Journal of Membrane Science

Published by Elsevier
Online ISSN: 0376-7388
Publications
Article
Nanoporous anodic aluminum oxide (AAO) tubular membranes were fabricated from aluminum alloy tubes in sulfuric and oxalic acid electrolytes using a two-step anodization process. The membranes were investigated for characteristics such as pore size, interpore distance and thickness by varying applied voltage and electrolyte concentration. Morphology of the membranes was examined using light optical and scanning electron microscopy and characterized using ImageJ software. Results showed that membranes having narrow pore size and uniform pore distribution with parallel channel arrays were obtained. The pore sizes were ranging from 14 to 24 nm and the wall thicknesses as high as 76 microm. It was found that the pore size increased in direct proportion with the applied voltage and inversely with the electrolyte concentration while the interpore distance increased linearly with the applied voltage. It was also observed that increase in acid concentration increased tubular membrane wall thickness that improved mechanical handling. By using anodic alumina technology, robust ceramic tubes with uniformly distributed pore-structure and parallel nano-channels of lengths and sizes practical for industrial applications were reliably produced in quantity.
 
Article
Current artificial lungs and respiratory assist devices designed for carbon dioxide removal (CO(2)R) are limited in their efficiency due to the relatively small partial pressure difference across gas exchange membranes. To offset this underlying diffusional challenge, bioactive hollow fiber membranes (HFMs) increase the carbon dioxide diffusional gradient through the immobilized enzyme carbonic anhydrase (CA), which converts bicarbonate to CO(2) directly at the HFM surface. In this study, we tested the impact of CA-immobilization on HFM CO(2) removal efficiency and thromboresistance in blood. Fiber surface modification with radio frequency glow discharge (RFGD) introduced hydroxyl groups, which were activated by 1M CNBr while 1.5M TEA was added drop wise over the activation time course, then incubation with a CA solution covalently linked the enzyme to the surface. The bioactive HFMs were then potted in a model gas exchange device (0.0084 m(2)) and tested in a recirculation loop with a CO(2) inlet of 50mmHg under steady blood flow. Using an esterase activity assay, CNBr chemistry with TEA resulted in 0.99U of enzyme activity, a 3.3 fold increase in immobilized CA activity compared to our previous method. These bioactive HFMs demonstrated 108 ml/min/m(2) CO(2) removal rate, marking a 36% increase compared to unmodified HFMs (p < 0.001). Thromboresistance of CA-modified HFMs was assessed in terms of adherent platelets on surfaces by using lactate dehydrogenase (LDH) assay as well as scanning electron microscopy (SEM) analysis. Results indicated HFMs with CA modification had 95% less platelet deposition compared to unmodified HFM (p < 0.01). Overall these findings revealed increased CO(2) removal can be realized through bioactive HFMs, enabling a next generation of more efficient CO(2) removal intravascular and paracorporeal respiratory assist devices.
 
Article
This paper describes the surface modification of macroporous membranes using ATRP (atom transfer radical polymerization) to create cation-exchange adsorbers with high protein binding capacity at high product throughput. The work is motivated by the need for a more economical and rapid capture step in downstream processing of protein therapeutics. Membranes with three reported nominal pore sizes (0.2, 0.45, 1.0 μm) were modified with poly(3-sulfopropyl methacrylate, potassium salt) tentacles, to create a high density of protein binding sites. A special formulation was used in which the monomer was protected by a crown ether to enable surface-initiated ATRP of this cationic polyelectrolyte. Success with modification was supported by chemical analysis using Fourier-transform infrared spectroscopy and indirectly by measurement of pure water flux as a function of polymerization time. Uniformity of modification within the membranes was visualized with confocal laser scanning microscopy. Static and dynamic binding capacities were measured using lysozyme protein to allow comparisons with reported performance data for commercial cation-exchange materials. Dynamic binding capacities were measured for flow rates ranging from 13 to 109 column volumes (CV)/min. Results show that this unique ATRP formulation can be used to fabricate cation-exchange membrane adsorbers with dynamic binding capacities as high as 70 mg/mL at a throughput of 100 CV/min and unprecedented productivity of 300 mg/mL/min.
 
Article
A family of heptapeptide-based chloride transporters (called synthetic anion transporters, SATs) were designed to insert into phospholipid membrane bilayers and form pores. Many of these compounds have proved to be chloride selective transporters. The transporters were designed to incorporate hydrophilic heptapeptides that could serves as headgroups and hydrocarbon tails that could serve as hydrophobic membrane anchors. Insertion of the SAT molecules into a bilayer requires approach to and insertion at the aqueous-membrane surface. The studies reported here were conducted to model and understand this process by studying SAT behavior at the air-water interface. A Langmuir trough was used to obtain surface pressure-area isotherm data. These data for amphiphilic SATs were augmented by Brewster angle microscopy (BAM), molecular modeling, and calculations of the hydrophobicity parameter log P. The analyses showed that the heptapeptide (hydrophilic) module of the SAT molecule rested on the water surface while the dialkyl (hydrophobic) tails oriented themselves in the air, perpendicular to the water surface. Brewster angle microscopy visually confirmed a high order of molecular organization. Results from these studies are consistent with the previously proposed mechanism of SAT membrane insertion and pore formation.
 
Flow diagram for membrane preparation procedure  
(a) Silver capture with different functionalized silica-cellulose acetate mixed matrix membrane (10% silica loading). Effect of total surface area and accessibility of surface −SH groups on silver ion capture capacity. (b) Silver capture data reported per gram of silica  
(a) Effect of residence time (t R ) on silver capture capacity for 40% MPTMS functionalized 874-85-1 silica -polysulfone mixed matrix membrane. The error bars indicate analytical error of measurement for Ag + concentration. Solid line ideal case of silver capture. (b) Silver capture data reported as a function of time. Dotted line indicates maximum silver capture capacity  
(a) Comparison of experimental and predicted data for silver breakthrough curves using thiol functionalized 874-85-1 silica-polysulfone MMM with 40% Silica Loading. (b) Data comparison for silver capture with 30% silica loading MMM. In all these cases, no adjustable parameter was used to predict the data. The volumetric mass transfer coefficient (k) is obtained from correlation based of membrane flux (J W ). Error bars indicate analytical error of measurement for Ag + concentration. Dotted lines show predicted data  
Article
The study deals with an aqueous phase application of Mixed Matrix Membranes (MMMs) for silver ion (Ag(+)) capture. Silica particles were functionalized with 3-mercaptopropyltrimethoxy silane (MPTMS) to introduce free thiol (-SH) groups on the surface. The particles were used as the dispersed phase in the polysulfone or cellulose acetate polymer matrix. The membranes were prepared by the phase inversion method to create more open and interconnected porous structures suitable for liquid phase applications. The effects of the silica properties such as particle size, specific surface area, and porous/nonporous morphology on the silver ion capture capacity were studied. It was demonstrated that the membranes are capable of selectively capturing silver from a solution containing significant concentrations of other metal ions like Ca(2+). The membranes were studied to quantify the dynamic capacity for silver ion capture and its dependence on residence time through the adjustment of transmembrane pressure. The thiol-Ag(+) interaction was quantified with Quartz Crystal Microbalance in a continuous flow mode experiment and the observations were compared with the membrane results. One dimensional unsteady state model with overall volumetric mass transfer coefficient was developed and solved to predict the silver concentration in the liquid phase and the solid silica phase along the membrane thickness at varying time. The breakthrough data predicted using the model is comparable with the experimental observations. The study demonstrates successful application of the functionalized silica-mixed matrix membranes for selective aqueous phase Ag(+) capture with high capacity at low transmembrane pressures. The technique can be easily extended to other applications by altering the functionalized groups on the silica particles.
 
