CO2 Permeability Coefficients Research Articles

Gas transport properties of poly(2,2,4,4‐tetramethyl cyclobutane carbonate)

AbstractGas transport properties of semicrystalline films of poly(2,2,4,4‐tetramethyl cyclobutane carbonate) (TMCBPC) were studied. Permeability coefficients for He, O2, N2, CH4, and CO2 at 35°C for pressures between 1 and 20 atm are reported as well as sorption isotherms for N2, CH4, and CO2 at the same conditions. The permeability coefficients for TMCBPC are larger than corresponding values for the aromatic bisphenol‐A polycarbonate (PC) and tetramethyl bisphenol‐A polycarbonate (TMPC), even though the TMCBPC films are semicrystalline. These results are explained on the basis of the larger free volume available for permeation in this polymer. Significant TMCBPC plasticization by CO2 was also observed and this causes typical time‐dependent behavior. The plasticization process starts at very low pressures compared with the behavior of aromatic polycarbonates PC and TMPC. This early onset of plasticization seems to be related also to the larger free volume in the amorphous phase of TMCBPC which favors high gas sorption. The diffusion coefficients for TMCBPC are also larger than those reported for the aromatic polycarbonates PC and TMPC. Ideal gas separation factors were found to follow the usual trend; that is, as permeability increases, the ideal separation factor decreases. © 1993 John Wiley & Sons, Inc.

Structure/permeability relationships of polyimides with branched or extended diamine moieties

AbstractMean permeability coefficients for CO2, O2, N2, and CH4 in seven types of 6FDA polyimides with branched or extended diamine moieties were determined at 35.0°C (95.0°F) and at pressures up to 10.5 atm (155 psia). In addition, solubility coefficients for CO2, O2, N2, and CH4 in six of these polyimides were determined at 35.0°C and at 6.8 atm (100 psia). Mean diffusion coefficients for the six gas/polyimide systems were calculated from the permeability and solubility data. The relationships between the chemical structure of the polyimides, some of their physical properties (glass transition temperature, mean interchain spacing, specific free volume), and their gas permeability, diffusivity, and solubility behavior are discussed. The 6FDA polyimides studied here exhibit a considerably lower selectivity for the CO2/CH4 and O2/N2 gas pairs than 6FDA polyimides with short and stiff aromatic diamines with comparable CO2 and O2 permeabilities. © 1993 John Wiley & Sons, Inc.

Concentration dependence of diffusivities of penetrants in glassy polymer membranes

AbstractSteady‐state permeation rates for pure CO2, CH4, and O2 through two kinds of homogeneous dense membranes, copolymer of methyl methacrylate and n‐butyl acrylate and poly(methyl methacrylate), were measured at three temperatures between 20 and 40°C and upstream pressures up to 3 MPa. The logarithms of the mean permeability coefficients for CO2 in both membranes increased linearly or quadratically with increasing upstream pressure, whereas the mean permeabilities for CH4 and CO2 were independent of pressure. The pressure dependence of the mean permeability coefficient for CO2 was interpreted in terms of a modified free‐volume model of diffusion for a Henry's law population in glassy polymers. © 1993 John Wiley & Sons, Inc.

Gas transport in polybutadiene treated with aqueous bromine

AbstractDiffusion, solubility, and permebility coefficients were measured for He, Co2, Ar, and CH4 in polybutadiene (PB) and in polybutadiene reacted in the solid state to various extents with aqueous bromine. Analysis of the sorption curves and X‐ray emission spectra showed that the bromination created a heterogeneous membrane with an outer brominated skin and an unreacted core. At relatively low extent of bromination, the diffusion and permeability coefficients for CO2, Ar, and CH4 decreased by two orders of magnitude, while the transport coefficients for He were virtually unchanged. The permeability coefficients for CO2, Ar, and CH4 became immeasurably small after about 3% bromination. The ideal separation factor for gas pairs with different molecular size increased with bromination, suggesting applications in gas separation processes. © 1993 John Wiley & Sons, Inc.

