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

Mean 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.     

The solubility of Carbon Dioxide and Methane in polyimides at elevated pressures

The solubility of CO₂ and CH₄ in five polyimides was measured at 35.0°C and at pressures up to 10 atm (147 psia). The concentration of the penetrant gases dissolved in the polymers can be represented satisfactorily as a function of penetrant pressure by the “dual-mode sorption” model. The solubility coefficients for CO₂ and CH₄, S(CO₂) and S(CH₄), increase in the polyimide order: The magnitude of the solubility coefficients appears to depend primarily on the intermolecular forces between the penetrant gases and the polymers. The values of these coefficients are greater for the polyimides with larger mean interchain spacings, but no one-to-one correspondence appears to exist in this respect. The lower solubility of CO₂ in PMDA-4,4'-m-APPS compared with that in the 6FDA polyimides may be due to the lower “excess” free volume of the former polymer. The ratio S (CO₂)/S (CH₄) varies relatively little for a variety of PMDA and 6FDA polyimides.     

Transport properties of polyimides containing phenylquinoxaline moieties

The gas permeabilities of a number of new structurally related polyimides containing phenylquinoxaline moieties were studied for the first time. The test polymers had different dianhydride units, whereas their diamine components differed in the presence of flexible ether bonds-O-in the main chain, a structure that is reflected in the effective packing of chains and, as a result, in transport parameters. The permeability, diffusion, and solubility coefficients for the gases H₂, He, O₂, N₂, CO, CO₂, and CH₄, as well as the ideal separation factors for gas pairs, were determined. The transport characteristics of polymers were compared within the given polymer series and with published data for other polymer series.     

Ending Aging in Super Glassy Polymer Membranes

Aging in super glassy polymers such as poly(trimethylsilylpropyne) (PTMSP), poly(4‐methyl‐2‐pentyne) (PMP), and polymers with intrinsic microporosity (PIM‐1) reduces gas permeabilities and limits their application as gas‐separation membranes. While super glassy polymers are initially very porous, and ultra‐permeable, they quickly pack into a denser phase becoming less porous and permeable. This age‐old problem has been solved by adding an ultraporous additive that maintains the low density, porous, initial stage of super glassy polymers through absorbing a portion of the polymer chains within its pores thereby holding the chains in their open position. This result is the first time that aging in super glassy polymers is inhibited whilst maintaining enhanced CO₂ permeability for one year and improving CO₂/N₂ selectivity. This approach could allow super glassy polymers to be revisited for commercial application in gas separations.     

Gas permeability and permselectivity of polyimides with large aromatic rings

Polyimides with large aromatic rings were prepared from 3,6-diaminocarbazole (CDA), N-ethyl-3,6-diaminocarbazole (ECDA), 2,7-diaminofluorene (DAF), 2,7-diaminofluorenon (DAFO), and dimethyl-3,7-diaminodibenzothiophene-5,5-dioxide (DDBT) with 2-bis(2,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). Their physical properties, including gas permeability and permselectivity, were investigated in comparison with those of the related polyimides from 1,3-phenylenediamine (mPD). Glass transition temperatures of the polyimides with large aromatic rings were much higher than those of the mPD-based polyimides as a result of increased rigidity of the former polymer chains. With changing diamine from mPD to the large aromatic diamines, charge transfer (CT) interaction between the moieties of acid anhydride and diamine seems to be enhanced, judging from the red shift of absorption edge of the polyimide films and the red shift of CT excitation band of the 6FDA-based polyimides in solution. Fraction of free space (V_F) was a little smaller for the polyimides with large aromatic rings except DDBT than for the mPD-based polyimides, probably because of enhancement in polymer chain-chain interactions as a result of the increased CT interaction. The DDBT-based polyimides had large V_F than the mPD-based polyimides because of the nonplanar structure of neighboring dibenzothiophene-5,5-dioxide and imide rings. For the 6FDA-based polyimides, permeability coefficients to H₂, O₂, N₂, CO₂, and CH₄ were in the order, DAFO < mPD ～ DAF < CDA < ECDA < DDBT. As for the membrane performance for H₂/CH₄, CO₂/CH₄, and O₂/N₂ systems, it is significant to change diamine from mPD to DDBT or CDA, but not to DAF or DAFO. The DDBT-based polyimides were excellent for H₂/CH₄ and CO₂/CH₄ separations. © 1995 John Wiley & Sons, Inc.     

