Gas Separation Membrane Materials Research Articles

Molecular dynamics simulation of diffusion of O 2 and CO 2 in amorphous poly(ethylene terephthalate) and related aromatic polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular simulation techniques have been applied to study the diffusion of oxygen and dioxide of carbon as small molecule penetrants in models of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions is generated into a unit cell whose dimensions are determined for each of the simulated aromatic polyesters in the cell to have the experimental density. The aim for this research is to explore and investigate the diffusion of gases through bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. The diffusion coefficients for O 2 and CO 2 were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 30 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, number of aromatic rings, ortho-, meta-, para-isomers, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume, temperature, number of aromatic rings, ortho-, meta- and para-isomers was found.

Simulation of diffusion of O 2 and CO 2 in amorphous poly(ethylene terephthalate) and related alkylene and isomeric polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular simulation techniques have been applied to study the diffusion of oxygen and carbon dioxide as small molecule penetrants in models of bulk amorphous poly(ethylene terephthalate) (PET) and related alkylene and isomeric polyesters. A bulk amorphous configuration with periodic boundary conditions made into a unit cell whose dimensions were determined for each of the simulated polyesters in the cell having the experimental density. The diffusion coefficients for O 2 and CO 2 were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 3 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, number of CH 2 groups, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume and number of CH 2 groups was found.

Structure-property relationships for film-forming poly(amide imide)s

Poly(amide imide)s from dichloro-dianhydrides of tri-mellitimide- and/or pyromellitimide-N-acetic acids (TMA and PMA), linear aromatic dimimines (LAD) and/or cardo diamine (CDA), 9,9′-bis-phenylfluorene (series PAI-1); a similar series (PAI-II) from dichloro-dianhydride of trimellitimide-N-benzoic acid (TMB), LAD and/or CDA; and a series of coPAIs from dichloro-dianhydride of TMA and mixtures of diphenylmethane diamine (DPA) with CDA were synthesized by low-temperature reaction in dimethyl acetamide (DMAA). The state of aggregation of PAIs and coPAIs in dilute and semi-dilute solutions of DMAA was characterized by measurements of specific heat capacity in the temperature interval 150-370 K, of heats of dilution in DMAA at room temperature, and of apparent viscosity in the temperature interval 288-373 K. The state of aggregation of PAIs and coPAIs in thin films cast from DMAA solutions was characterized by wide-angle and small-angle X-ray diffraction, dynamic mechanical analysis, broad-band dielectric spectroscopy and gas transport properties. It is concluded that film-forming PAIs and coPAIs have a reasonably good potential as high-temperature dielectrics and membrane materials for gas separation.

Energetic and Entropic Contributions to Mobility Selectivity in Glassy Polymers for Gas Separation Membranes

Gas transport and separation properties of a series of glassy polymers from different families are studied, and energetic and entropic contributions to diffusivity selectivity are estimated for the oxygen/nitrogen gas pair. Some polymers are found to entropically favor the diffusion of nitrogen over oxygen, despite an overall diffusivity selectivity favoring oxygen. General structural features of glassy polymeric matrices are identified which may be responsible for the magnitude of energetic and entropic contributions to diffusivity selectivity. Overly well-packed structures only result in entropic and energetic factors working to oppose each other leading to average overall diffusivity selectivities and low absolute diffusivities and permeabilities. On the other hand, rigidity of the polymer backbone introduced by substituent groups, hindering chain motion and disrupting packing, often leads to lower energetic contributions and similar entropic effects for both gas molecules. Increasing the entropic contributions to diffusivity selectivity while maintaining high energetic selectivities, observed for some polymers, would be the key to taking the next step in polymeric gas separation membrane materials. Polymers need to mimic the structure of molecular sieving media to successfully increase entropic selectivity values. This is a highly complex problem with subtle effects involving many structural features of the polymer matrix and the polymer−penetrant interaction.

