Gas diffusion and solubility in He-irradiated asymmetric polyimide membranes

In this study, the gas diffusion and solubility of the asymmetric polyimide membrane irradiated by He ions were investigated using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 °C. Specifically, we estimated their effects on the gas permeation properties of the asymmetric membranes. The asymmetric polyimide membranes were prepared by a dry-wet phase inversion process, and the surface skin layer on the membrane was irradiated by He ions at fluences of 1 × 10¹⁵ or 3 × 10¹⁵ ions/cm² at 150 keV. We demonstrated that the gas diffusion had a significant influence on the gas permeability and selectivity of the He⁺-irradiated asymmetric membrane.     

Gas permeability of fluorinated hyperbranched polyimide

The gas permeability of carbon dioxide, oxygen and nitrogen for hyperbranched polyimide (HBPI) containing trifluoromethyl groups were investigated. The HBPIs were prepared by condensation polymerization of a triamine monomer, 1,3,5‐tris(2‐trifluoromethyl‐4‐aminophenoxy) benzene (TFAPOB) and a commercially available dianhydride monomer 1,4‐bis(3,4‐dicarboxyphenoxy) benzene dianhydride (HQDPA). With different monomer addition methods and different monomer molar ratios, amine terminated HBPI (AM‐HQDPA) and anhydride terminated HBPI (AD‐HQDPA) were obtained, subsequently, the trifluoromethylphenyl amine terminated HBPI (CF₃‐HQDPA) was achieved by modifying the end groups of AD‐HQDPA with (3,5‐ditrifluoromethyl)aniline. The CF₃‐HQDPA exhibited a good mechanical and thermal stability as well as AM‐HQDPA, and showed better gas permeabilities than that of AM‐HQDPA because of increase of free volume contributed by the bulky trifluoromethyl group introduced, but, the selectivity ofCF₃‐HQDPA was lower than that of AM‐HQDPA. This result was consistent with the trade‐off relationship. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Vapor and gas permeability of asymmetric membranes

The membrane with continuously varying and pressure-dependent local permeability P may show a dependence of transport property for gases and vapors on the direction of flow. Such an asymmetry occurs only if the deviation of local permeability from ideality varies from layer to layer. In mathematical formulation, this means that the local permeability is an irreducible function of location and pressure, i.e., not a product of a function of location and a function of pressure. The membrane permeability is higher if the side with greater deviation from ideal, i.e., constant permeability, is exposed to the higher pressure. For two simple cases the currents in both directions and their ratio at constant pressure difference were calculated. It turns out that the asymmetry of permeability increases with increasing deviation from ideality up to a maximum, after which the membrane tends to return to symmetry. An additional result of this investigation is the conclusion that Fick's law, i.e., the proportionality of the diffusion current to the negative concentration gradient is inapplicable not only to inhomogeneous membranes but also to homogeneous not ideal membranes.     

Enhanced CO2 selectivity of polyimide membranes through dispersion of polyethyleneimine decorated UiO-66 particles

Branched polyethyleneimine (PEI) functionalized UiO-66 were synthesized and used as fillers to fabricated mixed-matrix membranes (MMMs) for CO₂/CH₄ separation. The purpose of introducing amino-functional groups in the filler is to improve the interfacial compatibility of the filler with the polymer through the formation of hydrogen bonds with the carbonyl group of 6FDA-ODA. Additionally, the amino group can facilitate CO₂ transport through a reversible reaction, enhancing the CO₂/CH₄ separation properties of MMM. The chemical structure and morphology of fillers and membranes were characterized by employing X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), thermogravimetric (TGA), Derivative thermogravimetry (DTG) and scanning electron microscope (SEM). Furthermore, the effects of filler loading and feed pressure on CO₂ permeability and CO₂/CH₄ selectivity have been investigated. MMMs present higher gas separation performance than pure 6FDA-ODA due to the presence of amino groups and the improvement of interface morphology. In particular, the MMM with 15 wt% loading of UiO-66-PEI shows optimum CO₂ permeability of 28.23 Barrer and CO₂/CH₄ selectivity of 56.49. Therefore, post-synthetic modification of UiO-66 particle with PEI is a promising alternative to improved membrane performance.     

Gas permeability of polyphosphazene membranes

A series of dense membranes for gas separation based on poly[bis(phenoxy)phosphazene] (PPOP) were prepared in flat sheet configurations. They were characterized by viscosimetry, differential scanning calorimetry (DSC), thermogravimetry (TG), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Permeation tests of O₂, N₂, He and CO₂ were made at five different temperatures below the T(1) transition of PPOP, which gave results that proved to be stable and reversible with temperature. The Arrhenius plots of permeabilities were linear, and showed that the faster permeability of CO₂ with respect to He was due to a larger solubility. Interesting O₂ to N₂ selectivity ratios were obtained.     

