Construction And Application Of Oil-water Separation Fiber Membrane Based On Layer-by-layer Continuous Gradient Structur

In recent years,electrospinning oil-water separation membranes have received much attention due to their controllable fiber diameter and surface wetting properties.However,current electrospinning oil-water separation membranes are usually monolithic in material and structure and thus lack internal driving force.In addition,some of the reported structures of Janus membrane still have liquid breakthrough resistance which limits their permeation/separation fluxes.Therefore,to address the low flux and external driving force dependence of electrospinning separation membranes,a material structure with continuous gradient of wettability between fiber layers was designed to provide continuous internal driving force for liquid transport by using continuous wettability difference between layers.We used two polymer spinning materials with similar chain segments but significantly different wettability to prepare fiber membranes with continuous wettability between fiber layers by program-controlled dual-channel electrospinning technology and investigated their performance for low-energy oil slick collection,oil-water separation at high/low gravity,and emulsion separation at low gravity.(1)Polycaprolactone(PCL)and a synthetic polycaprolactone-polyethylene glycol block copolymer(PCE-1)were selected as the spinning materials for the two channels,and continuous gradient membranes(PCL : PCE-1)with progressively increasing and decreasing hydrophilic/hydrophobic properties between the fiber layers were prepared by controlling the speeding speed of the spinning solution for the two channels separately using a desktop program.The effects of varying material components on fiber morphology and wettability in the direction of fiber membrane thickness were investigated.The results showed that the variation of the content of two polymer components with similar chain segments did not significantly affect the fiber morphology.And the variation of two components with significantly different wettability could build up a continuous gradient of wettability between the layers.The prepared continuous gradient membrane with smaller pore size and porosity than the homogeneous membrane has a significantly higher permeate flux compared to the homogeneous membrane.The continuous gradient membrane has unidirectional oil transport properties and potential applications in low-energy offshore oil slick collection.(2)Based on the above work,a block copolymer with further improved hydrophilicity was synthesized by adjusting the copolymer,and a continuous gradient membrane(PCL : PCE-2)was prepared by distributing PCL and the PCE-2 with improved hydrophilicity as the spinning materials for both channels.The prepared continuous gradient membranes exhibited unidirectional transport properties for both water and oil,and the water/oil permeation flux of the continuous gradient membranes at high/low gravity was significantly improved compared to other membranes.Based on the above characteristics,the continuous gradient membranes can achieve selective separation of oil and water with different surface tensions under low gravity conditions.(3)Graphene oxide(GO)and polyacrylonitrile(PAN)blended solution was selected as one of the channel spinning raw materials,and polystyrene-acrylonitrile block copolymer(PS-PAN)was synthesized as the other channel spinning raw material to improve the compatibility and regulate the hydrophilic/hydrophobic properties.And a continuous gradient membrane(GO/PAN : PS-PAN)with hydrophilic/oilic properties varying gradually in the fiber thickness direction was prepared.The continuous gradient enables the separation of oil-in-water emulsions without surfactant in a vertical device and the separation flux is significantly higher compared to the GO/PAN homogeneous membrane.In addition,the continuous gradient membrane enables the separation of different types of surfactant-stabilized oil-in-water emulsions in a horizontal device.In summary,PCL : PCE-1,PCL : PCE-2,and GO/PAN : PS-PAN continuous gradient membranes were prepared by using program-controlled dual-channel electrospinning technology to control two electrospinning materials with significantly different wettability to increment and decrement in the fiber membrane thickness direction respectively.This structure with continuous gradient of wettability in the direction of fiber membrane thickness provides internal driving force for liquid transport,endows the membranes with liquid unidirectional transport properties,significantly enhances the permeation/separation flux,and has favourable prospects for applications in low energy offshore oil slick collection,selective oil-water separation under low gravity,and water-in-oil emulsion separation in horizontal devices.