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.     

Addition-type polynorbornene with Si(CH3)3 side groups: Detailed study of gas permeation and thermodynamic properties

Polymerization of norbornene bearing Si(CH₃)₃ groups in the five position with the opening of double bonds was performed. By accurate selection of the ratios catalyst/co-catalyst and monomer/catalyst the samples with increased molecular mass (about 400,000) were obtained. Transport parameters of this, addition type poly(trimethylsilyl norbornene) (PTMSN) were measured using the gas chromatographic and mass spectrometric methods for different gases (H₂, He, O₂, N₂, CO₂, CH₄, C₂H₆, C₃H₈ and n-C₄H₁₀). Temperature dependence of the permeability coefficients (P) indicated that low activation energies of permeation (E_P) and diffusion (E_D) are characteristic for PTMSN. In some cases (CO₂, C₂H₆) negative E_P values were observed. Thermodynamics of vapor sorption in this polymer was studied using the inverse gas chromatography method. It was shown that PTMSN is characterized by very large solubility coefficients S similar to those of poly(trimethylsilyl propyne) (PTMSP). The comparison of the P, D, and S values of these highly permeable polymers showed that the greater permeability of PTMSP is determined by the larger D values. Application of different approaches for the determination of the size of microcavities in PTMSN indicated that this polymer is characterized by large size of microcavity (800–1200 ?³).     

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.     

Effects of film thickness on density and gas permeation parameters of glassy polymers

Dense films of poly(vinyltrimethyl silane) (PVTMS) and poly(trimethylsilyl norbornene) (PTMSNB) having different thicknesses in the range l = 5–150 μm were cast from hydrocarbon solutions. It was shown that a density is inversely proportional to the film thickness. The following equation holds for the density ?: 1/? = 1/?⁰ − b/l. Permeability and diffusion coefficients were determined using the time lag method in respect to different gases. For all the gases, diffusion coefficients decrease when film thickness decreases and film density increases. A correlation of diffusion coefficients with fractional free volume were demonstrated. On the other hand, permeability coefficients are nearly independent of the thickness and density. Possible mechanisms of this behavior are discussed.     

Preparation and characterization of a composite PDMS membrane on CA support

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

Gas permeation properties of phenylene oxide polymers

Gas permeability (P_i) and diffusion (D_i) coefficients in respect to several gases (H₂, O₂, N₂, CO, CO₂, CH₄) have been measured for poly(2,6-dimethylphenylene oxide) (PMPO), poly(2,6-diphenylphenylene oxide) (PPPO), and phenylene oxide copolymers containing methyl, phenyl, and allyl radicals as side groups. X-ray diffraction study shows that both homopolymers are semicrystalline materials, whereas all the copolymers are completely amorphous. The results show that a replacement of methyl by phenyl groups in PMPO/PPPO pair is accompanied by decrease in the P values. A transition from semicrystalline PMPO to amorphous copolymers results in a decrease in permeability and solubility coefficients and not in a growth of these parameters as can be expected on the basis of the behavior of other semicrystalline polymers (e.g. polyolefins). It is supposed that the crystallites of PMPO, and possibly of PPPO are packed loosely and, hence, take part in sorption and gas transport. This assumption is in agreement with numerous X-ray data as well as the results of positron annihilation study of these polymers.     

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     

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.     

Sorption and partial molar volume of gases in poly (dimethyl siloxane)

Sorption of N₂, O₂, Ar, CH₄, CO₂, C₂H₄, and C₂H₆ in poly (dimethyl siloxane) liquid and rubber and the dilation of the polymers due to sorption of the gases are studied at 25°C under pressures up to 50 atm. In the liquid, the sorption isotherms for low-solubility and high-solubility gases are described by Henry's law and the Flory–Huggins equation, respectively. Gas sorption in the rubber, which contains a 29 wt % silica filler, follows the dual-mode sorption model, though marked hysteresis is observed in the sorption of O₂ and CH₄. The dilation isotherms increase linearly or exponentially in both polymers with increasing pressure. Considering that gas molecules adsorbed into micropores of the filler particles do not participate in the dilation, partial molar volumes of the dissolved gases in the rubber are determined from data of sorption and dilation. The values are nearly equal to the partial molar volumes in the liquid (48–60 cm³/mol).     

