Molecular Simulation Of Polyetherimide With Side-group Modification To Tune Gas Permeation

Gas separation membrane technology was successfully industrialized in the 1970 s.It has attracted widespread attention because of its advantages of low investment,flexible operation,small footprint,stable operation,and low energy consumption.The membrane is the core component of the gas separation membrane device.The industrially large-scale membrane materials used for gas separation are mainly polymer materials,and common polymer membrane materials are classified into rubbery polymers and glassy polymers.Membranes prepared from glassy polymers have good selectivity and are widely used,but their permeation is low.Modification of glassy polymer materials is beneficial to improve gas separation performance and promote the development of membrane material.In this paper,molecular simulations were conducted to investigate the effect of pendant modified polyetherimide(PEI)on the solubility coefficients and diffusion coefficients of CO2,O2,N2,and CH4.The Materials Studio 2017 was used to build PEI stack structure.During the optimization of initial configurations,the evolution of PEI chains over time was found to have two consequences: closely packed configuration and non-closely packed configuration.After the compression and release process,the final configurations of closely packed and non-closely packed were obtained.The density,glass transition temperature,and diffusion coefficients of CO2,O2,N2,and CH4 in the final configurations of closely packed and non-closely packed were calculated.The results were compared with the experimental data.It was verified that the closely packed configuration of the simulated PEI chains was more in line with the actual situation.The Gaussian 09 was used to calculate weak interactions between gas molecules(CO2,O2,N2,and CH4)and functional groups.To calculate the energy of weak interaction bonds requires BSSE correction.The wave function was analyzed by Multiwfn,and the van der Waals surface penetration distances of CO2,O2,N2,or CH4 with functional group atoms were measured in conjunction with the visualization of VMD.According to the calculation results,the benzene ring structure was selected as the side-group,and the side-group was modified with ether oxygen linkages to the PEI.When benzene or biphenyl was used as the side-group to modify PEI,the solubility coefficients and diffusion coefficients of CO2,O2,N2,and CH4 were all reduced.When triptycence was used as the side-group to modify PEI,the solubility coefficients of CO2,O2,N2,and CH4 increased by 141.1%,43.3%,52.7%,and 37.2%,respectively.Their diffusion coefficients increased by 25.0%,37.9%,39.4%,and 88.9%,respectively.When the side-group volume is small,the voids formed by the polymer chains are filled,thereby hindering the diffusion of gas molecules and reducing the adsorption position of gas molecules.When modified with large side-group,the spaces between polymer chains are increased to provide more channels for the diffusion of gas molecules,and increasing the adsorption position of gas molecules.By increasing the rigidity of the side-group,the solubility coefficients and diffusion coefficients of O2,N2,and CH4 increased.However,the weak interaction between CO2 and rigid polymer chains increased and the solubility coefficient of CO2 increased significantly.Correspondingly,the diffusion of CO2 was also greatly constrained.As a result,the diffusion coefficient decreased.