IUPAC-NIST Solubility Data Series 70. Solubility of Gases in Glassy Polymers

Solubility of gases in polymers is an important property of polymeric materials relevant to many practical applications. Sorption of small molecules in polymers is a fundamental concern in such areas as food packaging, beverage storage, and polymer processing. However, by far the main interest in the solubility of gases in polymers, and especially in glassy polymers, is related to development of novel advanced materials for gas separation membranes. This is because the concentration gradient of a dissolved gas is the driving force of membrane processes. Development of these novel separation methods resulted in a rapid accumulation, in the recent literature, of thermodynamic data related to the solubility of gases in polymers at different temperatures and pressures. Polymers can be regarded as special cases of media intermediate between liquids and solids. As a consequence, modeling of gas sorption in polymers is very difficult and presents a permanent challenge to theoreticians and experimenters. The collection and critical evaluation of solubility data for various gas–polymer systems is relevant to both practical aspects of polymer applications and to fundamental studies of polymer behavior. This volume of the IUPAC-NIST Solubility Data Series summarizes the compilations and critical evaluations of the data on solubility of gases in glassy polymers. It is implied in this edition that “gases” are the components that are either permanent gases (supercitical fluids) or have saturated vapor pressure more than 1 atm at ambient conditions (298 K). The polymeric components of compilations and critical evaluations are primarily high molecular mass, amorphous, linear (noncross-linked) compounds that have the glass transition temperatures above ambient temperature. The data for each gas–polymer system have been evaluated, if the results of at least three independent and reliable studies have been reported. Where the data of sufficient accuracy and reliability are available, values are recommended, and in some cases smoothing equations are given to represent variations of solubility with changes in gas pressure and temperature. Referenced works are presented in the standard IUPAC-NIST Solubility Data Series format. Depending on the gas–polymer system, reported data are given in tabular form or in the form of sorption isotherms. The data included in the volume comprise solubilities of 30 different gases in more than 80 primarily amorphous homo and copolymers. Where available, the compilation or critical evaluation sheets include enthalpies of sorption and parameters for sorption isotherms. Throughout the volume, SI conventions have been employed as the customary units in addition to the units used in original publications.

Application of poly(phthalazinone ether sulfone ketone)s to gas membrane separation

A series of poly(phthalazinone ether sulfone ketone) (PPESK) copolymers containing different component ratios of bis(4-fluorodiphenyl) ketone and bis(4-chlorodiphenyl)sulfone with respect to a certain amount of 4-(4-hydroxyphenyl)-2,3-phthalazin-1-one were synthesized by polycondensation. Glass transition temperatures of these polymers were adjusted from 263°C to 305°C by changing the ratios of reactants. Gas permeability and selectivity of the dense membranes of the polymers for three kinds of gases (CO2, O2, and N2) were determined at different temperatures. The result indicated that the membrane of PPESK (S/K = 1/1, mol ratio) had an excellent gas separation property. Permeability of the polymer membranes for CO2, O2, and N2 was P = 4.121 barrier, P = 0.674 barrier, and P = 0.0891 barrier, respectively. Separation factors of α and α were 7.6 and 46, respectively. New material was made into a composite membrane with silicone rubber for blocking up leaks and defects on the surface of its nonsymmetrical membrane. As a result of the test, permeability of the composite membrane was J = 7.2 × 10−6 cm3 (STP) cm−2 S−1 cm−1 Hg and J = 0.99 × 10−6 cm3 (STP) cm−2 S−1 cm−1 Hg, whereas the α was still higher than 7. These showed that PPESKs had a bright prospect as the potential membrane material for high-temperature gas separation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2385–2390, 1999

Diffusion in gas separation membrane materials: A comparison and analysis of experimental characterization techniques

Typically, materials with high-performance transport properties such as zeolites, carbon molecular sieves, or hyper rigid polymers are inherently difficult or impossible to characterize by steady-state membrane permeation experiments used for conventional polymers. Diffusion coefficients determined by transient sorption, a measurement easily performed on brittle media, are analyzed here and compared to those determined by steady-state permeation/sorption and transient permeation for a glassy polymer and a carbon molecular sieve. Average and local diffusion coefficients are extrapolated to zero upstream partial pressure to eliminate effects caused by concentration dependence. Good agreement between the techniques was observed for the glassy polymer. On the other hand, carbon molecular sieves, possessing a more complex morphology, exhibit a greater difference in diffusion coefficients determined by the various techniques. Nevertheless, comparison of the analysis techniques is shown to provide potentially valuable insights into the morphological features of such carbon molecular sieves. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1747–1755, 1998

