CO2 permeability and amorphous fractional free-volume in uniaxially drawn HDPE

The CO₂ permeability of uniaxially drawn high-density polyethylene (HDPE) was investigated and drastic changes were observed as a function of draw ratio. An estimation of variations in both density and fractional free volume of the amorphous component in the drawn polymers was made on the basis of experimental data. The dramatic variations in permeability resulting from drawing were shown to be the consequence of changes of fractional free volume in the amorphous phase of the drawn polymer. Good correlation exists between measured solubility and diffusion coefficients of drawn samples and the estimated fractional free volume; this correlation agrees with the existing free volume theory for diffusion in polymers.     

Water barrier characteristics of plasma polymers of perfluorocarbons

Perfluorocarbon monomers such as C₂F₄, C₂F₆, C₄F₁₀, and mixtures thereof with H₂, were subjected to plasma polymerization and deposited onto low-density polyethylene (LDPE) substrates. The effect of plasma conditions, surface characteristics, and surface dynamics of plasma polymers on their ability to improve the resistance to water vapor permeation was investigated. An optimum discharge energy density was found for a monomer which provided the greatest reduction water vapor permeability. Although all of the plasma polymers show higher hydrophobicity than polyethylene, the reduction in water vapor permeability is not uniquely related to water contact angle. The surface-dynamic stability of a plasma polymer surface was found to be the key factor in determining the barrier performance of the plasma polymer. The extent of change of surface-configuration after water immersion strongly correlated with the improvement in the water vapor permeation resistance. Plasma polymers with the higher surface-dynamic stability provided the better water barrier coating applied on LDPE films. © 1996 John Wiley & Sons, Inc.     

The effect of pressure on gas permeation through semicrystalline polymers above the glass transition temperature

A method is proposed to analyze the effect of pressure on permeation of gases through semicrystalline polymers above the glass transition temperature. The method utilizes similarities in molecular diameters of the gases and differences in their solubilities. Two polymers, polyethylene and polypropylene, and a series of gases are chosen for an application of the method, and the effect of pressure on the permeabilities for 10 gases is measured in the pressure range 1–130 atm at 25°C. For polymers, the logarithm of the permeability coefficient is linear in the pressure for each gas, with negative slope for slightly soluble gases (He, Ne, H₂, N₂, O₂, and Ar) and positive slope for highly soluble gases (CH₄, Kr, CO₂, and N₂O). Analyzing these slopes by the method proposed permits contributions of hydrostatic pressure and concentration to the pressure dependence of permeation to be evaluated. On the basis of the results, the mechanism of gas permeation in rubbery films under high pressures is discussed.     

A dynamic approach to diffusion and permeation measurements

A dynamic method for investigating the mechanism of permeation and diffusion through polymers has been explored. The permeation cell consists of two compartments separated by the membrane. The permeant (gas, vapor, or liquid) is introduced into one compartment; a carrier gas (helium) flows at constant rate through the other and sweeps the permeant which diffuses through the membrane to the thermal conductivity detector. Both compartments are at atmospheric pressure; thus no or little membrane support is required, and leakage problems are minimal. Moreover, the same membrane can be used over a wide temperature range and for diverse permeants. The detector signal is at any instant proportional to the permeation rate. A simple mathematical formalism for deriving the diffusion coefficient from the transient permeation rates has been developed. The measured diffusion and permeability coefficients of CO₂, O₂, and N₂ through low-density polyethylene closely agree with literature values. Permeation of hexane and benzene through polyethylene follows a complex diffusion law, and the rate depends on the thermal history of the system. The dynamic method is particularly suited to the study of transitions in polymers. Changes in permeation rates, usually occurring at transition points, can easily be discovered by slow temperature scanning of the system.     

Tests of a “free-volume” model of gas permeation through polymer membranes. II. Pure Ar,SF6, CF4, and C2H2F2 in polyethylene

Permeability coefficients for Ar, SF₆, CF₄, and C₂H₂F₂ (1,1-difluoroethylene) in polyethylene membranes were determined from steady-state permeation rates at temperatures from 5 to 50°C, and at applied gas pressures of up to 15 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 parameters required by this model were determined from independent absorption (diffusivity) measurements with the above gases in polyethylene rods. The present work confirms the results of previous studies with CO₂, CH₄ C₂H₄ and C₃H₈ in polyethylene.     

