R. T. Chern

“Second component” effects in sorption and permeation of gases in glassy polymers☆

Abstract Data for CO 2 permeability through Kapton polyimide at 60°C are reported for upstream pressures up to 240 psia (16.33 atm) in the presence and absence of water vapor in the feed. The carbon dioxide flux was depressed by the presence of the water vapor. This phenomenon is analyzed in terms of the dual mode sorption and transport models. Together with other recent sorption and permeation data, this study suggests that competition of mixed penetrants for sorption sites and transport pathways associated with unrelaxed volume in glassy polymers is a general feature of gas/glassy polymer systems. The permselectivity of a membrane to a mixture of penetrants is strongly related to its ability to maintain a size and shape differentiating matrix, that is, to remain essentially unplasticized under operating conditions. Under such conditions, competition among penetrants for excess volume will be a generally important consideration for modeling gas permeation in permselective membranes.

Transport of gases in unmodified and arylbrominated 2,6-dimethyl- 1,4-poly (phenylene oxide)

Abstract Independent solubility and permeability data, measured at 35°C at up to 26 atm, are reported to show the influence of aryl-bromination on the transport of CO2, CH4, and N2 in 2,6-dimethyl-1,4-poly(phenylene oxide) (PPO). The permeability of PPO was found to vary with the extent of bromination, and the magnitude of change depends on the nature of the gas. The apparent solubility coefficients of all three gases at 20 atm in the polymer increased with the extent of bromination, and the percentage of increase was higher for the gas with lower condensability. The concentration-averaged diffusivities of CO2 and CH4 also showed some variation with the extent of bromination. In particular, there was a notable increase in the diffusivity of CO2 but a slight decrease in that of CH4 when the extent of bromination was increased to 91%. The gas-transport data were also analyzed according to the dual-mode model. The dual-mode parameters exhibit similar dependence on the extent of bromination as the apparent solubility coefficient and concentration-averaged diffusivity do. These observations are interpreted in terms of changes in the average packing, torsional mobility of the chain segments, and cohesive energy density of the polymer.

Gas separation properties of aromatic polyamides containing hexafluoroisopropylidene groups

Abstract The synthesis and gas transport properties of aromatic polyisophthalamides (PIPAs), based on isophthaloyl chloride derivatives bearing pendent groups and hexafluoroisopropylidene (6F) linkages in the main chain, are reported and compared with properties of a similar series of PIPAs containing sulfonyl (SO2) rather than 6F in the main chain. All of those polymers exhibit high glass transitions temperatures. The polymers containing 6F groups were markedly more permeable and somewhat less selective than their sulfonyl analogs. Polymers containing a t-butyl pendent group at the 5 position of the isophthaloyl linkage were much more permeable than those bearing only a hydrogen atom at this position, although a strong decrease in permselectivity accompanied the large increase in permeability. CO2/CH4 solubility selectivity values of the 6F-containing polymers were similar to values reported for other polymetric and non-polymeric organic materials with similar concentrations of polar carbonyl linkages. In contrast, the CO2/CH4 solubility selectivity in SO2-containing variants of these polymers was substantially lower than expected based on total polar group concentration. The low CO2/CH4 solubility selectivity is believed to be related to the extremely efficient chain packing in the SO2-containing polymers, which may lead to strong amide-amide linkage interaction, thereby inhibiting carbonyl groups in the amide linkage from interactions with CO2 molecules to increase CO2/CH4 solubility selectivity.

Aryl-nitration of poly(phenylene oxide) and polysulfone. Structural characterization and gas permeability

Abstract The gas permeability and permselectivity of unmodified and aryl nitrated poly(phenylene oxide) and polysulfone are reported for CO 2 , CH 4 , O 2 and N 2 . Aryl nitration decreases the permeability for all the gases, but increases the permselectivity of both polymers for either O 2 /N 2 or CO 2 /CH 4 gas pair. These results are related to changes in packing density, torsional mobility and interchain attractions due to the polar nitro group.