Gas permeability of composite membranes

The permeability of a composite membrane consisting of a homogeneous layer and a porous layer has been derived theoretically by assuming that the permeation through the homogeneous layer obeys Fick's law and that permeation through the porous layer is free molecular flow. The activation energy of the flow is described by three-dimensionless parameters, ? = |P₁₂/|P₂, θ = d₁/d₂, and σ = |P₁/|P₂. |P₁₂, |P₁, and |P₂ are the permeability coefficients of the composite membrane, the homogeneous layer and the porous layer, respectively, d₁ and d₂ are the thickness of the two layers. Once these parameters are determined, information on the structure of the membrane can be obtained (i.e., the pore size and the pore density). The permeabilities of various gases through homogeneous polycarbonate membrane, neutron-irradiated, nonsodium hydroxide-etched polycarbonate membrane, and their composite membrane were tested. A two-layer series model, incorporating the effect of neutron irradiation which produces some nonpenetrating pores in the porous membrane layer, is proposed and agreed quite well with the experimental data.     

Gas permeation and separation properties of sulfonated polyimide membranes

Permeability and selectivity of pure gas H₂, CO₂, O₂, N₂ and CH₄ as well as a mixture of CO₂/N₂ for sulfonated homopolyimides prepared from 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoro propane disulfonic acid (BAPHFDS) were measured and compared to those of the non-sulfonated homopolyimide having the same polymer backbone. The polyimide in a proton form (NTDA-BAPHFDS(H)) displayed higher selectivity of H₂ over CH₄ without loss of H₂ permeability. Strong intermolecular interaction induced by sulfonic acid groups decreased diffusivity of the larger molecules. The CO₂/N₂ (19/81) mixed gas permeation was investigated as a function of humidity. With increasing relative humidity from 0% RH to 90% RH, the CO₂ permeability for NTDA-BAPHFDS(H) polyimide increased by more than one order of magnitude, and the selectivity of CO₂/N₂ also increased twice or more. On the other hand, the gas permeability for the non-sulfonated polyimide slightly decreased with increasing humidity. NTDA-BAPHFDS(H) polyimide displayed a CO₂ permeability of 290×10⁻¹⁰ cm³ (STP) cm/(cm² s cmHg) and a separation factor of CO₂/N₂ of 51 at 96% RH, 50 °C and total pressure of 1 atm.     

Water transport mechanisms through asymmetric membranes

Permeability coefficients and activation energy values for the transport of water through asymmetric cellulose acetate membranes were determined in order to establish the mechanism of the process when different driving forces are applied. A stirred Lucite cell with controlled temperature was used to measure the membrane transport properties under hydraulic and osmotic pressure differences and also in the presence of a tracer concentration gradient across the membrane. The experimental results based on the temperature dependence of water flow show that the controlling step for water transport is diffusion with net flux in the dense zone of the membrane under hydraulic or osmotic pressure gradients. When a tracer concentration gradient is used, equimolar diffusion of water in the thicker, porous zone of the membrane is the controlling mechanism. A mass transport model based on the composed structure of the membrane is presented to provide a general framework for treating the particular cases. Finally, the difference in the controlling barriers, in agreement with a previous work by Hays,¹⁸ is shown to account for the much higher absolute values of osmotic than tracer water permeabilities determined here and frequently reported in the literature.     

Physical aging of 6FDA-based polyimide membranes monitored by gas permeability

The aging behaviors, as judged by gas permeability, of two glassy 6FDA-based polyimides, 6FDA-DAM and 6FDA-mPDA, in thick and thin film forms are reported. Their O₂ and N₂ gas permeabilities were monitored over several thousands of hours. In general, the properties of these thin films deviate dramatically from their bulk behavior, by showing much more rapid decrease in gas permeability and increase in selectivity. Owing to the high free volume, the 6FDA-DAM thin films have very high permeability at the very early aging time, followed by an order of magnitude decrease in permeability relative to the initial value over the course of 1000 h of aging. On the other hand, a thick 6FDA-DAM polymer film maintained a high permeability over thousands of hours of aging. 6FDA-mPDA thin films have moderate aging rates comparable to thin films made from other polyimides such as Matrimid^®. Effect of PDMS coating on gas permeability and aging was examined using a few 6FDA-mPDA thin film membranes and was found to be insignificant.     