Article
This paper deals with bimetallic (Fe/Pd) nanoparticle synthesis inside the membrane pores and application for catalytic dechlorination of toxic organic compounds form aqueous streams. Membranes have been used as platforms for nanoparticle synthesis in order to reduce the agglomeration, encountered in solution phase synthesis which leads to a dramatic loss of reactivity. The membrane support, polyvinylidene fluoride (PVDF) was modified by in situ polymerization of acrylic acid in aqueous phase. Subsequent steps included ion exchange with Fe(2+), reduction to Fe(0) with sodium borohydride and Pd deposition. Various techniques, such as STEM, EDX, FTIR and permeability measurements, were used for membrane characterization and showed that bimetallic (Fe/Pd) nanoparticles with an average size of 20-30 nm have been incorporated inside of the PAA-coated membrane pores. The Fe/Pd-modified membranes showed a high reactivity toward a model compound, 2, 2'-dichlorobyphenyl and a strong dependence of degradation on Pd (hydrogenation catalyst) content. The use of convective flow substantially reduces the degradation time: 43% conversion of dichlorobiphenyl to biphenyl can be achieved in less than 40 s residence time. Another important aspect is the ability to regenerate and reuse the Fe/Pd bimetallic systems by washing with a solution of sodium borohydride, because the iron becomes inactivated (corroded) as the dechlorination reaction proceeds.
 
Article
Microporous membranes are attractive for protein purification because convection rapidly brings proteins to binding sites. However, the low binding capacity of such membranes limits their applications. This work reports a rapid, aqueous procedure to create highly permeable, polymer brush-modified membranes that bind large amounts of protein. The synthetic method includes a 10-min adsorption of a macroinitiator in a hydroxylated nylon membrane and a subsequent 5-min aqueous atom transfer radical polymerization of 2-(methacryloyloxy)ethyl succinate from the immobilized initiator to form poly(acid) brushes. This procedure likely leads to more swollen, less dense brushes than polymerization from silane initiators, and thus requires less polymer to achieve the same binding capacity. The hydraulic permeability of the poly(acid) membranes is 4-fold higher than that of similar membranes prepared by growing brushes from immobilized silane initiators. These brush-containing nylon membranes bind 120 mg/cm(3) of lysozyme using solution residence times as short as 35 ms, and when functionalized with nitrilotriacetate (NTA)-Ni(2+) complexes, they capture 85 mg/cm(3) of histidine(6)-tagged (His-tagged) Ubiquitin. Additionally the NTA-Ni(2+)-functionalized membranes isolate His-tagged myo-inositol-1-phosphate synthase directly from cell extracts and show >90% recovery of His-tagged proteins.
 
Article
Membranes containing reactive nanoparticles (Fe and Fe/Pd) immobilized in a polymer film (polyacrylic acid, PAA-coated polyvinylidene fluoride, PVDF membrane) are prepared by a new method. In the present work a biodegradable, non-toxic -"green" reducing agent, green tea extract was used for nanoparticle (NP) synthesis, instead of the well-known sodium borohydride. Green tea extract contains a number of polyphenols that can act as both chelating/reducing and capping agents for the nanoparticles. Therefore, the particles are protected from oxidation and aggregation, which increases their stability and longevity. The membrane supported NPs were successfully used for the degradation of a common and highly important pollutant, trichloroethylene (TCE). The rate of TCE degradation was found to increase linearly with the amount of Fe immobilized on the membrane, the surface normalized rate constant (k(SA)) being 0.005 L/m(2)h. The addition of a second catalytic metal, Pd, to form bimetallic Fe/Pd increased the k(SA) value to 0.008 L/m(2)h. For comparison purposes, Fe and Fe/Pd nanoparticles were synthesized in membranes using sodium borohydride as a reducing agent. Although the initial k(SA) values for this case (for Fe) are one order of magnitude higher than the tea extract synthesized NPs, the rapid oxidation reduced their reactivity to less than 20 % within 4 cycles. For the green tea extract NPs, the initial reactivity in the membrane domain was preserved even after 3 months of repeated use. The reactivity of TCE was verified with "real" water system.
 
Article
Many industrial and biomedical devices (e.g. blood oxygenators and artificial lungs) use bundles of hollow fiber membranes for separation processes. Analyses of flow and mass transport within the shell-side of the fiber bundles most often model the bundle for simplicity as a packed bed or porous media, using a Darcy permeability coefficient estimated from the Blake-Kozeny equation to account for viscous drag from the fibers. In this study, we developed a simple method for measuring the Darcy permeability of hollow fiber membrane bundles and evaluated how well the Blake-Kozeny (BK) equation predicted the Darcy permeability for these bundles. Fiber bundles were fabricated from commercially available Celgard® ×30-240 fiber fabric (300 μm outer diameter fibers @ 35 and 54 fibers/inch) and from a fiber fabric with 193 μm fibers (61 fibers/inch). The fiber bundles were mounted to the bottom of an acrylic tube and Darcy permeability was determined by measuring the elapsed time for a column of glycerol solution to flow through a fiber bundle. The ratio of the measured Darcy permeability to that predicted from the BK equation varied from 1.09 to 0.56. A comprehensive literature review suggested a modified BK equation with the "constant" correlated to porosity. This modification improved the predictions of the BK equation, with the ratio of measured to predicted permeability varying from 1.13 to 0.84.
 
Article
Polymer transport through nanopores is a potentially powerful tool for separation and organization of molecules in biotechnology applications. Our goal is to produce aligned collagen fibrils by mimicking cell-mediated collagen assembly: driving collagen monomers in solution through the aligned nanopores in track-etched membranes followed by fibrillogenesis at the pore exit. We examined type I atelo-collagen monomer transport in neutral, cold solution through polycarbonate track-etched membranes comprising 80-nm-diameter, 6-μm-long pores at 2% areal fraction. Source concentrations of 1.0, 2.8 and 7.0 mg/ml and pressure differentials of 0, 10 and 20 inH(2)O were used. Membrane surfaces were hydrophilized via covalent poly(ethylene-glycol) binding to limit solute-membrane interaction. Collagen transport through the nanopores was a non-intuitive process due to the complex behavior of this associating molecule in semi-dilute solution. Nonetheless, a modified open pore model provided reasonable predictions of transport parameters. Transport rates were concentration- and pressure-dependent, with diffusivities across the membrane in semi-dilute solution two-fold those in dilute solution, possibly via cooperative diffusion or polymer entrainment. The most significant enhancement of collagen transport was accomplished by membrane hydrophilization. The highest concentration transported (5.99±2.58 mg/ml) with the highest monomer flux (2.60±0.49 ×10(3) molecules s(-1) pore(-1)) was observed using 2.8 mg collagen/ml, 10 inH(2)O and hydrophilic membranes.
 
