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

The 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.     

Die Gaspermeabilität von Polybutylenterephthalat in der Umgebung der Einfriertemperatur

The permeability of poly(butylene terephthalate) was examined for He, Ne, Ar and CO₂ at the temperature range of 293–338 K and an upstream pressure to 1,2 bar. The energies of activation of permeation and diffusion in case of Ne, Ar and CO₂ are higher above glass transition temperature than below, while they are nearly constant in case of He. The dependence of temperature of solubility coefficients does not change at the glass transition temperature.     

Gas transport properties of liquid crystalline poly (ethylene terephthalate-co-p-oxybenzoate)

Gas transport properties are reported for a series of films prepared from thermotropic poly (ethylene terephthalate-co-p-oxybenzoate), or PET/PHB, having compositions of 60 and 80 mol % PHB. The mesomorphic and crystalline morphology of the copolyester films was examined by cross-polarized light microscopy, differential scanning calorimetry (DSC), and x-ray diffraction. Melt-processed films of both compositions appeared to exhibit an entirely anisotropic morphology with low levels of conventional crystallinity. Solution-cast films prepared from the 60 mol % material were found to contain a large fraction of isotropic regions, which become ordered upon annealing above the glass transition. Permeability measurements were made for He, H₂, O₂, N₂, and CO₂ at 35°C and the diffusivities were computed from time-lag data. The largely anisotropic films exhibit good barrier properties resulting from very low solubility coefficients. The partially isotropic 60 mol % films show much higher permeability coefficients driven primarily by increased solubility coefficients, while diffusivity is affected to a lesser extent. These results appear to contrast with what is observed in semicrystalline systems where increased crystalline order results in more dramatic reductions in penetrant mobility.     

Gas permeability of cardo-poly(ether-ether-ketone) and poly(phenylene sulfide)

The gas permeability and sorption of CO₂ and N₂O was measured on cardo-poly(ether-ether-ketone) (C-PEEK) and poly(phenylene sulfide) (PPS) at 298 K. The results are discussed on the basis of the dual-mode model. Results obtained from measurements at 308 K are compared with literature data of poly(phenylene oxide) (PPO), poly(sulfone) (PSU) and poly(carbonate) (PC). While C-PEEK shows similar transport properties as PC and PSU, the values of PPS are distinctly lower. The solubility of CO₂ in the various polymers as well as the correlation of the permeability coefficients of the same polymers for He, Ar, CO₂, N₂, and CH₄ with the kinetic molecular diameter of the gases indicate a simple relation of the transport properties with the polymer density.     

Gas sorption in poly(vinyl benzoate)

High-pressure sorption (up to 50 atm) for CO₂, N₂, and Ar in poly(vinyl benzoate) (PVB) was studied at temperatures from 25 to 70°C by a gravimetric method utilizing an electromicrobalance. The results are described by Henry's law above the glass transition temperature T_g for all gases. The dual-mode sorption model, Henry's law plus a Langmuir isotherm, applies to the sorption isotherms of N₂ and Ar in the glassy state, and the dual-mode parameters are given. For CO₂, a new type of sorption isotherm is observed below T_g. The isotherm is concave to the pressure axis in the low-pressure region and turns into a straight line with increasing CO₂ pressure which can be extrapolated back to the coordinate origin. The linear part of the isotherm is characteristic of the rubbery state, while the nonlinear part stems from glassystate behavior. The “glass transition solubility” of CO₂, at which PVB film changes from the glassy to the rubbery state, decrease as the temperature increases. The disappearance of microvoids, that is, the decrease of the Langmuir capacity, may be due to a large plasticizing effect of sorbed CO₂. The difference between the N₂ and Ar isotherms and the CO₂ isotherm is discussed from this standpoint.     

