Gas permeability,diffusivity, and solubility of poly(4-vinylpyridine) film

Although poly(4-vinylpyridine) is believed to have good gas permselectivity, the intrinsic gas permeation property is rarely reported in the literature. The objective of this work is to study the the intrinsic gas permeation property of poly(4-vinylpyridine) using a free-standing film. Because of its brittleness and strong adhesion with most solid surfaces, a free-standing poly(4-vinylpyridine) film was therefore prepared from casting on a liquid mercury surface. The permeation behavior of He, H₂, O₂, N₂, CH₄, and CO₂ through the film was tested over a pressure range of 252 to 800 cm Hg at 35°C. The permeability and solubility decrease slightly with an increase in pressure, whereas the diffusivity increases as pressure increases. The pressure-dependent phenomenon can be explained using the partial immobilization model and the dual sorption model. An effective gas molecule diameter, which is defined as the square root of the product of gas collision and kinetic diameters, was used to correlate the diffusivity and gas molecule size, and an empirical equation was derived. Solubility is also a strong function of gas physical properties such as critical temperature and Lennard–Jones force constant, which are the measures of gas condensability and molecule interaction, respectively. In general, higher solubility in a polymer is obtained for gases with greater condensability and stronger interaction. Typical gas permeabilities of poly(4-vinylpyridine) measured at 619 cm Hg and 35°C are: 12.36 (He), 12.64 (H₂), 3.31 (CO₂), 0.84 (O₂), 0.14 (CH₄), and 0.13 (N₂) barrers. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2851–2861, 1999     

Ethylbenzene solubility,diffusivity, and permeability in poly(dimethylsiloxane)

The pure‐gas sorption, diffusion, and permeation properties of ethylbenzene in poly(dimethylsiloxane) (PDMS) are reported at 35, 45, and 55 °C and at pressures ranging from 0 to 4.4 cmHg. Additionally, mixed‐gas ethylbenzene/N₂ permeability properties at 35 °C, a total feed pressure of 10 atm, and a permeate pressure of 1 atm are reported. Ethylbenzene solubility increases with increasing penetrant relative pressure and can be described by the Flory–Rehner model with an interaction parameter of 0.24 ± 0.02. At a fixed relative pressure, ethylbenzene solubility decreases with increasing temperature, and the enthalpy of sorption is −41.4 ± 0.3 kJ/mol, which is independent of ethylbenzene concentration and essentially equal to the enthalpy of condensation of pure ethylbenzene. Ethylbenzene diffusion coefficients decrease with increasing concentration at 35 °C. The activation energy of ethylbenzene diffusion in PDMS at infinite dilution is 49 ± 6 kJ/mol. The ethylbenzene activation energies of permeation decrease from near 0 to −34 ± 7 kJ/mol as concentration increases, whereas the activation energy of permeation for pure N₂ is 8 ± 2 kJ/mol. At 35 °C, ethylbenzene and N₂ permeability coefficients determined from pure‐gas permeation experiments are similar to those obtained from mixed‐gas permeation experiments, and ethylbenzene/N₂ selectivity values as high as 800 were observed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1461–1473, 2000     

Gas solubility and permeability in MFA

An experimental analysis has been performed in this work, aimed to the characterization of thermodynamic and mass transport properties of a semicrystalline fluoro polymer (MFA) obtained from the copolymerization of tetrafluoroethylene (TFE) and perfluoromethylvinylether. Sorption and permeation experiments for two alkanes and corresponding perfluorinated compounds in MFA were performed at two different temperatures and solubility coefficients, as well as diffusivity and permeability, were determined. Experimental data were analyzed through different thermodynamic models to draw general conclusions about properties of MFA polymeric phases. Special attention was devoted to the glassy nature of MFA polymeric mixtures around room temperature. Indeed, analysis of experimental sorption data was performed through the use of specific models for glassy polymeric phases as well as by means of classical equilibrium models for fluid mixtures. Conclusions have been drawn from the aforementioned analysis, which significantly contributes to the discussion of correct location of glass‐transition temperature for PTFE and its copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1637–1652, 2007     