Synthesis, characterization, and gas permeation properties of 6FDA-2,6-DAT/mPDA copolyimides

The goal of this work is to explore new polyimide materials that exhibit both high permeability and high selectivity for specific gases. Copolyimides offer the possibility of preparing membranes with gas permeabilities and selectivities not obtainable with homopolyimides. A series of novel fluorinated copolyimides were synthesized with various diamine compositions by chemical imidization in a two-pot procedure. Polyamic acids were prepared by stoichiometric addition of solid dianhydride in portions to the diamine(s). The gas permeation behavior of 2,2′-bis(3,4′-dicarboxyphenyl) hexafluoropropane dianhydride(6FDA)-2,6-diamine toluene (2,6-DAT)/1,3-phenylenediamine (mPDA) polyimides was investigated. The physical properties of the copolyimides were characterized by IR, DSC and TGA. The glass transition temperature increased with increase in 2,6-DAT content. All the copolyimides were soluble in most of the common solvents. The gas permeability coefficients decreased with increasing mPDA content. However, the permselectivity of gas pairs such as H₂/N₂, O₂/N₂, and CO₂/CH₄ was enhanced with the incorporation of mPDA moiety. The permeability coefficients of H₂, O₂, N₂, CO₂ and CH₄ were found to decrease with the increasing order of kinetic diameters of the penetrant gases. 6FDA-2,6-DAT/mPDA (3:1) copolyimide and 6FDA-2,6-DAT polyimide had high separation properties for H₂/N₂, O₂/N₂, CO₂/CH₄. Their H₂, O₂ and CO₂ permeability coefficients were 64.99 Barrer, 5.22 Barrer, 23.87 Barrer and 81.96 Barrer, 8.83 Barrer, 39.59 Barrer, respectively, at 35°C and 0.2 MPa (1 Barrer = 10⁻¹⁰ cm³ (STP)·cm·cm⁻²·s⁻¹·cmHg⁻¹) and their ideal permselectivities of H₂/N₂, O₂/N₂ and CO₂/CH₄ were 69.61, 6.09, 63.92 and 53.45, 5.76, 57.41, respectively. Moreover, all of the copolyimides studied in this work exhibited similar performance, lying on or above the existing upper bound trade-off lines between permselectivity and permeability. They may be utilized for commercial gas separation membrane materials. __________ Translated from Acta Polymerica Sinica, 2008, 8 (in Chinese)     

Characterization and gas permeability of a three-component polyimide series

Packing density and gas permeability of a three-component copolymide series is presented. The three-component polyimides are prepared a via “stepwise” synthesis procedure that goes through the acid anhydride terminated pre-polymer. The procedure ensures the statistical distribution of segments of the polymers. The polyimide series is composed of contrasting segments: a bulky and rigid hexafluoroisopropylidene-2,2-bis(phthalic acid anhydride)/9,9,-bis(4-aminophenyl)fluorene and a flexible hexafluoroisopropylidene-2-2-bis(phthalic acid anhydride)/2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, with varying segment ratio. Generation of additional free volume by compolymerizing two segments is observed. The permeability of six pure gases—He, H₂, N₂, O₂, CH₄, and CO₂—to the polymides showed positive deviation from the simple additivity rule of segment weight ratio reflecting the generation of free volume. However, a conflicting result between free volume fraction and gas permeability is observed, which may be due to a difference of the nature of free volume of each segment. © 1994 John Wiley & Sons, Inc.     