Transport of organic vapors and liquids in poly(vinyl chloride)

This paper reviews research since 1980 on the equilibria and kinetics of transport of small organic molecules in rigid and plasticized PVC. The forms of both the solubility isotherms and the sorption kinetics are shown to change as the PVC/penetrant system undergoes a glass-rubber transition with an increase of either temperature or penetrant concentration. The isotherms are of “dual-mode” form (concave to the activity axis) for the glassy state, and show an inflection to Flory-Huggins form when the penetrant concentration exceeds C_g, the transition composition at the experimental temperature. The solubility at a given penetrant activity is governed primarily by the PVC/penetrant interaction parameter, χ. Sorption kinetics are Fickian for conditions producing small changes of concentration in either the glassy or rubbery state. For sorption into initially unplasticized PVC, kinetics are anomalous if the final penetrant concentration is between about C_g/2 and C_g, and Case II if C_g is exceeded. The magnitude of the Fickian diffusion coefficients depends largely on the geometric factors of molecular size and shape of the penetrant; this dependence is much steeper in the glassy than in the rubbery state. Recent results show that carbon dioxide displays both high diffusivity and substantial solubility in PVC under high pressure; this combination makes compressed CO₂ uniquely useful in accelerating the absorption of low-molecular-weight additives into PVC.     

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

Gas 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.     

Gas transport properties of thermotropic liquid-crystalline copolyesters. I. The effects of orientation and annealing

Gas transport properties are reported for two series of films prepared from copolyesters of 73 mol % hydroxybenzoic acid (HBA) and 27 mol % 2,6-hydroxynaphthoic acid (HNA) which systematically vary the degree of orientation and annealing time. Scanning electron microscopic (SEM) photomicrographs of the liquid-crystalline polymer (LCP) films showed evidence of a skin-core structure and polydomain texture. The degree of orientation in the films was quantified by analyzing the azimuthal intensity of the x-ray reflection associated with the lateral packing of the nematic mesophase. Using heat of fusion data from differential scanning calorimetry (DSC), the films were found to contain low levels of crystallinity estimated to be in the range of 5 to 15 wt %, which increased with annealing time. Permeability measurements were made for He, H₂, O₂, N₂, Ar, and CO₂ at 35°C and the diffusivities were computed from time-lag data. The films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. A moderate reduction in permeability was observed with increased orientation, which could be attributed directly to a decrease in the effective diffusivity. The effect of increased crystallinity from annealing on the permeability coefficients was smaller than would be expected for similar changes in the crystallinity of conventional polymers. Analysis using a simple two-phase model suggests that a mechanism dominated by transport in a small volume fraction of boundary regions possibly could account for the bulk transport properties of these materials.     

Gas transport properties of poly(ether‐b‐amide) segmented block copolymers

The permeation properties of H₂, N₂, and CO₂ were determined at 35 °C and pressures up to 15 atm in phase‐separated polyether‐b‐polyamide segmented block copolymers. These polymers contain poly(ethylene oxide) [PEO] or poly(tetramethylene oxide) [PTMEO] as the rubbery polyether phase and nylon‐6 [PA6] or nylon‐12 [PA12] as the hard polyamide phase. Extremely high values of polar (or quadrupolar)/nonpolar gas selectivities, coupled with high CO₂ permeability coefficients, were observed. CO₂/H₂ selectivities as high as 9.8 and CO₂/N₂ selectivities as high as 56 were obtained in polymers with CO₂ permeability coefficients of approximately 220 × 10⁻¹⁰ cm³(STP) cm/(cm² s cmHg). As the amount of polyether increases, permeability increases. Gas permeability is higher in polymers with less polar constituents, PTMEO and PA12, than in those containing the more polar PEO and PA6 units. CO₂/N₂ and CO₂/H₂ selectivities are higher in polymers with higher concentrations of polar groups. These high selectivity values derive from large solubility selectivities in favor of CO₂. Because CO₂ is larger than H₂ and has, therefore, a lower diffusion coefficient than H₂, the weak size‐sieving ability of the rubbery polyether phase, which is the locus of most of the gas permeation, also contributes to high CO₂/H₂ selectivity. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2051–2062, 2000     

