Glassy Polymer Membranes Research Articles

Gas permeation through a glassy polymer membrane: chemical potential gradient or dual mobility mode?

The flux equation for gas transport through a glassy polymer membrane is expressed in terms of a chemical potential gradient. It is an implicit function of the operating pressure. By studying the specific properties of this function, it can be simplified to an explicit form, which is mathematically identical to the expression proposed by the dual mobility model. Using this new model, all the data obtained by the dual mobility model can be described adequately. However, the physical significance of the parameters of the present model is completely different from the parameters of a dual mobility model. The present model assumes that only the molecules sorbed by the Henry mode contributed to the flux, and those sorbed by the Langmuir mode remained completely immobilized. This model provides an indirect method for the determination of partial molar volumes of gases in polymer membranes. It has been shown that the steady-state flux depends on the sorption rate at the interface, and not on the Henry constant. It has been found that the contribution of pressure in the diffusional transport of gases is of the same order and may be even higher than that of the concentration, which contradicts the popular perception that ‘the diffusion process is related basically to the concentration gradient’.

Sorbate-induced structural rearrangements and permeation of gases in the polyphenylene oxide copolymer

Rearrangements of the porous structure of the glassy polyphenylene oxide copolymer occurring in the course of its swelling in the atmosphere of several gases and the subsequent relaxations were investigated using the low temperature nitrogen adsorption technique. The values of porosity (ε) and BET surface area (SBET) were used to quantify the copolymer's free volume accessible to N2 molecules at 77K. The magnitude and the rate of changes of the copolymer's free volume invoked by its conditioning in vacuum, N2, O2, CH4, CO2 and C3H6, were analysed in terms of ε and SBET. These characteristics of the polymer structure were found to depend upon the sorbate nature, its pressure, exposure time and temperature. Swelling of the copolymer that occurs in certain atmospheres results in the expansion of the free volume, which is reflected in the increased values of ε and SBET. In accordance with the magnitude of the copolymer swelling under experimental conditions employed in this study, the gases can be qualitatively ranked as follows: C3H6>CO2>CH4>O2≈N2. The swollen polymer, being placed in the environment of a lower swelling ability, experiences structural relaxations. The relaxations result in a decrease of the free volume, which is reflected by a decrease in the values of ε and SBET. Gas permeability of glassy polymer membranes is directly related to the conditioning history of a polymer: the state of the glassy polymer characterised by the increased values of porosity and BET surface area corresponds to the more permeable state of a membrane.

Considerations on the performance of hollow-fiber modules with glassy polymeric membranes

Glassy polymeric membranes are widely used in the separation of gas mixtures. Typically, the permeability of these membranes has a pressure and composition dependence that is well described by the dual-mode transport model. Nevertheless, for simplicity, most of the hollow-fiber permeator models only consider constant permeability, which can lead to inaccurate results. A comparative theoretical study is performed on the influence of two different membrane mass transport models on gas separation hollow-fiber modules: the constant permeability and the dual-mode transport models. The comparison is performed in terms of the recovery and purity of the faster gas, under different design and operation conditions. Simulations are performed for the He/CH 4 separation in a polycarbonate membrane. It is shown that the differences in performance exhibited by the two models can go up to 10% for recovery and 5% for purity, particularly for high-permeate pressures. Pressure drop on the retentate and permeate sides is analyzed. Two pressure drop models based on Hagen–Poiseuille equation are considered and compared: constant viscosity, evaluated for feed conditions, and composition-dependent viscosity. It is concluded that neglecting viscosity composition dependency can lead to errors up to 20% in pressure drop calculations, consequently affecting the hollow-fiber performance in terms of recovery and purity.

O dyfuzji w membranach z polimerow szklistych w indukowanym polu naprezen

O dyfuzji w membranach z polimerow szklistych w indukowanym polu naprezen

Prediction of solvent solubility, diffusivity and permeability in glassy polymeric membranes

The filling-type membrane is composed of grafted polymer and solvent resistant substrate; calculation of solubility, diffusivity and the swelling-suppression effect of a substrate enabled us to predict solvent permeability. This approach, noted as membrane design, has previously been used in estimating permeability of aromatic compounds through rubbery polymeric membranes. In this study, the influence of plasticization on solubility and diffusivity is investigated theoretically and experimentally with poly(methyl methacrylate) (PMMA). Solubilities predicted by a group-contribution lattice-fluid equation of state (GCLF-EOS) model are consistent with the results of vapor sorption. A modified free volume model is proposed for calculating solvent diffusion in glassy polymers by taking into account the compositional dependence of depression on the glass transition temperature. The solvent diffusion coefficient has a clear transformation at an isothermal glass transition concentration, and predictions are consistent with those measured by vapor permeation. Fluxes of benzene and toluene through filling-type PMMA membrane are measured by vapor permeation experiments, and it was found that the predictions agreed with the experiments.