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.     

Gas sorption,diffusion, and permeation in poly(dimethylsiloxane)

The permeability of poly(dimethylsiloxane) [PDMS] to H₂, O₂, N₂, CO₂, CH₄, C₂H₆, C₃H₈, CF₄, C₂F₆, and C₃F₈, and solubility of these penetrants were determined as a function of pressure at 35 °C. Permeability coefficients of perfluorinated penetrants (CF₄, C₂F₆, and C₃F₈) are approximately an order of magnitude lower than those of their hydrocarbon analogs (CH₄, C₂H₆, and C₃H₈), and the perfluorocarbon permeabilities are significantly lower than even permanent gas permeability coefficients. This result is ascribed to very low perfluorocarbon solubilities in hydrocarbon‐based PDMS coupled with low diffusion coefficients relative to those of their hydrocarbon analogs. The perfluorocarbons are sparingly soluble in PDMS and exhibit linear sorption isotherms. The Flory–Huggins interaction parameters for perfluorocarbon penetrants are substantially greater than those of their hydrocarbon analogs, indicating less favorable energetics of mixing perfluorocarbons with PDMS. Based on the sorption results and conventional lattice solution theory with a coordination number of 10, the formation of a single C₃F₈/PDMS segment pair requires 460 J/mol more energy than the formation of a C₃H₈/PDMS pair. A breakdown in the geometric mean approximation of the interaction energy between fluorocarbons and hydrocarbons was observed. These results are consistent with the solubility behavior of hydrocarbon–fluorocarbon liquid mixtures and hydrocarbon and fluorocarbon gas solubility in hydrocarbon liquids. From the permeability and sorption data, diffusion coefficients were determined as a function of penetrant concentration. Perfluorocarbon diffusion coefficients are lower than those of their hydrocarbon analogs, consistent with the larger size of the fluorocarbons. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 415–434, 2000     

GAS SORPTION,DIFFUSION AND PERMEATION OF ORDERED POLYMERIC MEMBRANES

He, CO_2, O_2, N_2, CH_4, C_3H_8, and t-C_4H_(10) gas permeability coefficients and diffusion coefficients of poly(4-methylpentene-1) (PMP) with various degrees of crystallization were plotted against the degree of crystallization. The plotdemonstrated a linear relationship. The gas permeability coefficient and diffusion coefficient of pure amorphous and purecrystalline PMPs were evaluated by a linear extrapolation to zero and 100% crystallinity, respectively. The relationshipbetween the diffusion coefficient of crystalline parts of PMP and the kinetic diameter of penetrant gases was discussed.Syndiotactic polystyrene (SPS) could exist as δ form crystals complexed with organic solvents such as benzene, toluene,xylene, and ethylbenzene. The mesophase of SPS is prepared by annealing the δ form of crystalline complexes at a certaintemperature for 1 h. The desorption of solvent during annealing almost does not result in changes of both the conformation ofbackbone chains and the crystal lattice. We could prepare the mesophase containing molecular cavities with the size andshape of the organic solvent molecules. The mesophase could sorb the same solvent after the manner of Langmuir sorption atlow vapor pressure range while this would not be the case for solvents of different size and shape. This suggests a molecularrecognition of organic solvent, and mesophase SPS might be useful for separation membrane and adsorptive material.     

