Fabrication And Characterization Of Reverse Osmosis Membrane By Plasma Polymerization

Reverse osmosis is one of the most economical technologies for desalination of seawater and brackish water.Reverse osmosis membrane is the core of reverse osmosis technology.Among them,the aromatic polyamide reverse osmosis composite membrane prepared by interfacial polymerization is the most widely used.However,the traditional interfacial polymerization process limits the selection of monomers to a certain extent;the film-forming process is more complex;the solvent consumption is large,which is easy to cause environmental pollution.In order to develop new membrane materials,a thin active layer is synthesized on the surface of polysulfone(PSf)ultrafiltration membrane by low temperature plasma polymerization to achieve desalination.The characteristics of plasma polymerized films are as follows:(1)Polymer monomers are not required to contain unsaturated bonds or more characteristic functional groups;(2)Polymer films formed have good adhesion to the substrate;(3)The reaction process is dry reaction without solvent;(4)Polymer films are ultra-thin and without pinholes.Firstly,reverse osmosis composite membranes are prepared by traditional interfacial polymerization,which is compared with the subsequent plasma polymerization.M-phenylenediamine and trimesoyl chloride are water and oil monomers respectively.Polyamide layer is prepared on Polysulfone ultrafiltration membrane by interfacial polymerization.With the desalination rate and flux of composite membranes as evaluation indexes,the relationship between the influencing factors of interfacial polymerization and the separation performance of composite membranes is explored by orthogonal experiments.Eight factors are selected in the experiment,including molecular weight,water phase concentration,oil phase concentration,additive concentration,pH,reaction time,post-treatment temperature and post-treatment time.The optimum process conditions are determined by variance analysis: molecular weight of base membrane is 2000,1.5wt% m-phenylenediamine,0.15wt% trimesoyl chloride,0.15wt% sodium dodecyl sulfonate,pH=10,reaction time is 40 s at room temperature.The post-treatment temperature is 70 C and the time is 15 min.The morphology of the membrane surface is characterized by scanning electron microscopy(SEM).The results show that the surface of the composite membrane is rougher than that of the original membrane.With acrylonitrile as polymer monomer,plasma is produced by dielectric barrier discharge(DBD)and radio frequency discharge,respectively.Ultrathin polymer films are formed on the surface of polysulfone ultrafiltration membrane.The optimum polymerization conditions are determined by orthogonal experiment.The optimum conditions for plasma polymerization under atmospheric pressure are as follows: the output voltage is 28 V,the discharge time is 30 s,and the water bath temperature is 30?.The modified membrane has a slight removal of NaCl,but the removal efficiency is not significant,the highest removal rate is only 13.9%.SEM results show that the surface of the polymer film is smoother than that of the original film,and the pore size of the film is reduced but not completely disappeared.The optimum conditions for low-pressure plasma polymerization are as follows: discharge current is 5A,discharge time is 5 min,and grafting time is 30 min.The removal efficiency of divalent ions by polymer membranes is better than that of monovalent ions.The results of SEM analysis show that the pore size of the modified polysulfone membrane is smaller than that of the original membrane,but at the same time,needle-like pore is added.Methyl methacrylate(MMA)is grafted onto polysulfone ultrafiltration membrane by low pressure plasma polymerization.The optimum technological conditions of film preparation are optimized by orthogonal experiment: discharge current is 6A,discharge time is 3min,and grafting time is 60 min.The polymer membrane has a good removal effect on monovalent salt ions,and the highest desalination rate is 67.1%.At the same time,the effects of concentration of test solution and operating pressure on the separation performance of polymer membrane are investigated.SEM and EDS are used to obtain the information of surface morphology and composition.The results show that the surface of the polymer film is concave and convex compared with the original film,and the O/C ratio of the film surface is increased.