Gas‐transport properties of indan‐containing polyimides

A series of indan‐containing polyimides were synthesized, and their gas‐permeation behavior was characterized. The four polyimides used in this study were synthesized from an indan‐containing diamine [5,7‐diamino‐1,1,4,6‐tetramethylindan (DAI)] with four dianhydrides [3,3′4,4′‐benzophenone tetracarboxylic dianhydride (BTDA), 3,3′4,4′‐oxydiphthalic dianhydride (ODPA), (3,3′4,4′‐biphenyl tetracarboxylic dianhydride (BPDA), and 2,2′‐bis(3,4′‐dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)]. The gas‐permeability coefficients of these four polyimides changed in the following order: DAI–BTDA < DAI–ODPA < DAI–BPDA < DAI–6FDA. This was consistent with the increasing order of the fraction of free volume (FFV). Moreover, the gas‐permeability coefficients were almost doubled from DAI–ODPA to DAI–BPDA and from DAI–BPDA to DAI–6FDA, although the FFV differences between the two polyimides were very small. The gas permeability and diffusivity of these indan‐containing polyimides increased with temperature, whereas the permselectivity and diffusion selectivity decreased. The activation energies for the permeation and diffusion of O₂, N₂, CH₄, and CO₂ were estimated. In comparison with the gas‐permeation behavior of other indan‐containing polymers, for these polyimides, very good gas‐permeation performance was found, that is, high gas‐permeability coefficients and reasonably high permselectivity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2769–2779, 2004     

Predictions of cation and anion effects on solubilities,selectivities and permeabilities for CO2 in ionic liquid using COSMO based activity coefficient model

The solubilities and selectivities for CO₂, N₂ and CH₄ in ionic liquid were predicted using a COSMO based activity coefficient model, COSMO-SAC method. The 1-alkyl-3-methylimidazolium cations were focused in this work. The anion species include tetrafluoroborate [BF₄], hexafluorophosphate [PF₆], triflate [OTf], dicyanamide [dca] and bis(trifluoromethane)-sulfonimide [Tf₂N]. The predicted results of the solubilities of CO₂ in the ionic liquids by COSMO-SAC method are in agreement with the experimental data within the averaged deviation of 0.0017 in mole fraction. The predicted results of selectivities for CO₂/N₂ and CO₂/CH₄ represent the effects of anion species qualitatively. Permeability through supported liquid membrane can be presented by solubility and diffusion coefficients in the liquid. The permeabilities of CO₂ through the ionic liquid membranes were also predicted by a solution-diffusion model with COSMO-SAC method. The predicted results of the CO₂ permeabilities through the ionic liquids represent the experimental data within the order of the permeabilities.     

Effect of methyl substituents on permeability and permselectivity of gases in polyimides prepared from methyl-substituted phenylenediamines

Permeability and solubility coefficients for H₂, CO₂, O₂, CO, N₂, and CH₄ in polyimides prepared from 6FDA and methyl-substituted phenylenediamines were measured to investigate effects of the substituents on gas permeability and permselectivity. The methyl substituents restrict internal rotation around the bonds between the phenyl rings and the imide rings. The rigidity and nonplanar structure of the polymer chain, and the bulkiness of methyl groups make chain packing inefficient, resulting in increases in both diffusion and solubility coefficients of the gases. Polyimides from tetramethyl-p-phenylenediamine and trimethyl-m-phenylenediamine display very high permeability coefficients and very low permselectivity due to very high diffusion coefficients and very low diffusivity selectivity, as compared with the other polyimides having a similar fraction of free space. This suggests that these polyimides have high fractions of large-size free spaces.     

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 polymers based on spirobiindane bisphenol

Transport properties of pure gases in polycarbonates, polyesters, and polyetherimides based on 6,6′-dihydroxy-3,3,3′,3′-tetramethyl-1,1′-spiro biindane (SBI) and bisphenol-A (BPA) are compared at 35°C. The SBI monomer contains two spiro-linked five-membered rings which are fused to the phenyl rings at the meta and para positions to the hydroxyl groups. This molecular structure gives SBI-based polymers with higher fractional free volume (FFV) and lower intramolecular motions as compared to the BPA-based analogs. The inhibition of chain packing due to the SBI moiety yields polymers with much higher permeabilities for all the gases studied, despite the hinderance of mobility associated with the SBI structure. Simultaneous increase in selectivity was also observed for some gas pairs. Oxygen permeabilities up to 5.9-fold higher with increases of up to 13% in O₂/N₂ selectivities were observed for a polyester based on SBI as compared to its analog based on BPA. Higher permeabilities of up to 4.3-fold for He and up to 4.8-fold for CO₂ were observed due to the substitution of SBI for BPA. Not surprisingly, lower values of He/CH₄ and CO₂/CH₄ selectivities were obtained for the more open SBI-containing polymers. The changes in fractional free volume and inhibition of small-scale mobility for some materials caused by the SBI moiety were measured and used in the interpretation of the gas transport properties. The individual contributions of diffusivity and solubility to the overall transport behavior of the polymers are discussed and correlated to the structural alterations caused by the SBI substitution for BPA monomer. © 1995 John Wiley & Sons, Inc.     