Polar/nonpolar gas transfer through PEO-based copolymers membranes

Two PEG-based copolymers containing two different chain extenders, as hard segments, were synthesized by 4,4′-methylenediphenyl diisocyanate (MDI). The chain extenders were 1,4-butane diol (BDO) and 1,2-ethane diamine (EDA). The application of the polyurethane (PU) and poly(urethane-urea)s (PUU)s synthesized polymers, which were characterized by Fourier transform infrared spectrometer (FTIR), differential scanning calorimetry (DSC) and atomic Force Microscopy (AFM), in the gas permeability was investigated. The obtained results indicated that by replacing the urea linkage in the polymers, the microphase separation of hard and soft segments increased. The synthesized PEG-based copolymers were semi-crystalline at room temperature. According to the DSC results, the crystallinity of the synthesized polyurethanes decreased as temperature increased. In addition, a reduction in mean surface roughness could be seen based AMF information. The gas (carbon dioxide and methane) separation properties of the polymers revealed that by replacing the urea linkage, the diffusivity, permeability and selectivity of the gases increased slightly.

The solubility and diffusivity of gases indicated he solubility domination of gas transport in these membranes. However, the sorption coefficient (S) of a particular gas was surprisingly constant for the two synthesized polymers. The CO₂ permeability increased with increasing feed pressure, while CH₄ permeability remained almost constant at both temperatures of 25°C and 35°C. The increase in temperature led to an increase in the permeability of the gases and a decrease in the gas selectivity for the both synthesized polyurethanes.     

The diffusion of CO2, CH4, C2H4, and C3H8 in polyethylene at elevated pressures

Diffusion and solubility coefficients have been determined for the CO₂^?, CH₄^?, C₂H₄^?, and C₃H₈-polyethylene systems at temperatures of 5, 20, and 35°C and at gas pressures up to 40 atm. Diffusion coefficients were obtained from rates of gas absorption in polyethylene rods under isothermal-isobaric conditions by means of a new diffusivity apparatus. The concentration dependence of the diffusion coefficients was represented satisfactorily by Fujita's free-volume model, modified for semicrystalline polymers, while the solubility of all the penetrants in polyethylene was within the limit of Henry's law. Semiempirical correlations were found for the free-volume parameters in terms of physicochemical properties of the penetrant gases and the penetrant-polymer systems. These correlations, if confirmed, should permit the prediction of diffusion and permeability coefficients of other gases and of gas mixtures in polyethylene as functions of pressure and temperature.     

Comparison of three models for permeation of CO2/CH4 mixtures in poly(phenylene oxide)

Two models for the permeability of pure gases have been extended to include binary gas mixtures. The first is an extension of a pure gas permeability model, proposed by Petropoulos, which is based on gradients of chemical potential. This model predicts the permeability of components in a gas mixture solely on the basis of competition for sorption sites within the polymer matrix. The second mixed gas model follows an earlier analysis by Barrer for pure gases which includes the effects of saturation of Langmuir sites on the diffusion as well as the sorption processes responsible for permeation. This generalized “competitive sorption/diffusion” model includes the effect of each gas component on the sorption and diffusion of the other component in the mixture. The flux equations from these two models have been solved numerically to predict the permeability of gas mixtures on the basis of pure gas sorption and transport parameters. Both the mixed gas Petropoulos and competitive sorption/diffusion model predictions are compared with predictions from the earlier simple competitive sorption model based on gradients of concentration. An analysis of all three models is presented for the case of CO₂/CH₄ permeability in poly(phenylene oxide) (PPO). As expected, the competitive sorption/diffusion model predicts lower permeability than either of the models which consider only competitive sorption effects. The permeability depression of both CO₂ and CH₄ predicted by the competitive sorption/diffusion model is roughly twice that predicted by the competitive sorption model, whereas the mixed gas Petropoulos model predictions for both gases lie between the other two model predictions. For the PPO/CO₂/CH₄ system, the methane permeability data lie above the predictions of all three models, whereas CO₂ data lie below the predictions of all models. Consequently, the competitive sorption/diffusion model gives the most accurate prediction for CO₂, while the simple competitive sorption model is best for methane. The effects of mixed gas sorption, fugacity, and CO₂-induced dilation were considered and do not explain the inaccuracies of any of the models. The relatively small errors in mixed gas permeability predictions using either of the three models are likely to be related to “transport plasticization” of PPO owing to high levels of CO₂ sorption and its effect on polymer segmental motions and gas diffusivity.     

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.     

