Molecular Dynamics Simulation Study On The Diffusion Behavior Of CO₂ And Residual Molecules In TBDA-Polyimide Membranes

In this paper,molecular dynamics(MD)simulation was used to simulate the diffusion of CO2 gas molecules and membrane residue molecules in the TBDA-polyimide membranes.Three kinds of polyimide(TBDA1-6FDA-PI,TBDA1-ODPA-PI and TBDA2-6FDA-PI)membrane models were simulated by MD.and the chain spacing,glass transition temperature(Tg)and solubility parameter(3)were obtained.The former two agreed well with the experimental datas,while the solubility parameter was the predicted value.By analyzing the distributions of the dihedral angles between the dianhydride and diamine segments in the three polyimide separation membranes,the dihedral distribution between 6FDA and TBDA2 is the narrowest,the dihedral distribution between 6FDA and TBDA1 is slightly wider,the dihedral distribution between ODPA and TBDA1 is the broadest.Therefore,the molecular chain rigid size order:TBDA2-6FDA-PI>TBDA1-6FDA-PI>TBDA1-ODPA-PI.Because of the narrower dihedral distribution,the number of conformations are smaller and the molecular chains are more rigid.This is consistent with the trend of its glass transition temperature,free volume fraction and chain spacing.The above three models of polyimide and carbon dioxide composite system were simulated by MD.The shape factor of FVEs,histogram of eccentricity distribution and minimum threshold diameter distribution curve were obtained by analyzing the morphology of free volume elements(FVEs).TBDA1-ODPA-PI was found to have the lowest form factor value,and TBDA1-6FDA-PI was similar to the TBDA2-6FDA-PI form factor.This indicates that TBDA1-ODPA-PI has smaller free-volume pores and larger free volume pores of TBDA1-6FDA-PI and TBDA2-6FDA-PI;It was found that the relative numbers of eccentricity of FVEs in TBDA1-ODPA-PI close to 1 were the most,and the other two were similar,indicating that the number of FVEs of TBDA1-ODPA-PI near the sphere were the most;It was found that the number of FVEs minimum threshold values greater than CO2 diameter in the membrane was:TBDA2-6FDA-PI>TBDA1-6FDA-PI>TBDA1-ODPA-PI.The above results show that the volume of the free volume hole in the membrane is large with more minimum threshold FVEs larger than the diameter of CO2,which is favorable for the diffusion of CO2.In order to investigate the influence of the dihedral angle of the polymer chain on the diffusion coefficient,three TBDA2-ODPA-PI models were constructed with dihedral angles of 30°,90° and random angles between dianhydride and diamine.The diffusion behaviors of the residues of dimethylacetamide(DMAc),dimethylformamide(DMF)and N-methylpyrrolidone(NMP)in three membranes were simulated and DMAc and DMF were found to have the largest diffusion coefficient in the membrane with a polymer dihedral angle of 90°NMP was found to have the largest diffusion coefficient in the membrane with a polymer dihedral angle of random;This shows that diffusion properties of polymer system consists of residual liquid molecules in different dihedral angles in is not the same.