Gas transport behavior of mixed-matrix membranes composed of silica nanoparticles in a polymer of intrinsic microporosity (PIM-1)

Recently, high-free volume, glassy ladder-type polymers, referred to as polymers of intrinsic microporosity (PIM), have been developed and their reported gas transport performance exceeded the Robeson upper bound trade-off for O₂/N₂ and CO₂/CH₄. The present work reports the gas transport behavior of PIM-1/silica nanocomposite membranes. The changes in free volume, as well as the presence and volume of the void cavities, were investigated by analyzing the density, thermal stability, and nano-structural morphology. The enhancement in gas permeability (e.g., He, H₂, O₂, N₂, and CO₂) with increasing filler content shows that the trend is related to the true silica volume and void volume fraction.     

6FDA-containing polyimide-ionene + ionic liquid gas separation membranes

Polyimides (PI) synthesized from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with various diamines have been frequently studied as gas separation membranes. The use of 6FDA in polyimides creates a bent structure than can increase fractional free volume (FFV) and gas permeability. Here, we demonstrate that 6FDA is also a useful building block for PI-ionene materials, which contain cations directly within the polymer backbone. These new 6FDA-containing PI-ionenes were combined with several different imidazolium ionic liquids (ILs) to form thin membranes. The thermal properties of all the derivatives were investigated to determine the relationship between regiochemistry and degradation as well as the intermolecular forces that are present within these structures. The gas separation properties of these 6FDA-containing PI-ionene + IL materials were investigated, showing modest CO₂ permeabilities similar to other polyimide-ionenes and CO₂/CH₄ and CO₂/N₂ permselectivities that were relatively higher than other polyimide-ionenes.     

Thermally stable polyimide isomers for membrane-based gas separations at elevated temperatures

The temperature dependence of gas sorption and transport properties is examined for two polyimide isomers. The permeabilities and solubilities of five gases in these materials are reported over an extensive temperature range from 35 to 325°C. Also, the activation energies for permeation, the heats of sorption, and the activation energies for diffusion obtained for both polyimides are compared and correlated with physical properties of the polymers and penetrants. The influence of temperature on the selective properties of these membrane materials is discussed for three gas separations; He/N₂, CO₂/CH_4, and O₂/N_2. Thorough analysis of these data provides insight into the influence of the subtle difference in chain structure of the two isomers. The performance of the 6FDA-6F_p DA as a separation membrane at high temperatures suggests that it is an outstanding candidate for use in novel elevated temperature applications. ©1995 John Wiley & Sons, Inc.     

Determination of the upper limits, benchmarks, and critical properties for gas separations using stabilized room temperature ionic liquid membranes (SILMs) for the purpose of guiding future research

