Molecular characterization of polyOAPS-imide isomer hyper-cross-linked membranes: Free-volume morphologies and sorption isotherms for CH4 and CO2

Abstract

PolyPOSS-imides are hybrid hyper-cross-linked networks based on inorganic octa(aminopropyl)silsesquioxane (POSS) cages covalently bonded via organic imide bridges. They were initially developed for separations under harsh conditions, but they cannot be used above 300 °C due to the decomposition of their –(CH2)3– propyl linkers. Replacing the aliphatic-arm POSS with its phenyl-arm derivative, octa(aminophenyl)silsesquioxane (OAPS), should thus increase their resistance. However, OAPS has three possible isomers depending on the meta, para or ortho position of the –NH2 group on the phenyl ring. To pre-screen the most interesting structures before attempting synthesis, molecular dynamics (MD) simulations were recently used to compare twenty-two model networks based on the OAPS isomers, the initial POSS and several imide precursors at high temperatures. Six phenyl-arm polyOAPS-PMDA and polyOAPS-6FDA and two aliphatic-arm polyPOSS-PMDA and polyPOSS-6FDA were selected for further analyses of their CH4 and CO2 sorption isotherms at 35 °C. All of them have an average number of ~5 links per cage and cross-linking densities of ~3.8. In terms of connectivities and free-volume morphologies, both the metaOAPS and paraOAPS networks are found to be open structures with many low-energy sites and high penetrant solubilities. On the other hand, the ortho functionalization leads to denser morphologies associated to lower solubilities, which makes it closer to the POSS networks. Differences are enhanced when the inorganic precursor is associated to a rigid organic linker such as PMDA.The iterative GCMC (Grand Canonical Monte Carlo)-MD sorption-relaxation technique was used to predict the single-gas uptakes of CH4 and CO2 in the networks over an extended 0–60 bar pressure range at 35 °C. The sorption capacities varied in the order paraOAPS > metaOAPS > orthoOAPS > POSS for the inorganic precursor, and PMDA > 6FDA for the organic precursor (except for the orthoOAPS). All correlate directly to the accessible free-volume, but the CO2/CH4 ideal sorption selectivities remained essentially the same over the whole pressure range. Compared to POSS, OAPS showed less swelling at equivalent penetrant concentrations in the matrix. It is also more resistant to other mechanical deformations such as isotropic dilation or uniaxial tension. This confirms that the polyOAPS-imides can indeed be used under harsher conditions than the current polyPOSS-imides, without loss of the CO2/CH4 sorption selectivities.