Surface Antifouling Modification And Performance Study Of Polyimide Membranes

The sustainability of water resource plays a critical role for the human survival and the economic development.With the growing global population and severer water pollution,there is a pressing need to address the water scarcity issue.Trace irrigation technology is a novel water-saving technology with double filtration.The core device of the irrigation pipe is composed of the microfiltration membrane with relative larger area and smaller pore size,as well as bundles of capillary tubes with relative smaller area and larger pore size.Since common polymer membrane materials can't meet the high temperature requirement in the processing,polyimide membranes with with excellent thermal stability,high solvent resistance,and desirable mechanical property are studied in this work as the microfiltration membrnae for this application.However,similar with other membrane for the water treatments,the membrane fouling is always a critical issue against the stable performance and real applications of polyimide membranes,resulting in the decline in the separation performance and the membrane lifetime,as well as the increase in the operational cost.The membrane antifouling property can be enhanced via the alteration of the surface hydrophilicity,charge,roughness,chemical composition and porosity.However,the antifouling property and corresponding modification studies of polyimide membranes have seldom been reported so far.With the financial support of National Key Technology Support Program "Research and Large-scale Demonstration of Water-saving and Grain-Growing Irrigation Technology in Four Provinces in Northeast China",the preliminary study on the antifouling modification and property of PI membrane has been carried out in this thesis.By grafting the antibacterial materials(silver particles)and/or hydrophilic materials(zwitterions)onto the PI membrane surface,the improvement of the antifouling property of modified PI membranes is achieved.Various techniques including scanning electron microscopy,atomic force microscopy,energy-dispersive X-ray spectroscopy,fourier transform infrared spectroscopy,and so on,were employed to investigate the membrane surface morphology,hydrophilicity,charge,etc.The main methods and contents of this thesis can be summarized as follows:(1)By grafting silver particles onto the PI membrane surface via a simple three-step reaction,stable bonding of silver particles with the PI matrix is achieved.The effect of silver modification on the membrane morphology and physicochemical properties is studied.The antifouling property of modified membranes was also investigated by bovine serum albumin(BSA)fouling test,disk inhibition test and surface adhered aqueous suspension of Escherichia coli and Staphylococcus aureus.The temperature effect in the thermal treatment step is studied in terms of the membrane morphology,anti-fouling property as well as the silver stability in the membrane.(2)PI membrane is modified by grafting short-chain sulfonic-type zwitterions on the surface via a simple two-step reaction with N,N-diethylethylenediamine and 1,3-propane sultone sequentially,to improve its antifouling property.The chemical structures,compositions and morphology of the modified membranes were investigated with various characterization techniques.The anti-biofouling and antifouling properties of the zwitterion-modified membranes were evaluated by the bacterial suspension immersion tests and BSA fouling test systematically.(3)Surface modification of PI membrane by grafting AgNPs and SBMA simultaneously are also performed to improve the membrane hydrophilicity and antifouling properties.Polyethyleneimine with abundant amine groups on the main chains are used as the linker of PI membrane and AgNPs,to stabilize AgNPs through the formation of PEI-AgNPs coordination complexes.The reaction mechanism involved,the membrane morphology and physicochemical properties were investigated with various characterization techniques.The antifouling performance was studied by the BSA filtration test,FITC-BSA adsorption,zones of inhibition,and bacteria suspension immersion tests respectively.Through the above research,the main innovative works can be summarized as following:the introduction of silver particles onto the membrane surface can endow the antibacterial property to the PI membrane,and improve its anti-biofouling property;the introduction of hydrophilic zwitterionscan improve the membrane surface hydrophilicity significantly which makes it difficult for hydrophobic foulants to be adsorbed on the membrane surface;the introduction of both AgNPs and SBMA can improve the membrane surface hydrophilicity and anti-bacterial property simultaneously.Above studies show that,the antifouling property of all modified PI membranes has been improved significantly.This thesis should be able to provide theoretical and technical support for the preparation of polyimide membranes with high antifouling properties.