Gas transport properties of asymmetric polyimide membranes prepared by ion‐beam irradiation

Ion beam irradiation has been widely used to modify the structure and properties of membrane surface layers. In this study, the gas permeability and selectivity of an asymmetric polyimide membrane modified by He ion irradiation were investigated using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 °C. Specifically, we estimated the effects of the gas diffusion and solubility on the gas permeation properties of the asymmetric membranes with the carbonized skin layer prepared by ion irradiation. 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 to 5 × 10¹⁵ ions/cm² at 50 keV. The increase in the gas permeability of the He⁺‐irradiated asymmetric polyimide membrane is entirely due to an increase in the gas diffusion, and the gas selectivity increases of the membranes were responsible for the high gas diffusion selectivities. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 262–269, 2007.     

Gas transport properties of asymmetric polyimide membranes prepared by plasma‐based ion implantation

In this study, we report the gas permeance and selectivity of the asymmetric polyimide membrane prepared by plasma‐based ion implantation (PBII). The asymmetric polyimide membranes were prepared using a dry–wet phase inversion process, and the surface skin layer on the membrane was implantated by He ions at 2.5 keV. The asymmetric membranes treated by PBII were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg and 35°C. The (O₂/N₂) and (CO₂/CH₄) selectivities in the He⁺‐implanted asymmetric membrane at 60 sec resulted in 1.5 and 1.8 time increases, respectively, when compared to those of the asymmetric membrane before PBII. On the other hand, the O₂ and CO₂ permeances in the asymmetric membrane after PBII decreased with an increase in the He⁺ treatment time. In this paper, we addressed, for the first time, the gas permeation behavior of the asymmetric polyimide membranes prepared by PBII. Copyright © 2009 John Wiley & Sons, Ltd.     

Gas permeability in rubbery polyphosphazene membranes

The synthesis, characterization, and gas permeability of 10 new polyphosphazenes has been studied. Additionally, the first gas permeation data has been collected on hydrolytically unstable poly[bis-(chloro)phosphazene]. Gases used in this study include CO₂, CH₄, O₂, N₂, H₂, and Ar. CO₂ was the most permeable gas through any of the phosphazenes and a direct correlation between the T_g of the polymer and CO₂ transport was noted with permeability increasing with decreasing polymer T_g. To a lesser degree, permeability of all the other gases studied also yielded increases with decreasing polymer T_g. The trend observed for these new polymers was further supported by published data for other phosphazenes. Furthermore, permeability data for all gases were found to correlate to the gas condensability and the gas critical pressures, except for hydrogen, suggesting that the nature of the gas is also a significant factor for permeation through rubbery phosphazene membranes. Ideal separation factors () for the CO₂/H₂ and CO₂/CH₄ gas pairs were calculated. For CO₂/CH₄, no increase in was observed with decreasing T_g, however increases in were noted for the CO₂/H₂ pair.     

Gas transport properties of polypropylene/clay composite membranes

Polypropylene membranes modified with natural and organically modified montmorillonite clays were prepared. The permeability, diffusivity and solubility of helium, oxygen and nitrogen were determined for the unfilled and filled membranes over the temperature range 25-65 °C. Physical properties of polypropylene membranes were investigated using X-ray diffraction, thermogravimetric analyser, tensile testing and differential scanning calorimetry. The results showed that the filled membranes exhibit lower gas permeability compared to the unfilled polypropylene membrane. For helium, a reduced diffusivity is mainly responsible for the reduction in the permeability, in contrast, for nitrogen and oxygen, both diffusivity and solubility were reduced by the presence of fillers. The X-ray diffraction spectra showed that the incorporation of the unmodified and modified clay did not affect the crystallographic nature of polypropylene.     