Gas permeability of wet cation exchange membranes

The permeation of CO₂ across a Nafion cation exchange membrane in different hydrated forms is studied. The water solvent linked to the ion exchanging sites of the charged membranes forms a liquid supported membrane in which the transport of the polar gas is enhanced. This effect is still higher when the dry membrane is swollen in a Li₂CO₃ solution which increases the carbonate/bicarbonate anion concentration in the wet membranes and reduced the effectiveness of the Donnan co-ion exclusion from the membrane phase.     

Hindered diffusion of flexible polymers through polyimide ultrafiltration membranes

The hindered diffusion of polystyrene in dilute solutions of ethyl acetate through polyimide ultrafiltration membranes has been investigated. The present system did not show specific membrane‐solute interactions; furthermore, polystyrene can be considered as a flexible polymer coil. It is shown that the hindered diffusive permeability for monodisperse dilute solutions for a series of molecular weights can be compared well with the diffusive permeability curve of one polydisperse dilute polystyrene solution. In the case of very dilute solutions, the polymer coils have no interaction with each other, and the whole range of molecular‐weight‐dependent permeabilities can be determined from only one measurement. The diffusion behavior of polydisperse solutions through various polyimide membranes has been investigated as well. It was found that the diffusive permeability curve is strongly dependent on the type of membrane, that is, on the pore size distribution. It was not possible to calculate a pore size distribution from diffusion experiments due to mathematical limitations. Nevertheless, it was shown that hindered diffusion measurements are useful to estimate a maximum pore size for each membrane. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1180–1193, 2000     

Gas permselection properties in silicone-coated asymmetric polyethersulfone membranes

The gas permeation properties of H₂, He, CO₂, O₂, and N₂ through silicone-coated polyethersulfone (PESf) asymmetric hollow-fiber membranes with different structures were investigated as a function of pressure and temperature and compared with those of PESf dense membrane and silicone rubber (PDMS) membrane. The PESf asymmetric hollow-fiber membranes were prepared from spinning solutions containing N-methyl-2-pyrrolidone as a solvent, with ethanol, 1-propanol, or water as a nonsolvent-additive. Water was also used as both an internal and an external coagulant. A thin silicone rubber film was coated on the external surface of dried PESf hollow-fiber membranes. The apparent structure characteristics of the separation layer (thickness, porosity, and mean pore size) of the asymmetric membranes were determined by gas permeation method and their cross-section morphologies were examined with a scanning electron microscope. The results reveal that the gas pressure normalized fluxes of the five gases in the three silicone-coated PESf asymmetric membranes are nearly independent of pressure and did not exhibit the dual-mode behavior. The activation energies of permeation in the silicone-coated asymmetric membranes may be larger or smaller than those of PESf dense membrane, which is controlled by the membrane physical structure (skin layer and sublayer structure). Permselectivities for the gas pairs H₂/N₂, He/N₂, CO₂/N₂, and O₂/N₂ are also presented and their temperature dependency addressed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 837–846, 1997     

Gas permeation properties of new type asymmetric membranes

We have developed a new type of asymmetric membranes having a homogeneous hyperthin skin layer, which was used as a polyimide synthesized by 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 2,2-bis(4-amino phenyl) hexafluoro-propane (BAAF). The skin layer thicknesses of the 6FDA-BAAF polyimide asymmetric membranes were 40–60 nm, and the porosity was 10^-6% when a defect size was assumed as 5 nm. The permselectivity of 6FDA-BAAF polyimide asymmetric membranes after silicone coating had α of 40 for CO₂/CH₄ and a flux of 1.0 [Nm³/m²-h-atm] (=3.7 × 10⁻⁴ [cm³(STP)/cm² s cmHg]) for CO₂, α of 4.3 for O₂/N₂ and a flux of 2.0 × 10⁻¹ [Nm³/m²/h/atm] (=7.1 × 10⁻⁵ [cm³(STP)/cm²s cmHg]) for O₂. These values were constant for large-scale manufacturing. © 1996 John Wiley & Sons, Inc.     

Properties of asymmetric polyimide ultrafiltration membranes. I. Pore size and morphology characterization

Properties of asymmetric UF membranes made by solution casting of aromatic polyimides as a function of fabrication conditions are reported. The characteristic properties investigated include: the overall porosity, hydraulic permeability, the equivalent pore size of the skin layer (via a newly developed general diagram), and morphological features shown by the scanning electron microscope. It was found that as casting solution concentration and/or evaporation period was increased, the average pore size decreased while the skin thickness increased. At low casting solution concentrations the membranes were highly porous and the precipitated polymer phase had a granular structure consisting of aggregates of precipitated polymer micelles. While at high concentrations marcrovoid porosity was reduced but the precipitated polymer phase had a spongy structure. The interstitial openings of the granular skin structure of a membrane made from 15% polymer solution with no evaporation as revealed by SEM showed pore size values that were close to those calculated through pore models.     

