the antifouling property, composition, structure, and formation procedure of biomaterials, offers a new avenue for the design of functionalized materials. The target of this thesis is solving fouling problem of polymeric membranes in bio-separation process. Based on the understanding of the composition, structure, and performance of cell membranes, we designed modifying agents (amphiphilic block polymer and zwitterionic polymer) and membrane formation method (self-assemble technique) rationally, made the bioinspired fabrication of antifouling membranes.Starting from molecular design protocol, using polyethersulfone (PES) as base membrane material, we conducted chemical modification to PES. The synthesized material retained the excellent membrane forming property and physical, chemical stability of PES, while endowed PES with hydrophilic property and biocompatibility. Inspired by the structure of cell membrane (asymmetric superstructures with both hydrophilic and hydrophobic area), we grafted polyethylene glycol (PEG) onto PES backbone, made the formation of amphiphilic block polymer―pegylated PES. Inspired by the chemical composition of cell membrane (zwitterionic phospholipids), we prepared tertiary amine-modified PES (TA-PES), which had both positive and negative charges on its backbone, made the formation of zwitterionic membrane material. After doing chemical modification to PES successfully, we used phase inversion method to prepare ultrafiltration membranes. It was found that employing modified PES as membrane material or the composition of membrane material, the as prepared membranes showed dramatically increased hydrophilicity. The grafted hydrophilic functionalities had strong interaction with water on membrane surface, made the formation of hydration layer, which could repel protein adsorption and accumulation on membrane surface. Ultrafiltration results indicated the modified membranes showed improved antifouling property. The membranes could be used for several runs. Amino acids are natural zwitterionic materials. We chose amino acids as membrane modifying agent. Lysine (Lys), glycine (Gly), and serine (Ser) were grafted onto polyacrylonitrile (PAN)-based membrane surface via the formation of amide bonds between amine groups and carboxylic groups on hydrolyzed PAN membrane surface, respectively. The protein resistant property of PAN-amino acids membranes was studied in detail. The results showed that amino acids modified membrane with similar hydrophilicity might have different antifouling property. The net charges of amino acids grafted on membrane surface determined the protein resistance. Molecular simulation results confirmed that the hydrogen-bond between water molecules played a key role in fouling resistance of PAN-Lys membrane.Sum-frequency generation vibrational spectroscopy (SFG) and XPS were carried out to obtain fouling resistant mechanism at molecular level. The surface composition and structure of amphiphilic polymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronic) and zwitterionic polymer modified films were studied in situ. The results showed that in air the hydrophobic material or segment would cover the film surface, while the hydrophilic counterparts were buried within polymer matrix. Upon water exposure, the hydrophilic segments tend to segregate to surface and fully extend in water. The structure of water molecules near zwitterionic surface was also studied. The results provided direct evidence for the hydration layer antifouling mechanism.