Gas transport in a thermotropic liquid-crystalline polyester

A 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 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.     

Effect of isopropylidene replacement on gas transport properties of polycarbonates

The gas sorption and transport properties of a series of polycarbonates in which the isopropylidene unit of bisphenol A polycarbonate has been replaced with another molecular group are presented. Two new materials, bisphenol of norbornane polycarbonate (NBPC) and bisphenol Z polycarbonate (PCZ), are compared with several polymers which have been studied previously in this laboratory, including bisphenol A polycarbonate (PC), hexafluorobisphenol A polycarbonate (HFPC), and bisphenol of chloral polycarbonate (BCPC). The effect of molecular structure on chain mobility and chain packing is related to the gas transport properties. Dynamic mechanical thermal analysis and differential scanning calorimetry are used to judge chain mobility, while x-ray diffraction and free volume calculations give information about chain packing. Permeability measurements were made for He, H₂, O₂, N₂, CH₄, and CO₂ at 35°C over a range of pressures up to 20 atm. Sorption experiments were also done for N₂, CH₄, and CO₂ under the same conditions. The permeability coefficients of these polymers rank in the order HFPC ? NBPC>PC>BCPC ? PCZ for all of the gases. With the exception of BCPC, this order correlates well with fractional free volume. The low gas permeability of BCPC is attributed to a polarity effect. In general, bulky and relatively immobile substituents, as in HFPC and NBPC, can yield improved separation characteristics. The polar group of BCPC and the flexible cyclohexyl substituent of PCZ result in relatively low gas permeability.     

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.     

Gas sorption in polymers based on bisphenol-A

Equilibrium gas sorption measurements for CO₂, CH₄, and N₂ were made with three polymers based on bisphenol-A, namely a polyhydroxyether, a polyetherimide, and a polyarylate. These data plus previous results for two other bisphenol-A polymers, polycarhonate and polysulfone, were analyzed using the dual-mode sorption model and the more recent gas-polymer-matrix model. The models were compared on the basis of physical interpretations of the resulting parameters. The Langmuir capacity from the dual-model model was related to the unrelaxed volume of the glassy polymer. The Henry's law sorption parameter from the dual-mode model was related to the internal pressure of the polymer and to its tensile stress at yield. The work suggests a means for estimation of gas sorption levels from thermal and mechanical properties of the polymer.     

Characterization of gas transport in selected rubbery amorphous polyphosphazene membranes

Gas permeabilities for six different gases have been evaluated for a series of closely related polyphosphazenes. Polyphosphazenes are attractive polymers for use as gas separation membranes due to their inherent chemical, thermal, and radiation stability. Additionally, polyphosphazenes may be tailored for specific chemical affinities. In this report, polyphosphazenes with three different pendant groups with varied hydrophilicity were characterized for gas permeation. All polymers were characterized as having modest permeabilities for methane, oxygen, nitrogen, helium, and hydrogen. These gases were not observed to have a significant interaction with the polymer structure and transport is attributed to segmental chain motion. Carbon dioxide was found to have a significant intermolecular interaction with the polymer and the permeability was observed to be proportional to the percentage of hydrophilic 2-(2-methoxyethoxy)ethanol on the backbone. Thus, we report a promising method for the development of CO₂ selective membranes.     

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.     

Investigation of gas transport and other physical properties in relation to the bromination degree of polymers of intrinsic microporosity

In this work, the synthesis of novel polymers of intrinsic microporosity (PIMs) with different degrees of bromine substitution by a free-radical substitution reaction was performed. The synthesized polymers were thoroughly characterized and their bromination degree was verified via nuclear magnetic resonance. The brominated PIMs were investigated by infrared spectroscopy, X-ray diffraction, and density measurements and correlated with their gas transport properties. It was found that with an increase in the bromination degree, the synthesized PIMs exhibited a significant increase in polymer chain packing density which led to reduced fractional free volume and consequent decrease in gas diffusion and permeability coefficients. The change in permeability coefficients caused an improvement in the CO₂/N₂, CO₂/CH₄, and O₂/N₂ ideal permeability selectivities. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2752–2761     

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 a series of high Tg polynorbornenes with aliphatic pendant groups

A study of gas transport properties of novel polynorbornenes with increasing length of an aliphatic pendant group R (CH₃ , CH₃(CH₂)₃ , CH₃(CH₂)₅ , CH₃(CH₂)₉ ) has been performed. These polymers were synthesized using novel organometallic complex catalysts via an addition polymerization route. This reaction route maintained the bridged norbornene ring structure in the final polymer backbone. Gas permeability and glass transition temperature were found to be higher than those for polynorbornenes prepared by ring-opening metathesis and reported in the literature. It was shown that for noncondensable gases such as H₂ and He the selectivity over N₂ decreased when the length of the pendant group increased, but remained relatively stable for the more condensable gases (O₂ and CO₂). The permeability coefficient is correlated well to the inverse of the fractional free volume of the polymers. The more condensable gases showed a deviation from this correlation for the longest pendant group, probably due to an increase of the solubility effect. This polymer series demonstrated a simultaneous increase in permeability and selectivity, uncommon for polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 797–803, 1998     