Sorption and transport of organic vapors in poly[1-(trimethylsilyl)-1-propyne]

Abstract Sorption kinetics and apparent equilibrium isotherms were determined gravimetrically at 35°C for organic vapors in glassy poly[1-(trimethylsilyl)-1-propyne] (PMSP). The sorption kinetics are characterized by diffusion coefficients on the order of 10 −8 cm 2 /sec for the normal hydrocarbons (C 3 -C 9 ). These diffusivities are more than one million times larger than corresponding diffusivities characterizing the transport of corresponding penetrants in conventional glassy polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate). The sorption capacity of this polymer is also quite high compared with values observed for more conventional polymeric glasses. The sorption isotherms of the organic vapors in PMSP follow a generalized dual mode behavior which is often observed for vapors in polymeric glasses. Specifically, at low activities the isotherms are concave toward the activity axis, then become convex to the activity axis at higher activities. The transition separating these two portions of the isotherms does not become evident until the sorption level in PMSP reached more than 30 weight percent, compared with less than 10 weight percent for other glassy polymers. These unusual properties can be related to the low packing density of PMSP. However, the results also reveal that although the quantitative values of diffusivities and solubilities are extraordinary, the qualitative behavior regarding sorption and diffusion is consistent with comparable observations on conventional glassy polymers.

Gas separation properties of aromatic polyamides with sulfone groups

Abstract The gas transport properties of three aromatic polyisophthalamides based on isophthaloyl chlorides and 4,4′-diaminodiphenylsulfone are reported at 35°C. The effects of bulky t-butyl and phthalimide substituents, at the 5 position of the isophthaloyl chloride moiety, on CO 2 , CH 4 , O 2 , N 2 , H 2 and He permeability, solubility and diffusivity were determined and correlated with chain packing and thermal properties of the polymers. Gas permeability was higher in substituted polyisophthalamides than in the unsubstituted analogue. Polymers containing the pendent t-butyl substituent have substantially higher permeability than polymers bearing the phthalimide substituent, despite the fact that the phthalimide substituent appears to be more bulky than the t-butyl group, based on van der Waals volume estimations. The strong polarity of the phthalimide moiety may act to increase chain-chain cohesive forces, which would tend to enhance chain packing, thereby reducing the packing-disrupting ability of the bulky phthalimide group. The permeability increase of the substituted polymers was accompanied by a permselectivity decrease.

A note on the effects of mono-and di-bromination on the transport properties of poly(2,6-dimethylphenylene oxide)

Abstract The permeability of poly (2,6-dimethylphenylene oxide) (PPO) to carbon dioxide and methane at 35°C can be increased more than 100% by aryl-bromination, depending on the extent of bromination, whereas, the carbon dioxide/methane permselectivity of the polymer, which ranges from 17–20, is changed only slightly by bromination. The increase in permeability results primarily from a marked increase in the penetrant diffusivity, although the penetrant solubility is also moderately increased by bromination. These results are interpreted in terms of changes in packing density and chain torsional mobility brought about by aryl-bromination of the polymer.

The effects of aryl-halogenation on the gas permeabilities of poly (phenolphthalein terephthalate) and poly (bisphenol A phthalate)

Abstract Effects of aryl-halogenation on the permeability of poly(phenolphthalein terephthalate) and poly(bisphenol A phthalate) to several permanent gases are reported. Analyses based on separate solubility and diffusivity considerations are presented. The influence of reduced packing density of the polymer on the equilibrium gas sorption capacity and steady-state gas-permeability of the polymer is addressed. The results concur well with previous findings on poly(2,6-dimethyl phenylene oxide) and its aryl-brominated counterparts. Finally, the relationships among polymer chain rigidity, gas-diffusivity, gas molecular size, and permselectivity of the polymer are analyzed.

Unequivocal definition and determination of “dissolved gas” permeability and resistances of the liquid layers in contact lens applications

Abstract The conventional polarographic method for measuring “dissolved oxygen” permeability was modified and re-analyzed. An oxygen concentration probe with a large cathode covered by a layer of electrolyte and a Teflon membrane was used. The effective permeation area of the specimen matched that of the cathode. This design minimizes the contribution of the “edge effect” that often leads to errors in the measurement with smaller cathodes. Five permeation resistance layers were shown to exist when the lens specimen was installed on the polarographic probe. Manipulation of the governing equations led to an unequivocal definition of a “dissolved oxygen permeability”, P d , which could be calculated directly from the polarographic data once a “cell constant” was available. The cell constant was determined from the polarographic data for specimens of known intrinsic permeabilities. Finally, rationale was presented for our proposition that the compound parameter P d / L may be used to rank flat commercial lens materials regarding their in-situ oxygen transmissibilities.