Preparation And Molecular Dynamics Simulation Of Fluorosulfonated Polyimide Proton Exchange Membranes

Proton exchange membrane(PEM)is an important component of PEMFC,which is characterized by its light weight,high efficiency and low exhaust emission.Among which perfluorinated sulfonic acid membranes such as Nafion? are regarded as one of the most advanced PEMFC due to their excellent chemical and physical stability and high proton conductivity.However,high cost,high fuel permeability,and high temperature unavailability have limited Nafion? applications.Sulfonated polyimide film(SPI)is a substitute materials for perfluorinated sulfonic acid film with high thermal stability,high mechanical stability,low fuel penetration and high proton conductivity.However,the hydrolyzation-resistant stability of SPI membrane is poor,and the proton conductivity of SPI membrane has room for further improvement.In this paper,a series of novel fluorosulfonated polyimide films(FPI)were prepared by introducing fluorosulfonated groups into SPI molecular backbone starting from the chemical structure of membrane materials.The effects of fluorine unit content on the thermal stability,dimensional stability,proton conductivity,methanol permeability and hydrolysis resistance of the film were studied systematically.In order to ensure the feasibility of the experiment,the software Materials Studio 8.0 was used to simulate the molecular dynamics(MD)of polyimides with different fluorine unit contents before the experiment.The model was constructed,the energy and geometric optimization and dynamic balance were carried out to ensure the usability of the model,and the mechanical properties and proton conductivity of sulfonated polyimide with different fluorine unit contents were predicted,which provided the basis for the experiment.It is predicted that with the introduction of fluorine-containing groups,the tensile modulus,shear modulus and volume modulus will decrease,and the film rigidity will weaken and its flexibility will increase.The maximum proton conductivity was obtained when the fluorine content was predicted to be 10%.1,4,5,8-naphthalene tetradecanoic anhydride(NTDA)was used as the hexane diamine monomer,2,2'-benzidine disulfonic acid(BDSA)as the sulfonated diamine monomer,and 2,2'-bis(trifluoromethyl)-4,4' diaminobenzene(TFMB)as the fluorine-containing diamine monomer.The molar ratios of BDSA and TFMB were changed to synthesize FPI with different fluorine unit contents to explore the influence of fluorine unit content on FPI performance.The NMR and IR spectra of FPI indicate that fluoride unit is successfully introduced into the sulfonated polyimide.Thermal analysis and mechanical properties test show that the FPI film has good thermal stability and mechanical properties.The tensile strength range of FPI is 87.56-137.26 MPa,higher than that of Nafion?117 at 23 MPa.At 30?,the proton conductivity range of FPI is 0.054-0.131S/cm.Except for FPI50,the proton conductivity of FPI is higher than that of Nafion?117,which is the result of the synergistic effect of the sulfonic acid groups in the membrane and the introduced fluorine-containing groups.With the increase of fluorine unit content,the dimensional stability,anti-free radical oxidation stability and hydrolysis resistance stability of the membrane increase,and the proton conductivity decrease slightly,which is also consistent with the prediction results of molecular dynamics simulation.In order to explore methods to further improve the proton conductivity of PEM,MS 8.0 software was used to construct the carbon nanotubes(CNTs)composite FPI model to predict the effects of the addition of CNTs on the mechanical properties and proton conductivity of PEM.The addition of CNTs is predicted to increase the mechanical strength and modulus of the membrane within a certain content range,and the tensile modulus of the model reaches the maximum at 2 wt%.All proton conductivity values are higher than those without CNTs model.