Preparation Of Superwetting Polyurethane Nanofiber Composites And Their Application In Oil/Water Separation

Frequent oil spills accidents,the large amount of oily sewage in industrial production and daily life have caused seriously ecological environment pollution,and seriously threatened human daily life and health.It has become an increasing challenge to separate complex oily sewage with high speed and efficiency,which is benefit for the collection of high-purity water or oils from different oil-water mixtures.Recently,with the rapid development of bionics and interface theory,the application of the materials with special wetting behavior to the oil or water phase in oil-water separation has attracted more and more attention.In this dissertation,inspired by bionics,a series of polymer nanofiber membrane materials with special wetting behavior have been prepared by designing and controlling the morphology and chemical composition of the polymer nanofiber surface,and has realized the separation of diverse oil-water mixtures.The main research content in this dissertation are divided into the following five parts:1.The selective distribution of ACNTs and its influence on the wettability of PU composite nanofiber membraneThe ACNTs@PU composite nanofiber membrane has been prepared by ultrasonication induced adsorption of ACNTs onto electrospun PU nanofibers accompanying with hydrogen bonding.Compared with traditional hybrid co-spinning,the method of ultrasonication induced adsorption could ensure the homogeneous distribution of ACNTs on the PU nanofibers surface as much as possible.The formation of hydrogen bonds between ACNTs and imino groups or carbonyl groups in polyurethane molecular,combing with the interface sintering caused by ultrasonic cavitation,resulted in a strong interface interaction among the adsorpted ACNTs and the PU nanofibers.The adsorbed ACNTs on the fiber surface not only increases the surface roughness of the fiber,but also endows the composite nanofiber membrane with excellent electrical conductivity,at the same time,improves the mechanical properties of the nanofiber composite membrane.The increase of surface roughness is beneficial for the improvement of the surface wettability of the composite fiber membrane.It has been proved that the ACNTs decoration endows the porous membrane with superhydrophilicity and underwater superoleophobicity.When the ultrasonic treatment time reached 30 min,The WCAs value in air can reach 15°,and the underwater oil contact angles can reach more than 155°.More importantly,the hydrophilicity/underwater super oleophobicity remains stable even under 100%strain conditions.2.Preparation of superhydrophilic PDA/ACNTs@PU nanofiber composite membrane and its application in oil-water separationIn order to further improve the surface wettability of the ACNTs@PU nanofiber composite membrane prepared in the first part,in this section,a PDA/ACNTs@PU nanofiber composite membrane was fabricated by the PDA decoration deriving from the self-polymerization of dopamine under alkaline conditions.The multiple hydrogen bonds formed among PDA,ACNTs and PU nanofibers are benefical to further improve the interfacial interaction of the composite nanofiber membrane,thereby improving the Young's modulus and tensile strength of the composite nanofiber membrane.Because of the rough surface structure constructed by ACNTs and PDA,the nanofiber composite membrane possesses excellent superhydrophilicity/underwater superoleophobicity,which can be maintained under different strain or even after numerous cyclic tests.In addition,the as-prepared nanofiber composite membrane can realize separation of oil-water mixtures,including stratified light oil-water mixture and emulsified oil-in-water mixtures with high efficiency,and the separation flux,separation efficiency could reach more than 5000 L·m-2·h-1 and 99%,respectively.The separation flux and efficiency can still remain stable after cyclic separation.This flexible polymer nanofiber composite membrane material with a unique PDA/ACNTs shell and polymer nanofiber core structure has greatly potential application in the actual treatment of oily sewage.3.Preparation of superhydrophobic PSi/CNTs@PU nanofiber composite membrane and its application in oil-water separationIn the previous chapter,the as-prepared nanofiber composite membrane material with superhydrophilicity/underwater superoleophobicity can realize the efficient separation of stratified light oil-water mixtures and emulsified oil-in-water mixture,which is not suitable for the separation of heavy oil-water mixture and emulsified water-in-oil mixture.For this reason,in this chapter,the CNTs,which possesses hydrophobicity,was decorated onto the PU nanofiber surface by ultrasonication induced adsorption to obtain CNTs@PU nanofiber composite membrane,followed by the modification of a layer of low surface energy