Diffusion in gas separation membrane materials: A comparison and analysis of experimental characterization techniques

Diffusion in gas separation membrane materials: A comparison and analysis of experimental characterization techniques

Tailoring mixed matrix composite membranes for gas separations

Polymeric gas separation membrane materials have improved significantly over the past two decades due to systematic optimization in backbone structure. Recent evidence suggests, however, that advantages of a purely polymer-based approach are reaching diminishing returns for important separations such as O2 and N2. Zeolites, carbon molecular sieves (CMS), and rigid rod polymers offer attractive transport properties but are difficult and expensive to process. Mixed matrix composite (MMC) membranes, incorporating molecular sieving materials within polymeric substrates, may provide economical, high performance gas separation membranes if defects at the molecular sieve/polymer interface can be eliminated. In addition, careful matching of the intrinsic permeability and selectivity of the support matrix and the molecular sieve domains is necessary. Theoretical O2 permeability and O2/N2 selectivity predictions are presented for such optimized membranes of zeolite 4A and CMS in Ultem® and Udel® matrices using two idealized expressions based on classical and more recent results. Positive and desirable deviations from these idealized results can be anticipated if a continuous molecular sieving phase is formed, but this special case is not required if a proper selection of the polymer matrix is made.

Solubility and diffusivity of light gases in ethyl cellulose at elevated pressures Effects of ethoxy content

The solubility of He, O 2, N 2, CH 4, and CO 2 in ethyl cellulose (EC) membranes with ethoxy contents of 47.2, 47.9, and 49.6% was measured at 35.0°C and at pressures up to ca. 30 atm (3,040 kPa). Mean diffusion coefficients, D ̄ , for these gases in EC were obtained from the solubility measurements in conjunction with mean permeability coefficients determined in a previous study. The solubility coefficients, S, for the above-mentioned gases in EC increase in the gas order: S(He)⪡ S(N 2)< S(O 2)< S(CH 4)< S(CO 2); this is also the order of increasing critical temperatures of these gases. The values of D ̄ for He, O 2, N 2, and CH 4 at low concentrations in EC increase in the order of decreasing ‘kinetic’ molecular diameters: D ̄ ( CH 4 )< D ̄ ( N 2 )< D ̄ ( O 2 )< D ̄ ( He) . The values of D ̄ ( CO 2 ) are lower than those of D ̄ ( O 2 ) , even thrugh CO 2 has a smaller ‘kinetic’ diameter than O 2. The lower diffusivity of CO 2 in EC could be due to specific interactions between polar groups in the polymer and the polar CO 2 molecules. In general, the values of S and D ̄ for the five gases in EC tend to increase with increasing ethoxy content in the polymer, probably due to an increase in the polymer free volume. EC and cellulose acetate (CA) are compared as membrane materials for gas separations, with emphasis on the CO 2 CH 4 separation. EC has a higher gas permeability and a lower selectivity than CA because of its larger free volume and higher chain mobility.

Gas separation membranes from tailor‐made polymers

AbstractThe development of tailor‐made polymers as new gas separation membrane materials has lead to membranes allowing both higher selectivities as well as higher permeabilities. The increased productivity of these new membrane materials is explained in terms of a higher free volume available for gas transport.

Effect of structure on the temperature dependence of gas transport and sorption in a series of polycarbonates

AbstractThe permeabilities and solubilities of five gases are reported for bisphenol‐A polycarbonate (PC), tetramethyl polycarbonate (TMPC), and tetramethyl hexafluoro polycarbonate (TMHFPC) at temperatures up to 200°C. The temperature dependence of permselectivity is discussed in terms of solubility and diffusivity selectivity changes with temperature for CO2/CH4 and He/N2 gas separations. The activation energies for permeation and diffusion and the heats of sorption are also reported for each gas in the three polycarbonates. Analysis of these values provides a better fundamental understanding of the effect of polymer‐penetrant interactions and polymer backbone structure on the temperature dependence of the transport and sorption properties of gases in membrane separation processes. Important factors affecting the solubility and diffusivity selectivity losses or gains with increased temperature are also identified through correlation of these data with physical properties of the gases and polymers. These conclusions provide a framework for choosing the most promising membrane materials for particular gas separations at elevated temperatures. © 1994 John Wiley &amp; Sons, Inc.

