Molecular Modeling of Small-Molecule Permeation in Polyimides and Its Correlation to Free-Volume Distributions

Well-equilibrated molecular-packing models have been produced for 10 different polyimides. The Gusev−Suter transition-state theory was used to calculate gas solubilities and diffusion coefficients for nitrogen, oxygen, methane, and carbon dioxide. Good agreement with experiment (factors 1−4) was found, except for CO2. The difficulties in this comparison were discussed. A significant improvement from former results could be assessed for the predicted O2/N2 selectivity values. The simulated models allow an accurate determination of structural parameters, either as a single parameter, like the fractional free volume, or as size-distribution function of free-volume elements accessible for a certain penetrant. The 2,2‘-bis(3,4-dicarboxy-phenyl) hexafluoropropane polyimides with the highest oxygen permeability (50−130 Barrer) show a wider size distribution with an additional peak or shoulder at larger radii (>5−6 Å) than conventional polyimides. A constitutive structural element seems to be the o-methyl groups in the aromatic diamine moiety.