Polymer permeability and time lag at high concentrations of sorbate

A recently proposed theory of polymer sorption derived from thermodynamic analysis, was used to develop general analytical expressions for permeability, time lag and steady-state profile under non-linear Types I and II sorption equilibrium observed in glassy polymer membranes and Type III equilibrium observed in rubbers. Useful subclasses of these general analytical expressions are explored in order to demonstrate their utility for characterisation.

Gas permeation properties of thianthrene-5,5,10,10-tetraoxide-containing polyimides

A series of thianthrene-5,5,10,10-tetraoxide-containing polyimides with great chain stiffness were prepared and their gas permeation properties were investigated. TADATO/DSDA(1/1)–DDBT copolyimide, which was prepared from thianthrene-2,3,7,8-tetracarboxylic dianhydride-5,5,10,10-tetraoxide (TADATO), 3,3′,4,4′-diphenylsulfonyltetracarboxylic dianhydride (DSDA) and 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT), displayed much higher CO 2 permeability coefficient ( P CO 2 ) than both DSDA–DDBT and TADATO–DDBT homopolyimides, but their ideal selectivity ( P CO 2 / P N 2 ) were similar. The higher CO 2 permeability coefficient of TADATO/DSDA(1/1)–DDBT was attributed to the higher diffusion coefficient as well as a little higher solubility coefficient. The separation performance of TADATO/DSDA(1/1)–DDBT toward CO 2/N 2 fell above the “upper bound” line, indicating the highest separation performance among the glassy polymer membranes developed so far. Furthermore, this polyimide showed a significantly higher selectivity of CO 2 over N 2 for mixed gas permeation than for single gas permeation (single gas: P CO 2 / P N 2 =35, mixed gases: P CO 2 / P N 2 =49, at 35°C and 2 atm). The “dual-mode” model was used to describe gas sorption and transport behaviors in the membrane.

Sorption and pervaporation properties of sulfonyl‐containing polyimide membrane to aromatic/non‐aromatic hydrocarbon mixtures

Sorption of single-component vapors of benzene (Bz), n-hexane (Hx), and cyclohexane (Cx), and of binary liquid mixtures of Bz/Hx and Bz/Cx in a polyimide from 3,3′,4,4′-diphenylsulfone-tetracarboxylic dianhydride (DSDA) and 2,8(6)-dimethyl-3,7-diaminobenzothiophene-5,5-dioxide (DDBT) were investigated in detail at 333 K. Sorption and desorption of vapors followed the non-Fickian kinetics and the sorption isotherms were concave to the vapor activity. For the binary liquids, the sorption isotherms of the Bz component were concave to the Bz composition in feed, whereas those of Hx and Cx were convex because of competitive sorption. As a result, the solubility selectivity was much larger than the sorption ratio of pure liquids. The concentration-averaged diffusion coefficients of Bz (D̄Bz) and Hx (D̄Hx) were evaluated using the sorption and pervaporation data of the polyimide membrane toward the binary mixtures. A kind of coupling effect of the coexisting component on D̄ was observed. That is, D̄ of penetrant with smaller molecular size (Hx and Bz for Bz/Hx and Bz/Cx systems, respectively) was reduced by the presence of penetrant with larger molecular size (Bz and Cx, respectively) and vice versa. D̄Bz was similar to D̄Hx, but much larger than D̄Cx. The difference in PV behavior between Bz/Hx and Bz/Cx systems for glassy polymer membranes was understood based on the aforementioned features of sorption and diffusion. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2954–2964, 2000

Performances of glassy polymer membranes plasticized by interacting penetrants

The present paper deals with performances of glassy polymer membranes plasticized by interacting penetrants. The objects for investigation were polar gases and vapours such as ammonia, sulphur dioxide, freons, water as well as their mixtures with simple gases. The polymer were made of polyamides, polyheteroarylenes, derivatives of cellulose, polyurethanes, polyureas as well as block copolymers with flexible and rigid fragments containing active groups: polyether(ester) urethanes and polyether(ester) ureas. In the case of specific interaction between the penetrant and active groups of the polymer (donor–acceptor, ion–dipole and hydrogen bonding) plasticization of the polymer or rigid fragment of the block copolymer is observed producing a change in thermodynamical, diffusion and mechanical properties of the polymer as well as abnormal temperature dependence of diffusion properties of the polymer membranes. Comprehensive investigation of these properties has been carried out in a broad concentration/temperature range. General regularities of the physico-chemical processes observed have been generalized.