On the mechanism of gas transport in rigid polymer membranes

AbstractConventional polymers are compared as gas separation membrane materials with “tailormade” polymers. The increased permeability of the latter are due to their higher free volume available for gas transport. The increased free volume is associated with the rigidity polymer backbone. Free volume is obtained by subtracting the occupied volume, calculated using group contributions from the polymer specific volume. Wide Angle X‐ray techniques are used to obtain average d‐spacings that are interpreted in terms of average intermolecular space, and that are related to permeability data. These highly permeable rigid polymer membranes have high glass transition temperatures. The physical parameters, that is, Tg and the jump in heat capacity (ΔCp), are obtained with Differential Scanning Calorimetry, and are used to obtain an estimation of free volume. A good correlation for a series of random copoly[p, m‐phenylene(4‐phenyl)‐1,2,4‐triazoles] is obtained. A relationship between permeability and a free volume term, which can be estimated from thermodynamic properties, is equally valid for a wide variety of conventional polymers. © 1993 John Wiley &amp; Sons, Inc.

Transport of gases through polymeric membranes

AbstractIn this paper tailor‐made polymers like polyoxadiazoles, polytriazoles and polyimides are compared with conventional polymers as gas separation membrane materials. Because their permeation characteristics as a function of the free volume does not compare too well with that of the conventional ones the term free volume was reconsidered. Based on a unique relation between the diffusion coefficient and the jump of the heat capacity at the glass transition temperature Tg, the transport mechanism in the polymer matrix is considered to occur in jump motions from one cavity to an adjacent one. This mechanism was reconfirmed by a molecular dynamics study. It is shown that mass transport at high penetrant concentrations is not time‐independent but a phenomenon with transient characteristics. This transient character is also found in the volumetric behaviour during sorption. The polymer matrix is described as a network with topological constraints which changes its state upon high swelling stresses induced by the sorbed penetrant.

Synthesis and polymerization metathesis of fluorinated bicyclo[2.2.1]hept-2-enes. New materials for gas separation membranes

We report the synthesis and polymerization metathesis of fluorinated bicyclo [2.2.1]hept-2-enes Ia–IIIa: ▪ R 1=R 2=CF 3 ( I); R 2=F, R 2=OC 3F 7 ( II); R 1=F, R 2=OCF 2CF(CF 3)OC 3F 7( III);cat. - WCl 6 (WOCl 4) - organosilane. Monomers IIa and IIIa are novel compounds. Polycyclopentylenevinylenes Ib, IIlb easily form thin films which possess selective gas penetration. The best separation was obtained for the pair CH 4CO 2. Gas separation membranes are under development ( e.g. CU 4 from biogas). All results obtained will be discussed.

A Simple Method for Prediction of Gas Permeability of Polymers from Their Molecular Structure

A method for the prediction of gas permeabilities (P) through polymers from their chemical structure has been developed on the basis of the ratio of molar free volume to molar cohesive energy, V(f)/E(coh). The permeation of small gas molecules through polymer membranes is dependent on the chain packing density measured by V(f) and segmental motion of polymer chains measured by E(coh). But no simple relationship between P and V(f) or E(coh) alone was found. The permeability data of more than 60 polymers covering 7 orders of magnitude for six gases have been treated with linear regression analysis. All plots of log P vs. V(f)/E(coh) gave good straight lines. It is also found that a linear relationship holds when plotting both the intercepts and slopes of log P vs. V(f)/E(coh) lines against square of the diameters of gas molecules. Therefore, the permeabilities of all the non-swelling gases through a great variety of polymers can be estimated using two correlations above. Moreover, this method is more accurate than others in the literature and may found useful for the selection of gas separation or barrier membrane materials.

