Preparation Of Cellulose Reinforcedelectrospun Microfiltration Membranes And Its Oil-Water Emulsion Separation Performance

The study was to prepare a high-performance nanofiber membrane for oil-water separation,using cellulose nanocrystals（CNC）extracted from poplar wood as the raw material,and polyacrylonitrile（PAN）,polyvinylidene fluoride（PVDF）and other polymers as the precursor of the spinning solution.Through electrospinning technology,combined with chemical treatment,a novel nanofiber membrane with a hierarchical structure was prepared.Carry out systematic research on improving the mechanical properties,structural stability,surface wetting properties,and oil-water separation performance of membranes.Study the preparation mechanism and shape control of membranes to realize the controllable properties of membranes.On this basis,explore the relationship between functionality and oil-water emulsion separation,the influence of CNC on the wettability of membranes and the separation performance of oil-water emulsions were explored,which provide theoretical basis and technical support for electrospinning in the field of membrane filtration,and provide more opportunities for the preparation of super-wet membranes.This has important practical significance for the sustainable utilization and high-value conversion of wood resources.The research contents are as follows:（1）CNC-reinforced PAN nanofibers were successfully prepared by electrospinning.The effects of different CNC loadings on the microstructure and mechanical properties of nanofibers were investigated.The results showed that smooth nanofibers with a diameter of about 200-700 nm could be obtained.The addition of CNC improved the mechanical properties of PAN nanofibers.When the amount of CNC added was 5 wt.%,the membrane has the best mechanical properties.The tensile strength was increased by 5.9MPa,20.4%higher than that of PAN membrane.（2）The electrospun membranes were successfully synthesized based on hydrolyzed polyacrylonitrile（H-PAN）reinforced with CNC.The effects of CNC on the morphology,structural stability,surface wetting properties and separation of oil-in-water emulsions of membranes were investigated.The results showed that the membrane had a highly porous structure,and after the addition of CNC,the nanofibers were held together by cross-linking,which allowed rapid mass transport,while achieved high tensile strength and elongation at break.The membrane exhibited shape retention upon immersion in complex fluids.An underwater oil contact angle of 141°enables efficient emulsion separation(98.2%separation at a flux as high as 1293 L·m^-2·h^-1 for hexane-in-water emulsions)and reliable operation for at least 20filtration cycles.A similar performance was achieved in the separation of emulsions based on toluene,petroleum ether,diesel,and vegetable oils.Overall,the designed composite membranes endow stable three-dimensional structures,excellent durability,and separation performance.（3）An underwater oleophobic CNC/hydrolysis-polyacrylonitrile（H-PAN）membrane with a hydrogen bond interlocking effect and polydopamine（PDA）adhesion layer was fabricated.The synergistic mechanism of CNC and PDA was investigated,and its influence on the pore size,mechanical wettability and the separation performance of the membrane.The results showed that the surface roughness and pore structure of the membrane were improved by the self-aggregation of DA.The CNC/H-PAN/PDA nanofibers had an average diameter of 0.44 um.,PDA could still firmly adhere to the nanofiber surface after sonication or homogenization.The tensile strength of the CNC/H-PAN/PDA membrane was 4.19 MPa,the elongation at break was 44.4%,and the surface roughness was 2.86.The pore size distribution of membrane was centered around 0.67μm,which belongs to the single-layer reversible adsorption on the macro-porous solid surface.The specific surface area of membrane was 10.59m² g^-1,which was higher than that of CNC/PAN and CNC/H-PAN membranes.CNC/H-PAN/PDA membrane had excellent hydrophilicity,and underwater oleophobicity,and its special wettability and pore structure can trap water in the layered structure to form oil/water/solid interface,which reduces the contact between the oil droplet and membrane.Compared with the CNC/H-PAN membrane,the CNC/H-PAN/PDA membrane can effectively prevent the escape of tiny oil droplets and allowed water to permeate rapidly under the action of gravity and capillary suction.We tested three types of O/W emulsions（n-hexane,diesel,toluene）for phase separation,the total organic carbon content（TOC）value after filtration of the water-in-toluene emulsion was higher,163.4 mg/L.After 20 filtration cycles,the separation efficiency was 93%.（4）Through the electrostatic interaction of silica（Si O₂）particles,a hydrophilic and anti-swelling oil-water separation membrane was successfully prepared.The structural stability and durability of the fiber membrane were investigated.The influence of CNC and Si O₂ on the microscopic morphology and mechanical properties of the separation membrane,and the separation performance of the membrane on the oil-in-water emulsion were discussed.The results showed that the hydrolysis realized the hydrogen bond crosslinking between CNC and H-PAN nanofibers,and provided a sufficient number of reactive sites for grafted cationic polyethyleneimine（PEI）.And the electronegative Si O₂ was successfully adsorbed on the fiber surface.The CNC/H-PAN/Si O₂ film had good swelling resistance,and the swelling ratio was only 6.7 compared to 25.4 measured for a CNC/PAN membrane.In addition,the fibrous membrane contains highly interconnected channels,they were non-swellable in aqueous media.By contrast with untreated H-PAN membranes,those obtained CNC/H-PAN/Si O₂ after modification displayed high structural integrity and allowed regeneration and cyclic operation.Finally,wettability and oil-in-water emulsion separation tests demonstrated remarkable oil rejection and separation efficiency in aqueous media.（5）Through coaxial electrospinning technology,a series of nanofiber membranes regulated by CNC and polydimethylsiloxane（PDMS）were successfully prepared,including CNC/PVDF,CNC/PVDF/PDMS,CNC/PVDF-co-PDMS.The effects of CNC,PDMS,solvent,and humidity on the microstructure of PVDF-based membranes were investigated.The results showed that CNC/PVDF was composed of independent fibers,while CNC/PVDF/PDMS and CNC/PVDF-co-PDMS had a cross-linked network structure,and the fiber surface had a multi-level rough structure.Spinning solvent and humidity can directly affect the fiber morphology,produce a spindle/nanosphere structure.PVDF was responsible for spinnability,CNC regulates the mechanical strength and surface microstructure of the membrane by inducing phase separation between the polymer and the rich-phase solvent,and PDMS was responsible for regulating the degree of crosslinking of the fiber and the multi-level structure of the membrane surface.When adding 4 wt.%CNC,（relative to PVDF）,the CNC/PVDF membrane had the maximum strain value,while the CNC/PVDF/PDMS membrane could maintain a multi-level structure.The coaxial CNC/PVDF-co-PDMS fiber membrane had a WCA of 158°,an average pore size of 1.93μm,and oil droplets can quickly penetrate the membrane.Based on the microstructure,mechanics,and wettability of membranes,CNC/PVDF-co-PDMS was the best choice for oil-water separation membranes.Taken the water-in-toluene emulsion as an example,during the first separation process,the apparent state of the solution changed from turbid to transparent,and the separation flux and separation efficiency of the membrane was 2890 L m^-2 h^-1 and 94%,respectively.After the 20-second separation cycle,the separation efficiency remained above 94%.