Permeation of binary gas mixture through glassy polymer membranes with concentration‐dependent diffusivities

AbstractAn analytical solution has been obtained for the modified dual‐mode mobility model for a single gas proposed by Zhou and Stern and extended to a binary gas mixture to describe the pressure dependence of mean permeability coefficients for CO2 and CH4 mixtures in homogeneous cellulose triacetate membranes. The permeabilities calculated from the model fitted the corresponding experimental results quite well. Permeation experiments for equimolar CO2 and CH4 mixture in a homogeneous membrane of methyl methacrylate and n‐sbutyl acrylate copolymer were performed along with sorption experiments for pure CO2 and CH4 to test the applicability of the model. The experimental permeabilities were close to those calculated from the model.

Gas transport in poly [bis(trifluoroethoxy) phosphazene] above the T(1) transition

AbstractGas permeation measurements were performed by the time‐lag method for 13 gases in poly [bis(trifluoroethoxy) phosphazene] above the mesophase transition, T (1) transition, of the crystals. Hysteresis was found in the temperature dependence of the permeability coefficients for CO2, O2, and N2 in the transition region as observed by dilatometry and differential scanning calorimetry. Independently the solubility of CO2 was determined gravimetrically above and below T(1). Noticeable change and hysteresis were also observed in the temperature dependence of the solubility coefficients. Diffusion of CO2 in the measophase is suggested from the permeation and sorption measurements. Gas transport parameters of CO2 in the mesophase are estimated, and the contributions of the mesophase and the crystalline phase to gas transport are considered.

Permeation of mixed gases in glassy polymer membranes

AbstractSteady‐state permeation rates for pure CO2 and CH4 and the binary mixture through homogeneous dense polysulfone and polycarbonate membranes have been measured at 40°C and upstream pressures up to 26 atm. The pressure dependence of the mean permeability coefficients for CO2 and CH4 for all of the systems was simulated equally well in terms of the dual‐mode mobility models driven by gradients of chemical potential and concentration. The dual‐mode sorption and transport parameters used for evaluation of the mobility models are only those determined from sorption and permeation runs for a single gas. The approximate solution for the mean permeability coefficient of each component in a binary mixture was derived on the basis of the same mobility model driven by gradients of chemical potential. The deviation of approximate solutions from numerical ones remains less than 0.3%.

Sorption and transport of pure gases in random styrene/methyl methacrylate copolymers

AbstractPrevious work has shown that permeability coefficients for CO2 can increase with pressure for poly(methyl methacrylate) (PMMA); whereas, those for polystyrene decrease slightly as found for many glassy polymers. This response is attributed to a greater propensity for PMMA to be plasticized by CO2. This issue is considered in detail here by examining the behavior of a series of random styrene/methyl methacrylate copolymers in order to learn how the plasticization response varies with MMA content. At low pressure, the sorption and transport of CO2 and other gases in these copolymers depend on copolymer composition in ways expected from simple theories for multicomponent polymers. The change in CO2 permeability coefficient upon pressurization from 1 to 20 atm ranged from –6% for polystyrene to +57% for poly (methyl methacrylate). Furthermore, upon holding at 20 atm of CO2 driving pressure, there was a significant increase in the CO2 permeability coefficient with time for PMMA; whereas this conditioning effect was much smaller for polystyrene. Conditioning and plasticization effects seem to be related to the same molecular causes. The responses change progressively for the copolymers but not directly in proportion to MMA content. The greater effects of CO2 for PMMA are, to a significant extent, but not entirely, due to its much higher level of CO2 sorption compared with that of polystyrene. The results are discussed in terms of relevant theories.