Article
Rejection characteristics of organic and inorganic compounds were examined for six reverse osmosis (RO) membranes and two nanofiltration (NF) membranes that are commercially available. A batch stirred-cell was employed to determine the membrane flux and the solute rejection for solutions at various concentrations and different pH conditions. The results show that for ionic solutes the degree of separation is influenced mainly by electrostatic exclusion, while for organic solutes the removal depends mainly upon the solute radius and molecular structure. In order to provide a better understanding of rejection mechanisms for the RO and NF membranes, the ratio of solute radius (r(i,s)) to effective membrane pore radius (r(p)) was employed to compare rejections. An empirical relation for the dependence of the rejection of organic compounds on the ratio r(i,s)/r(p) is presented. The rejection for organic compounds is over 75% when r(i,s)/r(p) is greater than 0.8. In addition, the rejection of organic compounds is examined using the extended Nernst-Planck equation coupled with a steric hindrance model. The transport of organic solutes is controlled mainly by diffusion for the compounds that have a high r(i,s)/r(p) ratio, while convection is dominant for compounds that have a small r(i,s)/r(p) ratio.
 
Article
Microdialysis is a well-developed membrane-based tool relying on diffusion to sample diffusible constituents of complex media, such as biological tissue. The objective of this research is to expand the utility of microdialysis by combining transmembrane convection with diffusion to enhance solute exchange between microdialysis probes and the surrounding medium. We have developed a mathematical model to describe probe performance and performed validation experiments utilizing tracer solutes and commercially available probes with 100-kDa molecular weight cutoff membranes. Diffusive and fluid permeabilities of the probe membranes are evaluated for probes immersed in well-stirred bathing media in vitro. Transmembrane convection alters the solute extraction fraction, i.e., the fractional loss of a solute from the probe perfusate during delivery and the fractional gain by the perfusate during sampling. The extraction fraction change depends upon the magnitude and direction (inward or outward) of fluid movement across the membrane. However, for solutes with zero reflection coefficients, equality is maintained between these delivery and sampling extraction fractions. This equality is a prerequisite for probe calibration approaches that rely on analyte delivery from the perfusate. Thus, we have provided the theoretical and experimental basis for exploiting convection in a quantitative manner to enhance solute delivery and sampling in microdialysis applications.
 
Article
The material properties of silk are favorable for drug delivery due to the ability to control material structure and morphology under ambient, aqueous processing conditions. Mass transport of compounds with varying physical-chemical characteristics was studied in silk fibroin films with control of β-sheet crystalline content. Two compounds, vitamin B12 and fluorescein isothiocynate (FITC) labeled lysozyme were studied in a diffusion apparatus to determine transport through silk films. The films exhibited size exclusion phenomenon with permeability coefficients with contrasting trends with increases in β-sheet crystallinity. The size exclusion phenomenon observed with the two model compounds was characterized by contrasting trends in permeability coefficients of the films as a function of β-sheet crystallinity. The diffusivity of the compounds was examined in the context of free volume theory. Apart from the β-sheet crystallinity, size of the compound and its interactions with silk influenced mass transfer. Diffusivity of vitamin B12 was modeled to define a power law relationship with β-sheet crystallinity. The results of the study demonstrate that diffusion of therapeutic agents though silk fibroin films can be directed to match a desired rate by modulating secondary structure of the silk proteins.
 
Article
Microporous membranes are widely utilized in cell biology to study cell-cell signaling and cell migration. However, the thickness and low porosity of commercial track-etched membranes limit the quality of cell imaging and the degree of cell-cell contact that can be achieved on such devices. We employ photolithography-based microfabrication to achieve porous membranes with pore diameter as small as 0.9 μm, up to 40% porosity, and less than 5% variation in pore size. Through the use of a soap release layer, membranes as thin as 1 μm can be achieved. The thin membranes minimally disrupt contrast enhancement optics, thus allowing good quality imaging of unlabeled cells under white light, unlike commercial membranes. In addition, the polymer membrane materials display low autofluorescence even after patterning, facilitating high quality fluorescence microscopy. Finally, confocal imaging suggests that substantial cell-cell contact is possible through the pores of these thin membranes. This membrane technology can enhance existing uses of porous membranes in cell biology as well as enable new types of experiments.
 
Article
Layer-by-layer polyelectrolyte adsorption in porous polymeric membranes provides a simple way to create ion-exchange sites without greatly decreasing hydraulic permeability (<20% reduction in permeability). At 80% breakthrough, membranes coated with 3-bilayer poly(styrene sulfonate) (PSS)/polyethyleneimine (PEI) films bind 37±6 mg of negatively charged Au colloids per mL of membrane volume. The binding capacity of membranes coated with 1-bilayer films decreases in the order PSS/PEI>PSS/poly(diallyldimethyl ammonium chloride)>PSS/poly(allylamine hydrochloride). Films terminated with a polyanion present cation-exchange sites that bind lysozyme, and the lysozyme-binding capacities of (PSS/PEI)(3)/PSS films increase with the ionic strength of the solution from which the last PSS layer is deposited. Charge screening during deposition of the terminal PSS layer gives rise to a larger number of ion-exchange sites and lysozyme binding capacities as high as 16 mg per mL of membrane. At 10% breakthrough, a stack of 3 membranes binds 3 times as much lysozyme as a single membrane, showing that stacking is an effective way to increase capacity.
 
Article
The aim of this study was to develop polyurethane (PU) based fibro-porous membranes and to investigate the size-effect of hierarchical porous structure on permeability and surface properties of the developed electrospun membranes. Non-woven Selectophore™ PU membranes having tailored fibre diameters, pore sizes, and thickness were spun using electrospinning, and their chemical, physical and glucose permeability properties were characterised. Solvents, solution concentration, applied voltage, flow rate and distance to collector, each were systematically investigated, and electrospinning conditions for tailoring fibre diameters were identified. Membranes having average fibre diameters - 347, 738 and 1102 nm were characterized, revealing average pore sizes of 800, 870 and 1060 nm and pore volumes of 44, 63 and 68% respectively. Hydrophobicity increased with increasing fibre diameter and porosity. Effective diffusion coefficients for glucose transport across the electrospun membranes varied as a function of thickness and porosity, indicating high flux rates for mass transport. Electrospun PU membranes having significantly high pore volumes, extensively interconnected porosity and tailorable properties compared to conventional solvent cast membranes can find applications as coatings for sensors requiring analyte exchange.
 
