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a b s t r a c tPermeability coefficients for pure CO 2 , pure CH 4 , and CO 2 /CH 4 mixtures containing 50% CO 2 are reported for a polyimide synthesized from 3,3 0 -dihydroxy-4,4 0 -diamino-biphenyl (HAB) and 2,2 0 -bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and for three thermally-rearranged (TR) derivatives thereof. Permeability measurements were made at 35 1C for fugacities ranging from 4 to 25 atm. The permeability of CO 2 and CH 4 increased as the degree of TR conversion increased. For example, CO 2 permeability at 10 atm increased by a factor of 30 between the unconverted polyimide and its TR analog converted at 450 1C. In pure-gas experiments, CO 2 was observed to plasticize the unconverted polyimide, but it did not appear to plasticize the TR polymers. In mixed-gas experiments, dual-mode competitive sorption caused a depression in CH 4 permeability, with very little change in CO 2 permeability. In addition, plasticization by CO 2 was evident in the CH 4 mixed-gas permeability trends, but its impact was small in contrast with dual-mode competitive effects. Consequently, CO 2 /CH 4 mixed-gas permeability selectivity was higher than the ideal selectivity, calculated as the ratio of pure gas permeability coefficients. The dual-mode sorption and permeation model was fit to the experimental data. Dual-mode model parameters and model predictions are reported, along with their confidence intervals. By comparing the dual-mode model predictions with the experimental mixed-gas data, the degree of CO 2 -induced plasticization was observed to decrease as the degree of TR conversion increased and was completely absent (within experimental uncertainty) for the TR polymer converted at 450 1C.

The permeability–selectivity upper bounds show that perfluoropolymers have uniquely different separation characteristics than hydrocarbon-based polymers. For separating helium from hydrogen, these differences are particularly dramatic. At a given helium permeability, the upper bound defined by perfluoropolymers has helium/hydrogen selectivities that are 2.5 times higher than that of the upper bound defined by hydrocarbon-based polymers. Robeson hypothesized that these differences in transport properties resulted from the unusual sorption relationships of gases in perfluoropolymers compared to hydrocarbon-based polymers, and this paper seeks to test this hypothesis experimentally. To do so, the gas permeability, sorption, and diffusion coefficients were determined at 35 °C for hydrogen and helium in a series of hydrocarbon-, silicon-, and fluorocarbon-based polymers. Highly or completely fluorinated polymers have separation characteristics above the upper-bound for helium/hydrogen separation because they maintain good diffusivity selectivities for helium over hydrogen and they have helium/hydrogen sorption selectivities much closer to unity than those of hydrocarbon-based samples. The silicon-based polymer had intermediate sorption selectivities between those of hydrocarbon-based polymers and perfluoropolymers. Comparisons of hydrogen and helium sorption data in the literature more broadly extend the conclusion that helium/hydrogen sorption selectivity is rather different in hydrocarbon and fluorocarbon-based media.

Ultrathin graphene oxide (GO) (<5 nm) membranes were prepared by spin-casting onto microporous polymeric support membranes. GO membranes exhibited a highly CO2 permeable character, which is suitable for CO2 separation. In the presence of water vapour, high CO2 selectivity (e.g., CO2/H2, CO2/N2, and CO2/CH4) was achieved by enhanced CO2 sorption.

Thermally rearranged (TR) polymers have been the subject of many fundamental studies, but the effect of TR conversion on temperature-dependent transport properties is largely unexplored. Sorption isotherms for N 2 , CH 4 , and CO 2 in HAB-6FDA polyimide and its TR analogs were measured at temperatures ranging from-10 °C to 50 °C and pressures up to 27 atm. Solubilities increase with decreasing temperature for each gas and sample tested. At low TR conversions, the sorption process initially becomes less exothermic. However, enthalpies of sorption do not significantly change with TR conversion after the initial stages of rearrangement. Enthalpies of sorption in TR polymers are qualitatively similar to those of other high free volume materials. Solubility selectivity for CO 2 /CH 4 at 10 atm did not change with temperature due to similar enthalpies of sorption for CO 2 and CH 4. Sorption data were fit to the dual mode model at different temperatures, and model parameters were correlated with polymer and penetrant properties.