Gas permeation through glassy polymer membranes with relatively low glass-transition temperature

When the glass-transition temperature of the polymer is not so much higher than the experimental temperature, the pressure dependence of the mean permeability coefficient of the poly-mer membrane to a gas is apt to deviate from the prediction by the conventional dual-mode mobility model, and to obey a similar model with concentration-dependent diffusivities because of the plasticization action of sorbed gas in the polymer membrane. In this work, sorption and permeation for oxygen and carbon dioxide in a membrane of polystyrene whose glass-transition temperature is 95°C, were measured to discuss the mechanism of gas diffusion in glassy polymer membranes with relatively low glass-transition temperature at 30, 40 and 50°C respectively.     

Evidence of dual mobility in sulfur dioxide-polyarylate system: An integral sorption analysis

Integral sorption/desorption measurements were carried out for the sulfur dioxide-glassy polyarylate polymer system at 25°C, 40°C, 55°C, and 63°C. The transport of sulfur dioxide in the glassy polyarylate polymer was governed by Fickian diffusion. The effective diffusion coefficient of sulfur dioxide increased with increasing penetrant concentration. The concentration dependence of the effective diffusion coefficient is explained on the basis of the partial-immobilization model developed by Paul and Koros. The mobility of the molecules sorbed in the Langmuir mode is shown to be significantly lower than the mobility of the molecules in Henry's law dissolution mode. The predictions of permeability values as a function of upstream gas pressure are presented. The equilibrium sorption isotherms for this system are well represented by the dualmode sorption model. The eltergetics and the temperature dependence of the dual-mode parameters are also discussed.     

Molecular Simulation of Oxygen Sorption and Diffusion in the Poly (lactic acid)

Grand canonical Monte Carlo and molecular dynamics simulation methods are used to simulate oxygen sorption and diffusion in amorphous poly(lactic acid) (PLA). The simulated solubility coefficient of oxygen is close to experimental data obtained from the quartz crystal microbalance but much higher than those from the time-lag method. This discrepancy is explained by using the dual-mode sorption model. It is found that oxygen sorption in PLA is predominantly Langmuir type controlled, em.e., through the process of filling holes. The time-lag method only takes into account oxygen molecules that participate the diffusion process whereas a large proportion of oxygen molecules trapped in the void have little chance to execute hopping due to the glassy nature of PLA at room temperature. The simulated diffusion coefficient of oxygen is reasonably close to the data obtained from the time-lag method. The solubility coefficient of oxygen decreases linearly with increasing relative humidity while its diffusion coefficient firstly decreases and then increases as a function of relative humidity.     

Residual porosity in polymeric latex films

Solubility and diffusivity measurements of a probe gas (CO₂), which has an inherently low solubility in the polymer, have been used to characterize residual porosity in polymeric latex films. Sorption isotherms resembling those of a glassy polymer were obtained, even though the glass transition temperature of the polymer was 1°C, about 30°C below the experimental temperature. Solvent cast films of the same polymer exhibited much lower solubilities, and followed the expected Henry's law behavior. CO₂ solubility and diffusivity were found to decrease with aging time for the latex films, but did not quite reach the values of the solvent cast films, even after 75 days at room temperature. The sorption data could be described by the dual-mode sorption model, which is commonly employed in the analysis of glassy polymer sorption isotherms. Estimates of the amount of porosity were made from the sorption data, and values ranging from 0.6–0.03% were obtained for latex films aged from 62 h to 75 days, respectively. Our results suggest that permeability differences noted by others for latex and solvent cast films of the same polymer are due to the substantial solubility differences for low-solubility penetrants in these two types of films.     

气体在均质高分子膜中渗透过程的研究

本文研究了气体在玻璃态聚合物膜中的渗透行为,提出了Langmuir环境和Henry环境的渗透分离系数的概念,推导出它们与表观分离系数间的关系,分析了压力,组成和温度及膜的玻璃化转变对选择渗透性的影响,验证了双吸着双迁移模型及其扩展形式的适用性.通过理论分析和实验证明探讨了实际中出现的“快气变慢,慢气变快”及“纯气渗透系数之比往往高于实际的混合气渗透分离系数”,指出其并非一般性结论,有时会出现与此相反的结果.     