Structural characterization and gas‐transport properties of brominated matrimid polyimide

The commercial polyimide Matrimid was modified by bromination for the purpose of improving its membrane‐transport properties as a gas‐separation material and providing functional group reactivity for further modifications. The unmodified and brominated polymers were characterized by elemental analysis and one‐dimensional and two‐dimensional NMR, which revealed that one bromine atom per repeat unit was incorporated regioselectively onto the indane aromatic ring. The thermal and physical properties of the polymers before and after bromination were also investigated. The gas‐transport properties of the unmodified and brominated polymers were compared. © 2002 Government of Canada. Exclusive worldwide publication rights in the article have been transferred to Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4193–4204, 2002     

Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyimide hollow fiber

Asymmetric carbon hollow fiber membranes were prepared by pyrolysis of an asymmetric polyimide hollow fiber membrane, and their mechanical and permeation properties were investigated. The carbon membrane had higher elastic modulus and lower breaking elongation than the polyimide membrane. Permeation experiments were performed for single gases such as H₂, CO₂, and CH₄, and for mixed gases such as H₂/CH₄ at high feed pressure ranging from 1 to 5 MPa with or without toluene vapor. The permeation properties of the carbon membranes and the polyimide membrane were compared. There was little change in the properties of the carbon membranes with a passage of time. The properties were hardly affected by the feed pressure, whether the feed was accompanied with the toluene vapor or not, because the carbon membranes were not affected by compaction and plasticization.     

Gas transport property of polyallylamine–poly(vinyl alcohol)/polysulfone composite membranes

Polyallylamine (PAAm) was synthesized by free radical polymerization and characterized by Fourier transform infrared resonance (FT-IR) spectroscopy, hydrogen nuclear magnetic resonance (¹H NMR) spectroscopy and differential scanning calorimetry (DSC). The composite membranes were prepared by using PAAm–poly(vinyl alcohol) (PVA) blend polymer as the separation layer and polysulfone (PSF) ultrafiltration membranes as the support layer. The surface and cross-section morphology of the membrane was inspected by environmental scanning electron microscopy (ESEM). The gas transport property of the membranes, including gas permeance, flux and selectivity, were investigated by using pure CO₂, N₂, CH₄ gases and CO₂/N₂ gas mixture (20 vol% CO₂ and 80 vol% N₂) and CO₂/CH₄ gas mixture (10 vol% CO₂ and 90 vol% CH₄). The plots of gas permeance or flux versus feed gas pressure imply that CO₂ permeation through the membranes follows facilitated transport mechanism whereas N₂ and CH₄ permeation follows solution–diffusion mechanism. Effect of PAAm content in the separation layer on gas transport property was investigated by measuring the membranes with 0–50 wt% PAAm content. With increasing PAAm content, gas permeance increases initially, reaches a maximum, and then decreases gradually. For CO₂/N₂ gas mixture, the membranes with 10 wt% PAAm content show the highest CO₂ permeance of about 1.80 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ KPa⁻¹ and CO₂/N₂ selectivity of 80 at 0.1 MPa feed gas pressure. For CO₂/CH₄ gas mixture, the membranes with 20 wt% PAAm content display the highest CO₂ permeance of about 1.95 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ KPa⁻¹ and CO₂/CH₄ selectivity of 58 at 0.1 MPa feed gas pressure. In order to explore the possible reason of gas permeance varying with PAAm content, the crystallinity of PVA and PAAm–PVA blend polymers was measured by X-ray diffraction (XRD) spectra. The experimental results show an inverse relationship between crystallinity and gas permeance, e.g., a minimum crystallinity and a maximum CO₂ permeance are obtained at 20 wt% PAAm content, indicating that the possibility of increasing CO₂ permeance with PAAm content due to the increase of carrier concentration could be weakened by the increase of crystallinity.     