Gas separation membrane material selection criteria: Differences for weakly and strongly interacting feed components

Insights into the basic principles governing sorption and transport in glassy polymers has recently led to large improvements in the materials used for membrane-based gas separation processes. Examples of such advanced materials taken from the polyimide and polycarbonate families are compared to conventional membrane materials to illustrate the value of a systematic structure-property approach to this topic. The current thinking regarding optimization strategies for gas separation applications will be summarized in two apparently simple rules for selecting candidate materials for gas separation membranes in the absence of strong interactions between the polymer and the penetrating gas mixture. Detailed consideration of a series of polycarbonates in terms of these two rules is presented. Finally, preliminary insights into more complex cases involving high pressure carbon dixoide, where strong interactions cannot be neglected, are also discussed for the polycarbonate family. The most familiar example of such interactions, “plasticization”, is generally undesirable since it tends to reduce the selectivity of the membrane for permeating components. The onset point of plasticization for the polycarbonates is shown to be related to the achievement of a critical sorbed concentration, so the less strongly sorbing materials resist plasticization up to higher pressures.

Gas permeability and permselectivity in polyimides based on 3,3',4,4'-biphenyltetracarboxylic dianhydride

Permeation rates of hydrogen, carbon monoxide, carbon dioxide and methane were measured for amorphous films of a series of polyimides prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and each of the following diamines: 4,4′-oxydianiline (ODA), 4,4′-methylenedianiline (MDA), 4,4′-diaminodiphenyl sulfone (DDS), and dimethyl-3,7-diaminodibenzothiophene 5,5′-dioxide (DDBT). The permeability coefficients ( P) at 10 atm and 50°C were in the order BPDA-DDBT > BPDA-DDS ≥ BPDA-MDA > BPDA-ODA. On going from BPDA-ODA to BPDA-DDBT, P increased by 6.0- and 9.4-fold for hydrogen and carbon dioxide, respectively. This was due to 1.4- and 3.2-fold increases in the diffusion coefficients accompanied by appreciable decreases in activation energies, and also to a 3.7-fold increase in the solubility coefficients. These results suggest that both high rigidity and bulkiness of a polymer backbone may prevent efficient chain packing and result in a more open structure for gas transport. BPDA-DDBT displayed high permselectivities for hydrogen/methane and carbon dioxide/methane in addition to high permeability coefficients, and has a high potential for use as a membrane material for gas separation.

Simplified analysis of gas/polymer selective solubility behavior

AbstractInteractions of penetrants with gas separation membrane materials can have both desirable and undesirable consequences. Improved permselectivity resulting from specific polymer/penetrant interactions must be balanced against potentially undesirable plasticization effects resulting from such interactions. Consideration is given to the utility of gas solubility measurements in low‐molecular‐weight solvents as an aid to understanding solubility data for polymer/gas systems. The solubility coefficients of gases in low‐molecular‐weight liquids and in polymeric media of similar chemical structure tend to be quite different in magnitude. “Equation‐of‐state” and combinatorial entropy differences for the free energy of mixing of gases with low‐molecular‐weight solvents and high‐molecular‐weight polymers presumably cause this discrepancy. The above complications are minimized and useful trends identified by considering the so‐called solubility selectivity comprised of ratios of absolute gas solubilities in both low‐and high‐molecular‐weight media of similar chemical natures. For example, both acetone and methyl acetate are highly sorptive of CO2 and display a high degree of solubility selectivity for CO2 relative to CH4. These two solvents have mass densities of carbonyl groups that are similar to that in cellulose acetate and poly(methyl methacrylate). Both of these polymers are also highly sorptive of CO2 and have solubility selectivities for CO2 relative to CH4 that are similar to that of acetone and methyl acetate. Anomalous behavior is encountered in the case of Kapton poly(ether diimide), which contains a high density of carbonyl groups (four per repeat unit). On the basis of the behavior of the other carbonyl‐rich polymers, it was expected that Kapton would show high solubility selectivity for CO2 relative to CH4, but this was not observed. A hypothetical explanation for the Kapton behavior is presented. In addition, possible advantages of the Kapton‐type behavior are discussed in the context of the reported good plasticization resistance of polyimides.