Transport properties of poly(phenylquinoxalines) containing heterocyclic moieties

The transport behavior of a new class of membrane materials—a series of poly(phenylquinoxalines) containing heterocyclic fragments in the backbone—has been studied. These polymers contain moieties of a common chemical structure. Therefore, it is possible to follow how the transport parameters change upon introduction of various moieties into the backbone chain. The coefficients of permeability, diffusion, and solubility for H₂, He, O₂, N₂, CO, CO₂, and CH₄ along with the separation factors for the corresponding pairs of gases have been determined. The results are compared with the data for previously studied polymers of the poly(phenylquinoxaline) series.     

Isomer effects on transport properties of polyesters based on bisphenol-A

Pure gas sorption and transport properties of polyesters based on bisphenol-A and both pure isophthalic and pure terephthalic acid chloride were obtained for He, N₂, O₂, CH₄, and CO₂ at 35°C. The polymers were synthesized in our laboratory and amorphous films were prepared with a specialized solvent casting procedure. The polymer containing m-phenylene groups shows higher permselectivity for most of the gas pairs. The ideal selectivity of O₂/N₂ was increased by 33% when p-phenylene units were replaced by m-phenylene ones. On the other hand, the polyester containing only p-phenylene groups, shows higher permeability to all the gases studied. The polymer based on pure terephthalic acid chloride has a 75% higher oxygen permeability and a 1.1-fold higher carbon dioxide permeability than the isophthalic acid derivative. The polyester containing meta-phenylene units has lower T_g, higher permselectivity, lower permeability, lower fractional free volume (FFV), and lower d-spacing. The values of FFV, and lower d-spacing. The values of FFV and d-spacing were only slightly different between the two isomers. Moreover, for the sub-T_gγ transition the maximum in tan δ occured at essentially the same temperature (?55°C). The polymer with a higher concentration of p-phenylene units shows somewhat larger area under the γ-peak, indicating slightly more sub-T_g motion. The Distribution of FFV is considered to be the determining factor for the differences in transport properties observed. © 1993 John Wiley & Sons, Inc.     

Effects of packing density on the gas-transport properties of poly(phenolphthalein phthalate)s

The gas transport and other pertinent properties of three aromatic polyesters, made by reacting phenolphthalein with terephthaloyl chloride (PPha-tere), 50:50 terephthaloyl and isophthaloyl chlorides (PPha-50:50), and isophthaloyl chloride (PPha-iso), are reported. The glass transition temperatures of PPha-tere, PPha-50:50, and PPha-iso are 299, 279, and 249°C, respectively. Unexpectedly, the densities are 1.297, 1.298, and 1.304 g/cm³ in the same order; PPha-iso, therefore, has a higher packing density than PPha-tere. The permeability of PPha-tere to CO₂, CH₄, N₂, and O₂ is roughly 25% higher than that of PPha-50:50, and roughly 100% higher than that of PPha-iso. PPha-tere is roughly five times as permeable as polysulfone but has slightly higher permselectivities for the above gases. On the other hand, the solubilities of each gas in these three aromatic polyesters are roughly the same, suggesting that the higher permeability of PPha-tere is the result of a much larger gas diffusivity in the polymer. It is shown here that the significant difference in gas-diffusivity is a manifestation of the effects of packing density on the transport properties of glassy polymers.     

Permeation and sorption in polynorbornenes with organosilicon substituents

Gas sorption properties, permeability coefficients, and diffusion coefficients of a series of norbornene polymers are presented. Introduction of the Si(CH₃)₃ group into the polynorbornene (PNB) backbone chain results in significant increases in glass transition temperature, permeability, and diffusion coefficient for a number of gases (H₂, O₂, N₂, CO₂, CH₄, C₂H₆). The transport properties and sorption isotherms for poly(5-trimethylsilyl norbornene) (PTMSNB) are very similar to those for poly(vinyltrimethyl silane) (PVTMS), which contains the same side-chain group but differs from PTMSNB by the structure of its main chain. For another silicon-containing polymer poly[5-(1,1,3,3-tetramethyl-1,3-disilabutyl) norbornene] (PDSNB) having a bulkier side-chain group, the glass-transition temperature is decreased in comparison with that of PNB, presumably owing to self-plasticization. Both silicon-containing norbornene polymers (PTMSNB and PDSNB) have permeability coefficients for “rapid” gases like H₂ or CO₂ of about 10² Barrer. The high values of the Langmuir sorption capacity C′_H for PTMSNB and PVTMS, as well as the high diffusivity and mobility of spin probes in these polymers, were attributed to a large free volume related to the bulky Si(CH₃)₃ groups attached directly to the main chain. © 1993 John Wiley & Sons, Inc.     

