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

Abstract

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 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.

Simulations of gas transport in membranes based on polynorbornenes functionalized with substituted imide side groups

“…In this latter case, the permeability coefficient is more than one order of magnitude below the experimental one as a consequence of the low solubility coefficient obtained by simulation. The same behavior was obtained by Fried et al [70] and Heuchel et al [71] in Table 7 Comparison between the experimental permeability coefficient, P exp , and the simulated results using the volume V, P*, and the free volume, P**, for the simulations of the solubility coefficient The results were measured and computed at T = 300 K and p = 1 bar.…”

“…Kucukpinar et al [69] used this parameters in permeability study of these gases in copolymers of styrene, Fried et al in poly(2,6-dimethyl-1,4-phenylene oxide) Table 3 Lennard-Jones (6, 9) parameters employed to compute the interaction between polymer matrix and diffusant gas molecules appearing in Eqs. (6) and (7) Difussant [71]. Good agreement between simulated and experimental results was found.…”

Using Fractional Charges for Computing Fukui Functions in Molecular and Periodic Systems

“…Additionally some of the polyimides have a second maximum for cavities greater than 5 which might be important for the permeability as stated in the literature [30], [31]. Table 6 Summarising the main result from the FAV analysis, polyimides containing silicon have a higher free volume and cavities of bigger radii than their carbon containing counterparts.…”

Advanced proton-exchange materials for energy efficient fuel cells.

“…Decreases in the FFV have been modeled and correlated to decreases in gas permeability. [75,76,77,78] This phenomena has been observed with poly(trimethylsilylpropyne) (PTMSP), another high free volume polymer. [79] The fractional free volume in the polymer film can be indirectly monitored by measuring the bulk density of the film.…”