Mixed-matrix membranes comprising graphene-oxide nanosheets for CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> separation: A comparison between glassy and rubbery polymer matrices

The effect of graphene oxide (GO) nanosheets on the CO₂/CH₄ separation performance of a rubbery (poly(dimethylsiloxane), PDMS) as well as a glassy (polyetherimide, PEI) polymer is studied. Interfacial interactions between the nanosheets and both polymers are revealed by FTIR and SEM. The results of gas permeation through the membranes demonstrate that GO nanosheets enhance CO₂/CH₄ diffusivityselectivity of PEI and CO₂/CH₄ solubility-selectivities of the PEI and PDMS polymers, while diminish CO₂/CH₄ diffusivity-selectivity of PDMS. Furthermore, the possibility of overcoming the common tradeoff between CO₂ permeability and CO₂/CH₄ selectivity of rubbery and glassy polymers by incorporating very low amounts of graphene oxide nanosheets is addressed. In other words, at 0.25 wt % GO loading, the PEI membrane shows simultaneous enhancement of CO₂ permeability (16%) and CO₂/CH₄ selectivity (59%). Also, for the PDMS membrane simultaneous enhancement of CO₂ permeability (29%) and CO₂/CH₄ selectivity (112%) is occurred at 0.5 wt % GO loading. Finally, the capability of the well known Nielsen model to predict the gas permeability behavior of the nanocomposites is investigated.     

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.     

Transport of water and gases through EVA copolymer films,EVA70/PVC,and EVA70/PVC/gluten blends

The transport of water vapor and gases (oxygen or carbon dioxide) through poly(ethylene‐co‐vinyl acetate) (EVA) films of different VA contents and through EVA₇₀/PVC and EVA₇₀/PVC/gluten blend films, was analysed by permeation measurements. In the case of water, a plasticization effect on the material is observed for EVA films with more than 33percnt; wt. of VA content and also for the EVA₇₀/PVC blend. For EVA of 19 wt.percnt; VA, there is no plasticization, while for LDPE (low density polyethylene) and EVA of 4.5 wt.percnt; VA, the water diffusion coefficient decreases with increasing the water content. A negative plasticization effect was accounted for by an empirical model and attributed to the formation of water clusters in the non polar polymers. The increase in water sorption extent with the VA content leads to a steady increase in the water permeability in the EVA copolymers while for the EVA₇₀/PVC blend, the reduced water permeability is explained by the interaction between chlorinated units and polar groups. In the case of gas permeation, both for O₂ and CO₂ and whatever the VA content of the copolymer used, the experimental curves are characterized by a constant diffusion coefficient. This result is not surprising as it is generally admitted that, gases sorb into rubbery polymers according to Henry's law. By mixing PVC with the EVA of 70percnt; wt. VA, the diffusion coefficients of CO₂ and O₂ are greatly reduced (6 times).     

Gas transport properties of poly(ether‐b‐amide) segmented block copolymers

The permeation properties of H₂, N₂, and CO₂ were determined at 35 °C and pressures up to 15 atm in phase‐separated polyether‐b‐polyamide segmented block copolymers. These polymers contain poly(ethylene oxide) [PEO] or poly(tetramethylene oxide) [PTMEO] as the rubbery polyether phase and nylon‐6 [PA6] or nylon‐12 [PA12] as the hard polyamide phase. Extremely high values of polar (or quadrupolar)/nonpolar gas selectivities, coupled with high CO₂ permeability coefficients, were observed. CO₂/H₂ selectivities as high as 9.8 and CO₂/N₂ selectivities as high as 56 were obtained in polymers with CO₂ permeability coefficients of approximately 220 × 10⁻¹⁰ cm³(STP) cm/(cm² s cmHg). As the amount of polyether increases, permeability increases. Gas permeability is higher in polymers with less polar constituents, PTMEO and PA12, than in those containing the more polar PEO and PA6 units. CO₂/N₂ and CO₂/H₂ selectivities are higher in polymers with higher concentrations of polar groups. These high selectivity values derive from large solubility selectivities in favor of CO₂. Because CO₂ is larger than H₂ and has, therefore, a lower diffusion coefficient than H₂, the weak size‐sieving ability of the rubbery polyether phase, which is the locus of most of the gas permeation, also contributes to high CO₂/H₂ selectivity. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2051–2062, 2000     