Gas transport in polymers based on bisphenol-A

Permeation measurements for CO₂, CH₄, O₂, N₂, and He were made with three polymers based on bisphenol-A, namely a polyhydroxyether, a polyetherimide, and a polyarylate. Measurements were also made for CO₂ and CH₄ in polysulfone. The data for CO₂, CH₄, and N₂ plus previous data for these gases in polycarbonate and polysulfone were combined with equilibrium gas sorption data and analyzed with the dual mode/partial immobilization model and the more recent gas-polymer-matrix model. A comparison of the two models was done on the basis of physical interpretations of the resulting parameters. The diffusion coefficient for the Henry's law population was related to the kinetic diameter of the gas. The infinite dilution, Henry's law, and Langmuir diffusion coefficients were related to the free volume of the polymer. The work suggests a means for order-of-magnitude estimation of diffusion coefficients from polymer density and molecular structure.     

Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles

The fundamental gas transport properties of thin films of six high performance polymers were evaluated in the presence of silicon dioxide particles. The silica particles were brought in close contact with the polymer inside the 200 Å. (DIA) pores of Anopore™ aluminum oxide membranes. This unique environment allows intimate contact between the polymer and the silica particles. The presence of silica improves the gas separation properties of the permselective layer, particularly for oxygen and nitrogen. The increase in O₂/N₂ selectivity for some membranes is accompanied by an increase in oxygen permeability. The oxygen/nitrogen separation properties of the polymers in the presence of silica falls above the so-called “upper limit” of performance reported for polymeric materials. The observed significant increases in the glass transition temperature suggest restriction of chain segmental mobility possibly due to adsorption of polymer to silica surface. The increase in the activation energy of permeation points to increases in energetics of diffusion as the reason for the improved selective permeation. The observed behavior was not limited to oxygen and nitrogen as demonstrated by the results for other gas pairs tested.     

Applications of ESCA to polymer chemistry. XVII. Systematic investigation of the core levels of simple homopolymers

The core levels of a series of 83 homopolymers have been studied by electron spectroscopy for chemical analysis (ESCA). Comparisons of the experimentally determined core-level binding energies with theoretical calculations using the ground-state potential model in the complete neglect of differential overlap (CNDO/2) self-consistent field molecular orbital (SCF MO) formalism have been made on the C_1s and O_1s core levels for the oxygen-containing polymers in the series. A comparison of the ground-state potential model (GPM) and relaxation potential model (RPM) on a series of six model compounds representative on the series of polymers is given. Compilations are given of binding energies of C_1s, O_1s, N_1s, Cl_2p, S_2p, Si_2p, and Br_3d levels for typical structural features of common occurrence in polymer systems. These data, taken in conjunction with that previously published on fluoropolymers, provide a sound basis for the development of ESCA as a fingerprint tool in the elaboration of features of structure and bonding in polymers in general.     

Gas‐Separation Membranes Loaded with Porous Aromatic Frameworks that Improve with Age

Porosity loss, also known as physical aging, in glassy polymers hampers their long term use in gas separations. Unprecedented interactions of porous aromatic frameworks (PAFs) with these polymers offer the potential to control and exploit physical aging for drastically enhanced separation efficiency. PAF‐1 is used in the archetypal polymer of intrinsic microporosity (PIM), PIM‐1, to achieve three significant outcomes. 1) hydrogen permeability is drastically enhanced by 375 % to 5500 Barrer. 2) Physical aging is controlled causing the selectivity for H₂ over N₂ to increase from 4.5 to 13 over 400 days of aging. 3) The improvement with age of the membrane is exploited to recover up to 98 % of H₂ from gas mixtures with N₂. This process is critical for the use of ammonia as a H₂ storage medium. The tethering of polymer side chains within PAF‐1 pores is responsible for maintaining H₂ transport pathways, whilst the larger N₂ pathways gradually collapse.     

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.     

Polypyrrolones for membrane gas separations. I. Structural comparison of gas transport and sorption properties

Despite efforts by the membrane community to develop polymeric materials with improved O₂/N₂ separation performance, limited progress has occurred for almost a decade. Molecular sieving media, which can exhibit gas separation properties superior to polymers, tend to be brittle and uneconomical to produce for large‐scale membrane separation processes. Considering this, the polymer structures investigated in this work were designed to mimic aspects of the structure of molecular sieving media such as zeolites and carbon molecular sieves while maintaining the processability associated with polymers. Significantly attractive gas separation material properties were obtained using hyper rigid polypyrrolone copolymers with controlled packing disruptions between flat, packable segments. The gas transport properties in the materials changed dramatically as a result of different average interchain spacing. Moreover, all of the polypyrrolones studied in this work exhibited performance lying on or above the existing O₂/N₂ upper bound trade‐off line between permselectivity and permeability. These results, therefore, may point the way to a new cycle of membrane materials improvements for gas separations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1235–1249, 1999