The transport properties of polyimide isomers containing hexafluoroisopropylidene in the diamine residue

The effect of modification of the central moiety of the dianhydride residue and isomerism on the gas transport and physical properties were compared for six polyimides containing the hexafluoroisopropylidene group in the diamine residue. Substitution of bulkier groups within the dianhydride residue resulted in disruption of chain packing and slight increases in resistance to chain motions which led to an increase in permeability with little loss in selectivity. The permeabilities and diffusivities in the meta connected polyimide isomers were considerably lower than in the para connected polyimide isomers. Similarly, the permselectivities in the meta connected isomers were consistently higher than in the para connected isomers. These lower permeabilities and higher permselectivities were a result of the more dense packing and a significant suppression of small scale motions in the meta connected isomers. The suppression of segmental mobility in the meta connected isomers was indicated by an increase in the sub Tg transition temperatures in these materials relative to the para connected isomers. The differences in transport properties for these polyimides were attributed to contributions by several factors, including: (1) total free volume (2) distribution of free volume (3) intersegmental resistance to chain motions, and (4) intrasegmental resistance to chain motions. © 1994 John Wiley & Sons, Inc.     

Surface orientation effect of asymmetric polyimide hollow fibers on their gas transport properties

In this study, we focused on the shear stress effects within a spinneret during hollow fiber spinning on the formation of the hollow fibers and their gas transport properties. We fabricated asymmetric polyimide hollow fibers with a completely defect-free thin skin layer using a dry/wet phase inversion process. The apparent calculated skin layer thickness of the hollow fiber was 280 nm and the O₂ permeance was 2.9×10⁻⁵ cm³ (STP)/(cm² s cmHg). Interestingly, the skin layer thickness was reduced at the high shear rate. In addition, the gas permeances and selectivities of the hollow fibers increased with the increasing shear rate. We concluded that the oriented skin layer of the hollow fiber induced by shear stress had a significant influence on the formation of the skin layer and its gas transport properties. From the ATR-IR spectra results, it was clear that the surface skin layer of the hollow fiber was parallel oriented.     

Gas transport properties of poly(ethylene oxide-co-epichlorohydrin) membranes

Gas permeation properties of crosslinked membranes prepared from a series of poly(ethylene oxide-co-epichlorohydrin) (P(EO/EP)) copolymers with different contents of ethylene oxide are determined by using the constant-volume and pressure-increase method. In addition to the chemical composition, the transport properties are related to the main characteristics of copolymers like the glass transition temperature, crystallinity and crosslinking ratio. Permeation measurements of He, H₂, N₂, O₂, CO₂ and CH₄ show that the permeabilities are nearly constant up to an EO content of about 75–80 mol%, then increase rapidly up to a maximum around 90 mol% of EO in the copolymers. The same behavior is observed for the diffusion coefficient and the CO₂ sorption coefficient. The presence of an optimal EO composition is explained by the competition between crystalline and amorphous EO sequences. The copolymers present very high CO₂ permeability and selectivity respect to other permanent gases even in gas mixtures and under high pressures.     

Gas and liquid permeation properties of modified interfacial composite reverse osmosis membranes

Gas permeation tests using nitrogen, oxygen, hydrogen, helium and carbon dioxide were performed to assess how membrane modification procedures affect the separating layer morphology of thin-film composite reverse osmosis membranes. Gas selectivity data provided evidence for the presence of nanoscale separating layer defects in dry samples of six commercial membrane types. These defects were eliminated when the membrane surface was coated with a polyether–polyamide block copolymer (PEBAX 1657), as indicated by a 25-fold decrease in gas permeance and at least a 2-fold increase in most selectivity values. Treatment with n-butanol followed by drying reduced water flux and gas flux by 30% and 75%, respectively, suggesting that using n-butanol as a solvent for applying coatings negatively affects membrane performance. The results of this study demonstrate that gas permeation measurements can be used to detect morphological features that impact gas and water membrane flux.     