Effect of crystallinity and swelling on the permeability and selectivity of polymer membranes

The conventional equations describing mass transport through membranes predict proportionality between flux and driving force. Experimental results inconsistent with this prediction are usually attributed to a concentration dependency of the diffusion coefficients of the species involved. It has now been found that, in addition to this effect, simplifications in the models and the neglect of crystallinity and swelling of the membrane material contribute in no small way to the noted deviations. On the basis of the well-known “solution-diffusion” theory a new permeation rate equation has been derived, which accounts for the various shortcomings mentioned and which is capable of predicting the large effects of crystallinity and swelling of the membrane material on permeability. This equation also expresses the mutual effects of permeants on their separation without the introduction of an arbitrary coupling coefficient, which is required in the conventional equations.A simplified procedure is proposed for calculating membrane separation of multicomponent mixtures, which is based on the assumption of an exponential concentration gradient inside the membrane in accordance with reported experimental observations. The exponential form, which differs for each composition and set of conditions, can be calculated from the boundary conditions of the membrane.     

Zeolite filled polyimide membranes for dehydration of isopropanol through pervaporation process

Six mixed matrix membranes (MMMs) were prepared using zeolites of 4A and ZSM-5 incorporated in polyimide of Matrimid 5218. Effects of filler type on membrane morphology and pervaporation performance of MMMs were investigated using isopropanol dehydration. In addition, effects of operating temperature (30, 40, 50, and 60 °C), feed water concentration (10, 20, 30, and 40 wt.%) and permeate side pressure (0 and 15 torr) on pervaporation performance were studied. Scanning electron microscopy (SEM) analysis showed there were good adhesion between the fillers and the polymer matrix. Zeolite 4A has a better contact with the polymer phase and thereby nearly no void is formed in the MMM structure. Pervaporation were performed based on L16 array of Taguchi method for design of experiments. The results showed that the best separation condition is achieved at temperature, feed water concentration, and permeate pressure of 30 °C, 10 wt.% water and 0 torr, respectively. Selectivities of zeolites 4A and ZSM-5 filled MMMs were calculated as 8991 and 3904 compared with 1276 measured for the neat Matrimid 5218 membrane. Permeation rates of the zeolite 4A and ZSM-5 filled MMMs and the neat polymeric membrane were found to be 0.018, 0.016, and 0.013 kg/m² h, respectively.     

Preparation and permeability of solutes through acrylic acid-g-methylcellulose copolymer membranes

Graft copolymer membranes from the methylcellulose and the acrylic acid were prepared and their properties and the permeability of four solutes were estimated. Acrylic acid-g-methylcellulose (AA-g-MC) copolymer dissolved in aqueous acetone solvent was cast to prepare membranes followed by the subsequent crosslinking either with aluminium potassium sulfate or by the thermal-curing method. The equilibrium water content in the membrane increased with the volume fraction of acetone in the aqueous acetone solvent system. Membrane, the ionically crosslinked with the aluminum potassium sulfate, showed the water content in the range of 38.5 and 58.4% and 0.25–0.33 kg/mm² of the tensile strength in the wet state. Compared with ionically crosslinked membranes, thermally dried membranes exhibited a more dense structure, resulting in lower water contents and higher mechanical strength. Experimental results on the permeation of four small and midsize solutes through the graft copolymer membranes revealed the molecular weight dependence of the permeability coefficient. The higher the degree of swelling, the greater the permeability coefficient. Ionically crosslinked membranes had higher solute permeability than the commercial Cuprophane membrane had. © 1993 John Wiley & Sons, Inc.     

Gas permeability and permselectivity of plasma‐treated polyethylene membranes

The effects of NH₃‐plasma and N₂‐plasma treatments on rubbery polyethylene (PE) membranes on the permeation behavior for carbon dioxide (CO₂), O₂, and N₂ were investigated with permeability measurements. The NH₃‐plasma and N₂‐plasma treatments on PE membranes increased both the permeation coefficient for CO₂ and the ideal separation factor for CO₂ with respect to N₂. For O₂ transport, both the permeation coefficient for O₂ and the ideal separation factor for O₂ with respect to N₂ were increased. NH₃‐plasma and N₂‐plasma treatments on polymer membranes possibly bring about an augmentation of permeability and permselectivity simultaneously. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 383–387, 2006