Molecular Dynamics Simulation Study Of Proton Diffusion In Polymer Electrolyte Membranes Based On Sulfonated Poly (Phenylene Oxide)

Sulfonated poly（phenylene oxide）（SPPO）is a kind of polymer material with stable physical and chemical characteristics and very good electrical properties.The theoretical study of microcosmic mechanism of SPPO membrane proton conduction is significant for improving the conductivity of proton exchange membrane materials.In this study,the transmission properties of water molecules（H₂O）and hydronium ions（H₃O⁺）in the SPPO membranes were investigated by the molecular dynamics（MD）simulation method.The work is as follows:（1）We built four different SPPO cells and its number of water molecules per sulfonic acid groupλvalues are 5,8,11 and 14,respectively.All initial cells were minimized by energy optimization,geometry optimization and annealing,and then stable equilibrium configurations were obtained.After MD simulation,the parameters such as densityρ,radial distribution function RDF,mean square displacement MSD,diffusion coefficient D and conductivityσof the cells will be analyzed.The results show that,theρof SPPO film decrease with the increase ofλ.The RDF of SPPO cells shows that,the carbon skeletons of SPPO molecular chain are hydrophobic group and sulfonic acids are hydrophilic group,the electrostatic force between H₃O⁺and sulfonic acids makes the distance between H₃O⁺and sulfonic acids more close.H₃O⁺is transported under the package of H₂O.The water molecules diffusion coefficient D_w and hydronium ions diffusion coefficient D_h increase with the increase ofλ,and whenλchanges from 5 to 14,D_w increases from1.33×10^-10 m²·s^-1 to 10.67×10^-10 m²·s^-1 and D_h also increases from 0.50×10^-10 m²·s^-1 to 5.00×10^-10 m²·s^-1.The conductivity of films also increases from 0.0056 S·cm^-1 to 0.0382 S·cm^-1.The diffusion coefficients of H₂O and H₃O⁺and conductivity of films greatly increase,Thus,it is obvious that water content has a great influence on the transmission performance of membrane.（2）We built three different SPPO cells and its sulfonation degree DS values are15%,30%and 50%,respectively.The properties of membrane transport were studied by MD simulation.The parameters such as density,radial distribution function,mean square displacement,diffusion coefficient and conductivity of the cells were analyzed.The results show that,theρof SPPO film increase with the increase of DS.With increase of DS,the distances between sulfur atoms in SPPO models get shorter.The D_w and D_h increase with the increase of DS.When DS changes from 15%to 50%,D_w increase from 4.26×10^-10 m²·s^-1 to 10.67×10^-10 m²·s^-1 and D_h also increase from0.76×10^-10 m²·s^-1 to 5.00×10^-10 m²·s^-1.The conductivity of films also increases from0.0026 S·cm^-1 to 0.0382 S·cm^-1.Therefore it is obvious that sulfonation degree has a great influence on the transmission performance of membrane.