Sorption and transport of pure C02 and CH4 in miscible bisphenol chloral polycarbonate/poly (methyl methacrylate) blends

AbstractSorption and permeation behavior of CO2 and CH4 was examined in detail for a series of miscible blends based on bischloral polycarbonate (BCPC) and poly (methyl methacrylate) (PMMA). The former pure polymer exhibits decreases in the permeability coefficients for both CO2 and CH4 as the upstream driving pressure increased as predicted by the dual‐mode sorption model. On the other hand, PMMA shows a decrease for CH4 but an increase for CO2. The latter is attributed to plasticization by this more soluble gas. The blends show intermediate behavior. CO2 permeation was examined on pressurization, at constant pressure for two weeks, and then on depressurization. The pattern of behavior progressively deviated between that of BCPC and PMMA as the MMA content of the blends was varied. These trends were contrasted with those previously described for a series of styrene/methyl methacrylate copolymers. Some differences in behavior between the blends and the copolymers were noted when compared at constant MMA content. These results have important ramifications for the design and selection of polymers for membranes to separate the CO2/CH4 gas pair.

The Conductance of the Plasmalemma for CO2

Permeability coefficients (PS values) for CO2 of the plasma membrane (PM) of the unicellular green algae Eremosphaera viridis, Dunaliella parva, and Dunaliella acidophila, and of mesophyll protoplasts isolated from Valerianella locusta were determined from 14CO2 uptake experiments using the rapid separation of cells by the silicone oil layer centrifugation technique. The experimental PS values were compared with calculated numbers obtained by interpolation of Collander plots, which are based on lipid solubility and molecular size, for D. parva cells, mesophyll protoplasts isolated from Spinacia oleracea, mesophyll cells and guard cells of Valerianella, and guard cell protoplasts isolated from Vicia faba. The conductivity of algal plasma membranes for CO2 varies between 0.1 and 9 × 10−6 m s−1, whereas for the plasmalemma of cells and protoplasts isolated from leaves of higher plants values between 0.3 and 11 × 10−6 m s−1 were measured. By assuming that these measurements are representative for plants and algae in general, it is concluded that the CO2 conductivity of algal PM is of the same order of magnitude as that of the higher plant cell PM. Ps values of plasma membranes for CO2 are lower than those for SO2, but are in the same order of magnitude as those measured for H2O. On the basis of these results it is concluded that theoretical values of about 3000 × 10−6 m s−1 believed to be representative for higher plant cells (Nobel, 1983) and which are frequently used for computer-based models of photosynthesis, lack experimental confirmation and represent considerable overestimations. However, with several systems, including higher plant cells, the conductance of the PM for CO2 was significantly higher in light than in darkness. This suggests that in light, additional mechanisms for CO2 uptake such as facilitated diffusion or active uptake may operate in parallel with diffusional uptake.

The Effect of Morphology on Gas Permeability and Permselectivity in Polyimide Based on 3,3′,4,4′-Biphenyltetracarboxylic Dianhydride and 4,4′-Oxydianiline

The permeability and diffusion coefficients for H2, CO, CO2, and CH4 through polyimide films based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA) decreased, but their activation energies increased with decrease in specific volume, which reflected the average degree of molecular aggregation of the polymer. The changes were greater for gases with larger molecular diameters. The solubility coefficients also decreased with decrease in specific volume but the sorption enthalpies hardly changed. The decreases were hardly dependent on the molecular size of the gases and were similar to the decreases in the diffusion coefficient for H2. With decrease in specific volume, the permselectivity was enhanced significantly for the H2/CO and H2/CH4 systems at the sacrifice of the permeability, but not for the CO2/CH4 system. There was no appreciable difference in the permeation properties attributed to the difference in the film-casting method.

Effect of tacticity on permeation properties of poly(methyl methacrylate)

AbstractThe effect of stereoregularity on gas permeation properties of poly(methyl methacrylate) (PMMA) was investigated. The gas permeability coefficients for He, H2, O2, N2, Ar, CH4, and CO2 at 35°C near atmospheric pressure have been measured for three different PMMAs. Apparent diffusion and solubility coefficients were obtained from time lag data, and these were compared with data for a commercial PMMA previously reported. The permeability, solubility, and diffusion coefficients increase as the content of syndiotactic sequences increases. These observations are consistent with more dense packing of the isotactic form in the glassy state that stems in part from its lower glass transition temperature. The transport behavior for a 50:50 isotactic/syndiotactic blend was also studied. These so‐called stereocomplexes exhibit permeation behavior comparable to other weakly interacting miscible blend systems.