Permeability and morphology of poly(cis‐butadiene)/ester‐ether liquid crystal composite membranes

Various amounts of diethylene glycol bis[4‐(4′‐ethoxybenzoyloxy)benzoate] (DEBEB) were added into a poly(cis‐butadiene) (PB) membrane to improve its gas permeation ability. This type of rubber/liquid crystal (LC) composite membranes showed two obvious characteristics that are different from the gas permeation behavior reported in previous literature: (1) The permeabilities to O₂, N₂, and CO₂ were enhanced at room temperature, for example, the permeabilities for the PB/DEBEB (90/10) membrane were higher above six times than those of PB membrane under the same conditions. It is suggested that the interface microvoids probably existed on pontes between polar crystal domains and nonpolar PB matrix. (2) All relationships between the permeability coefficient (P) and temperature (T) were characterized by N‐shape, that is, there were the peaks and valleys on the P‐T curves. Furthermore, morphology studies demonstrated that when DEBEB content in the membranes was above 10 wt %, it was spherically dispersed and embedded in the PB matrix in a crystal domain state. Gas permeation characteristics of the composite membranes were appropriately interpreted as together influence results of DEBEB phase transition behavior and the membrane morphology. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1833–1840, 2000     

Gas permeability,diffusivity, solubility,and aging characteristics of 6FDA-durene polyimide membranes

We have determined the intrinsic gas transport properties of He, H₂, O₂, N₂, CH₄, and CO₂ for a 6FDA-durene polyimide as a function of pressure, temperature and aging time. The permeability coefficients of O₂, N₂, CH₄, and CO₂ decrease slightly with increasing pressure. The pressure-dependent diffusion coefficients and solubility coefficients are consistent with the dual-sorption model and partial immobilization. All the gas permeabilities increase with temperature and their apparent activation energies for permeation increase with increasing gas molecular sizes in the order of CO₂, O₂, N₂, and CH₄.The percentages of permeability decay after 280 days of aging are 22, 32, 36, 40, 42, and 30% for He, H₂, O₂, N₂, CH₄, and CO₂, respectively. Interestingly, except for H₂ (kinetic diameter of 2.89 Å), the percentages of permeability decay increase exactly in the order of He (kinetic diameter of 2.6 Å), CO₂ (3.30 Å), O₂ (3.46 Å), N₂ (3.64 Å), and CH₄ (3.80 Å). The apparent activation energies of permeation for O₂, N₂, CH₄, and CO₂ increase with aging because of the increases in activation energies of diffusion and the decreases in solubility coefficients. The activation-energy increase for diffusion is probably due to the decrease in polymeric molar volume because of densification during aging. The reduction in solubility coefficient indicates the available sites for sorption decreasing with aging because of the reduction of microvoids and interstitial chain space.     

Effect of polyethyleneglycol (PEG) on gas permeabilities and permselectivities in its cellulose acetate (CA) blend membranes

The effect of polyethyleneglycol (PEG) on gas permeabilities and selectivities was investigated in a series of miscible cellulose acetate (CA) blend membranes. The permeabilities of CO₂, H₂, O₂, CH₄, N₂ were measured at temperatures from 30 to 80°C and pressures from 20 to 76 cmHg using a manometric permeation apparatus. It was determined that the blend membrane having 10 wt% PEG20000 exhibited higher permeability for CO₂ and higher permselectivity for CO₂ over N₂ and CH₄ than those of the membranes which contained 10% PEG of the molecular weight in the range 200–6000. The CA blend containing 60 wt% PEG20000 showed that its permeability coefficients of CO₂ and ideal separation factors for CO₂ over N₂ reached above 2 × 10⁻⁸ [cm³ (STP) cm/cm² s cmHg] and 22, respectively, at 70°C and 20 cmHg. Based on the data of gas permeability coefficients, time lags and characterization of the membranes, it is proposed that the apparent solubility coefficients of all CA and PEG blend membranes for CO₂ were lower than those of the CA membrane. However, almost all the blend membranes containing PEG20000 showed higher apparent diffusivity coefficients for CO₂, resulting in higher permeability coefficients of CO₂ with relation to those of the CA membrane. It is attributed to the high diffusivity selectivities of CA and PEG20000 blend membranes that their ideal separation factors for CO₂ over N₂ were higher than those of the CA membrane in the range 50–80°C, even though the ideal separation factors of almost all PEG blend membranes for CO₂ over CH₄ became lower than those of the CA membrane over nearly the full range from 30° to 80°C.     

