| In recent years,humans have achieved rapid development in economic construction.However,due to their weak awareness of ecological environment protection,the over-exploitation of natural resources and the discharge of a large number of pollutants have caused the imbalance of the ecological environment.This makes the recovery ability of the ecological environment lower than its own purification ability,resulting in a series of air pollution problems.The main components of air pollutants are small particles,which are often suspended in the atmosphere as aerosols.Because of the high surface energy,they can easily carry viruses into the bronchus and lungs with human breath,which poses serious threats to human health.Living in this air pollution environment for a long time is likely to cause diseases such as respiratory system,heart disease,stroke,cancer,and even death.Especially in the epidemic period of infectious virus,serious air pollution environment accelerates the spread of the virus.Wearing a mask made of fibers is an effective way to protect humans from air pollution.The fibers prepared by electrospinning are much smaller in diameter than traditional fibers,and the formed fiber membrane has the excellent characteristics of high porosity and good pore connectivity.The fiber membrane with different properties and structures can be prepared by adjusting the spinning process parameters such as voltage and distance,the properties of the spinning solution and the spinning environment parameters.This fiber membrane can effectively separate the particles from the air.Therefore,the electrospun fiber membrane has a wide application prospect in the field of air filtration.Cellulose acetate(CA),a cellulose derivative obtained by acetylation of cellulose,can be dissolved in multiple organic solvents such as N,N-dimethylformamide(DMF),formic acid and acetic acid.Moreover,it is a non-toxic polymer with excellent characteristics such as biodegradability and biocompatibility.The CA fiber membrane prepared by electrospinning not only retains the advantages of CA itself,but also has the characteristics of electrospun fiber membrane.However,the spinnability of CA in a single solvent is so poor that it is not suitable for spinning alone.In addition,the electrospun fiber membrane generally has the problem of excessive pressure drop,which limits its practical application.In this dissertation,in view of the problems of the poor spinnability of CA and high pressure drop of electrospun fiber membrane,poly(ethylene oxide)(PEO)was blended with CA for electrospinning,and a series of CA composite fiber air filtration membranes with adjustable structure were designed and prepared.The research work of this dissertation mainly includes the following five aspects:(1)The CA/PEO fiber membrane was prepared by adding PEO into CA solution,and the spinnability of the solution and the forming mechanism of the fiber were analyzed.It was found that the addition of PEO improved the spinnability of CA in DMF.As PEO concentration increased,CA products gradually changed from compressed round beads to bead fibers and then to smooth fibers.When PEO concentration was 1.6 wt%,smooth fibers can be obtained.The DMF/dichloromethane(DCM)solvent system of CA/PEO was studied by the solubility parameter theory,and the appropriate solvent system was determined as DMF/DCM solvent with a ratio of 1/1.The relationship between solution concentration and fiber apparent morphology was investigated.The results showed that there was a linear relationship between fiber diameter and solution concentration.The solution with a concentration lower than entanglement concentration(Ce)can only form round beads.The solution with a concentration above Ce produced bead fibers and gradually turned into smooth fibers.When the solution concentration reached 6.0 wt%,i.e.2.85 times of Ce,a smooth fiber membrane can be obtained.(2)The functional CA/PEO fiber membranes with bead structure were prepared by adding rutin into the CA/PEO solution under changing the relative humidity of spinning environment.The effects of rutin content on their apparent morphology,antioxidant and antibacterial properties were investigated,and the effects of rutin content and relative humidity on their filtration performance were studied.The results showed that with the increase of rutin content,the antioxidant and antibacterial properties of the fiber membrane gradually enhanced.The decrease in fiber diameter improved the filtration performance of fiber membrane.The fiber diameter gradually increased with increasing relative humidity,and the bead fibers were easy to appear at low relative humidity.The bead structure can increase the spatial distance of the fiber membrane and inhibit the rapid increase of its pressure drop,thus improving the filtration performance.The composite fiber membranes were prepared by using the difference of solution viscosity to adjust the spinning order of solutions,and their filtration performance was studied.The results showed that the filtration path of the composite fiber membrane was wavy to improve the filtration efficiency,and its filtration performance was better than that of a single structure fiber membrane.