Effects of physical aging on solubility,diffusivity, and permeability of propane and n-butane in poly(4-methyl-2-pentyne)

The effects of physical aging on the solubility, diffusivity, and permeability of propane and n-butane in a hydrocarbon-based disubstituted polyacetylene, poly(4-methyl-2-pentyne) (PMP), were studied. As the relative pressure of propane and n-butane increased, the solubility of both hydrocarbons increased. Like other glassy polymers, the sorption isotherms for propane and n-butane in all PMP films were concave to the relative pressure axis, indicating dual-mode sorption behavior. The diffusion of propane and n-butane in PMP followed typical Fickian diffusion in a plane sheet. The propane diffusivity in both the unaged and aged films increased with increasing concentration of propane sorbed in the film. The n-butane diffusivity in aged films also increased with increasing n-butane concentration. However, unaged films showed the opposite behavior: the diffusivity decreased with increasing n-butane concentration. These diffusion phenomena are a consequence of the interplay between thermodynamic and mobility factors. The permeabilities of propane and n-butane decreased monotonically with increasing penetrant concentration, similar to the behavior observed in other common glassy polymers. The relaxation of the nonequilibrium excess free volume in PMP films induced the decrease in both solubility and diffusivity. As a result, the permeability of propane and n-butane in PMP decreased upon physical aging. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2407–2418, 2004     

The physical aging phenomenon of 6FDA‐durene polyimide hollow fiber membranes

The aging phenomenon of asymmetric 6FDA‐durene polyimide hollow fibers spun with different shear rates for gas separation has been investigated. The permeances and selectivities of different gases, such as H₂, O₂, N₂, CH_4, and CO₂, were experimentally determined as a function of time for around five months at room temperature. It was found that the gas permeation fluxes of the uncoated and silicone rubber‐coated hollow fibers decreased significantly during the first 30 days following fabrication and then slightly deteriorated thereafter. In the early stage of aging, because of different molecular orientations and skin morphologies induced by shear rates, the percentage of permeance drop for uncoated fibers increased with increasing shear rates, then decreased with increasing shear rates. The permeance of 6FDA‐durene hollow fibers coated with silicone rubber dropped more significantly than the uncoated fibers, implying that silicone rubber coating did affect the aging behavior. This might be due to the fact that silicone rubber layer hindered the molecular relaxation and tightened interface molecules between the dense selective layer and silicone rubber, thus the selectivity increased with aging. Thermal analysis data suggest two processes occurring simultaneously during the aging: one is the relaxation of shear oriented chains, and the other is the densification of chain packing through the reduction of interstitial space among chains. The former has been confirmed by an increase in CTE, while the latter was confirmed by an increase in the peak of β‐relaxation temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 765–775, 2000     

Gas transport properties of hyperbranched polyimide/hydroxy polyimide blend membranes

Gas transport properties of novel hyperbranched polyimide/hydroxy polyimide blends and their silica hybrid membranes were investigated. Gas permeability coefficients of the blend membranes showed positive deviation from a semilogarithmic additive rule. The enhanced gas permeability were resulted from the increase in free volume elements caused by the intermolecular interaction between terminal amine groups of the hyperbranched polyimide and hydroxyl groups of the hydroxy polyimide backbone. Additionally, CO₂/CH₄ separation ability of the blend membranes was markedly promoted by hybridization with silica. The remarkable CO₂/CH₄ separation behavior was considered to be due to characteristic distribution and interconnectivity of free volume elements created by the incorporation of silica. For the hyperbranched polyimide/hydroxy polyimide blend system, polymer blending and hybridization techniques synergistically provided the excellent CO₂/CH₄ separation ability.     

