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 CO 2 . The difficulties in this comparison were discussed. A significant improvement from former results could be assessed for the predicted O 2 /N 2 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.
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Heuchel et al. (2003) studied this question.
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