Water-salt Transport Of Polybenzimidazoles

The shortage of water resources and the energy consumption issue restricting global economic development,membrane separation technology as a separation process with low energy consumption,is widely applied in many industries.Desalination processes rely on polymer to control the transport of water/salt.Polybenzimidazole(PBI)has attracted much attention due to its excellent high-temperature resistance,chemical stability and good mechanical strength.In this work,the water/salt transfer behavior of PBI and N-substitution modified polybenzimidazole polymer films were analyzed under the guidance of solution-diffusion mechanism.To better understand the influence of chlorination on chemical structure,membranes were characterized by Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),etc.The effects of chlorination on the chemical structure and mass transfer properties of polymer films were investigated under three pH(4,7,10)conditions.The research results show that:(1)After chlorination,the FTIR characteristic peak of PBI films decreased,and the most significant changes occurred under acidic conditions.The characteristic peak of C=O appears,supposing due to the ring open of imidazole ring.XPS result showed that Cl~-was introduced into the PBI matrix after chlorination.In addition,after chlorination under acidic conditions,the content of Cl is the highest,which is caused by the chlorination of strongly oxidized hypochlorous acid to benzene ring and secondary amine and the doping of Cl~-to PBI.There was not C=O peak observed in FTIR spectrum of SPBI after experiencing the same chlorine process with PBI,indicating that N-substitution modification inhibited the oxidation of PBI membrane.However,SPBI membranes doped with almost the same amount of Cl compared with PBI after chlorinated at pH 4,indicating that direct chlorination of benzene ring and doping of Cl-into the membrane matrix at acidic solution.(2)With the increase of chlorine exposure,the water permeability of PBI increased,especially after chlorination in acidic and neutral solutions.Under neutral and alkaline solution,the SPBI membrane could maintain similar properties with pristine SPBI membrane.However,the mass transfer behavior of SPBI membrane was similar to that of PBI membrane after chlorination under acidic conditions,which is due to the chlorination of the benzene ring by the strongly oxidizing hypochlorous acid.(3)Owing to the tight structure and negative surface of PBI,which can reject salt ions by electrostatic repulsion,the salt permeability coefficient of PBI film has little change after chlorination under all three pH conditions.However,the salt partition coefficient in the chlorinated film has a linear relationship with the water partition coefficient,indicating that the dissolution of salt ions depends on the water absorption capacity of the film.After N-substitution modification,the absorption of water and salt of SPBI films decreased,the introduced groups limited the dissolution of salt ions.After chlorination,the salt partition coefficient of SPBI films remained almost unchanged,thus the water/salt solubility selectivity of polymer films increased.(4)Chlorination can improve the water/salt permeation selectivity and diffusion selectivity of PBI membrane,because the chlorinated membrane can maintain a lower salt ion permeability and improve the water permeability coefficient.The diffusion coefficient is the main factor affecting permeability.