Molecular insight into transport properties of Troger's base based polyimide membrane.

Molecular Dynamics (MD) simulations were employed to investigate the transport properties of three polyimides comprising various diamines and dianhydrides. The diamines were 2,8-diamine-4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TB1) and 3,9-diamine-4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TB2); the dianhydrides were 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 4,4'-oxydiphthalic anhydride (ODPA). And the three polyimides were denoted as p-TB1-6FDA, p-TB1-ODPA and p-TB2-6FDA, respectively. The simulations provided the properties of the bulk polyimides such as glass transition temperature, fractional free volume, and solubility parameter. The results obtained were generally in consistent with experimental findings, which validated the quality of the model construction. Diffusion coefficient of carbon dioxide (CO2) in polyimide membrane was extracted based on mean square displacement analysis. Further research on backbone dihedral distribution and radial distribution function unveiled that Troger (TB1 or TB2) bases exerted strong influence on intra-chain mobility, 6FDA components were evenly distributed in polyimide matrix thus inhibiting inter-chain packing and CO2 molecules in free volume are surrounded by layers formed by Oxygen/Nitrogen atoms.