Study of transport of pure and mixed CO2/N2 gases through polymeric membranes

The permeations of pure CO₂ and N₂ gases and a binary gas mixture of CO₂/N₂ (20/80) through poly(dimethylsiloxane) (PDMS) membrane were carried out by the new permeation apparatus. The permeation and separation behaviors were characterized in terms of transport parameters, namely, permeability, diffusion, and solubility coefficients which were precisely determined by the continuous‐flow technique. In the permeation of the pure gases, feed pressure and temperature affected the solubility coefficients of CO₂ and N₂ in opposite ways, respectively; increasing feed pressure positively affects CO₂ solubility coefficient and negatively affects N₂ solubility coefficient, whereas increasing temperature favors only N₂ sorption. In the permeation of the mixed gas, mass transport was observed to be affected mainly by the coupling in sorption, and the coupling was analyzed by a newly defined parameter permeation ratio. The coupling effects have been investigated on the permeation and separation behaviors in the permeation of the mixed gas varying temperature and feed pressure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 179–189, 2000     

Permeation through CO2selective glassy polymeric membranes in the presence of hydrogen sulfide

Minor components present in feed gas streams can have a significant influence on the separation performance of polymeric membranes. Hydrogen sulfide is present in many of the processes where CO₂ capture is possible and can therefore undergo competitive sorption with CO₂ for transport through the membrane, as well as influence the membrane morphology inducing plasticization. This study investigates the change in CO₂ permeability and CO₂/N₂ selectivity of two glassy polymeric membranes; polysulfone and 6FDA‐TMPDA, when 500 ppm H₂S is present in the gas mixture. The outcomes of this study reveal that H₂S in trace amounts has a strong influence on the separation performance of both membranes. For both membranes, a plasticization partial pressure ～0.5–0.6 kPa H₂S is observed. H₂S competitive sorption is also observed and is modeled by competitive dual‐sorption theory. Results suggest that mixed gas permeation influences the amount of each gas immobilized within the Langmuir voids in addition to the expected competitive sorption effects. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Nitric oxide and carbon monoxide permeation through glassy polymeric membranes for carbon dioxide separation

Minor components present in polymeric membrane gas separation can have a significant influence on the separation performance. Carbon monoxide and nitric oxide exist in post-combustion gas streams and can therefore influence CO₂ transport through membranes designed for that application. Here, the permeability of nitric oxide (NO) through three glassy polymeric membranes (polysulfone, Matrimid 5218 and 6FDA-TMPDA) was determined and found to be less than the CO₂ but greater than the N₂ permeability in each membrane. This study also investigated the influence of 1000 ppm CO on the mixed gas permeability of CO₂ and N₂ for two glassy polymeric membranes; polysulfone and 6FDA-TMPDA. For both membranes, CO competitive sorption resulted in a reduction in the measured permeability of CO₂ and N₂ even though present at only low concentration.     

High gas permeability in open-structure membranes

For most polymeric membranes, the gas permeability coefficient (P) is often interpreted as the product of diffusivity (D) and solubility (S) of a penetrant gas in the polymer (P=D S). The basic assumption is that molecular diffusion is primarily responsible for mass transport in the membrane permeation process. However, for some open structure membranes, such as poly(1-trimethylsilyl-1-propyne) [PTMSP] or poly(dimethylsiloxane) [PDMS], the high permeabilities of some gases yield much higher diffusivities when calculated from the above relationship (P=D S) than when calculated by using the direct kinetic measurement of diffusivity. It is hypothesized that this discrepancy is due to the convective transport of gas molecules through such open structured polymers. In most cases, the convective contribution to mass transport through membranes is negligible. However, for polymer membranes with high free volume, such as PTMSP, whose free volume fraction is 20 to 25%, the convective term may dominate the permeation flux. In this study, a non-equilibrium thermodynamic formalism is employed to properly treat the diffusion term and convective term that constitute the Nernst-Planck equation. The current analysis indicates that the total permeation flux, which consists of a diffusion term and a convective term, agrees well with the experimental data for several permeation systems: pure components propane and n-butane/PTMSP, pure gas hydrogen/PTMSP, and mixed gas hydrogen/PTMSP. Also, the permeation systems of a nonporous rubbery membrane, PDMS, and eight organophosphorus compounds were included in the study. It is recommended that the proposed model be validated by using other polymers with high free volumes and high permeabilities of gases and vapors, such as poly(1-trimethylgermyl-1-propyne) [PTMGeP] and poly(4-methyl-2-pentyne) [PMP]. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Effect of sorption behavior on transport properties of gases in polymeric membranes

The purpose of this work was to study the relationships between the gas sorption and transport properties in polymeric membranes. The intrinsic gas transport properties: permeation, diffusion, and sorption in a series of dense membranes with various carbonyl group densities were investigated. The poly(methyl methacrylate) (PMMA), polycarbonate (PC), and cyclic olefin copolymer (COC) membranes have similar helium permeability, but the helium permeability of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) membrane was very high. The variation of permeability for these four membranes consists with their fraction free volume. In this study, a direct relationship was found between the carbonyl group density and Langmuir affinity constant. Furthermore, dependence of the fractional free volume on the membrane Langmuir capacity constant was observed.     