Article
Diffusion based separations are essential for laboratory and clinical dialysis processes. New molecularly thin nanoporous membranes may improve the rate and quality of separations achievable by these processes. In this work we have performed protein and small molecule separations with 15 nm thick porous nanocrystalline silicon (pnc-Si) membranes and compared the results to 1- and 3- dimensional models of diffusion through ultrathin membranes. The models predict the amount of resistance contributed by the membrane by using pore characteristics obtained by direct inspection of pnc-Si membranes in transmission electron micrographs. The theoretical results indicate that molecularly thin membranes are expected to enable higher resolution separations at times before equilibrium compared to thicker membranes with the same pore diameters and porosities. We also explored the impact of experimental parameters such as porosity, pore distribution, diffusion time, and chamber size on the sieving characteristics. Experimental results are found to be in good agreement with the theory, and ultrathin membranes are shown to impart little overall resistance to the diffusion of molecules smaller than the physical pore size cutoff. The largest molecules tested experience more hindrance than expected from simulations indicating that factors not incorporated in the models, such as molecule shape, electrostatic repulsion, and adsorption to pore walls, are likely important.
 
Article
Rotating cylindrical filtration displays significantly reduced plugging of filter pores and build-up of a cake layer, but the number and range of parameters that can be adjusted complicates the design of these devices. Twelve individual parameters were investigated experimentally by measuring the build-up of particles on the rotating cylindrical filter after a fixed time of operation. The build-up of particles on the filter depends on the rotational speed, the radial filtrate flow, the particle size and the gap width. Other parameters, such as suspension concentration and total flow rate are less important. Of the four mechanisms present in rotating filters to reduce pore plugging and cake build-up, axial shear, rotational shear, centrifugal sedimentation and vortical motion, the evidence suggests rotational shear is the dominant mechanism, although the other mechanisms still play minor roles. The ratio of the shear force acting parallel to the filter surface on a particle to the Stokes drag acting normal to the filter surface on the particle due to the difference between particle motion and filtrate flow can be used as a non-dimensional parameter that predicts the degree of particle build-up on the filter surface for a wide variety of filtration conditions.
 
Article
The ability to fabricate flexible filtration membranes that can selectively separate particles of different sizes is of considerable interest. In this article, we describe a facile, reproducible and simple one-step method to produce pores in polydimethylsiloxane (PDMS) membranes. We embedded micron-sized NaHCO(3) particles in 50 micron thick PDMS films. After curing, the membranes were immersed in concentrated HCl acid. Pores were generated in the membrane by the evolution of CO(2) gas from the reaction of NaHCO(3) and HCl. High resolution Scanning Electron Microscope images clearly reveal the presence of openings on the surface and the cross-section of the membranes. Fluorescence and back-scattered electron imaging of porous PDMS membrane with embedded gold nanoparticles and comparison with non-porous PDMS membranes provided unambiguous evidence of pores in the membrane. Transport studies of molecular fluoresceinate ions, ions (sodium and chloride) and 240 nm polystyrene nanoparticles through these membranes demonstrate passable pores and existence of channels within the body of the membrane. Mechanically stretching the porous PDMS membrane and comparing the flow rates of fluoresceinate ions and the polystyrene beads through the stretched and unstretched membranes allowed a direct proof of the modulation of transport rate in the membranes. We show that stretching the membranes by 10% increases the flow rate of fluorescein molecules by 2.8 times and by a factor of approximately ~40% for the polystyrene nanoparticles.
 
Article
The effects of pore size on the performance of ultrafiltration membranes are fairly well understood, but there is currently no information on the impact of pore geometry on the trade-off between the selectivity and permeability for membranes with pore size below 100 nm. Experimental data are presented for both commercial ultrafiltration membranes and for novel silicon membranes having slit-shaped nanopores of uniform size fabricated by photolithography using a sacrificial oxide technique. Data are compared with theoretical calculations based on available hydrodynamic models for solute and solvent transport through membranes composed of a parallel array of either cylindrical or slit-shaped pores. The results clearly demonstrate that membranes with slit-shaped pores have higher performance, i.e., greater selectivity at a given value of the permeability, than membranes with cylindrical pores. Theoretical calculations indicate that this improved performance becomes much less pronounced as the breadth of the pore size distribution increases. These results provide new insights into the effects of pore geometry on the performance of ultrafiltration membranes.
 
Article
Silicon micromachining provides the precise control of nanoscale features that can be fundamentally enabling for miniaturized, implantable medical devices. Concerns have been raised regarding blood biocompatibility of silicon-based materials and their application to hemodialysis and hemofiltration. A high-performance ultrathin hemofiltration membrane with monodisperse slit-shaped pores was fabricated using a sacrificial oxide technique and then surface-modified with poly(ethylene glycol) (PEG). Fluid and macromolecular transport matched model predictions well. Protein adsorption, fouling, and thrombosis were significantly inhibited by the PEG. The membrane retained hydraulic permeability and molecular selectivity during a 90 hour hemofiltration experiment with anticoagulated bovine whole blood. This is the first report of successful prolonged hemofiltration with a silicon nanopore membrane. The results demonstrate feasibility of renal replacement devices based on these membranes and materials.
 
Article
A dehumidification system for low gravity plant growth experiments requires the generation of no free-liquid condensate and the recovery of water for reuse. In the systems discussed in this paper, the membrane is a barrier between the humid air phase and a liquid-coolant water phase. The coolant water temperature combined with a transmembrane pressure differential establishes a water flux from the humid air into the coolant water. Building on the work of others, we directly compared different hydrophilic membranes for humidity control. In a direct comparison of the hydrophilic membranes, hollow fiber cellulose ester membranes were superior to metal and ceramic membranes in the categories of condensation flux per surface area, ease of start-up, and stability. However, cellulose ester membranes were inferior to metal membranes in one significant category, durability. Dehumidification systems using mixed cellulose ester membranes failed after operational times of only hours to days. We propose that the ratio of fluid surface area to membrane material area (approximately = membrane porosity) controls the relative performances among membranes. In addition, we clarified design equations for operational parameters such as the transmembrane pressure differential. This technology has several potential benefits related to earth environmental issues including the minimization of airborne pathogen release and higher energy efficiency in air conditioning equipment. Utilizing these study results, we designed, constructed, and flew on the space shuttle missions a membrane-based dehumidification system for a plant growth chamber.
 
Article
Traditionally, the pervaporation of water-solvent mixtures where the solvent is the major component is performed using hydrophilic membranes (such as PVA or zeolites). In the present paper a new type of pervaporation membrane (amorphous perfluorinated polymer, hydrophobic) was studied for separation of water-solvent mixtures. This membrane has high free volume and is inert for all solvents, and has a remarkable mechanical, chemical and thermal stability. The water is transported by solution diffusion model and the separation of solvent is primarily based on molecular sieving (size exclusion) principles. The membrane shows a high stability for operation over a broad range of feed concentrations without swelling; the operating temperature does not have a significant effect on membrane separation performance. Separation factors as high as 349 and 500 for water-ethanol and water-IPA mixtures (2-98 % wt water-solvent) and fluxes of 0.15 and 0.05 kg/m(2)h, respectively were obtained at 22 °C. The permeance-based selectivities were also calculated, and the selectivity is approximately constant for a wide range of feed concentrations. The pervaporation of more complex (ternary) mixtures of water-ethanol-ethyl acetate showed that this system could be successfully applied for solute separation based on size exclusion.
 