CO2-induced plasticization phenomena in glassy polymers

A typical effect of plasticization of glassy polymers in gas permeation is a minimum in the relationship between the permeability and the feed pressure. The pressure corresponding to the minimum is called the plasticization pressure. Plasticization phenomena significantly effect the membrane performance in, for example, CO₂/CH₄ separation processes. The polymer swells upon sorption of CO₂ accelerating the permeation of CH₄. As a consequence, the polymer membrane loses its selectivity. Fundamental understanding of the phenomenon is necessary to develop new concepts to prevent it.In this paper, CO₂-induced plasticization phenomena in 11 different glassy polymers are investigated by single gas permeation and sorption experiments. The main objective was to search for relationships between the plasticization pressure and the chemical structure or the physical properties of the polymer. No relationships were found with respect to the glass-transition temperature or fractional free volume. Furthermore, it was thought that polar groups of the polymer increase the tendency of a polymer to be plasticized because they may have dipolar interactions with the polarizable carbon dioxide molecules. But, no dependence of the plasticization pressure on the carbonyl or sulfone density of the polymers considered was observed. Instead, it was found that the polymers studied plasticized at the same critical CO₂ concentration of 36±7 cm³ (STP)/cm³ polymer. Depending on the polymer, different pressures (the plasticization pressures) are required to reach the critical concentration.     

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.     

Amine‐Appended Hierarchical Ca‐A Zeolite for Enhancing CO2/CH4 Selectivity of Mixed‐Matrix Membranes

An amine‐appended hierarchical Ca‐A zeolite that can selectively capture CO₂ was synthesized and incorporated into inexpensive membrane polymers, in particular polyethylene oxide and Matrimid, to design mixed‐matrix membranes with high CO₂/CH₄ selectivities. Binary mixture permeation testing reveals that amine‐appended mesoporous Ca‐A is highly effective in improving CO₂/CH₄ selectivity of polymeric membranes. In particular, the CO₂/CH₄ selectivity of the polyethylene oxide membrane increases from 15 to 23 by incorporating 20 wt % amine‐appended Ca‐A zeolite. Furthermore, the formation of filler/polymer interfacial defects, which is typically found in glassy polymer‐zeolite pairs, is inhibited owing to the interaction between the amine groups on the external surface of zeolites and polymer chains. Our results suggest that the amine‐appended hierarchial Ca‐A, which was utilized in membrane fabrication for the first time, is a good filler material for fabricating a CO₂‐selective mixed‐matrix membrane with defect‐free morphology.     

Permeation of pure carbon dioxide and methane and binary mixtures through cellulose acetate membranes

Steady-state permeation rates for pure CO₂ and CH₄ and their binary mixtures through homogeneous dense cellulose triacetate membranes have been measured at three temperatures between 20 and 40°C and pressures up to 2.8 MPa. The pressure dependence of the mean permeability coefficient for CO₂ can be described by the total immobilization model in conjunction with a modified free-volume model. No appreciable pressure dependence of the permeability coefficient for CH₄ is observed, while the permeability coefficients for CH₄ in binary mixture of CO₂ and CH₄ depend on applied gas pressure. The pressure dependences of the mean permeability coefficients for the components in the binary mixture are discussed in terms of the above mobility model. Membrane plasticization induced by CO₂ affects permeation by both gases.     

