| China has gradually become a major force in the global oil market as world energy demand and downstream markets have grown.But while the development of the oil industry has brought huge economic benefits,the oily wastewater generated by it also has an impact on the ecological environment and human health that could not be ignored.Researchers are working to develop oil-water and oil-oil separation membrane materials with superior performance,strong operational stability,corrosion resistance and superior mechanical properties.Organic polymer membranes are currently the most widely used substrate for membrane separation due to the advantages of good membrane formation properties,strong mechanical properties,and low cost.However,traditional polymer membrane materials,such as poly(vinylidene fluoride)(PVDF)membrane and poly(phenylene sulfone)(PPSU)membrane,exhibit poor solvent resistance and cannot stably separate oil-water mixtures such as chloroform-in-water emulsion for a long time.This limits the wider application of the conventional polymer membrane materials in the separation field.To solve the problem of poor solvent resistance of the traditional polymer membrane materials,an effective strategy is to process poly(ether ether ketone)(PEEK)and polyimide(PI)and other special engineering plastics to make membranes.Because of its crystalline structure,PEEK is resistant to almost any chemical corrosion and have excellent solvent resistance.With its strong mechanical properties,excellent thermal stability and extremely low water uptake compared to other specialty polymers,PEEK has great potential for applications in the area of solvent-resistant separation membranes.However,the strong solvent tolerance of PEEK makes it difficult to process the membrane via solution processing,and the limited adjustable parameters of commonly used thermally processed membranes make it difficult to accurately regulate the pore structure of the PEEK membrane to meet the requirements of molecular sieving.To solve this problem,the researchers use strong acid such as concentrated sulfuric acid to dissolve the PEEK and then make the membrane,but in the process the PEEK would undergo the sulfonation reaction and the crystallinity of the PEEK membrane would experience irreversible changes.Therefore,it is necessary to control the degree of sulfonation of the PEEK in a certain range to ensure the solvent resistance of the PEEK membrane,so that the parameter tuning space of the strong acid membrane method is limited.It is difficult to regulate the membrane pore structure accurately on demand.To solve this problem,the precursor polymer membrane is prepared by non-solvent induced phase separation(NIPS),and then the solvent-resistant PEEK separation membrane is successfully prepared by acid-catalyzed hydrolysis,solving the bottleneck of PEEK membrane difficult to be processed by solution.The advent of the theory of special wettability at interfaces has provided a very inspiring research idea for the realization of high-quality and multi-functional separation membrane materials.Through surface modification and other interfacial engineering,the separation membrane surface is endowed with different structural properties from the bulk and a functional separation layer is constructed.Not only that,but the problem of polymer separation membranes being highly oleophilic and the separation process being susceptible to the contamination by oil can be improved by the construction of the hydrophilic interfaces.In summary,the interface could be constructed on the surface of the PEEK membrane as required to improve the antifouling performance,extend the membrane lifespan,and improve the quality of the PEEK membrane material while realizing a versatile PEEK membrane.The main findings of this paper are as follows:1.The Schiff base bis(4-fluorophenyl)-N-phenylmethanimine monomer is prepared by modifying the carbonyl group of 4,4’-difluoro diphenylmethanone with aniline,and then through the nucleophilic polycondensation reaction of Schiff base monomer and hydroquinone monomer,the polymer precursor PAEN containing ketoimide groups is prepared.Benefiting from its amorphous morphology,PAEN has excellent solubility and could be processed into membranes via NIPS method.Subsequently,under acid catalysis conditions,the aniline group in the PAEN membrane is converted to the carbonyl group and a series of characterization confirms that the polymer membrane is transformed into the PEEK membrane with the crystalline structure.Different from the hot processing method and the strong acid solution method,the method has the advantages of simple operation,high flexibility,and accurate adjustment of the membrane pore structure on demand,which can be used for the large-scale production of solvent resistant PEEK separation membranes.Subsequently,a series of PEEK-g-PAA-x separation membranes are obtained through interface