Preparation And Structure Control Of Polyvinylidene Fluoride (PVDF) Membrane

Offshore oil spills and industrial oily wastewater are a seriously threaten to our ecological environment.The emulsified oil in oily wastewater contains stabilized oil droplets of submicron to micrometer,which are difficult to be removed by conventional separation methods.In contrast,membrane separation technology has excellent separation effect due to the controllable pore size.Among all the membrane processes,microfiltration membranes has been widely adopted for the treatment of oil/water emulsions due to low energy consumption.PVDF is a microfiltration membrane material with excellent performance due to excellent thermal stability and chemical stability.However,there are still challenges in preparing high-performance PVDF microfiltration membranes.To the best of our knowledge,this is the first report of a quaternary polyvinylidene fluoride（PVDF）dope solution system,which is characterized by a lower critical solution temperature（LCST）without any temperature-sensitive polymer.The thermodynamic properties of the LCST system were studied and the LCST mechanism was proposed.On this basis,PVDF microfiltration membranes were prepared by nonsolvent thermally induced phase separation（NTIPS）and vapor induced phase separation（VIPS）using dope solution system with LCST,respectively.The effect of the membrane preparation parameters in NTIPS and VIPS on the membrane structure and performance was investigated,and a systematic study on membrane structure control was carried out.After that,membranes prepared by NTIPS and VIPS were characterized by porosity,flux,mechanical strength,average pore diameter,etc.,and were used for oil-water separation.This thesis mainly starts from the following three aspects.Firstly,the effect of the concentration of each component in the quaternary LCST system on the cloud point of the system was studied.And the results showed that the concentration of MgCl₂has the greatest impact on the cloud point of the system.The research of FTIR showed that ion-dipole and dipole-dipole interactionswere the main mechanisms dominating the solution behavior of the LCST solution.The interaction between the Mg²⁺cations and DMAc becomes weaker when PVP and PVDF are present in the solution while the increase in temperature from 30°C to 90°C may have further reduce the solvation number of the Mg²⁺cation and DMAc complexes,resulting in less DMAc molecular in the complex and creating opportunity for the polymer chains of PVP and PVDF to interact with the Mg²⁺.As the temperature is raised to above a critical point（i.e.,the LCST）,polymer aggregation or shrinking in the solution may occur and lead to phase separation.Secondly,the PVDF-MgCl₂-PVP-DMAc quaternary LCST system with the composition ratio of 12-8-12-68 was used to producea highly porous and high strength microfiltration membrane without a dense skin layer based on a facile NTIPS approach.The analysis of membrane formation kinetic showed that when the temperature of the coagulation bath is lower than the LCST（50℃）of the system,the finger-like macrovoids structure is formed by NIPS,and when it is higher than the LCST,the bicontinuous network pores are formed by NTIPS.As the temperature of the coagulation bath increases,the porosity of the membranebecomes higher,the pore size distributionbecomes narrower,the cross-section structure tends to be a bi-continuous network pore structure,and the tensile strength is up to 3.5 MPa.The membrane prepared at coagulation bath temperature of 70℃exhibited superior filtration performance in both pure water testing with an high flux of 6870 L·m^-2·h^-1·bar^-1,and filtration of surfactant-stabilized oil/water emulsion in terms of superior permeability of 1608 L·m^-2·h^-1·bar^-1,99.1%rejection of oil contaminants.In addition,it exhibited excellent fouling performance with a flux recovery of 85%.Finally,based on the PVDF-MgCl₂-PVP-DMAc quaternary LCST system with acomposition ratio of 14-8-4-74,the PVDF microfiltration membrane was prepared by the VIPS method.The phase separation process based on the LCST system showed a high degree of controllability of the membrane structure.Adjusting the vapor temperature,membrane thickness,PVDF concentration and exposure time can regulate the membrane structure from finger-like macrovoids to sponge-like pores or connected cell-like pores structure.By adjusting the vapor temperature,membrane thickness,PVDF concentration and exposure time,a microfiltration membrane with no skin layer,high surface porosity,and sponge-like pore structure or bicontinuous cell-like pore structure but without finger-like pore structure was successfully prepared.The membrane prepared with a casting solution with 14 wt%PVDF exposed to vapor temperature of 80℃for 80 s has the best performance with a pure water flux of 13186 L·m^-2·h^-1·bar^-1,a tensile strength of 3.0 MPa and a elongation of 95.3%.Besides,the membrane exhibited outstanding separation efficiency in filtration of surfactant-stabilized oil/water emulsion with a permeation flux of 3028 L·m^-2·h^-1·bar^-1 and a rejection of oil contaminants of 99%.