Microstructure Regulation During The Preparation Of PVDF Membrane Hydrophilization Modification

With the worsening global water pollution problem,how to recycle wastewater has become a focus in current society.Membrane separation technology with high efficiency,energy saving,flexible operation and processing capabilities and other characteristics show great potential.The properties of membrane materials play an important role in the membrane separation process.Polyvinylidene fluoride(PVDF)is a polymer with excellent mechanical strength,chemical stability and membrane-forming properties.It is an excellent raw material for the preparation of microfiltration separation membrane.However,the strong hydrophobicity of PVDF makes it have two major problems when it is used to treat aqueous solution.One is that the membrane has a great resistance to water permeation and therefore requires a higher working pressure during operation.The other is that during the use of the membrane,it is easy to be contaminated by the adsorption of protein,which leads to a sharp drop in the permeation flux.Hydrophilic modification of the membrane is the main method to solve the above drawbacks,improve membrane flux,reduce membrane fouling and prolong membrane service life.In order to improve the hydrophilicity of the PVDF membrane,polyvinyl butyral(PVB)with good hydrophilicity,less susceptible to bacteria erosion and good membrane-forming properties is blended.In this paper,PVDF/PVB blend hollow fiber microfiltration membrane was prepared by thermal induced phase inversion(TIPS).The structure and properties of the composite membranes were characterized by SEM,fourier transform infrared spectroscopy,differential scanning calorimetry,DSC,flux and rejection,and contact angle measurements.The solid-liquid(S-L)phase separation occurs in the PVDF/PVB/PC system during cooling,and the different compatibilities of PVDF/PVB affect the crystallization of PVDF,which resulted in different structures.As a result,membrane formation and enhancement mechanisms were proposed.It was found that the middle layer of membrane was spherical particle structure,with the increase of the mass ratio of PVDF/PVB the size of spherical particle was increased,and stick-shaped structure began to grow in PVDF/PVB 10/3.In addition,there was no inner layer and the outer layer became thinner and denser with the increase of the amount of PVB.The outer skin layer is very thin as the mass ratio of PVDF/PVB was 10/1,so the flux of pure water is as high as 926.26 L/(m2·h),and the retention rate of carbon ink reaches 99.60%.As the mass ratio of PVDF/PVB is 10/5,the breaking strength of the membrane is up to 17.08 MPa,and the strength is very high for the obvious role of PVB adhesive.Thus,PVB enhancement mechanism and outer skin layer growth mechanism are proposed.Then,the interaction force between PVDF and PC was decreased by adding non-diluent-1(TOH)of PVDF,and a better connectivity structure was formed by adding an appropriate amount of PVB.Finally,the non-diluent of PVDF was changed to non-diluent-2(GOH).By changing the mass ratio of PC/GOH in order to make the phase separation liquid-liquid phase separation,and then a better connectivity membrane has been obtained by adding appropriate amount of PVB.