Gas transport properties of soluble poly(phenylene sulfone imide)s

Gas transport of helium, hydrogen, carbon dioxide, oxygen, argon, nitrogen, and methane in three soluble poly(phenylene sulfone imide)s based on 2,2-bis(3,4-decarboxyphenyl) hexafluoropropane dianhydride (6FDA) has been investigated. The effects of increasing length of well-defined oligo(phenylene sulfone) units on the gas permeabilities and diffusivities were determined and correlated with chain packing of the polymers. Activation energies of diffusion and permeation were calculated from temperature-dependent time-lag measurements. The influences of the central group in the diamine moiety of 6FDA-based polyimides on physical and gas transport properties are discussed. The incorporation of a long oligo(phenylene sulfone) segment in the polymer backbone decreases gas permeability and permselectivity simultaneously. The decreases in permeability coefficients can be mainly related to decreases in diffusion coefficients. Changing the central group of diamine moiety from  S to  SO₂ leads to a 45–50% decrease in CO₂ and O₂ permeabilities without appreciable increase in the selectivities. This is considered to be due to the formation of charge transfer complexes. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1855–1868, 1997     

Preparation and characterization of a composite PDMS membrane on CA support

Polydimethylsiloxane (PDMS) is the most commonly used membrane material for the separation of condensable vapors from lighter gases. In this study, a composite PDMS membrane was prepared and its gas permeation properties were investigated at various upstream pressures. A microporous cellulose acetate (CA) support was initially prepared and characterized. Then, PDMS solution, containing crosslinker and catalyst, was cast over the support. Sorption and permeation of C₃H₈, CO₂, CH₄, and H₂ in the prepared composite membrane were measured. Using sorption and permeation data of gases, diffusion coefficients were calculated based on solution‐diffusion mechanism. Similar to other rubbery membranes, the prepared PDMS membrane advantageously exhibited less resistance to permeation of heavier gases, such as C₃H₈, compared to the lighter ones, such as CO₂, CH₄, and H₂. This result was attributed to the very high solubility of larger gas molecules in the hydrocarbon‐based PDMS membrane in spite of their lower diffusion coefficients relative to smaller molecules. Increasing feed pressure increased permeability, solubility, and diffusion coefficients of the heavier gases while decreased those of the lighter ones. At constant temperature (25°C), empirical linear relations were proposed for permeability, solubility, and diffusion coefficients as a function of transmembrane pressure. C₃H₈/gas solubility, diffusivity, and overall selectivities were found to increase with increasing feed pressure. Ideal selectivity values of 9, 30, and 82 for C₃H₈ over CO₂, CH₄, and H₂, respectively, at an upstream pressure of 8 atm, confirmed the outstanding separation performance of the prepared membrane. Copyright © 2009 John Wiley & Sons, Ltd.     