Gas sorption and transport in miscible blends of tetramethyl bisphenol‐A polycarbonate and polystyrene

AbstractThe permeability coefficients for He, O2, N2, CH4, and CO2 in miscible blends of polystyrene (PS) and tetramethyl bisphenol‐A polycarbonate (TMPC) at 35°C and 1 atm driving pressure are reported. Sorption isotherms for CO2 and CH4 are also presented. The isotherms were fitted to the dual sorption model. The Langmuir capacity factor was found to follow an earlier correlation based on unrelaxed volume. For each gas, the permeability was found to go through a minimum when plotted against blend composition. This behavior is primarily the result of the volume change on mixing observed for this system. The attractive interaction between TMPC and PS is relatively strong as indicated by density and solubility data. The value of the binary interaction parameter was found to be of the same magnitude as that for poly(phenylene oxide) (PPO)‐polystyrene (PS) blends. Considering the similarity of structure between PPO and TMPC, it is concluded that similar phenyl‐phenyl interactions and conformational changes on blending may prevail in TMPC/PS blends.

Gas transport in a thermotropic liquid‐crystalline polyester

AbstractA preliminary report is given on the gas transport characteristics of a thermotropic liquid‐crys talline copolyester that is fully aromatic. At 35°C the permeability coefficients for He, H2, O2, Ar, N2, and CO2 in this polymer are comparable to or smaller than those for polyacrylonitrile, which is one of the least permeable polymers known. This low permeability seems to stem from low solubility of these gases in the liquid‐crystalline polymer rather than low transport mobility. Activation energies for transport are somewhat larger than those found for conventional amorphous polymers. Determination of the detailed mechanism of transport in polymers with liquid‐crystalline order requires further study.

Gas permeation in miscible blends of poly(methyl methacrylate) with bisphenol chloral polycarbonate

The miscibility of poly(methyl methacrylate) (PMMA) with bisphenol chloral polycarbonate (BCPC) has been studied using differential scanning calorimetry (DSC), optical indication of phase separation on heating (i.e., lower critical solution temperature (LCST) behavior), density measurement, and gas permeation. All evidence indicates that PMMA is miscible with BCPC over the whole blend composition range. Single composition-dependent glass transition temperature and LCST behavior have been observed for each blend. The specific volumes of the blends follow closely the simple additivity rule indicating the interaction between PMMA and BCPC is weak. Gas permeability coefficients for He, H2, O2, Ar, N2, CH4, and CO2 measured at 35°C under 1 to 2 atm upstream pressure are lower than those calculated from the semilogarithmic additivity rule. The difference between this calculated permeability and the measured one increases with gas molecular size. As a result, the ideal gas separation factors for He/CH4, CO2/CH4, and O2/N2 gas pairs estimated from the ratio of pure gas permeabilities are higher than predicted from the semilogarithmic additivity rule. These permeation results were interpreted in terms of the free volume theory and the activated state theory, which have been proposed to describe gas transport behavior in polymer mixtures.

The differential permeation rate measurement of gases in PVAc

AbstractThe temperature dependence of the CO2 permeability coefficient in poly(vinyl acetate) (PVAc) was measured continuously by a differential permeation rate method. Tg was changed at upstream pressure in PVAc/CO2 system, and curious behavior of Ar was observed after CO2 conditioning. The differential permeation rate experiment successfully exhibits the effects of continuous temperature change for membranes with different histories.

Gas transport in partially fluorinated low‐density polyethylene

AbstractLow‐density polyethylene film was subjected to direct fluorination on one surface by exposure to a dilute fluorine gas stream for various periods of time. Various analyses indicate partial fluorination of a thin surface layer. The permeability coefficients for He, CO2, and CH4 were measured at 35°C. The permeability of He was not changed by fluorination; whereas, values for CO2 and CH4 were decreased by as much as two orders of magnitude. The selectivity of transport for gas pairs of different molecular size was greatly improved, suggesting applications of this technique for membrane separation processes.

