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

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 aliphatic-arm POSS with its phenyl-arm derivative, octa(aminophenyl)silsesquioxane (OAPS), should thus increase resistance. However, OAPS has three possible isomers depending meta, para or ortho position –NH2 group phenyl ring. To pre-screen most interesting structures before attempting synthesis, molecular dynamics (MD) simulations recently compare twenty-two model isomers, initial and several precursors at high temperatures. Six polyOAPS-PMDA polyOAPS-6FDA two polyPOSS-PMDA polyPOSS-6FDA selected further analyses CH4 CO2 sorption isotherms 35 °C. All them have an average number ~5 links per cage cross-linking densities ~3.8. In terms connectivities free-volume morphologies, both metaOAPS paraOAPS found open many low-energy sites penetrant solubilities. On other hand, functionalization leads denser morphologies associated lower solubilities, which makes it closer networks. Differences enhanced when precursor is a rigid linker such as PMDA. The iterative GCMC (Grand Canonical Monte Carlo)-MD sorption-relaxation technique was predict single-gas uptakes in over extended 0–60 bar pressure range capacities varied order > orthoOAPS precursor, PMDA 6FDA (except orthoOAPS). correlate directly accessible free-volume, CO2/CH4 ideal selectivities remained essentially same whole range. Compared POSS, showed less swelling equivalent concentrations matrix. It also more resistant mechanical deformations isotropic dilation uniaxial tension. This confirms that polyOAPS-imides can indeed harsher conditions than current polyPOSS-imides, without loss selectivities.