Effect of molecular association on solubility,diffusion, and permeability in polymeric membranes

The filling‐type membrane is composed of grafted polymer and solvent‐resistant substrate; the calculation of solubility, diffusivity and swelling‐suppression effect by the substrate permits the prediction of solvent permeability. As noted in our previous article, the use of this approach, called membrane design, resulted in accurate prediction of the permeability of aromatic compounds. In this study, the influence of hydrogen bonding on solubility and diffusivity is investigated both theoretically and experimentally. The solubility of chloroform and dichloromethane in poly(acrylate)s increases, and their diffusivity decreases, compared with that estimated without considering the hydrogen‐bonding effect. Solubilities predicted by the lattice‐fluid hydrogen‐bonding (LFHB) model show good agreement with the results of vapor sorption. Comparison of diffusion coefficients measured by vapor permeation with those predicted from free volume theory reveals that the decrease of solvent diffusion coefficient is approximately proportional to the fraction of associated molecules. Fluxes of chloroform and dichloromethane were measured by vapor permeation experiments through filling‐type acrylate membranes, and predictions agree well with experiments. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 171–181, 2000     

Gas permeability in rubbery polyphosphazene membranes

The synthesis, characterization, and gas permeability of 10 new polyphosphazenes has been studied. Additionally, the first gas permeation data has been collected on hydrolytically unstable poly[bis-(chloro)phosphazene]. Gases used in this study include CO₂, CH₄, O₂, N₂, H₂, and Ar. CO₂ was the most permeable gas through any of the phosphazenes and a direct correlation between the T_g of the polymer and CO₂ transport was noted with permeability increasing with decreasing polymer T_g. To a lesser degree, permeability of all the other gases studied also yielded increases with decreasing polymer T_g. The trend observed for these new polymers was further supported by published data for other phosphazenes. Furthermore, permeability data for all gases were found to correlate to the gas condensability and the gas critical pressures, except for hydrogen, suggesting that the nature of the gas is also a significant factor for permeation through rubbery phosphazene membranes. Ideal separation factors () for the CO₂/H₂ and CO₂/CH₄ gas pairs were calculated. For CO₂/CH₄, no increase in was observed with decreasing T_g, however increases in were noted for the CO₂/H₂ pair.     

Gas transport properties in thermally cured aromatic polyimide membranes

Aromatic polyimide derived from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 3,3′-diaminodiphenylsulfone (m-DDS) has been synthesized to facilitate the study of relationships between the polymer structure and the gas transport properties (permeability and selectivity). The gas permeability and selectivity of CO₂, O₂, N₂, and CH₄ for the 6FDA-m-DDS membranes cured at 150, 200, and 250°C have been determined at 35°C and at pressures up to 760 cmHg. The packing density and the fluorescence intensity of the 6FDA-m-DDS polyimide increased sharply with the increasing curing temperature. We propose that this behavior is associated with an increase in intermolecular and/or intramolecular interactions by a charge transfer complex formed in 6FDA-m-DDS containing an alternating sequence of electron donor and electron aceptor molecules. The effect of the microstructure of the thermally cured 6FDA-m-DDS membranes on their gas transport properties is discussed.     

Permselectivity, solubility and diffusivity of propyl propionate/water mixtures in poly(ether block amide) membranes

This work is concerned with the separation of propyl propionate/water mixtures by pervaporation using PEBA membranes, which is relevant to aroma compound recovery from dilute aqueous solutions. The solubility and diffusivity pertinent to the permselectivity were investigated. The effects of feed concentration and the operating temperature on the separation performance were studied. Under the experimental conditions tested, the permeate concentration was much higher than the solubility limit, and upon phase separation substantially pure propyl propionate could be achieved. The diffusivity of propyl propionate through the membrane from its dilute aqueous solutions was affected by the solution concentration exponentially. It was shown that the permselectivity of the membrane for propyl propionate/water separation was mainly derived from its sorption selectivity due to the organophilicity of the membrane. The diffusivity of pure propyl propionate in the membrane was about 28 times higher than pure water diffusivity.     