Article
The dense dual phase composite membrane made from strontium-stabilized bismuth oxide and silver, (Bi2O3)0.74(SrO)0.26–Ag (40% v/o), was investigated. The composite was found to exhibit very high electrical conductivity at the room temperature, revealing that the silver phase has formed electron-conducting networks in the oxide matrix. The composite shows much improved oxygen permeability compared with the bismuth oxide alone. An oxygen flux of 5×10−8 mol cm−2 s−1 was observed for a 1.00 mm thick composite at 700°C with oxygen partial pressures of the feed and permeate side at 0.209, 0.0024 atm, respectively. Combination of electrical conductivity and oxygen permeation measurements reveals that oxygen-ion conduction through the oxide phase of the composite is the rate-limiting step for oxygen permeation.
 
Article
A new supported liquid membrane (SLM) system for the selective transport of silver ion is introduced. The SLM used is a thin porous membrane impregnated with a recently synthesized mixed aza-thioether crown containing a 1,10-phenanthroline sub-unit dissolved in nitrophenyl octyl ether. In the presence of thiosulfate as a metal ion acceptor in the strip solution, the transport of silver occurs almost quantitatively after 3 h. The selectivity and efficiency of silver transport from aqueous solution containing excess amounts of Mg2+, Ca2+, Sr2+, Ba2+ Co2+, Ni2+, Cu2+ Zn2+, Pb2+, Cd2+ and Hg2+ ions were investigated.
 
Article
A flow-through membrane bioreactor was developed for the enantioselective catalytic hydrolysis of racemic 1,2-epoxyoctane (epoxide) to (S)-1,2-epoxyoctane and (R)-1,2-octanediol by an enzyme from the yeast Rhodosporidium toruloides. The effect of the low solubility on access of the epoxide to the yeast cells was improved by adding 20% ethanol (EtOH) to the aqueous buffer in the feed. Two process parameters were investigated to improve selectivity. The separation factor of the racemic epoxide (αepoxide) was increased from 2.3 to 6.3 by breaking the yeast cells. While the influence of the amount of yeast cells on selectivity was negligible, decreasing the flow rate from 0.4 to 0.2 ml min−1 resulted in an increase of αepoxide from 4.4 to 17.7.
 
Article
Crosslinked oligosilylstyrene–polydimethylsiloxane composite membranes were used to separate trace 1,2-dichloroethane (1,2-DCE) from the mixtures of 1,2-DCE and water by pervaporation. The performances of pervaporation were evaluated by low feed flow rates (0.35–1.0 l/min), corresponding to Reynolds number from 79 to 220. The resistance-in-series model and semi-empirical Sherwood correlations were employed to study the transfer characteristics. For the given hydrodynamic conditions, both mass transfer resistances of the boundary layer and the membrane were found to be important to the overall mass transfer. The transfer resistance of 1,2-DCE through the boundary layer and the membrane decreased as the decrease of thickness of boundary layer. The permeation rate and the separation factor of 1,2-dichloroethane increased, but the permeation rate of water kept unchanged as the feed flow rate increased. The theoretical data from resistance-in-series model were compared with experimental data. The good correlation between the theoretical and experimental data can work as the predication of pervaporation performance and the management of pervaporation process.
 
Article
The facilitated transport of americium(III) (Am(III)) through a supported liquid membrane (SLM) impregnated with dimethyldibutyltetradecyl-1,3-malonamide (DMDBTDMA) in n-dodecane has been studied. Microporous PTFE membrane was used as the polymeric support. 241Am tracer in nitrate medium constituted the feed phase and 0.01 M nitric acid was used as the strip phase. A model describing the transport mechanism which considers all possible transport resistances has been proposed. An attempt has been made to explain the experimental data quantitatively with the help of wellknown diffusion rate equations. The chemical conditions and diffusional parameters affecting transport have been investigated. Under identical conditions, permeability of Fe(III) is found to be significantly lower than that of Am(III). Durability/chemical stability of the membranes has also been studied.
 
Article
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.
 
Article
1,3-1H-Dibenzimidazole-benzene (DBImBenzene) has been synthesized using phosphorus pentoxide-methanesulfonic acid (PPMA) as a solvent and dehydration agent and investigated as an additive (up to 2.0 wt.%) in sulfonated polysulfone (SPSf) membranes to promote proton conduction via acid–base interactions. The SPSf/DBImBenzene blend membranes with various DBImBenzene contents (0–2.0 wt.%) have been prepared and characterized by proton conductivity measurement and electrochemical polarization and methanol crossover measurements in direct methanol fuel cells (DMFCs). The blend membranes with DBImBenzene content of 0.5 and 1.0 wt.% show higher proton conductivities (3.4 and 2.9 × 10−4 S/cm, respectively) than plain SPSf (2.4 × 10−4 S/cm) even though the blend membranes have lower ion exchange capacity (0.81 and 0.75 mequiv./g, respectively) than plain SPSf (0.86 mequiv./g). The blend membranes exhibit better electrochemical performance in DMFC than plain SPSf membrane due to an enhancement in proton conductivity through acid–base interactions and lower methanol crossover.
 
Article
Diffusion of two small penetrant molecules, O2 and N2, in the bulk amorphous polyimide of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 2,2-dimethyl-1,3-(4-aminophenoxy) propane is studied with molecular dynamics simulations. This polyimide is abbreviated as PI-2. The temperature of the simulation is chosen to be slightly below the experimental glass transition temperature of PI-2 (230°C, or 503 K). The ratio of the diffusion coefficients of O2 and N2 in PI-2 from the simulation compares favorably with experimental results of the same ratio in structurally similar glassy polyimides. Detailed analysis is performed for the diffusion of O2 in PI-2. Throughout the molecular dynamics trajectory, O2 for most of the time is trapped in certain locations (voids) of the polymer matrix. The residence time is on the order of 100 ps. Occasionally, an O2 goes into a fast motion and translates rapidly to a neighboring void. Some voids are visited by different O2 at different times, indicating that although the thermal fluctuations of the polymer matrix play an important part in the diffusion process, conformational relaxation of the polymer chains may not be important for estimating the diffusion coefficient. The chemical and geometric environment of O2 is studied via the pair correlation functions of the diffusant molecules and the atoms in the polymer. For some of the atoms in PI-2, the peaks in the pair correlation functions do not correspond exactly to the sum of the van der Waals radii of the atom in the diffusant molecule and the atom in the polymer chain. This finding indicates that the local packing of the amorphous polymer tends to shield some of the atoms from close contact with the diffusant molecules in the diffusion process. The atomic grouping in PI-2 that have the greatest exposure to O2 are identified.
 