Characterization of the transport properties of membranes of uncertain macroscopic structural homogeneity

The present paper confirms and extends previous applications of an original method of time-lag analysis to (concentration-independent) gas permeation through porous pellets produced by uniaxial compaction of fine graphite powder. It is shown that this approach to the study of membrane permeability enables one to (i) detect unambiguously a macroscopic structural inhomogeneity affecting transport across the membrane (often present as an unsuspected artifact of the membrane fabrication process); (ii) determine appropriate average values of the resulting space-dependent diffusion, D(x), and sorption, S(x), coefficients (without recourse to equililibrium sorption measurements); (iii) secure substantial information about the functional form, as well as the degree, of variability of D(x) and S(x); and ultimately (iv) link this information to the underlying membrane structural inhomogeneity. In the present context, the salient underlying structural feature is nonuniform porosity across the membrane, represented by (x), and the link between D(x), S(x) and (x) is provided by the simple dual-mode (intrapore gas-phase + adsorbed-phase) model of sorption and transport in porous media. The effect of increasing overall pellet porosity (_obs) by reducing applied compacting pressure, was specifically studied and found to entail marked enhancement of the degree of structural inhomogeneity without material changes in the functional form of D(x), S(x) and hence of (x). The conclusions drawn from time-lag analysis were shown to be consistent with the observed behavior of apparent diffusion coefficients derived from transient-state sorption or permeation measurements and with the results of dual-mode steady-state permeation analysis. The latter results showed additionally that, for a more complete interpretation of observed transport behavior, the variability across the pellet of the orientation of graphite particles should also be taken into account.     

A molecular dynamics simulation study of surface effects on gas permeation in free-standing polyimide membranes

A 141100-atom model of a glassy ODPA–ODA polyimide free-standing membrane, corresponding to a thickness of two average radii of gyration for the 40-mers chains, has been studied using molecular dynamics (MD) simulations. Due to the large-scale of the fully atomistic model, a parallelized particle-mesh technique using an iterative solution of the Poisson equation had to be implemented for the efficient evaluation of the electrostatic interactions. With flattened-chain configurations, the density was found to adjust itself naturally in the middle of the membrane to 95% of the ODPA–ODA experimental value. At the free-standing surfaces, the density profile became sigmoïdal, indicating surface roughness. For comparison, two isotropic bulk models, one at the “normal” density as obtained for ODPA–ODA under ambient conditions and the other one at 95% of the normal-density, were built. Small gas probes were inserted into all three models in order to investigate whether the interfacial structure of the glassy free-standing membrane can influence penetrant transport. Gas diffusion in the low-density part of the interface was found to be very fast. The limiting value for the gas diffusion coefficient D_membrane is only attained when the probes enter more dense regions in the membrane. Indeed, D_membrane compares well with D_bulk obtained for the 95%-density bulk system, i.e. about twice that in the normal-density bulk. Solubility is larger in the membrane than in both bulk models, thus suggesting an effect of chain flattening in addition to the density. Adsorption is particularly high at the free-standing interfaces.     

Charge mosaic membranes prepared from laminated structures of PVA-based charged layers: 1. Preparation and transport properties of charged mosaic membranes

A charge mosaic (CM) membrane has high permselectivity for electrolytes. While there are many reports of attempts to prepare such membranes, it is difficult to make CM membranes for practical applications. We report the preparation of CM membranes from laminated structures of charged-poly(vinyl alcohol) (PVA) membranes. The membranes were prepared by alternately stacking negatively charged base membranes and positively charged base membranes and by cutting the stack of charged layers. Permeation experiments were performed in a dialysis system consisting of the membrane and mixed solutions of KCl and sucrose. Although the salt flux through the membrane was about 30 times less than that through the charge mosaic membrane Desalton^® (Tosoh Co. Ltd.), which was prepared using microphase separation, the permselectivity for salt of our membrane is more than 30 times higher than that of Desalton^®.     

