Gas transport in membranes based on polynorbornenes with fluorinated dicarboximide side moieties

This work reports a comparative study of gas transport in membranes based on polynorbornenes and polyoxanorbornenes containing fluorinated dicarboximide side moieties, specifically, poly(N-pentafluorophenyl-exo-endo-norbornene-5,6-dicarboximide), poly(exo-N-pentafluorophenyl-7-oxanorbornene-5,6-dicarboximide), poly(N-3,5-bis(trifluoromethyl)phenyl-exo-endo-norbornene-5,6-dicarboximide) and a random copolymer of N-pentafluorophenyl-exo-endo-norbornene-5,6-dicarboximide-co-N-phenyl-exo-endo-norbornene-5,6-dicarboximide with 50/50 M composition. The gases studied were hydrogen, oxygen, nitrogen, carbon dioxide, methane, ethane, ethylene and propylene. The presence of fluorine atoms in the membranes increases their permeability in slight detriment of the permselectivity, the increase being higher for F–C bonds with carbon hybridization sp3 than sp2. The substitution of the CH2 groups of the norbornene moieties decreases the permeability as a consequence of the decrease of the solubility. A thorough study is carried out on the variation of the permeability coefficient and concentration of condensable gases, such as carbon dioxide and propylene, in the poly(N-pentafluorophenyl-exo-endo-norbornene-5,6-dicarboximide) and poly(N-3,5-bis(trifluoromethyl)phenyl-exo-endo-norbornene-5,6-dicarboximide) membranes finding that the values of the effective diffusion coefficient are similar to those obtained by the dual-mode model but significantly higher than those estimated by the time-lag from permeation results. The results also show that conditioning the membranes with condensable gases under high pressure increases the diffusivity.