Article
We have determined the effect of temperature on intrinsic permeation properties of 6FDA-Durene/1,3-phenylenediamine (mPDA) 50/50 copolyimide dense film and fabricated high performance hollow fiber membranes of the copolyimide for CO2/CH4 separation. The hollow fiber membranes were wet-spun from a tertiary solution containing 6FDA-Durene/mPDA (PI), N-methyl-pyrrolidone (NMP) and tetrahydrofuran (THF) with a weight ratio of 20:50:30 at different shear rates within the spinneret. We observed the following facts: (1) the CO2/CH4 selectivity of the copolyimide dense film decreased significantly with an increase in temperature; (2) the performance of as-spun fibers was obviously influenced by the shear rate during spinning. For uncoated fibers, permeances of CH4 and CO2 decreased with increasing shear rate, while selectivity of CO2/CH4 sharply increased with shear rate until the shear rate reached 2169 s−1 and then the selectivity leveled off; (3) After silicone rubber coating, permeances of CH4 and CO2 decreased, the selectivity of CO2/CH4 was recovered to the inherent selectivity of its dense film. Both the permeances and selectivity with increasing shear rate followed their same trends as that before the coating; (4) there was an optimal shear rate at which a defect-free fiber with a selectivity of CO2/CH4 at 42.9 and permeance of CO2 at 53.3 GPU could be obtained after the coating; and (5) the pressure durability of the resultant hollow fiber membranes could reach 1000 psia at room temperature.
 
Article
Boric acid is selectively transported by 1,3-diols from a source aqueous phase into a receiving alkaline aqueous phase through a phase of organic solvent (o-dichlorobenzene). Measurements of the transport rates were made using a U-tube apparatus. The process was adapted to liquid membranes supported on polypropylene films. The factors which influence the stability of the supported liquid membrane are discussed. Assuming that the transport of the neutral 1:1 diol-boric acid complex within the organic phase is diffusion-limited, a model is proposed that realistically represents the fluxes of boric acid as a function of the initial concentrations of boric acid and diol. The experimental diffusion coefficients were determined and agreed satisfactorily with the calculated values.
 
Article
We have identified that 1,3-cyclohexanebis(methylamine) (CHBA) can be effectively used as a new cross-linking agent for the chemical modification of polyimide membranes. We have also observed that the combined effects of diamino cross-linking and thermal annealing would significantly change the chemical compositions, micro-structure, gas sorption, gas transport properties and plasticization resistance of polyimide membranes. The membrane's physicochemical changes after diamino cross-linking and thermal annealing were characterized by FTIR-ATR, XPS, gel content, UV, SEM-EDX and TGA and the possible reaction mechanisms during chemical modification and annealing have been proposed. Interestingly, it is found that the chemical reactions between diamino and polyimides are reversible during thermal annealing and the membrane micro-structures are modified and charge transfer complexes (CTCs) are formed during the post-treatments. The gas sorption and gas transport properties of membranes before and after modifications are reported and discussed. The gas sorption concentration continuously decreases with an increase in the degree of cross-linking and the subsequent thermal annealing. Experimental results illustrate that thermal annealing not only improves CO2/CH4 selectivity of the cross-linked membranes but also greatly enhances the plasticization resistance by the formation of CTCs. The critical plasticization pressure is significantly improved from about 300 psia of the original samples to more than 720 psia of the cross-linked and 200 °C thermal treated samples. Based on the experimental results, a novel approach to enhance plasticization resistance of polyimide membranes by means of diamino cross-linking and followed by thermal annealing is elucidated.
 
Article
Single and mixed gas permeation experiments for 1,3-butadiene and 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, , of 4–200 Barrer and very large ideal separation factors (permeability ratios of 1,3-butadiene over for single gas permeation), , of 30–200 at 1 atm. The large 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 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 as compared with due to the strong plasticization effect of 1,3-butadiene. However, the polyimides had still high performance.
 
Article
A series of random copolyesters was prepared by replacing up to 10 wt.% of the dimethyl terephthalate (DMT) in poly(ethylene terephthalate) (PET) with dimethyl 2,6-naphthalene dicarboxylate (NDC), isophthalic acid (IPA), or 2,5-bis-(4-carboxyphenyl)-1,3,4-oxadiazole (ODCA). Solution cast films of the resulting copolymers were prepared and characterized. Modification of PET with NDC and ODCA led to copolymers with glass transition temperatures higher than that of PET, while modification with IPA decreased the glass transition temperature. Copolymerization decreased crystallinity levels in all cases. The acetone solubility and acetone diffusion coefficient were determined by integral kinetic gravimetric sorption experiments at 35°C and 5.4 cm Hg acetone pressure. PET containing low levels of NDC had lower amorphous phase acetone diffusivity and solubility than PET, while PET modified with IPA had amorphous phase acetone diffusivity and acetone solubility similar to that of PET. PET modified with 5% ODCA had amorphous phase acetone diffusivity similar to that of PET, while PET modified with 10% ODCA had an amorphous phase acetone diffusivity value slightly lower than that of PET. Copolymers containing ODCA had somewhat higher acetone solubilities that PET, due mainly to the lower levels of crystallinity in the ODCA-containing polymers than in PET.
 
Article
Fluorene-containing polymers were synthesized followed by sulfonation into a series of fluorene-containing sulfonated poly(arylene ether oxadiazole)s (SPAEO) using chlorosulfonic acid. The sulfonic acid groups can be controlled to be only attached onto the fluorenyl rings by optimizing the reaction conditions. Their properties of ionic exchange capacity, sulfonation degree, water-uptake, mechanical property, thermal and oxidative stabilities as well as proton conductivity of the synthesized SPAEO membranes were fully investigated. It was found that the synthesized SPAEO with the sulfonic acid groups on fluorenyl rings exhibited superior oxidative stability and highly proton conductivity when compared with other reported sulfonated poly(arylene ether)s. Therefore, they showed potential alternatives as the proton-exchange membrane for fuel cell application.
 
Article
The mechanical strength of polymeric membranes is one of the limitations in their applications. Carbon nanotubes (CNTs) are very effective in reinforcing polymeric materials, but it is unknown whether they degrade the membranes’ gas separation performance. Using brominated poly(2,6-diphenyl-1,4-phenylene oxide) (BPPOdp) as an example, we show that pristine single-wall CNTs (SWNTs) and multi-wall CNTs (MWNTs) formed polymeric nanocomposite membranes with BPPOdp. The composite membranes had an increased CO2 permeability but a similar CO2/N2 selectivity compared to the corresponding pure-polymer membrane. The CO2 permeability increased with increasing the CNT content and reached a maximum of 155 Barrer at 9 wt% of SWNTs, or 148 Barrer at 5 wt% of MWNTs. The CO2/N2 separation performance was insensitive to the MWNT diameter or length. Carboxylic acid-functionalized SWNTs (COOH-SWNTs) dispersed more uniformly in BPPOdp, and neither increased the gas permeability nor deteriorated the gas separation performance. Thus, it is feasible to add CNTs to polymeric membranes for improved mechanical strength without deteriorating the gas separation performance of the membranes. The pristine CNT-enhanced gas permeability was attributed to the formed nanogaps surrounding the CNTs.
 