engineering by using the“self-initiation"property of PEEK membrane and introducing poly(acrylic acid)on the PEEK membrane surface by UV irradiation to construct a pH-responsive negatively charged interface.Among them,the PEEK-g-PAA-2 membrane with the best performance could effectively separate a series of oil-in-water emulsions and organic liquid mixtures,and the separation flux and rejection of the hexane-in-water emulsion are 132.9 L·m-2·h-1 and 99.9%,respectively.The separation flux and rejection of the formamide/hexane mixture are 71.65 L·m-2·h-1 and 99.1%,respectively.The PEEK-g-PAA-2 membrane has excellent reversible pH response,and its separation flux can be tuned in different pH environments.Benefiting from the good solvent resistance of the PEEK membrane substrate,the PEEK-g-PAA-2 membrane exhibits good solvent resistance to the solvents,and its surface morphology does not change significantly after 6 h of immersion in chloroform.2.The temperature responsive functional polymer poly(N-isopropyl acrylamide)(PNIPAm)is grafted onto the PEEK separation membrane surface after ultraviolet irradiation,and the temperature responsive functional interface is constructed to prepare the PEEK-g-PNIPAm separation membrane.When the temperature is lower than LCST,intermolecular hydrogen bonds are formed between the amide groups in PNIPAm and the water molecules,so the PEEK-g-PNIPAm membrane surface is superhydrophilic/underwater superoleophobic.The PEEK-g-PNIPAm membrane could effectively separate a series of oil-in-water emulsions,and the flux and rejection of the hexane-in-water emulsions are 393.3 L·m-2·h-1 and 99.9%,respectively.When the temperature is higher than LCST,the hydrogen bonds between the amide group and water are broken and the PEEK-g-PNIPAm membrane surface is hydrophobic/superoleophilic,the PEEK-g-PNIPAm membrane could effectively separate a series of water-in-oil emulsions.The separation flux and rejection of the water-in-hexane emulsions are 289.0 L·m-2·h-1 and 99.7%,respectively.In addition,benefitted from the solvent resistant PEEK membrane matrix and temperature responsive interface,the PEEK-g-PNIPAm membrane can also be used to separate different types of organic liquid mixtures at different temperatures,effectively separating formamide/hexane mixtures at the temperature below LCST,the separation flux and rejection are 117.7 L·m-2·h-1 and 99.2%,respectively.When the temperature is higher than the LCST,the PEEK-gPNIPAm membrane could effectively separate the carbon tetrachloride/formamide mixture,the separation flux is 156.6 L·m-2·h-1,and the rejection is 99.8%.This work is the first to apply the temperature response property to the oil-oil separation field.Compared to the conventional membrane distillation methods,this work has the advantage of low energy consumption,small footprint and short time.At the same time,the PEEK-g-PNIPAm membrane could also be applied to strong acid/organic product systems for efficient separation of target products with the rejection up to 99.4%.In summary,the temperature-responsive PEEK-g-PNIPAm membrane could be used for the separation of more complex systems and has promising applications in oil-water and oil-oil separation.3.Due to the small pore size of the PEEK membrane matrix,the PEEK membrane flux obtained by the first two issues remains low even after the hydrophilic interface construction modification.Aiming at this bottleneck,this work optimizes the pore size of the PAEN porous membranes to increase the membrane flux.The PAEN porous membrane is transformed into the PEEK separation membrane by acid catalyzed hydrolysis.After UV irradiation,poly(acrylic acid)is grafted on the PEEK membrane surface.By using electrostatic attraction,poly(dimethyl diallyl ammonium chloride)and poly(sodium-p-styrenesulfonate)self-assemble layer by layer on the poly(acrylic acid)grafted membrane surface,creating a multilayer hydrophilic interface and improving the mechanical stability of the hydrophilic layer on the membrane surface.The service stability of the membrane is enhanced,and the pore size is further reduced to ensure the efficient emulsion separation.When the number of self-assembled layers is 6,the M6 membrane shows the best comprehensive emulsion separation performance,the separation flux of the hexanein-water emulsion flux is 731.2 L·m-2·h-1,and the rejection is more than 99.0%.After 15 consecutive separations of the hexane-in-water emulsion,the flux recovery rate of the M6 membrane is 96.9%,demonstrating the excellent antifouling capability.Moreover,thanks to the hydrophilic modification of the layer-by-layer self-assembly,the anti-fouling capability of the self-assembled membrane is enhanced and it has excellent anti-oil adhesion and self-cleaning properties.The high-flux layer-by-layer self-assembled modified membranes obtained in this work have promising applications in oil-water separation. |
