Preparation,Structural Regulation And Functionalization Of Electrospun Stereocomplex Polylactide Fibrous Membranes

Water is the origin of all things,and the development of human society greatly depends on water.However,the water resources existing on the earth have been seriously polluted by industrial development.Oil stains,organic dyes and heavy metal ions in wastewater pose great risks to human health and the environment.In order to protect the ecological system,harmless wastewater treatment is particularly important.Membrane adsorption/separation methods have the advantages of no secondary pollution,simple operation,cheap and so on.Contaminants from wastewater can be effectively adsorbed and separated by membrane adsorption/separation method,which achieves the purpose of wastewater purification.As a green biodegradable polymer,Polylactide（PLA）completely meets the requirements of sustainable development of human society.Therefore,it is significant to develop PLA-based membranes for wastewater treatment.However,there are still some problems while using PLA as adsorption/separation membrane.Firstly,PLA shows low crystallinity,poor mechanical property,and poor solvent resistance so that the durability of the PLA-based membranes is poor;Secondly,PLA does not own adsorption functional groups,so the dyes and heavy metal ions in wastewater can not be effectively treated by the PLA-based membranes.Therefore,the preparation of the PLA-based membranes with excellent adsorption/separation performance and durability through modification is of important research significance.In this paper,it is proposed to improve the mechanical properties,durability and adsorption/separation performance of poly（L-lactic acid）（PLLA）membrane by introducing the stereocomplex crystallites of PLA（sc-PLA）,special multistage pore structure,nanoparticles and functional groups aiming at the problems of low crystallinity,poor mechanical properties and poor adsorption and separation ability of PLLA membrane.Firstly,the sc-PLA/γ-Fe₂O₃hollow porous fiber membranes were prepared by introducing poly（D-lactic acid）（PDLA）andγ-Fe₂O₃ into PLLA through using coaxial electrospinning technology and solvent etching method.The effects of“shell”and“core”velocity ratio on fiber morphology,mechanical properties,porosity and oil-water separation performance during coaxial electrospinning were investigated.Secondly,the sc-PLA/polydopamine（PDA）/MXene composite fiber membranes were prepared by in-situ self-polymerization modification of dopamine（DA）to introduce PDA onto the surface of sc-PLA fiber membrane,and MXene was coated with binder PDA.Then the effects of different DA concentrations on micromorphology,wettability,adsorption,and oil/water separation properties of sc-PLA/PDA/MXene fiber membranes were investigated.Finally,the sc-PLA/PDA/polyethyleneimine（PEI）composite membranes were prepared by in-situ loading of PDA particles and grafting modification of PEI.The effects of the relative molecular weight of PEI on the grafting rate and adsorption properties of sc-PLA/PDA/PEI composite membranes were investigated.The main research contents and achievements are as follows:（1）Aiming at the problems of poor mechanical properties,oil adsorption capacity and cycling stability of PLLA membranes,the tubular hollow porous sc-PLA/γ-Fe₂O₃ fiber was prepared by introducing PDLA andγ-Fe₂O₃ into PLLA through coaxial electrospinning technology and solvent stenography.The microstructure characterizations show that the composite fibers exhibit tubular hollow and porous structure.The special tubular hollow porous structure endows the composite membranes with high porosity.In addition,the relatively low flow rate provides more opportunities for PLLA and PDLA to form sc-PLA,which leads to the increasing crystallinity of the composite membranes.At the same time,the sample 40-10-17 exhibits the best mechanical property,which is consistent with the change of crystallinity.Further studies show that the composite membrane exhibit excellent oil adsorption capacity and cycling stability.The maximum adsorption capacity of the membrane for castor oil reaches 219.5 g/g,and the adsorption capacity remains 85.7%after 10 cycles of adsorption,which are attributed to the special hollow porous structure and good mechanical property of the membrane.The hollow structure plays a key role in improving the porosity of the membrane and reducing the negative effect of oil surface tension on the membrane;The improved mechanical properties result in the membrane remaining intact during recycling.In addition,the special hollow and porous structure also endows the membrane with good permeability and excellent oil/water separation performance.The measured pure hexane flux is up to 57324.8 L/（m²·h）under gravity.In addition,the sc-PLA formed after the introduction of PDLA endows the membrane with excellent solvent resistance,even in acidic,alkaline,and high salt environment,and the fiber membrane also shows excellent oil/water separation ability and cycling stability.（2）In view of the problems of poor hydrophilicity and weak adsorption capacity of PLLA membrane,the sc-PLA/PDA/MXene composite fiber membrane was successfully prepared by PDA in-situ self-polymerization method and coating MXene by PDA bonding effect.The results show that PDA and MXene are both introduced into the fibers.In addition,the composite membranes modified by PDA and MXene are hydrophilic and lipophilic due to the presence of many hydrophilic groups such as amino and hydroxyl groups on PDA and MXene.The modified composite membrane shows hydrophobicity under water and hydrophobicity under oil,so that the emulsified n-hexane-in-water mixture（E-N/W）and the emulsified water-in-n-hexane mixture（E-W/N）can be separated by the membrane.The maximum water flux and oil flux of E-N/W and E-W/N are 1526.95 L/（m²·h）and 2514.84 L/（m²·h）,respectively.In addition,photothermal materials PDA and MXene endow the sc-PLA/PDA/MXene composite membrane with good photothermal conversion performance,enabling it to carry out efficient oil/water separation experiment under sunlight.Dye adsorption tests show that the composite fiber membrane with PDA and MXene can adsorb the cationic dye methylene blue（MB）through charge interaction,and the maximum experimental adsorption capacity of MB is 302.01 mg/g.Meanwhile,the maximum theoretical adsorption value calculated by Langmuir model is up to 434.78 mg/g.（3）Aiming at the issues of weak adsorption capacity and function of PLLA membrane,sc-PLA/PDA/PEI composite membranes were prepared by grafting different molecular weights of PEI onto sc-PLA/PDA fibers using GA as crosslinking agent.The microstructure characterization show that the fiber structure of sc-PLA/PDA/PEI composite membranes do not change significantly,and the rough PDA particles on the fibers remained intact after grafting.Microstructural characterizations show that PEIs are successfully grafted,and the graft rates increase with the increase of the relative molecular weight of PEI.The dye adsorption results show that sc-PLA/PDA/PEI composite fiber membrane exhibits good adsorption capacity for methyl orange（MO）,and the maximum theoretical adsorption value calculated by Langmuir model reaches 724.64 mg/g,which is due to the rich active groups such as amino groups on the surface of PEI grafted composite membrane and charge attraction between the membrane and MO.In addition,the sc-PLA/PDA/PEI composite fiber membrane exhibits the highest adsorption capacity（352.24 mg/g）in hexavalent chromium（Cr（Ⅵ））solution at pH=2.The theoretical maximum adsorption value of the membrane toward Cr（Ⅵ）calculated by Langmuir model is up to 434.78 mg/g.Further studies show that the LDF-3-70000 membrane grafted with PEI is rich in amino groups on the surface,which could produce many protonated amino groups under acidic conditions for desorption and reduction of Cr（Ⅵ）to trivalent chromium（Cr（Ⅲ））.Moreover,the sample LDF-3-70000 shows strong reducibility,and 64.79%of the adsorbed Cr（Ⅵ）was reduced to Cr（Ⅲ）.