Article
A novel route for preparing anion exchange membranes from a linear engineering plastics polymer, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), is presented in this paper. This method employed Friedel–Crafts chloroacetylation to avoid the use of the carcinogenic chemicals: chloromethylmethylether (CME) or bis-chloromethylether (BCME). They both are broadly used in the conventional chloromethylation process. Furthermore, the method presented in the paper extended the polymers for anion exchange membrane preparation from those containing benzyl groups, such as PPO, to general aromatic polymers. The reaction conditions in chloroacetylation processes such as the quantity of the added catalyst and the reaction temperature, and the amination processes such as amine concentration, amination temperature and amination time were fully investigated. The products of Friedel–Crafts reaction were identified by both FTIR and 1H NMR. The final anion exchange membranes were characterized by ion exchange capacity, water content and membrane area resistance. It was founded that the medium degree of substitution, e.g. 50%, provided a flexible, mechanically useable membrane. The optimum reaction conditions were found to be: amination time = 48–50 h, amination concentration = 0.91 mol/l and amination temperature = 35–45 °C. The intrinsic properties of the membranes under these conditions were ion exchange capacity [IEC] = 1.15 mequiv./g dried membrane, water content = 0.4–0.6 g/g wet membrane and area resistance = 0.2 Ω cm2.
 
Article
A new series of positively charged asymmetrical membranes were prepared by in situ amination and phase inversion in which the dimethylformamide (DMF) solution of trimethylamine (TMA) and brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) was cast and immerged into an ethanol coagulation bath. The membranes were characterized in terms of pure water flux, rejection, streaming potential and SEM. The effect of the amount of amine and evaporation time on the membrane performance and structure were investigated, respectively. The separation properties of the membrane were exemplified with gelatin solution of different pH values. The results showed that with an increase in amount of amine, the flux of pure water enhanced. But the rejection to gelatin depended on both the electrostatic interaction and membrane microstructure. For the membrane at the same ratio of BPPO to TMA, the rejection decreased with an increase in pH value due to electrostatic effect; while for the membrane of different ratio of BPPO to TMA, the rejection increased to some extent at all the considered pH values with an increase in ratio of BPPO to TMA due to sieve effect. In addition, shorter evaporation time was beneficial to the flux and negative to the rejection.
 
Article
Poly(vinyl alcohol)-based mixed matrix membranes loaded with 5 and 10 wt.% of sodium montmorillonite (Na+MMT) clay particles, i.e. (PVA/Na+MMT-5 and PVA/Na+MMT-10), were fabricated by solvent casting method. The uncrosslinked membranes were used in pervaporation (PV) dehydration of aqueous solutions of isopropanol and 1,4-dioxane at 30 °C. Membrane morphology was characterized by scanning electron microscopy. Differential scanning calorimetry, thermogravimetric analysis and dynamic mechanical thermal analysis were used to understand thermal properties. Universal testing machine was used to study mechanical strength properties. Separation factor to water has increased from 1116 to 2241 for PVA/Na+MMT-5 and PVA/Na+MMT-10 mixed matrix membranes when tested for 10 wt.% water-containing isopropanol feed, but the corresponding flux values of 0.051 and 0.075 kg/m2 h were somewhat lower than those observed for pristine PVA membrane. On the other hand, for water + 1,4-dioxane feed mixtures, separation factors were quite lower and ranged from 216 to 369 with the corresponding flux values of 0.076 and 0.093 kg/m2 h, respectively, for PVA/Na+MMT-5 and PVA/Na+MMT-10 membranes at 10 wt.% water in the feed. Membranes of this study could extract up to 99.20 and 99.60 wt.% water on the permeate side from water–isopropanol mixture, while from water–1,4-dioxane mixture, only 96.0 and 97.62 wt.% of water were removed for 10 wt.% water-containing feed. The membranes were further tested for PV at 40, 50 and 60 °C for 10 wt.% of water-containing feeds of aqueous isopropanol and 1,4-dioxane solutions to confirm their stability at higher temperatures. Plots of ln Jp versus 1/T are linear in the studied range of 30–60 °C for both the feed mixtures, indicating that flux follows an Arrhenius trend. PV results are discussed in terms of water flux, separation factor, permeation separation index, enrichment factor and activation energy of permeation. Sorption and PV experiments were carried out for the thermally crosslinked and glutaraldehyde crosslinked PVA/Na+MMT mixed matrix membranes for 10 wt.% water-containing feed mixtures at 30 °C. A small increase in separation factor with a slight sacrifice in flux was observed without showing any great difference as compared to uncrosslinked mixed matrix membranes. The hydrophilic nature of Na+MMT clay and the formation of PVA/Na+MMT mixed matrix membranes are responsible to offer such increased separation to water over the organic components of the feed mixtures.
 
Article
Self-diffusion and solubility coefficients of four gas molecules (O2, N2, CH4, and CO2) in poly(2,6-dimethyl-1,4-phenylene oxide) have been investigated by means of molecular simulation using the COMPASS molecular mechanics force field. Diffusion coefficients were obtained from molecular dynamics (NVT ensemble) using up to 2 ns simulation times. Solubility coefficients were obtained by means of the Widom particle insertion method. Simulation values for O2, N2, and CH4 agree reasonably well with published data. Agreement was less satisfactory for CO2. Possible explanations for the CO2 results are discussed on the basis of the partial immobilization model and considerations of simulation time and the size of the simulation box.
 
Article
Independent solubility and permeability data, measured at 35°C at up to 26 atm, are reported to show the influence of aryl-bromination on the transport of CO2, CH4, and N2 in 2,6-dimethyl-1,4-poly(phenylene oxide) (PPO). The permeability of PPO was found to vary with the extent of bromination, and the magnitude of change depends on the nature of the gas. The apparent solubility coefficients of all three gases at 20 atm in the polymer increased with the extent of bromination, and the percentage of increase was higher for the gas with lower condensability. The concentration-averaged diffusivities of CO2 and CH4 also showed some variation with the extent of bromination. In particular, there was a notable increase in the diffusivity of CO2 but a slight decrease in that of CH4 when the extent of bromination was increased to 91%. The gas-transport data were also analyzed according to the dual-mode model. The dual-mode parameters exhibit similar dependence on the extent of bromination as the apparent solubility coefficient and concentration-averaged diffusivity do. These observations are interpreted in terms of changes in the average packing, torsional mobility of the chain segments, and cohesive energy density of the polymer.
 