Free Volume Distributions in Glassy Polymer Membranes: Comparison between Molecular Modeling and Experiments

We compare molecular modeling results of two glassy polymer membranes and one rubbery polymer membrane with gas transport parameters and free-volume-related quantities from positronium annihilation. A simple geometric model reveals hole size distributions of asymmetric shape. Among glassy polymers, the distribution parameters show a good correlation with average hole sizes determined by positron annihilation lifetime spectroscopy. Higher permeability is measured in the glassy polymer with the higher mean value of the hole size distribution. The permselectivity of the membranes for permanent gases can be interpreted in terms of the distribution broadness via free-volume-controlled diffusion selectivity. A comparison with the rubbery polymer shows that the permeation behavior is not determined only by the free volume concentration. The thermal fluctuations of the polymer matrix play an important role for gas transport properties.

Sorption and pervaporation properties of sulfonyl-containing polyimide membrane to aromatic/non-aromatic hydrocarbon mixtures

Sorption of single-component vapors of benzene (Bz), n-hexane (Hx), and cyclohexane (Cx), and of binary liquid mixtures of Bz/Hx and Bz/Cx in a polyimide from 3,3',4,4'-diphenylsulfone-tetracarboxylic dianhydride (DSDA) and 2,8(6)-dimethyl-3,7-diaminobenzothiophene-5,5-dioxide (DDBT) were investigated in detail at 333 K. Sorption and desorption of vapors followed the non-Fickian kinetics and the sorption isotherms were concave to the vapor activity. For the binary liquids, the sorption isotherms of the Bz component were concave to the Bz composition in feed, whereas those of Hx and Cx were convex because of competitive sorption. As a result, the solubility selectivity was much larger than the sorption ratio of pure liquids. The concentration-averaged diffusion coefficients ofBz (D Bz ) and Hx (D Hx ) were evaluated using the sorption and pervaporation data of the polyimide membrane toward the binary mixtures. A kind of coupling effect of the coexisting component on D was observed. That is, D of penetrant with smaller molecular size (Hx and Bz for Bz/Hx and Bz/Cx systems, respectively) was reduced by the presence of penetrant with larger molecular size (Bz and Cx, respectively) and vice versa. D Bz was similar to D Hx , but much larger than D Cx . The difference in PV behavior between Bz/Hx and Bz/Cx systems for glassy polymer membranes was understood based on the aforementioned features of sorption and diffusion.

A transport model for sorption and desorption of penetrants in glassy polymer membrane

The transport of penetrants in glassy polymer membranes is assumed to be a process of mixing of penetrant and polymer molecules accompanied by a creep strain of polymer membrane. The stress–strain relation of polymer membrane and its contribution to total Gibbs free energy are calculated by the Voigt creep model of linear viscoelastic solids. The contribution due to mixing of polymer and sorbed penetrant is calculated by Flory–Huggins model. Combining with the mass conservation equation and phenomenological diffusive flux expression, a transport model of penetrant in glassy polymer membrane is then established. The effective diffusion coefficient of penetrants in membrane which is time- and place-dependent is obtained by comparing with Fickian diffusion law. The numerical simulation of integral sorption shows that this model can describe various sorption and desorption curves including sigmoid-type curves. In general case, three adjustable model parameters are used to correlate the sorption curve. Corresponding desorption curve can then be predicted. If the Young's modulus and Flory–Huggins interaction parameter are obtained from other independent experiment, only one adjustable parameter is needed. The correlated and predicted results agree well with the experimental sorption and desorption curves of water and ethanol in polyimide membranes. It is shown that this model is more flexible and require fewer model parameters as comparing with Crank model.

Permeation and separation properties of polyimide membranes to 1,3-butadiene and [formula omitted

Single and mixed gas permeation experiments for 1,3-butadiene and n- butane were carried out for several polyimide and other glassy polymer membranes at pressures up to 1.5 atm and 323 K. They displayed large permeation coefficients to 1,3-butadiene, P C 4H 6 , of 4–200 Barrer and very large ideal separation factors (permeability ratios of 1,3-butadiene over n- butane for single gas permeation), α id , of 30–200 at 1 atm. The large P C 4H 6 was attributed to large diffusion and solubility coefficients to 1,3-butadiene. The diffusion coefficient to 1,3-butadiene for each polymer was a little larger than or similar to that to propylene, whereas the diffusion coefficient to n- butane was much smaller than that to propane, indicating the relatively small effective diameter for diffusion of 1,3-butadiene probably because of its rigid and straight molecular shape. For mixed gas permeation, the separation factors, α, were reduced by a factor of 1 3 – 1 6 as compared with α id due to the strong plasticization effect of 1,3-butadiene. However, the polyimides had still high performance.