Transport of C1–C3 hydrocarbons in poly(phenylene oxides) membranes

Transport of CH₄, C₂H₄ and C₂H₆ in poly(phenylene oxides) membranes at low pressures has been studied. The relation between the free volume and permeability of the polymers was analyzed in terms of the `dual-sorption' model. The accessible free volume of the polymers was estimated assuming the density of a sorbed fluid is equal to the density of the corresponding liquid. Transient separation of the three-component mixture CH₄/C₂H₄/C₂H₆ was studied.     

Gas sorption and transport in substituted polycarbonates

The effects of molecular structure manipulation of polycarbonates on sorption and transport of various gases were studied using tetramethyl, tetrachloro, and tetrabromo substitutions onto the aromatic rings of bisphenol A polycarbonate. Solubility and permeability measurements were made at 35°C over the pressure range of 1–20 atm for a variety of gases, namely CO₂, CH₄, O₂, N₂, and He. A threefold to fourfold increase in permeability was caused by the tetramethyl substitution, whereas the tetrachloro and tetrabromo substitutions reduced the permeability relative to the tetramethyl substitution. Lower activation energies for transport were found for the tetramethyl polycarbonate relative to the unsubstituted polycarbonate. Permeability coefficients were factored into solubility and diffusion coefficients. Sorption levels increased for all substitutions, but among the substituted polymers the levels remain practically the same. Solubility data were analyzed in terms of the dual sorption model. The Henry's law solubility coefficients obtained from this analysis were found to be consistent with a predictive equation developed for rubbery polymers. The usual correlation for predicting the Langmuir sorption capacity of the model overestimates the values for the substituted polycarbonates, and a proposal for the cause of this is offered. Thermal expansion of these polymers was measured using dilatometry, and the results are used in the interpretation of the sorption data. Diffusion phenomena are explained by segmental mobility and free volume considerations. The effects of CO₂ exposure history on sorption and transport were also investigated.     

Membrane separation of multicomponent mixture of alkanes C1–C4

The membrane separation of the four-component mixture of gaseous alkanes C₁–C₄ is studied. Homogeneous films based on two high-permeable polymers, namely, addition-type poly[3-(trimethylsilyl)tricyclononene-7] and poly[3,4-bis(trimethylsilyl)tricyclononene-7], are used as membranes. Separation of the multicomponent mixture of hydrocarbons on these polymers follows the same trends as separation of binary mixtures CH₄-C₄H₁₀ on polyacetylenes. In the presence of higher hydrocarbons, the permeability coefficients of methane decrease and the permeates become enriched with higher hydrocarbons. During separation of the multicomponent mixture, permeability coefficients P(C₄H₁₀) attain high values (up to 12000 Barrers).     

Poly(ether urethane) and poly(ether urethane urea) membranes with high H2S/CH4 selectivity

The objective of this study was to synthesize rubbery polymers with a high H₂S/CH₄ selectivity for possible use as membrane materials for the separation of H₂S from ‘low-quality’ natural gas. Two poly(ether urethanes), designated hereafter PU1 and PU3, and two poly(ether urethane ureas), designated PU2 and PU4, were synthesized and cast in the form of ‘dense’ (homogeneous) membranes. PU1 and PU2 contained poly(propylene oxide) whereas PU3 and PU4 contained poly(ethylene oxide) as the polyether component. The permeability of these membranes to two ternary mixtures of CH₄, CO₂, and H₂S was measured at 35°C, and for a PU4 membrane also at 20°C, in the pressure range from 4 to 13.6 atm (4.05–13.78×10⁵ Pa). PU4 is a very promising membrane material for H₂S separation from mixtures with CH₄ and CO₂, having a H₂S/CH₄ selectivity greater than 100 at 20°C as well as a very high permeability to H₂S. Permeability measurements were also made with commercial PEBAX^TM membranes for comparison. The possibility of upgrading low-quality natural gas to US pipeline specifications for H₂S and CO₂ by means of membrane processes utilizing both highly H₂S-selective and CO₂-selective polymer membranes is discussed.