Article
Anion exchange membranes with triethylbenzylammonium group were prepared and their relative permselectivity of monovalent anions to chloride ion was examined. The membranes were prepared from linear engineering plastics poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by conducting the processes of benzyl bromination and then quaternary amination with triethylammonium (TEA) in alcohol solution of different compositions. The ion exchange capacity, water content, membrane area resistance and permselectivity to monovalent anions were studied and compared with the membranes with trimethylbenzylammonium group that have been serially developed in our lab recently. The results show that the membrane intrinsic properties in some manner vary with the concentration of TEA in ethanol: the IEC firstly increases with a decrease in TEA concentration and then decreases; At a TEA–ethanol ratio of about 1:1, IEC attains the maximum plateau value about 0.30 mmol/g dry membrane. The water content and membrane area resistance are changed accordingly. Compared with the membrane with trimethybenzyllammonium group, the prepared membranes have extremely low water content and IEC, but high area resistance. The permselectivity to monovalent anions are largely dependent on the membrane hydrophobicity and ionic hydrated radii. The hydrated anion has a much lower permselectivity value referenced to chloride in membranes with triethylbenzylammonium than that with trimethylbenzylammonium and vice versa for hydrophobic anions. All of these are understood by a compromise effect of ionic affinity to the membrane and its size in solution.
 
Article
A series of PPO based organic–inorganic hybrid membranes with both strong and weak base groups were prepared though sol–gel process of 3-aminopropyl-trimethoxysilane (A1110) and polymer precursors PPO–Si(OCH3)3(+), which were obtained by reacting bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) with A1110 and trimethylamine (TMA). The chemical structure of the step reaction products was confirmed by FT-IR measurements. The final hybrid membranes were characterized by TGA, IEC, SEM, water flux, water uptake, rejection rate as well as streaming potential. Results showed that the prepared hybrid membranes possessed higher thermal stability than usual polymeric charged membranes, anion exchange capacity range (2.44–2.95 mmol/g dry membrane), low water flux (5.09–12.26 L/(m2 bar h)), positively streaming potential even at high pH value, and decreasing water uptake and rejection rate towards gelatin when pH value increases.
 
Article
Poly(1,4-phenylene sulfide) was sulfonated with chlorosulfonic acid in 1,2-dichloroethane. The product (IEC = 2.38 mequiv./g) was ground and sieved (mesh size 63 μm) to obtain small particles. The particles and linear polyethylene were mixed in various ratios and the resulting blends were press-molded at 150 °C to obtain the membranes. Membranes containing up to 66 wt.% of sulfonated particles could be prepared without any problem in mechanical strength. The membranes were characterized by their stability in oxidative environment, ionic conductivity, and diffusive permeability to methanol. The membrane containing 66 wt.% of sulfonated particles was almost as conductive as Nafion 117; it exhibited, however, much lower diffusive permeability to methanol. In a strongly oxidative environment (3% aqueous H2O2 at 70 °C), the prepared membranes were less stable than Nafion 117, but much more stable than membranes with sulfonated poly(styrene-co-divinylbenzene) particles. In preliminary laboratory tests with H2/O2 and direct methanol fuel cells, the prepared membranes with high concentrations of sulfonated particles performed similarly to Nafion 117.
 
Article
Pervaporation (PV) separation of water + isopropanol and water + 1,4-dioxane mixtures has been attempted using the blend membranes of poly(vinyl alcohol) (PVA) with 5 wt.% of poly(methyl methacrylate) (PMMA). These results have been compared with the plain PVA membrane. Both plain PVA and PVA/PMMA blend membranes have been crosslinked with glutaraldehyde in an acidic medium. The membranes were characterized by differential scanning calorimetry and universal testing machine. Pervaporation separation experiments have been performed at 30 °C for 10, 15, 20, 30 and 40 wt.% of feed water mixtures containing isopropanol as well as 1,4-dioxane. PVA/PMMA blend membrane has shown a selectivity of 400 for 10 wt.% of water in water + isopropanol feed, while for water + 1,4-dioxane feed mixture, membrane selectivity to water was 104 at 30 °C. For both the feed mixtures, selectivity for the blend membrane was higher than that observed for plain PVA membrane, but flux of the blend membrane was lower than that observed for the plain PVA membrane. Membranes of this study are able to remove as much as 98 wt.% of water from the feed mixtures of water + isopropanol, while 92 wt.% of water was removed from water + 1,4-dioxane feed mixtures at 30 °C. Flux of water increased for both the feed mixtures, while the selectivity decreased at higher feed water concentrations. The same trends were observed at 40 and 50 °C for 10, 15 and 20 wt.% of water mixtures containing isopropanol as well as 1,4-dioxane feed mixtures, which also covered their azeotropic composition ranges. Membrane performance was studied by calculating flux (Jp), selectivity (α), pervaporation separation index (PSI) and enrichment factor (β). Permeation flux followed the Arrhenius trend over the range of temperatures investigated. It was found that by introducing a hydrophobic PMMA polymer into a hydrophilic PVA, the selectivity increased dramatically, while flux decreased compared to plain PVA, due to a loss in PVA chain relaxation.
 
Article
A further investigation on the effect of solvent-system composition on microscopic structure and reverse-osmosis performance of sulphonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO)-polyethersulphone (PES) composite membranes is presented. Charged composite membranes are prepared by coating PES substrate ultrafiltration membranes with dilute solutions of hydrogen-form SPPO (SPPOH) having an ion-exchange capacity of 1.93 meq g−1. Methanol and methanol-chloroform mixtures containing 18, 42 and 66 mass% chloroform are used as solvents for making 1.0 mass% SPPOH coating solutions. Reverse-osmosis performance of the composite membranes is investigated by measuring the membrane permeation rates and rejection for various electrolyte solutions. The effect of the solvent, used in making the coating solution, is also studied through intrinsic-viscosity measurements. The microscopic structure of the SPPOH-PES composite membranes is explored by employing a scanning electron microscope (SEM) and an atomic force microscope operated in the tapping mode (TM AFM). The reverse-osmosis performance is explained in terms of the observed TM AFM skin-layer topographs, which are in turn correlated with the intrinsic-viscosity measurements.
 
Article
In this paper, the water splitting properties of bipolar membranes with different types of functional groups are discussed. To do this, the anion-exchange layers with different ammonium groups were prepared in advance which can be controlled by reacting the bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) base membrane with different amines such as methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), triethylamine (TEA), tri-n-propylamine (TPA), tri-n-butylamine (TBA) and dimethyethanolamine (DMEA) to form the corresponding secondary, tertiary and different quaternary ammonium groups. And then the corresponding bipolar membranes were prepared by casting the sulphonated PPO solution on these anion-exchange layers. The fundamental characteristics of bipolar membranes, current–voltage curves, are fully discussed based on the functional group type of anion-exchange layers. The results demonstrate that the bipolar membrane with strongly basic groups has larger water dissociation ability than those with weakly basic groups. For membranes with weakly basic groups (secondary or tertiary ammonium groups) or strongly basic groups (quaternary ammonium groups), water dissociation mainly depend on the pKb value of amines to form the anion-exchange layers if the mono-polar ion-exchange layers processes the close intrinsic properties such as water uptake and conductivity: voltage at a given current density roughly decreases with an increase in pKb values.
 
Top-cited authors
A.G. Fane
  • UNSW Sydney
Tai-Shung Chung
  • National University of Singapore
Menachem Elimelech
  • Yale University
Matthias Wessling
  • RWTH Aachen University
William Koros
  • Georgia Institute of Technology