Gas sorption and transport in poly(phenylene oxide) and comparisons with other glassy polymers

The sorption and transport of several gases in poly(phenylene oxide) were measured at 35°C, and the results have been analyzed in terms of the dual sorption/mobility models which have been successfully employed for this purpose for other glassy polymers. Both the extent of sorption and rate of permeation of gases are quite large for poly(phenylene oxide) compared to other glassy polymers with rigid chain backbones. It is shown that the high extent of sorption is owing to the high glass transition temperature of this polymer, but this is not a significant factor in its high permeability to gases. The latter stems from large diffusion coefficients. It is shown that the capacity of the Langmuir mode of sorption inherent to glassy polymers is related to the value of the glass transition temperature in a general way for a wide variety of polymers. Observations about the diffusion coefficients for numerous gas–polymer pairs are discussed.     

Transport phenomena in gas permeation through glassy polymer membranes with concentration-dependent sorption and diffusion parameters

The modified dual-mode mobility model for permeation of a gas in glassy polymer membranes was combined with the extended dual-mode sorption model to take account of the plasticization effect of sorbed gas molecules on both sorption and diffusion processes. The combined model was further simplified by the introduction of a concentration of the mobile gas species. However, the observed pressure dependence of mean permeability coefficients of carbon dioxide in methylmethacrylate-n-butyl acrylate copolymer and polymethylmethacrylate films at 30°c and also that of oxygen in a polycarbonate film at 50°C and 60°C showed that a plasticization action of sorbed gas species has an influence on the diffusion process rather than on the sorption process, that is, were simulated by the modified dual-mode mobility model combined with the conventional dual-mode sorption model.     

Analysis of the solvent diffusion in glassy polymer films using a set inversion method

Within the framework of solvent diffusion in glassy polymers, this paper concerns an experimental study of toluene sorption and desorption in P(MMA/nBMA) copolymer films. Gravimetric experiments (quartz microbalance) are performed in a pressure and temperature controlled chamber. Coupling between solvent diffusion and viscoelastic relaxation is taken into account through the time-dependent solubility model, based on the Fick diffusion equation inside the film and a time variable boundary condition at the film/vapor interface. Viscoelastic relaxation is described by a first order model or by a stretched exponential. In the present paper, a special focus is given on the set inversion method used to analyze the data and to derive well-defined uncertainty intervals upon each determined quantity, taking all the uncertainties on the weight measurements into account. We find that the mutual diffusion coefficient strongly decreases in the glassy state, of about two orders of magnitude for a 0.05 decrease in the solvent weight fraction.     

Energetics of gas sorption in glassy polymers

The molar enthalpy for gas sorption in glassy polymers at a fixed concentration, often called the isosteric enthalpy of sorption, exhibits a clearly discernable minimum when plotted as a function of penetrant concentration. This unexpected behaviour has been observed in several glassy polymer systems including poly(ethylene terephthalate), polyacrylonitrile and polycarbonate. The behaviour can be modelled by analysing the temperature dependence of the various equilibrium parameters comprising the so-called dual mode sorption model for gas sorption in glassy polymers. The fundamental significance of the various enthalpies describing the temperature dependence of the Henry's law solubility constant, the Langmuir affinity constant and the Langmuir capacity constant are included in the discussion. Provision is made for non-ideal vapours and gases by introduction of the compressibility factor in the expression for the isosteric enthalpy. Application of relationships for calculating both the isosteric and the isothermal enthalpies of sorption is made to the case of CO₂ in poly(ethylene terephthalate) in the temperature range 35° to 115°C. These results and analyses complement the wealth of equilibrium and transport data which are consistent with the dual mode sorption model for penetrant sorption in glassy polymers.     

Water vapor sorption and transport in dense polyimide membranes

The sorption and transport of water vapor in five dense polyimide membranes were studied by thermogravimetry. The sorption isotherms of water vapor in the polyimides could be successfully interpreted by both the dual‐mode sorption model and the Guggenheim–Anderson–de Boer equation. The water vapor diffusion behavior was found to be nearly Fickian at higher water vapor activities, whereas non‐Fickian diffusion was observed at lower water activities. The phenomena could be well described by the mechanism of combined Fickian and time‐dependent diffusion. The diffusion coefficient and water vapor uptake in the polyimides were strongly dependent on the polymer molecular structure. Except for the polyimide prepared from 3,3′,4,4′‐diphenylsulfone tetracarboxylic dianhydride and 1,3‐bis(4‐aminophenoxy) benzene, the permeability of water vapor in the dense polyimide membranes predicted from the sorption measurement at 30°C corresponded well with the water vapor permeability measured at 85°C. Among the polyimides studied, pyromellitic dianhydride–4,4′‐diaminophenylsulfone (50 mol%)/4,4′‐oxydianiline (50 mol%) showed both high water sorption and diffusion and, therefore, high water vapor permeability, which for vapor permeation membranes is necessary for the separation of water vapor from gas streams. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2306–2317, 2003     