Correlation and prediction of gas permeability in glassy polymer membrane materials via a modified free volume based group contribution method

Over the past decade or more an extensive amount of data on the permeation of gases such as helium, hydrogen, oxygen, nitrogen, methane, and carbon dioxide in a wide array of glassy polymers has been published. Much of this work has been motivated by the search for materials with high permeability and high selectivity for potential use as gas separation membranes. This paper attempts to develop a method for correlating this data in a way that permits prediction of permselectivity behavior of other polymer structures. The method used involves an empirical modification of a free volume scheme that has been used in the past with some success. The previous method requires an experimental density of the polymer and an estimate of occupied volume from a group contribution method developed by Bondi. The present method actually predicts the density and uses a refined estimate of occupied volume specific to each gas. The parameters in the model were deduced from a database including over one hundred polymers. The new method significantly improves the accuracy of correlation and of prediction.

Estimation of diffusion and permeability coefficients of CO2 in polymeric membranes by FTIR method

Infrared spectra of CO2 sorbed in rubbery and glassy polymeric membranes were measured to examine the relationships between the spectroscopic data and the physical properties of the membranes. The two peaks observed in the spectra of CO2 were attributed to the R branch and P branch of CO2 sorbed in the membranes based on the consideration that both peaks were observed at a temperature above the glass transition temperature of the membranes. Apparent diffusion coefficients of CO2 in the membranes were measured from the desorption kinetics of CO2 detected by FTIR spectroscopy. The solubility coefficients of CO2 were also estimated from absorbance spectra of CO2 sorbed in the membranes using Lambert-Beer's rule. The permeability, solubility, and diffusion coefficients estimated by the FTIR method were found to correlate well with the coefficients obtained by conventional methods such as vacuum-pressure or sorption isotherm methods. © 1996 John Wiley & Sons, Inc.

Sorption and permeation behavior for a gas in glassy polymer membrane near the glass transition temperature

The deviation from the conventional dual-mode sorption and mobility model for a gas in glassy polymer membranes has separately been studied thus far, and to simulate sorption and diffusion behavior, an extended dual-mode sorption model and a modified dual-mode mobility model, respectively, have been proposed independently. However, simultaneous deviation from the conventional dual-mode sorption and mobility model was observed in cases of CO2in poly-4-methyl-1-pentene membrane at 20°C and in polystyrene membrane at 60 and 70°C. The plasticization effect of sorbed CO2 on both the sorption and diffusion processes tends to be brought about in glassy polymer membranes near the glass transition temperature. The behavior was simulated based on the concept that only one population of sorbed gas molecules exists. © 1996 John Wiley & Sons, Inc.

Molecular modeling studies of polymeric gas separation and barrier materials: structure and transport mechanisms

AbstractMolecular modeling studies have been completed on cis‐PTBA(poly(tert‐butylacetylene)) and Sixef44 polyimide, two glassy polymers that can be used to form gas separation membranes. The modeling studies show that polymer backbone bond rotations in PTBA are not thermally allowed. This leads to a helical structure for the cis‐PTBA chains which pack as if the helices were rigid rods. Here, polymer free volume is formed by the interstitial space between adjacent helices, and gas transport occurs via continuous diffusion through the resulting channel‐like free volume. On the other hand, Sixef44 exhibits a flexible polymer backbone, which leads to the formation of irregular voids. In this case, gas molecules are free to move within the voids, but transport occurs only by hopping to an adjacent void, or by void diffusion. In either case, gas transport is closely coupled to polymer backbone motion. Thus, these studies suggest two different types of free volume and gas transport mechanisms. The diffusion mechanism in glassy polymer membranes will depend on the nature of the free volume (e.g. the type of chain packing), and the polymer backbone chain flexibility.

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.

Solution and Diffusion Behavior of Pure Gases and Gas Mixtures in Glassy Polymer Membranes

A general model for the solution and diffusion behavior in pure gas-polymer membrane systems and gas mixture-polymer membrane systems has been developed. Proved by experiments on different glassy and rubbery polymer membranes at various temperatures and pressures, this model can achieve the prediction of permeation behavior of pure gases and gas mixtures in polymer membranes only using the model parameters obtained from experiments on pure gases. The calculated results are in good agreement with experimental.

Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes

This study examines the effect of treatment of defective glassy polymer membranes with a variety of vapors and liquids which have varying solvency power for the polymer. The pure-gas oxygen/nitrogen selectivities of defective, asymmetric membranes are shown to be permanently increased, in special cases, by treatment with certain solvents which have adequate solvency power to cause a critical level of swelling in the membrane skin layer. Three distinct types of membranes have been treated; asymmetric polysulfone membranes formed by dry/wet phase inversion, spin-coated poly(phenylene oxide)-ceramic composite membranes and solution-deposited polyimide-ceramic composite membranes. While the detailed fundamental processes controlling the elimination of surface defects are complex, our results suggest that plasticization of the selective skin layer, coupled with surface-tension driven cohesive forces are likely to be the key factors at play.