Surface Hydrophilization Tailoring Of PVDF Separation Membrane And Properties Investigation

Global water stress is a major challenge to the development of modern society and is amplifying due to climate change,population growth,water pollution,and other factors.Membrane separation has gradually become a critical technology in the field of water treatment by virtue of absence of phase change,low energy consumption,and high integration degree.At present,the most widely used separation membranes are mostly made of polymer materials,but the hydrophobicity of polymer membranes such as poly（vin ylidene fluoride）（PVDF）makes them vulnerable to the adsorption and aggregation of pollutants during water treatment,leading to membrane pollution,resulting in the reduction of membrane separation efficiency and the increase of operation cost,meanwhile,hydrophobic polymers with chemical inertness lead to hard interface modification.Thus,effective interface modification strategies are necessitated to improve membrane hydrophilicity and antifouling capacity.Inspired by mussel bio-adhesion,a series of biomimetic coating methods were developed for membrane interface hydrophilization.The coating mechanisms and the effects on the hydrophilicity,surface morphology,separation performance,antifouling capacity,and stability of the modified membrane were systematically studied.Herein,dopamine（DA）and tannic acid（TA）were used on the interface modification of PVDF microfiltration and ultrafiltration membranes.The phenolic hydroxyl groups and oxidized quinone groups of TA could crosslink with amino groups in the precursor of polydopamine.The hydrophilicity of DA/TA co-deposited microfiltration membrane was greatly improved when compared with membranes modified by the single component;its water contact angle was significantly reduced to 7.8°.The optimized hydrophilic microfiltration membrane has excellent underwater super-oleophobicity,the initial flux during toluene-in-water emulsion separation was up to 2126 L m^-2 h^-1 bar^-1,and demonstrated excellent cycling separation efficiency,and the oil reject ion rate was more than 99.6%.The DA/TA co-deposited coating had excellent mechanical stability and could withstand bending and water shearing tests.The rejection rate of BSA protein solution by modified ultrafiltration membrane was about 92.3%,the water flux was about 498.3 L m^-2 h^-1 bar^-1,and the adsorption capacity against BSA pollution was increased by 83.3%.Inspired by mussel adhesive byssus proteins,and considering the limited hydrophilicity of polydopamine coating itself and the high price of do pamine monomer,we proposed the biomimetic“phenol&crosslinker”composite coating concept of dopamine“split-and-adjust”,and developed a series of economic and universal composite coatings based on various phenols.Biomimetic composite coating of gallic acid（GA）andγ-amino-propyl-triethoxysilane（APTES）was realized by covalent crosslinking reaction of phenol hydroxyl structure（catechol and pyrogallol groups）with amino groups as well as the synchronous hydrolysis of siloxane.The hydrophilic silicon oxide was in situ introduced,and the hydrophilicity of modified microfiltration membrane was greatly improved.The initial water contact angle could reach 7.5°.The initial flux of separation can reach2406 L m^-2 h^-1 bar^-1,and the oil rejection is more than 99.5%.The water absorption rate was about 81.4%,the BSA rejection rate of the modified ultrafiltration membrane was about 96.6%,the water flux was about 278.2 L m^-2 h^-1 bar^-1,and the BSA anti-adsorption capacity was increased by 70.8%.In addition,the composite coating system based on highly hydrophilic TA was studied,the biomimetic composite coating of TA and cysteamine（Cys）was constructed by the covalent crosslinking reaction of phenolic hydroxyl structure and amino and thiol groups.The water contact angle of the modified microfiltration membrane could reach 5.5°,and the initial flux of emulsion separation could reach 2036 L m^-2 h^-1 bar^-1,the oil rejection rate was more than 99.5%.The rejection rate of BSA protein solution of modified ultrafiltration membrane was about 95.4%,the water flux was about 400.7L m^-2 h^-1 bar^-1,and the adsorption resistance capacity to BSA was increased by78.0%.In addition,the construction of TA and 3-glycidyl propyl trimethoxy silane（KH560）biomimetic composite coating was realized.The initial flux for oil-in-water emulsion separation was up to 2690 L m^-2 h^-1 bar^-1,the oil rejection rate was more than 99.2%,the rejection rate of modified ultrafiltration membrane to BSA protein solution was about 94.1%,and the water flux was about 424.4 L m^-2 h^-1bar^-1,and the resistance to BSA adsorption was increased by 73.2%.Based on the adhesion and reactivity of polyphenols and the high hydrophilicity of metal oxides,and considering the low efficiency of atomi c layer deposition（ALD）reaction on polymer films,a strategy of polyphenol pretreatment sensitized atomic layer deposition reaction for membrane interface modification was proposed.When the concentration of polyphenol TA aqueous solution was 10 mg/m L,the sensitized effect of ALD coating was the optimized,and the dynamic contact angle results showed that the hydrophilicity of TA sensitized ALD modified membrane with more than 30 ALD cycles was greatly improved.The mechanism of"island nucleation and film intergrowth"of sensitized ALD for polymeric membrane interface enhancement coating was proposed,the excellent hydrophilicity and stability of the modified coating were explained.The molecular interaction mechanism of sensitized coating was verified by molecular dynamics simulations and density functional theory calculations.The simulation results showed that phenolic structure and ALD metal organic precursor demonstrated close interactions,the order of interaction strength was carbonyl oxygen>hydroxyl oxygen>ether bond oxygen,which provides a reference for the exploitation of universal sensitized ALD coatings.The small molecular phenol gallic acid was used to verif y the universality of polyphenol sensitized Ti O₂ ALD strategy,and the highly hydrophilic polymeric membranes were successfully prepared.The TA sensitized ALD modified membrane demonstrated excellent separation performance and antifouling capacity to the crude oil-in-water emulsion.The mechanical stability test results showed that the TA sensitized Ti O₂ ALD coating had good adhesion.