Relationships between chemical structures and solubility,diffusivity, and permselectivity of 1,3-butadiene and n-butane in 6FDA-based polyimides

The solubility, diffusivity, and permselectivity of 1,3-butadiene and n-butane in seven different polyimides synthesized from 2,2-bis (3,4-carboxyphenyl) hexafluoropropane dianhydride (6FDA) were determined at 298 K. The influence of chemical structures on physical and gas permeation properties of 6FDA-based polyimides was studied. Solubility of 1,3-butadiene in 6FDA-based polyimides can be described by a dual-mode sorption model. 1,3-Butadiene-induced plasticization is considered to be associated with the increasing permeabilities of 1,3-butadiene and n-butane and the decreasing permselectivity of 1,3-butadiene vs. n-butane in the mixed gas system containing a high concentration of 1,3-butadiene. It was found that controlling the solubility of 1,3-butadiene in an unrelaxed volume in 6FDA-based polyimides is very important to maintain the high permselectivity of 1,3-butadiene vs. n-butane in the mixed gas system. Changing the  C(CF₃)₂ linkage to a  CH₂ ,  O linkage, removing methyl substituents at the ortho position of the imide linkage, and changing the p-phenylene linkage to an m-phenylene linkage in the main chains in some 6FDA-based polyimides are effective to decrease fractional free volume and restrict the solubility of 1,3-butadiene in the unrelaxed volume of a polymer matrix. The 6FDA-based polyimides restricting the solubility of 1,3-butadiene in an unrelaxed volume exhibit high separation performance in the 1,3-butadiene/n-butane mixed gas system compared with conventional glassy polymers and, therefore, are potentially useful membrane materials for the separation of 1,3-butadiene and n-butane in the petrochemical industry. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2941–2949, 1999     

Olefin/paraffin gas separations with 6FDA-based polyimide membranes

Pure gas permeation and sorption experiments were carried out for the gases ethylene, ethane, propylene and propane using polyimides based on 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA). Composite membranes and free films were used. Experiments were performed at 308 K and feed pressures up to 17 atm for ethylene and ethane and 9 atm for propylene and propane. Mixed gas permeation experiments were carried out with 50 : 50 olefin/paraffin feed mixtures. For all investigated polyimides, the ideal ethylene/ethane separation factor ranged between 3.3 and 4.4 and the ideal propylene/propane separation factor ranged between 10 and 16 at a feed pressure of 3.8 atm and 308 K. In mixed gas permeation experiments, up to 20% lower selectivity was found for the ethylene/ethane separation and up to 50% reduced selectivity for the propylene/propane separation compared to the ideal selectivity. The influence of feed temperature on separation and permeation properties will be discussed based on pure gas permeability data at 298 and 308 K.     

Structure/permeability relationships of polyimide membranes. Applications to the separation of gas mixtures

The permeability of nine different polyimide membranes to H₂, N₂, O₂, CH₄, and CO₂ has been determined at 35°C and at applied pressures of up to 9 atm. The dianhydride monomers used for the synthesis of the polymides were PMDA and 6FDA, whereas the diamine monomers were ODA, BDAF, and p-PDA. The selectivities of the 6FDA polymides toward CO₂ relative to CH₄ are higher than those of the PMDA polyimides at comparable CO₂ permeabilities. Both types of polyimides exhibit significantly higher CO₂/CH₄ selectivities than more common glassy polymers, such as cellulose acetate, polysulfone, and polycarbonate. The selectivities of the PMDA and 6FDA polyimides to O₂ relative to N₂ are of the same magnitude and generally higher than those of common glassy polymers with similar O₂ permeabilities. The polymides are more permeable to N₂ than to CH₄, whereas the opposite is true for many other glassy polymers. Possible factors responsible for the above behavior, such as segmental mobility, mean interchain distance, and formation of charge transfer complexes, are examined. The relevance of the study to the development of more highly gas-selective and permeable membranes for the separation of gas mixtures is also discussed.     