Tests of a “free-volume” model of gas permeation through polymer membranes. II. Pure Ar,SF6, CF4, and C2H2F2 in polyethylene

Permeability coefficients for Ar, SF₆, CF₄, and C₂H₂F₂ (1,1-difluoroethylene) in polyethylene membranes were determined from steady-state permeation rates at temperatures from 5 to 50°C, and at applied gas pressures of up to 15 atm. The temperature and pressure dependence of the permeability coefficients was represented satisfactorily by an extension of Fujita's free volume model of diffusion of small molecules in polymers. The parameters required by this model were determined from independent absorption (diffusivity) measurements with the above gases in polyethylene rods. The present work confirms the results of previous studies with CO₂, CH₄ C₂H₄ and C₃H₈ in polyethylene.     

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

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.     

Estimation of diffusion and permeability coefficients of CO2 in polymeric membranes by FTIR method

Infrared spectra of CO₂ sorbed in rubbery and glassy polymeric membranes were measured to examine the relationships between the spectroscopic data and the physical properties of the membranes. The two peaks observed in the spectra of CO₂ were attributed to the R branch and P branch of CO₂ sorbed in the membranes based on the consideration that both peaks were observed at a temperature above the glass transition temperature of the membranes. Apparent diffusion coefficients of CO₂ in the membranes were measured from the desorption kinetics of CO₂ detected by FTIR spectroscopy. The solubility coefficients of CO₂ were also estimated from absorbance spectra of CO₂ sorbed in the membranes using Lambert-Beer's rule. The permeability, solubility, and diffusion coefficients estimated by the FTIR method were found to correlate well with the coefficients obtained by conventional methods such as vacuum-pressure or sorption isotherm methods. © 1996 John Wiley & Sons, Inc.     

Gas permeation through hydrogels : II. Poly(vinylalcohol-co-itaconic acid) membranes

Permeability and diffusion coefficients of O₂, He, CO₂ and C₄H₆ were measured in water,swollen poly(vinylalcohol-co-itaconic acid) membranes having various water contents from 0.48 to 0.83. The permeability coefficients of CO₂ and C₄H₆ were found to depend on the upstream pressure, while the permeability coefficients of O₂ and He were independent of the pressure. With decreasing pressure the permeability coefficients of CO₂ and C₄H₆ increased, and the pressure dependence became larger with decreasing water content of the membranes. A parallel permeation model based on the two states of water in the water-swollen membranes could be applied successfully to CO₂ and C₄H₆.     

Tests of a “free-volume” model of gas permeation through polymer membranes. I. Pure CO2, CH4, C2H4, and C3H8 in polyethylene

Permeability coefficients have been measured for CO₂, CH₄, C₂H₄, and C₃H₈ in polyethylene membranes at temperatures of 5, 20, and 35°C and at applied gas pressures of up to 30 atm. The temperature and pressure dependence of the permeability coefficients was represented satisfactorily by an extension of Fujita's free-volume model of diffusion of small molecules in polymers. The results of the present steady-state permeability measurements provide further support for the conclusion reached from previous unsteady-state diffusivity measurements that Fujita's model is applicable to the transport of small molecules, such as CO₂, CH₄, C₂H₄, and C₃H₈, in polyethylene. It was previously thought that this model is applicable only to the transport of larger molecules, such as of organic vapors, in polymers.     

Permeation of high-pressure gases in poly(ethylene-co-vinyl acetate)

A previously proposed theoretical treatment to elucidate the pressure dependence of gas permeability is improved in order to apply it to polymer-gas systems in which gas dissolution follows the Flory-Huggins equation. Permeation rates of N₂, CH₄, and CO₂ in poly(ethylene-co-vinyl acetate) are measured in the pressure range below 90 atm at 10–40°C, and the effect of pressure on permeability is found for each gas. The data are analyzed using the improved method to estimate the contributions of concentration and hydrostatic pressure to the pressure dependence of permeability. The concentration effect decreases with increasing temperature, whereas the hydrostatic-pressure effect is almost independent of temperature. Crystallinity dependence of the concentration effect is discussed in connection with high-pressure permeation data of other semicrystalline polymers reported elsewhere. © 1993 John Wiley & Sons, Inc.