(3)The bead structure of CA/PEO/rutin fiber membrane was regulated by ferrous sulfate,and a multilayer structure was constructed by inserting nonwovens into the fiber membrane.The effect of ferrous sulfate on the bead structure and filtration performance of the fiber membrane was studied.The results showed that a small amount of ferrous sulfate can promote the formation of more beads.As the concentration of ferrous sulfate increased,the number of beads gradually decreased,and the diameter of the fibers first decreased and then increased.The bead structure can regulate the packing density of fiber membrane and reduce the pressure drop,while small diameter can increase the filtration efficiency.The filtration performance of fiber membrane was improved under the combined action of beads and small diameter.The influence of areal density of the membrane on filtration performance of the fiber membrane was studied.It was found that the increase of the areal density increased the packing density of the fiber membrane,which made the growth rate of the pressure drop higher than that of the filtration efficiency,thereby reducing the filtration performance.Based on this,nonwoven was inserted into the fiber membrane with high filtration efficiency to adjust the internal structure to construct multilayer structure fiber membranes with different segmented layers,and the influence of segmented layers on filtration performance was studied.The results showed that inserting nonwoven into the fiber membrane can not only improve the filtration efficiency,but also reduce the pressure drop.When ferrous sulfate concentration was 0.050 wt%and the relative humidity was 30%,the 4-layer structure fiber membrane had the best filtration performance.(4)A multiscale diameter CA/PEO/polyamide 6(PA)fiber filtration membrane with photocatalytic activity was prepared by adding silicon dioxide(SiO2)/titanium dioxide(TiO2)into CA/PEO/PA solution.The effect of CA/PEO/PA concentration on the apparent morphology and filtration performance of fiber membranes was studied.It was found that the multiscale diameter fiber membranes composed of coarse fibers and a small number of fine fibers(less than 100 nm),accompanied by some protrusion nodes,can be constructed at different concentrations of CA/PEO/PA.Fine fibers can keep high filtration efficiency of fiber membrane,while coarse fiber can greatly reduce the pressure drop of fiber membrane.These helped to improve filtration performance.The average diameter of the fibers gradually increased with increasing CA/PEO/PA concentration.When the concentration of CA/PEO/PA was 9 wt%,the fiber membrane had the best filtration performance.Based on this,different ratios of SiO2/TiO2 were added to the solution,and the effects of SiO2/TiO2 ratios on the apparent morphology,photocatalytic performance and filtration performance of the fiber membrane were investigated.The results showed that the change of SiO2/TiO2 ratios affected the fiber diameter.When the ratio of SiO2/TiO2 was 5/5,the fiber membrane had better filtration and photocatalytic performance.(5)The forming mechanism of the multiscale CA/PEO/PA fiber membrane composed of coarse and fine fibers was studied.The effects of the ratios of formic acid/acetic acid solvent and CA/PEO/PA polymer on the apparent morphology of fiber membrane were investigated.It was found that the increase of PA and acetic acid content contributed to the formation of multiscale structure,resulting in a smaller surface pore size.When the ratio of formic acid/acetic acid and CA/PEO/PA was 5/5 and 5/1/5,respectively,a multiscale structure fiber membrane composed of a large number of fine fibers was formed.The effect of the multiscale structure on filtration performance was studied.The results showed that the multiscale fiber membrane contained many fibers with a diameter of less than 100 nm,which was beneficial to achieve high filtration efficiency and low pressure drop under ultrathin membrane thickness,resulting in excellent filtration performance.The fiber membrane with an areal densityof 0.855 g/m2 showed the best air filtration performance.The fiber membrane performed deep filtration during the filtration process,which inhibited the rapid growth of pressure drop and improved the filtration performance.This provided a new idea for the research to develop the high efficiency and low pressure drop fiber membrane. |
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