The literature reports that supported ionic liquid membranes (SILMs) outperform standard polymers for the separations of CO₂/N₂ and CO₂/CH₄, even under continuous flow mixed gas conditions. Before the expenditure of more resources to develop new room temperature ionic liquids (RTILs) and SILMs, it is time to consider what benchmarks for SILM performance exist and if upper limits could be projected based on the physical chemistry of RTILs. At this juncture, we should ask if the current research efforts are properly focused based on the successes and failures in the literature. We summarize literature data, along with adding new data, on the SILM permeabilities and selectivities for the following gas pairs: CO₂/N₂, CO₂/CH₄, O₂/N₂, ethylene/ethane, propylene/propane, 1-butene/butane, and 1,3-butadiene/butane. The analysis predicts a maximum CO₂-permeability for SILMs and an upper bound for permeability selectivity vs. CO₂-permeability with respect to the CO₂/N₂ and CO₂/CH₄ separations. Also summarized are the representative successes and failures for improving the separation performance of SILMs via functionalization and facilitated transport in the context of the CO₂/N₂, CO₂/CH₄, and olefin/paraffin separations. In the context of the CO₂-separations, the analysis recommends a number of future research foci including research into SILMs cast from RTILs with smaller molar volumes. In the context of olefin/paraffin separations, the preliminary data is encouraging when considering the use of facilitated transport via silver carriers. Since RTIL-solvent/solvent interactions dominate in terminating the overall SILM performance, past attempts at enhancing solute/solvent interactions via the addition of functional groups to the RTILs have not produced SILMs with better separation performance compared to the unfunctionalized RTILs. Future research into functionalized RTILs needs to consider the changes to the dominant solvent/solvent interactions and not just the solute/solvent interactions.     

Sorption of carbon dioxide in fluorinated poyimides

Sorption isotherms of CO₂ for ten fluorinated polyimides measured at 35°C and up to about 25 atm are analyzed according to the dual-mode sorption model. Sorption properties for these polyimides are compared with those for other glassy Polymers including unfluorinated polyimides. The glassy polymers with higher glass transition temperatures T_g tend to show greater CO₂ sorption. Introduction of a ? C (CF₃)₂? linkage into the repeat unit of the main chain increases the sorption by 20–80%. For glassy polymers, including the fluorinated and unfluorinated polyimides, the Langmuir affinity constant b and Henry's law solubility constant k_D are correlated with the content of functional (carbonyl or sulfonyl) groups [FG], and composite parameter reflecting the magnitude of both [FG] and free-space fraction V_F, respectively, with some exceptions. The Langmuir capacity constant C′_H is correlated with T_g, but there are two correlation lines; one for unfluorinated polyimides and a different one for other glassy polymers including fluorinated polyimides. The slope of the former group is smaller probably because of smaller differences in thermal probably because of smaller differences in thermal expansion coefficients in rubbery and glassy states. Most fluorinated polyimides show greater solubility of CO₂ than unfluorinated polyimides and other glassy polymers, because of their larger C′_H and k_D. © 1993 John Wiley & Sons, Inc.     

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     

Gas sorption and transport in substituted polystyrenes

The effects of pendant groups on gas transport were investigated using a series of substituted polystyrenes. Permeability coefficients were measured at 35°C and 1 atm for He, N₂, O₂, CH₄, and CO₂, and diffusion coefficients were calculated from time lag data. The absolute permeabilities for the polystyrenes are correlated reasonably well using a free volume model. All pendant group substitutions resulted in a reduction of the mobility selectivity for CO₂/CH₄ separation relative to polystyrene, although there was very little effect on the O₂/N₂ selectivity. The effects of the various substituents were individually analyzed in terms of their size, rigidity, and polarity. The addition of a methyl group to the backbone significantly decreases transport, while attachment to the para ring position increases permeation. Bulky rigid groups, such as t-butyl, enhance permeation even more. Methoxy and acetoxy substitutions provided an excellent means of examining plasticization of polymers by CO₂, such as cellulose acetate, which contain these same moieties. The response of these polymers indicates that the degree of plasticization is related to the polarity and flexibility of the pendant group.     

Gas transport properties of 6FDA-TMPDA/MOCA copolyimides

The gas permeation behavior of 2,2′-bis(3,4′dicarboxyphenyl) hexafluoropropane dianhydride(6FDA)-2,4,6-trimethyl-1,3-phenylenediamine (TMPDA)/4,4′-methylene bis(2-chloroaniline) (MOCA) copolyimides was investigated by systematically varying the diamine ratios. All the copolyimides were soluble in most of the common solvents. The gas permeabilities and diffusion coefficients decreased with increasing MOCA content; however, the permselectivity of gas pairs such as H₂/N₂, O₂/N₂, CO₂/CH₄ was enhanced with the incorporation of MOCA moiety. Moreover, all of the copolyimides studied in this work exhibited performance near, lying on or above the existing upper bound trade-off line between permselectivity and permeability.