Tests of a free-volume model for the permeation of gas mixtures through polymer membranes. CO2-C2H4, CO2-C3H8, and C2H4-C3H8 mixtures in polyethylene

Steady-state permeability coefficients have been measured for equimolar mixtures of CO₂-C₂H₄, CO₂-C₃H₈, and C₂H₄-C₃H₈, as well as for a mixture of 74.9 mol % CO₂ and 25.1 mol % C₂H₄ in polyethylene membranes. The measurements were made at 20, 35, and 50°C and at pressures of up to 28 atm. Each component of the permeating mixtures studied had the effect of increasing the permeability coefficient for the other component. Furthermore, at equal partial pressures and at the same temperature, the component exhibiting the highest solubility in the polymer had the largest effect in increasing the permeability coefficient of the other component. This behavior is in agreement with the predictions of a free-volume model for the permeation of gas mixtures proposed by Fang, Stern, and Frisch. From a quantitative viewpoint, the permeability coefficients for the components of the mixtures agreed, on the average, to better than 25% with the predicted values. The theoretical permeability coefficients can be estimated from the model by using parameters determined with the pure components only.     

High‐Performance Polymers for Membrane CO2/N2 Separation

This Concept examines strategies to design advanced polymers with high CO₂ permeability and high CO₂/N₂ selectivity, which are the key to the success of membrane technology for CO₂ capture from fossil fuel‐fired power plants. Specifically, polymers with enhanced CO₂ solubility and thus CO₂/N₂ selectivity are designed by incorporating CO₂‐philic groups in polymers such as poly(ethylene oxide)‐containing polymers and poly(ionic liquids); polymers with enhanced CO₂ diffusivity and thus CO₂ permeability are designed with contorted rigid polymer chains to obtain high free volume, such as polymers with intrinsic microporosity and thermally rearranged polymers. The underlying rationales for materials design are discussed and polymers with promising CO₂/N₂ separation properties for CO₂ capture from flue gas are highlighted.     

Crosslinking of gelatin membranes with ferulic acid or glutaraldehyde: Relationship between gas permeability and renaturation level of gelatin triple helices

This study investigated the gas separation membranes made with gelatin, crosslinked with ferulic acid (FA) and blended with polyethylene glycol (PEG) 200, by using a solvent‐free procedure. Gas permeation properties (He, N₂, O₂, and CO₂) of these “green membranes” were studied and discussed in relation with their structure. Differential scanning calorimetric measurements were carried out to determine the gelatin triple helical renaturation level. The lowest permeability values [He and CO₂ permeability (4.5 × 10^?2 Barrer) with CO₂/O₂ selectivity of 14.5] were reached with gelatin/PEG 200 uncrosslinked membranes showing the highest renaturation level (40%). Crosslinking with FA lead to less rigid and brittle materials than GTA and to 10 times more permeable membranes compared with uncrosslinked membranes. Membranes crosslinked with glutaraldehyde broke during gas permeation measurements. Results demonstrated that higher gas permeability values were closely related to lower renaturation level of gelatin. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 280–287     

碳纳米管/聚氨酯杂化膜的原位聚合构建及气体渗透性能

采用H2SO4/HNO3混酸处理得到不同氧化程度的多壁碳纳米管(MWCNT-COOH),再通过与4,4′-二苯基甲烷二异氰酸酯(MDI)、1,4-丁二醇(BDO)的预聚和扩链反应构建碳纳米管/聚氨酯(MWCNT-COOH/PU)杂化膜。利用傅里叶红外光谱(FTIR)、拉曼光谱(Raman)和透射电子显微镜(TEM)等分析表征多壁碳纳米管结构;探讨了多壁碳纳米管氧化程度和填充量对MWCNTCOOH/PU杂化膜的形貌和CO2、N2渗透性能的影响。结果表明,混酸处理后的多壁碳纳米管带有一定的含氧基团,并随氧化程度的提高,多壁碳纳米管的拉曼光谱G峰和D峰的强度之比(ID/IG)有所增大;氧化程度对多壁碳纳米管在溶剂和杂化膜中的分散性有较大影响,氧化程度越高,分散性越好;杂化膜的CO2、N2渗透性及CO2/N2渗透选择性随多壁碳纳米管氧化程度的增加有所增大,而随多壁碳纳米管填充量的增加表现出先增大后减小的趋势,当氧化程度较高的多壁碳纳米管(H-MWCNT-COOH)填充量为1.0wt%时,H-MWCNT-COOH/PU杂化膜的CO2渗透系数为67.8 Barrer,CO2/N2渗透选择性可达45,表明适量填充MWCNT-COOH能显著提高MWCNT/PU杂化膜的CO2渗透性及CO2/N2的渗透选择性。     

