The Pore Size Regulation And Surface Modification And Application Of PVDF Membranes Were Prepared By TIPS Method

In this paper,hydrophobic polymer PVDF was used as membrane material and PVDF flat membrane was prepared by TIPS?Thermally Induced Phase Separation?method.The effects of different quenching bath conditions,dopamine deposition modification and the spray modification of hydrophilic/hydrophobic spraying solution on the final membrane structure and its properties were studied.And the application of oil/water separation was carried out.This study is mainly carried out from the following three aspects.First of all,the unmodified PVDF membrane has low mechanical strength,small flux,and is easily contaminated by protein or some oil pollution,resulting in blockage of membrane pores and poor anti-pollution performance.Based on this,we studied the effects of different quenching bath media?water,air?and quenching bath temperatures on membrane structure and properties,and the application of oil/water separation was carried out.Through the experiment,we found that the quenching bath condition could regulate the pore size of the membrane,which has a direct effect on the permeability and separation performance of the membrane.Compared with the membrane prepared by the traditional TIPS method using cold water as the quenched medium(PVDF_T-water),PVDF membrane quenching in air(PVDF_T-air)at the optimum quenching bath temperature?30°C?was not only more energy-saving and eco-friendly,but also larger pore size and higher pure water flux,and its mechanical strength can be greatly improved,which eventually can break through the"trade-off"line of pure water flux and tensile strength.In the oil/water separation application,oil in water emulsion can be well trapped,which is a good improvement for the PVDF membrane to be used for oil/water separation.Secondly,dopamine as a kind of functional material with good adhesion,dopamine has abundant hydrophilic hydroxyl groups and amino functional groups.When it is applied to the modified membrane material,it can rapidly form a film on the surface of the membrane through an oxidation self-polymerization reaction.Its rich hydrophilic groups and amino groups can significantly enhance surface hydrophilicity and chemical functionality.At present,dopamine is used for membrane material modification by many researchers,and most of them have achieved very good results.However,PVDF membrane is prepared by TIPS are rarely reported.PVDF flat membrane is prepared by the TIPS method of quenching in air,and then it completely immersed in dopamine solution for deposition modification.By changing the deposition time of dopamine?24,48,72,and 96 h?,the effects of different dopamine deposition time on the structure and properties of PVDF membrane were studied.Through the research,we found that the PVDF_T-air membrane modified by dopamine could exhibit excellent mechanical strength and ultra-high permeation flux with no reduction in oil rejection,when separating three types of oil-in-water emulsions?hexane,toluene,petroleum ether?,oil retention rate can exceed 98.5%.It can significantly improve the surface hydrophilicity,underwater oleophobic and pure water permeation flux at the appropriate dopamine deposition time,and the pure water flux can reach3855.6 L·m^-2·h^-1·bar^-1 at the optimal conditions?48 h?.In addition,the anti-pollution performance of PVDF membrane prepared by this novel method has been increased to over 90%,which shows potential application prospect in the field of oil/water separation.Finally,based on the previous two studies,we found that most of the oil/water separation materials could only be"one-way"separation,the hydrophilic membrane material is usually separated from the oil in water emulsion,and the hydrophobic membrane material separates the oil in water emulsion.By spraying the hydrophilic/hydrophobic coating solution on the top and bottom surfaces of the PVDF flat membrane,we constructed a PVDF Janus membrane that can simultaneously separate“oil-in-water”and“water-in-oil”emulsions.The biggest advantage of this Janus membrane is that it can combine the two seemingly"contradictory"properties of hydrophilic/hydrophobic properties into one membrane so that it can meet the two requirements of oil/water separation application simultaneously.We first add a proper amount of silicon dioxide to the casting solution and then prepare the PVDF flat membrane by the"air bath"TIPS method.Hydrophilic surface is sprayed with improved"dopamine"solution,which greatly shortens the dozens of hours of time required for deposition in the past,and hydrophilic effect has been greatly improved,truly achieving the purpose of“superhydrophilic”.The hydrophobic surface was sprayed with micro-nanostructured silica and a silane coupling agent,which two materials combined by chemical bond and then fixed on the membrane surface.Modified membrane meet the"superhydrophobic"material conditions including contact angle and sliding angle.Through experiments,it was found that the PVDF Janus membrane prepared by this method can simultaneously efficiently separate“oil-in-water”and“water-in-oil”emulsions and oil-water mixtures.The maximum oil rejection rate can reach 96.09%,and the highest water rejection rate can reach 99.08%.Besides,its excellent acid and alkali resistance,heat and cold resistance,and abrasion resistance make it well applicable in the oil/water separation field.