Transport of styrene monomer through natural rubber

The diffusion and transport of inhibitor-free styrene through crosslinked natural rubber (NR) have been studied at various temperatures. NR has been vulcanized by conventional, efficient, peroxide and mixed vulcanization techniques. The dependence of diffusion coefficient on the crosslinking system has been studied for all the systems. The influence of temperature on the sorption and the activation energies of sorption have been calculated. The interaction parameter, permeability, sorption coefficient and molecular weight between crosslinks have been evaluated using the diffusion data. The effect of degree of crosslinking on the sorption characteristics of styrene through NR has also been investigated for the different crosslinking systems. The peroxide system showed lowest uptake and the conventional system showed highest uptake.     

Synthesis and evaluation of aromatic polyamide membranes for desalination in reverse-osmosis technique

Reverse-osmosis membrane-grade aromatic polyamides have been synthesized by reacting 3,5-diaminobenzoic acid separately with three different acylchlorides, viz. isophthaloyl chloride, terephthaloyl chloride, and 4,4′-diphenyldicarboxylic acid chloride. Using these polyamides, asymmetric membranes were developed and characterized for various physical parameters, such as Staverman coefficient, membrane potential, and percent salt rejection using sodium chloride solution under high pressure. The effects of pressure, feed concentration, and feed flow rate have been studied on membrane transport parameters, viz. pure water permeability constant, product rate, solute transport parameter, and separation factor. The effects of annealing temperature and solvent evaporation time on the performance of the membranes were also studied. The analysis of the reverse-osmosis data revealed that the membranes prepared from the 3,5-diaminobenzoic acid and 4,4′-diphenyldicarboxylic acid chloride are superior to the membranes prepared from other polymeric materials. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 643–653, 1997     

Predictive calculations of gas solubility and permeability in glassy polymeric membranes: An overview

The possibility to evaluate in a predictive way the relevant transport properties of low molecular weight species, both gases and vapors, in glassy polymeric membranes is inspected in detail, with particular attention to the methods recently developed based on solid thermodynamic basis. The solubility of pure and mixed gases, diffusivity and permeability of single gases in polymer glasses are examined, considering in particular poly(2,6-dimethyl-1,4-phenylene oxide) as a relevant test case. The procedure clearly indicates what are the relevant physical properties of the polymer matrix and of the penetrants required by the calculations, which can be obtained experimentally through independent measurements. For gas and vapor solubility, the comparison with direct experimental data for mixed gases points out also the ability to account for the significant variations that solubility-selectivity experiences upon variations of pressure and/or feed composition. For gas and vapor permeability, the comparison with direct experimental data shows the possibility to account for the various different trends observed experimentally as penetrant pressure is increased, including the so-called plasticization behavior. The procedure followed for permeability calculations leads also to clear correlations between permeability and physical properties of both polymer and penetrant, based on which pure predictive calculations are reliably made.

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纯气体在玻璃态高分子膜中吸收过程机理的研究

基于双方式吸收的基本概念,并假设气体在膜中的溶解过程类似于在溶液中的溶解过程,得出了用于描述气体在玻璃态高分子膜中吸收过程的普遍化双方式吸收模型.该模型考虑了气体分子与膜之间的相互作用,并认为主要是溶解浓度影响了溶解度系数.     

Separation of Azeotropic Dimethylcarbonate/Methanol Mixtures by Pervaporation: Sorption and Diffusion Behaviors in the Pure and Nano Silica Filled PDMS Membranes

In this article, polydimethylsiloxane and hydrophobic nano-silica filled polydimethylsiloxane membranes were prepared and employed in dimethylcarbonate (DMC) removal from the DMC/methanol mixture via pervaporation. The sorption and diffusion behavior of the binary molecule were discussed separately to provide qualitative estimation of the PV performance in both membranes. Compared with the polydimethylsiloxane membrane, hydrophobic nano-silica filled polydimethylsiloxane membranes had a little lower sorption selectivity but higher diffusion selectivity. In both membranes, the sorption value was obtained by both experimentation and model prediction. The Flory-Huggins model was performed to predict the solvent uptakes and the sorption concentrations based on the experimental results. The sorption behavior of DMC was predictable, while the methanol solubility was a little higher than the experimental results. Moreover, the diffusion behavior was studied by Fick's law, the calculated diffusion coefficients of the permeates demonstrated a diffusion-coupling phenomenon, especially in hydrophobic nano-silica filled polydimethylsiloxane membranes. Methanol molecules diffused faster than DMC, suggesting the low diffusion selectivity. The PV performance was affected by both sorption and diffusion. Sorption was demonstrated to be a decisive factor in this pervaporation process.