Gas permeability of cupric ion containing HTPB based polyurethane membranes

For the purpose of oxygen enrichment from air, the gas permeability and selectivity of an ionic polyurethane membrane was under investigation. Membranes of ionic polyurethane were prepared by step-growth polymerization of hydroxyl terminated polybutadiene (HTPB) and 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI). The ionic group was introduced by adding N-methyldiethanolamine (MDEA) as the chain extender of which the tertiary amines were complexed with cupric ions. The effect of hard segment content, polymerization method, peroxide introduction, and the amount of cupric ion on gas permeability were investigated. It was found that the binding of hard segment and the flexibility of soft segments had subtle effects on gas permeability. Membranes of the same composition were synthesized through two different procedures, one- and two-stage polymerization. The former contains large hard segment of cluster aggregation and flexible soft segments had a higher gas permeation rate. When a crosslinker, benzoyl peroxide, was added, the crosslinkage within soft segments hindered cluster formation by hard segment aggregation, the permeability increased. Furthermore, CuCl₂ addition enhanced hard segment aggregation, more hard segments formed cluster aggregates and less dispersed in soft segment region, which also increased permeability. However, excess CuCl₂ addition resulted in CuCl₂ piling up in the soft segment region, which restricted the movement of soft segments and therefore reduced the gas permeability.     

Gas separation characteristics of isomeric polyimide membrane prepared under shear stress

We synthesized the isomeric polyimides, 6FDA-m-DDS and 6FDA-p-DDS, and investigated the gas selectivity of the asymmetric polyimide membranes with an oriented surface skin layer. Particularly, we focused on the effect of the chemical structure of the polyimide on the molecular orientation. The asymmetric membranes with the oriented skin layer were prepared by a dry–wet phase inversion process at different shear stresses. The gas permeances of the asymmetric polyimide membranes were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg. The molecular orientation in the asymmetric polyimide membranes was measured using polarized ATR–FTIR spectroscopy. The gas selectivity of the asymmetric 6FDA-m-DDS membrane increased with an increased in the shear stress and were greater than that of the dense membrane. In contrast, the gas selectivities of the asymmetric 6FDA-p-DDS membrane did not depend on the shear stress and were similar to those of the dense membrane. We clarified that a parallel oriented surface formed on the asymmetric 6FDA-m-DDS membrane caused the enhanced gas selectivity of the membrane.     

Hydrocarbon solubility,permeability, and competitive sorption effects in polymer of intrinsic microporosity (PIM‐1) membranes

The sorption and permeation of pentane, hexane, and toluene through highly permeable polymer of intrinsic microporosity (PIM‐1) membranes were investigated. It was established that the hydrocarbons sorbed strongly within the micro‐void regions of the PIM‐1 membrane. The sorption concentration was similar for the paraffins, pentane and hexane, but greater for aromatic toluene at high vapor activities. The magnitude of the hydrocarbon permeability was associated with the critical temperature of the hydrocarbon. The PIM‐1 membrane displayed selectivity for the three hydrocarbons over CO₂. As a consequence, the presence of the three hydrocarbons dramatically reduced the permeability of CO₂ and N₂ under mixed gas–vapor conditions to 68%–95% below the dry gas value. For all three hydrocarbons the N₂ permeability was more strongly impacted than CO₂ permeability, and hence the ideal CO₂/N₂ selectivity of PIM‐1 increased. It was determined that CO₂ and N₂ solubility decreased because of hydrocarbon competitive sorption while CO₂ and N₂ diffusivity also decreased because of anti‐plasticization, which was due to the presence of hydrocarbon clusters within the membrane structure. There was a clear correlation between the magnitude of anti‐plasticization and the critical temperature of the hydrocarbon. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 397–404     