Electrospun nanofibrous membranes embedded with aerogel for advanced thermal and transport properties

The primary purpose of cold weather clothing is to shield the wearer from the extremities of the external environment. The thermal properties of nanofibers and their potential applications have tremendous scope and application in this area. The objective of this study was to investigate the mechanisms of heat transfer through fibrous insulation where the fiber diameter was less than 1 μm. Electrospinning process was used to produce flexible polyurethane and polyvinylidene fluoride nanofibers embedded with silica aerogel. The thermal and transport behavior of the samples was evaluated, and results were statistically analyzed. Presence of aerogel particles were confirmed through microscopic examination. Thermal behavior was investigated by using thermogravimetric analysis and differential scanning calorimetry. The results showed that the polyvinylidene fluoride nanofibrous membranes embedded with aerogel obtained a good thermal stability with lower weight loss than polyurethane nanofibrous membranes. The glass transition and melting point was not affected by the aerogel content in the layers, validating that polymers are not miscible. The increase in duration of electrospinning led to higher web thickness, which resulted in considerable decrease in air permeability. Considerable improvement of thermal insulation was observed by increasing the number and the weight per unit area of both nanofibrous membranes. The results confirmed increase in thermal insulation by embedding silica aerogel in nanofibrous membranes. With reference to the results, it could be concluded that nanofibers embedded with aerogel are good for thermal insulation in cold weather conditions. Thermal insulation battings incorporating nanofibers could possibly decrease the weight and bulk of current thermal protective clothing.     

Synthesis and characterization of new aromatic poly(ether imide)s and their gas transport properties

A series of processable fluorinated poly(ether imide)s (PIs) were synthesized by reacting a diamine monomer, 1,4-bis-[{2′-trifluromethyl 4′-(4″-aminophenyl)phenoxy}] benzene (HQA) with six different aromatic dianhydrides e.g., BPADA, 6FDA, ODPA, BPDA, BTDA and PMDA. The polyimides showed reasonably high glass-transition temperature (T_g up to 280 °C) and high thermal stability (T_d,10 up to 558 °C). The membranes of these polymers showed tensile strength up to 107 MPa with elongation at break up to 15%, low water absorption (0.61–1.29%), low dielectric constant (2.10–3.13 at 1 MHz) and high optical transparency (λ_cut-off up to 466 nm). The PI membrane prepared from 6FDA exhibited high permeability and permselectivity for O₂/N₂ (P_O2 = 11.8 and P_O2/P_N2 = 9.44) gas pair which eventually surpassed the present upper boundary limit drawn by L.M. Robeson.     

Gas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylene

Teflon AF 2400 (Du Pont) is an amorphous, glassy perfluorinated copolymer containing 87 mol% 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole and 13 mol% tetrafluoroethylene. The polymer has an extremely high fractional free volume of 0.327. Permeability coefficients for helium, hydrogen, carbon dioxide, oxygen, nitrogen, methane, ethane, propane, and chlorodifluoromethane (Freon 22) were determined at temperatures from 25 to 60°C and pressures from 20 to 120 psig. Permeation properties were also determined at a feed pressure of 200 psig at 25°C with a 2 mol% n-butane/98 mol% methane mixture. Permeabilities of permanent gases in Teflon AF 2400 are among the highest of all known polymers; the oxygen permeability coefficient at 25°C is 1600 × 10⁻¹⁰ cm³ (STP) cm/cm² s cmHg and the nitrogen permeability coefficient is 780 × 10⁻¹⁰ cm³ (STP) cm/cm² s cmHg. The permeabilities of organic vapors increase up to 20-fold as the vapor activity increases from 0.1 to unity, indicating that Teflon AF 2400 is easily plasticized. Although Teflon AF 2400 is an ultrahigh-free-volume polymer like poly(1-trimethylsilyl-1-propyne) [PTMSP], their gas permeation properties differ significantly. Teflon AF 2400 shows gas transport behavior similar to that of conventional, low-free-volume glassy polymers. PTMSP, on the other hand, acts more like a nanoporous carbon than a conventional glassy polymer.     