碳纳米管/聚氨酯杂化膜的原位聚合构建及气体渗透性能

采用H₂SO₄/HNO₃混酸处理得到不同氧化程度的多壁碳纳米管（MWCNT-COOH）,再通过与4,4’-二苯基甲烷二异氰酸酯（MDI）、1,4-丁二醇（BDO）的预聚和扩链反应构建碳纳米管/聚氨酯（MWCNT-COOH/PU）杂化膜。利用傅里叶红外光谱（FTIR）、拉曼光谱（Raman）和透射电子显微镜（TEM）等分析表征多壁碳纳米管结构;探讨了多壁碳纳米管氧化程度和填充量对MWCNT-COOH/PU杂化膜的形貌和CO₂、N₂渗透性能的影响。结果表明,混酸处理后的多壁碳纳米管带有一定的含氧基团,并随氧化程度的提高,多壁碳纳米管的拉曼光谱G峰和D峰的强度之比（I_D/I_G）有所增大;氧化程度对多壁碳纳米管在溶剂和杂化膜中的分散性有较大影响,氧化程度越高,分散性越好;杂化膜的CO₂、N₂渗透性及CO₂/N₂渗透选择性随多壁碳纳米管氧化程度的增加有所增大,而随多壁碳纳米管填充量的增加表现出先增大后减小的趋势,当氧化程度较高的多壁碳纳米管（H-MWCNT-COOH）填充量为1.0wt%时,H-MWCNT-COOH/PU杂化膜的CO₂渗透系数为67.8Barrer,CO₂/N₂渗透选择性可达45,表明适量填充MWCNT-COOH能显著提高MWCNT/PU杂化膜的CO₂渗透性及CO₂/N₂的渗透选择性。     

Poly(amide-6-b-ethylene oxide)/SAPO-34 mixed matrix membrane for CO2 separation

In this paper, poly(amide-6-b-ethylene oxide) (Pebax1657)/SAPO-34 mixed matrix membranes (MMMs) were prepared by solvent-evaporation method with acetic acid as a novel solvent. CO₂, N₂, CH₄ and H₂ permeation properties were investigated, and the physical properties of Pebax/SAPO-34 MMMs were characterized by XRD and SEM. At low SAPO-34 content, it was homogeneously distributed in the Pebax matrix, and then precipitated and agglomerated at high SAPO-34 content. The crystallinity of Pebax phase in Pebax/SAPO-34 MMMs decreased initially and then rebounded as a result of phase separation. With the increase of transmembrane pressure difference, CO₂ permeability was enhanced due to the effect of pressure-induced plasticization. Owing to the happening of stratification, the CO₂ permeability of Pebax/SAPO-34 MMMs (50 wt% SAPO-34) increased to 338 Barrer from 111 Barrer of pristine Pebax, while the selectivities of CO₂/CH₄ and CO₂/N₂ were almost unchanged. Compared with the pristine Pebax, the gas separation performances of Pebax/SAPO-34 MMMs were remarkably enhanced.     

Effects of extension on the gas permeability of hard elastic polypropylene film

Permeabilities of He, N₂, O₂, Ar, and CO₂ in stretched hard-elastic polypropylene film exhibiting extraordinarily high elasticity were measured as functions of temperature, pressure, and extension ratio. The permeability decreases slightly for strains of 10% or less, increases abruptly for strains from 15% to 30%, and then increases gradually for strains up to 100%. In recovery from extension, changes in permeability are similar to those in extension. The gas permeation mechanism is of the solution-diffusion type for extensions less than 15%. Above 15% extension, the mechanism involves permeation through voids created during deformation. The apparent mean pore size, estimated from the gas permeation, is very large at comparatively small extensions, and decreases with increasing extension. The change in specific volume accompanying extension of the sample indicates that only a very small fraction of the voids are interconnected to form channels that traverse the thickness of the film.