Gas transport properties of asymmetric polyimide membranes prepared by ion‐beam irradiation

Ion beam irradiation has been widely used to modify the structure and properties of membrane surface layers. In this study, the gas permeability and selectivity of an asymmetric polyimide membrane modified by He ion irradiation were investigated using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 °C. Specifically, we estimated the effects of the gas diffusion and solubility on the gas permeation properties of the asymmetric membranes with the carbonized skin layer prepared by ion irradiation. The asymmetric polyimide membranes were prepared by a dry–wet phase inversion process, and the surface skin layer on the membrane was irradiated by He ions at fluences of 1 × 10¹⁵ to 5 × 10¹⁵ ions/cm² at 50 keV. The increase in the gas permeability of the He⁺‐irradiated asymmetric polyimide membrane is entirely due to an increase in the gas diffusion, and the gas selectivity increases of the membranes were responsible for the high gas diffusion selectivities. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 262–269, 2007.     

Relationships between the chemical structures and the solubility,diffusivity, and permselectivity of propylene and propane in 6FDA‐based polyimides

The solubility, diffusivity, and permselectivity of propylene and propane in 40 different polyimides synthesized from 2,2‐bis(3,4‐decarboxyphenyl)hexafluoropropane dianhydride (6FDA) were determined at 298 K. The influence of the chemical structures on the physical and gas permeation properties of the 6FDA‐based polyimides was studied. The solubility of propylene in an unrelaxed volume of a polymer matrix mainly contributes to the total solubility of propylene for various 6FDA‐based polyimides. The diffusivity, the permeability of propylene, and the permselectivity in the propylene/propane mixed‐gas system depend on the solubility of propylene. This is thought to be associated with the penetrant‐induced plasticization effect. 6FDA‐based polyimides, which have a high glass‐transition temperature and a large fractional free volume, exhibit a high permeability with a relatively low permselectivity. Changing the number of  CH3 substituents in the phenylene linkage and changing the connectivity in the main chain are good ways of controlling the solubility of propylene and the corresponding permselectivity in the propylene/propane mixed‐gas system. Some 6FDA‐based polyimides restrict the solubility of propylene through the introduction of a  CONH linkage between the phenylene linkage; the  Cl substituent in the phenylene linkage at the diamine moiety exhibits a high separation performance in the mixed‐gas system. The polyimides are potentially useful membrane materials for the separation of propylene and propane in the petrochemical industry. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2525–2536, 2000     

Thermal treatment of dense polyimide membranes

A study has been conducted to clarify the relationship between polymer structure, annealing temperature, and the extent of plasticization by high‐pressure CO₂ for two typical polyimide membranes; BTDA‐DAPI (poly(3,3′‐4,4′‐benzophenone tetracarboxylic–dianhydride diaminophenylindane) and 6FDA‐TMPDA (poly(2,2′‐bis(3,4′‐dicasrboxyphenyl) hexafluoropropane dianhydride–2,3,5,6‐tetramethyl‐1,4‐phenylenediamine). Both membrane materials are exposed to varying levels of thermal annealing at 200 and 250 °C. The effect of this heat treatment on free volume is examined using positron annihilation lifetime spectroscopy (PALS), whereas fluorescence spectroscopy is used to monitor changes in electronic structure. Results show that thermal annealing causes a reduction in both the size and number of free volume elements. A strong relationship is found between the fluorescence peak intensity for 6FDA‐TMPDA and both the membrane gas permeability and plasticization pressure. This correlation is most likely the result of the formation of charge transfer complexes, particularly at 250 °C. However, the formation of covalent crosslinks at these temperatures cannot be discounted. No fluorescence is observed for BTDI‐DAPI. Although thermal annealing has a significant effect on the extent of plasticization in both polymers, it is found that the rate of plasticization is unaffected by the annealing temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1879–1890, 2008