Simultaneous studies of pressure effect on charge transport and photophysical properties in organic semiconductors: A theoretical investigation

High-mobility and strong luminescent materials are essential as an important component of organic photodiodes, having received extensive attention in the field of organic optoelectronics. Beyond the conventional chemical synthesis of new molecules, pressure technology, as a flexible and efficient method, can tune the electronic and optical properties reversibly. However, the mechanism in organic materials has not been systematically revealed. Here, we theoretically predicted the pressure-depended luminescence and charge transport properties of high-performance organic optoelectronic semiconductors, 2,6-diphenylanthracene (DPA), by first-principle and multi-scale theoretical calculation methods. The dispersion-corrected density functional theory (DFT-D) and hybrid quantum mechanics/molecular mechanics (QM/MM) method were used to get the electronic structures and vibration properties under pressure. Furthermore, the charge transport and luminescence properties were calculated with the quantum tunneling method and thermal vibration correlation function. We found that the pressure could significantly improve the charge transport performance of the DPA single crystal. When the applied pressure increased to 1.86 GPa, the hole mobility could be doubled. At the same time, due to the weak exciton coupling effect and the rigid flat structure, there is neither fluorescence quenching nor obvious emission enhancement phenomenon. The DPA single crystal possesses a slightly higher fluorescence quantum yield ～ 0.47 under pressure. Our work systematically explored the pressure-dependence photoelectric properties and explained the inside mechanism. Also, we proposed that the external pressure would be an effective way to improve the photoelectric performance of organic semiconductors.     

Transport properties of ions through temperature-responsive charged membranes prepared using poly(vinyl alcohol)/poly(N-isopropylacrylamide)/poly(vinyl alcohol-co-2-acrylamido-2-methylpropane sulfonic acid)

Temperature-responsive charged membranes were prepared from the polymer mixture of poly(vinyl alcohol) (PVA), in situ polymer of N-isopropylacrylamide (NIPAAm) and PVA, and a polyanion [poly(vinyl alcohol-co-2-acrylamido-2-methylpropane sulfonic acid)]. The membranes were cross-linked under several conditions. The relationship between the preparation conditions and the water content response to temperature change, rH, and the charge density response to temperature change, rCx, was investigated. The membrane cross-linked with glutaraldehyde after annealing has the highest rH and rCx in all the membranes. rCx decreases with increasing polyanion content, and increases with increasing poly(NIPAAm) content. Permeation experiments in a dialysis system consisting of the membrane and mixed KCl and CaCl₂ solutions show that the transport modes of Ca²⁺ ions through the membrane are controlled by temperature changes in two ways: downhill transport (transport along their own concentration gradient in a system) at temperatures below the lower critical solution temperature (LCST) of poly(NIPAAm); uphill transport (transport against their concentration gradient) at temperatures above the LCST.     

Charge transport properties in microcrystalline KDyFe(CN)6

Microcrystalline solid dysprosium(III) hexacyanoferrate(II) was synthesized by co-precipitation in aqueous solution. The resulting solid has been studied by Fourier transform infrared spectroscopy, X-ray analysis and solid state electrochemistry. The use of a cavity microelectrode was necessary to explore a wide range of time scale and minimize the (undesired) capacitive currents. Cyclic voltametric experiments were very helpful to understand the kinetic of charge transfer in such microstructure. A structure-properties relationship has been established from the crystallographic and the electrochemical properties. A square-scheme is presented to explain the unique electrochemical behavior of hexacyanoferrate containing dysprosium since this compound exhibits a second redox system. The solid presents an open channel-like morphology in which the motion of charged species occurs during the redox processes. Precisely, the electronic transfer is accompanied by a cation diffusion inside the microcrystalline structure. The size of these channels strongly suggests that the kinetic of charge transfer is limited by the cation transport into these structures.