Direct evaluation of the permeability of the rat proximal convoluted tubule to CO2

Conflicting data exist regarding the ability of the rat proximal convoluted tubule to maintain a transepithelial gradient for CO2 and the effects of carbonic anhydrase on CO2 permeability. The present in vivo microperfusion experiments were designed to assess the ability of the rat proximal tubule to sustain a CO2 gradient between tubule lumen and peritubular blood. Tubules were perfused at rates ranging from 10 to 100 nl/min with isotonic sodium chloride containing no CO2. Peritubular capillary and intraluminal PCO2 was measured during microperfusion with PCO2 microelectrodes to allow determination of the transepithelial CO2 gradient. The mean PCO2 measured in peritubular capillaries of control rats was 60.6 +/- 1.9 mmHg. Since the perfusion solution initially contained no CO2, a gradient of 60 mmHg was imposed across the tubule epithelium. Intraluminal PCO2 rapidly approached that of the surrounding capillaries. At a tubule perfusion rate of 20 nl/min, the gradient between lumen and blood decreased to 0.9 mmHg, a value not significantly greater than zero. The calculated CO2 permeability coefficient (KCO2) was 3.69 X 10(-5) cm2/s. Addition of either 10(-4) M acetazolamide or benzolamide did not prolong the rapid dissipation of the imposed CO2 gradient. The KCO2 during carbonic anhydrase inhibition was not significantly different from control values. It is concluded that the rat proximal tubule does not present a physiologically significant diffusion barrier to CO2 either in the presence or absence of carbonic anhydrase activity. The previously demonstrated acid disequilibrium pH in the proximal tubule during inhibition of carbonic anhydrase represents an intraluminal accumulation of carbonic acid rather than of carbon dioxide gas.

Effects of photo‐degradation on the permeability and diffusivity characteristics of poly(p‐methylstyrene)

AbstractDiffusion and permeability coefficients for O2 and CO2 were determined for poly‐(p‐methylstyrene) films which have been exposed to short wave (λ=253,7 nm) ultra violet radiation, both in vacuum and at varying pressures of oxygen. It was found that both diffusion and permeability coefficients decrease with increasing exposure to the radiation, the decrease in the vacuum irradiated films being significantly greater than those observed during irradiation in oxygen, presumably a result of higher cross‐linking in the absence of oxygen.Solubility coefficients for polymers irradiated both in oxygen and in vacuum increase non‐linearly on irradiation time. Activation energies and pre‐exponential factors for diffusion increase with increasing degradation. Decreased permeabilities and diffusivities and increasing Arrhenius parameters are discussed in terms of the free volume theory of diffusion—the free volumes decreasing as a result of polymer modification, e.g. crosslinking during degradation—and the energy required to create the necessary critical free volume for a gas molecule to make a diffusive jump, increasing with degradation.

A dynamic approach to diffusion and permeation measurements

AbstractA dynamic method for investigating the mechanism of permeation and diffusion through polymers has been explored. The permeation cell consists of two compartments separated by the membrane. The permeant (gas, vapor, or liquid) is introduced into one compartment; a carrier gas (helium) flows at constant rate through the other and sweeps the permeant which diffuses through the membrane to the thermal conductivity detector. Both compartments are at atmospheric pressure; thus no or little membrane support is required, and leakage problems are minimal. Moreover, the same membrane can be used over a wide temperature range and for diverse permeants. The detector signal is at any instant proportional to the permeation rate. A simple mathematical formalism for deriving the diffusion coefficient from the transient permeation rates has been developed. The measured diffusion and permeability coefficients of CO2, O2, and N2 through low‐density polyethylene closely agree with literature values. Permeation of hexane and benzene through polyethylene follows a complex diffusion law, and the rate depends on the thermal history of the system. The dynamic method is particularly suited to the study of transitions in polymers. Changes in permeation rates, usually occurring at transition points, can easily be discovered by slow temperature scanning of the system.