Research Of Nanocellulose-doped Purine Modified Chitosan Proton Exchange Membranes

Chitosan is a well-known abundant natural polymer with excellent biodegradability,biocompatibility,non-toxicity and antibacterial biological properties,which widely used in areas such as the pharmaceutical industry,fuel cells and wastewater treatment.In addition,chitosan-based membranes have excellent hydrophilicity,low methanol permeability and environmental protection.However,compared with the commercial Nafion perfluorosulfonic acid proton exchange membrane,they also suffer from low proton conductivity as proton exchange membranes.In recent years,nitrogen heterocyclic compounds have been evaluated for use in PEMs.Nitrogen heterocycles have a unique structure that makes them both proton donors and proton acceptors,and thus have proton conduction potential.At present,studies on the modification of nitrogen heterocyclic compounds?especially purine?onto chitosan have not been reported.In this paper,a novel purine-containing chitosan based proton exchange membrane was synthesized by chemically grafting adenine and adenosine monophosphate with proton conduction potential on the chitosan side chain.In order to reduce the swelling property of chitosan membrane and increase the mechanical property,the nano-cellulose with high specific surface area and good mechanical property was used to modify the material to synthesize a proton exchange membrane with excellent performance of whole biomass.The membrane was tested for proton conductivity and other comprehensive properties.In this paper,formaldehyde was used as a grafting agent.At 90?,chitosan reacts with a one-step Schiff base with formaldehyde to form an imine intermediate.Because the system is an acidic aqueous system,the imine is prone to protonation.Therefore,the electron-rich primary amine on the adenine undergoes a nucleophilic addition reaction with the protonated imine intermediate to form a purine-grafted chitosan.The structure of the polymer was characterized by FT-IR and ¹³C NMR,which proved the success of adenine grafting.The graft ratio of adenine was calculated to be 0.2926 by the carbon to nitrogen ratio of the polymer molecules.The tensile strength of the A-CS membrane is slightly higher than that of the pure CS membrane,and the thermal stability is lowered,but it is still stable at the working temperature.However,the A-CS membrane has a fairly good water uptake property to make the membrane swelling.Electrochemical test results show that the A-CS membrane can reach 0.1138 S·cm^-1 at 100?,which is much higher than the chitosan membrane.Moreover,the membrane also maintains excellent alcohol resistance,and the methanol permeability is about 1.2593×10^-6 cm²·s^-1.Adenosine monophosphate was used as a modification reagent for the problem that the proton conductivity of A-CS membrane is not high at room temperature.The structure of the polymer was characterized by FT-IR and ¹³C NMR,which proved the success of adenine grafting.The graft ratio of adenine was calculated to be 0.2926 by the carbon to nitrogen ratio of the polymer molecules.The AMP-CS membrane is relatively brittle and has poor mechanical properties.Compared with chitosan,the membrane is stable at operating temperature despite a decrease in thermal stability.The AMP-CS membrane is too hydrophilic and easy to swell.Electrochemical test results show that the proton conductivity of AMP-CS membrane is higher than that of A-CS membrane at 20?,and reaches 0.1867S·cm^-1 at 100?.This is even higher than the proton conductivity of commercial Nafion 117membranes.Meanwhile its methanol permeability was much lower than Nafion(7.888×10^-7cm²·s^-1).In view of the lack of mechanical properties and easy swelling of A-CS membrane and AMP-CS membrane,we prepared nanocellulose-doped modified chitosan proton exchange membrane by doping TEMPO-oxidized nanocellulose in modified chitosan matrix.As the doping amount of nanocellulose increases,the tensile strength and the thermal stability of both membranes increase.The results of water absorption and swelling ratio show that after doping with nanocellulose,the water absorption of A-CS membrane and AMP-CS membrane will decrease,and the swelling ratio will be below 30%,which improves the dimensional stability of the membrane.Electrochemical test results show that the proton conductivity increases with the increase of CNF doping amount of A-CS membrane.However,the proton conductivity decreases with the increase of CNF doping amount of AMP-CS membrane,which is related to the different microscopic morphology of the two membranes.