matter-polysiloxane(PSi),which derivated from the hydrolysis reaction of methyltrichlorosilane(MTS)to prepare a superhydrophobic/superoleophilic PSi/CNTs@PU composite nanofiber membrane material.The introduction of CNTs and PSi not only improves the surface roughness of the PU fiber,but also significantly enhances the mechanical properties of the nanofiber composite membrane,and gives the composite nanofiber membrane a certain electrical conductivity.The improvement of surface roughness and the modification of low surface energy substances significantly improve the superhydrophobicity/superoleophilicity of the composite nanofiber membrane.The PSi/CNTs@PU nanofiber composite membrane not only possesses good mechanical stability,but also possesses excellent chemical corrosion resistance,which could withstand various organic solvents,strong acids,strong alkalis,and aqueous solutions with high salt content.More importantly,the as-prepared PSi/CNTs@PU nanofiber composite membrane can efficiently separate oil-water mixtures containing strong acid,strong alkali,and high salt content,and has excellent adaptability to the harsh working environment of oil-water separation.4.Preparation of asymmetric superwetting PU nanofiber composite membrane and its application in oil-water separationIn the previous chapters,the prepared super-wet composite nanofiber membrane can only retain the water phase or the oil phase from the different oil-water mixtures singly,and allow the oil or water phase to pass through the separation membrane material with high efficiency during the separation of various oil-water mixture.In this chapter,inspired by the opposite wettability on both sides of the lotus leaf,a PU nanofiber composite membrane with asymmetric superwettability was prepared.The specific preparation method could be simply descripted as follows:Firstly,a layer of PU nanofiber was electrospun onto the surface of PDA/ACNTs@PU nanofiber composite membrane,which possesses superhydrophilicity/underwater superoleophobicity.Secondly,the electrospun PU nanofiber layer was modified with CNTs decoration to construct the hydrophobic layer.On the one hand,the surface modification of PDA/ACNTs significantly improves the superhydrophilic/underwater superoleophobicity on one side of the nanofiber composite membrane;on the other hand,the surface adsorption of CNTs on the other side improves greatly the hydrophobicity of asymmetric superwetting membrane.The introduction of PDA,ACNTs and CNTs improve the mechanical properties of Janus film to a certain extent.Thanks to the completely opposite wettability on both sides of the Janus membrane,the as-prepared nanofiber composite membrane showed high separation flux and excellent separation efficiency during the separation of various oil-water mixtures,which could be kept relatively stable after cyclic separations.The as-prepared Janus membrane is expected to be widely used in the treatment of different oily wastewater.5.Preparation of photothermal reversible super-wetting PNIPAM/CNTs@PU nanofiber composite membrane and its application in oil-water separationIn the fourth chapter,the Janus composite membrane was fabricated by organically combining composite nanofiber membrane materials with completely opposite wettability to realize the separation of various oil-water mixtures.However,during the separation process,it is necessary to adjust the orientation of the hydrophobic layer or the hydrophilic layer according to the properties of the separated oil-water mixture to achieve effective separation of the oil-water mixture.For this reason,in this section,combining the photothermal conversion property of CNTs and the temperature-sensitive properties of PNIPAM,a PNIPAM/CNTs@PU nanofiber composite membrane material with reversible photothermally controlled wettability transition properties was prepared.The specific method is that PNIPAM with temperature-sensitive property was grafted to the surface of the CNTs@PU nanofiber composite membrane by using the atom transfer radical polymerization(ATRP)reaction of NIPAM.Under infrared light irradiation,the heat absorbed by CNTs can be effectively transferred to the PNIPAM molecular chain grafted on its surface,so that the structure of the PNIPAM molecular chain is changed,and then resulting in the reversible transformation of the surface wettability.Benefiting from the reversible transformation of the super wettability of the composite nanofiber membrane,the prepared PNIPAM/CNTs@PU nanofiber composite membrane exhibits high separation flux and separation efficiency when separating different types of oil-water mixtures,which could be kept relatively stable after cyclic separations.The as-prapared composite membrane with reversible photothermally controlled wettability transition properties is expected to be widely used in the intelligent and controllable separation of different oily wastewater.