RESEARCH TRENDS
Research Trends Morphology and performance of hollow-fiber membrane for UF The objective of this study was to determine the effects of dope extrusion rate (DER) on polyethersulfone hollow-fiber ultrafiltration (UF) membrane performance and morphology. The hollow-fiber UF membranes were produced by using a simple dry/wet spinning process with forced convection in the dry air gap. A newly developed dope solution, consisting of 20.6 wt% polyethersulfone (PES), 63.6 wt% 1-methyl-2-pyrrolidone (NMP), 9.3 wt% polyethylene glycol (PEG) and a mixture of potassium acetate and water (20/80 wt%) was used as a bore fluid in this study. This dope solution was designed to be very close to its cloud point (binodal line) in order to speed up the coagulation process so that the relaxation effect on molecular orientation can be reduced. The dope extrusion rate was varied from 2.0 to 4.0 cm3/min, with 0.5 cm3/ min increments, in order to study the effect of DER on fiber performance and morphology. The experimental results showed that the flux of the hollow-fiber UF membranes decreases while the separation performance for particular solute increases with an increase in dope extrusion rate. This suggests that the outer skin layer of the membranes becomes apparently thicker and denser with increasing dope extrusion rate. Once the separation performance reaches maximum (critical point), the rejection decreases with increasing dope extrusion rate, observed possibly because of the formation of a less tight outer-skin structure at the high dope extrusion rate. Plane-polarized Fourier transform infrared spectroscopy revealed that membrane spun at a dope extrusion rate of 3.5 cm3/min showed the largest spectrum difference, suggesting that higher molecular orientation is responsible for an enhanced separation performance. A.F. Ismail, M.I. Mustaffar, R.M. Illias and M.S. Abdullah: Separation and Purification Technology 49(1) 10–19 (1 April 2006). DOI: 10.1016/j.seppur.2005.08.001
Sodium chloride rejection by UF ceramic membrane The rejection of salt (NaCl) by a ceramic (titanium dioxide-alumina layered) ultrafiltration (UF) membrane, having a nominal pore
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size of 5 nm and operated in the cross-flow mode, was investigated in this research study. Measurements were undertaken using different concentrations of salt over a range of pH at relatively low trans-membrane pressure (TMP). The rejection was assessed by determining the concentrations of sodium and chloride ions in the permeate and retentate, and comparing them with the feed concentration. Rejections in the range 15–40% were found, and at the same pH lower rejections are found at higher salt concentrations. Increased concentration reduces the rejection by up to 15%. Overall, the rejection is seen to be mainly a function of pH which causes the rejection to vary by up to 50%. The observed rejection characteristics were compared with both filtration potential and ζ-potential data over the same range of pH and salt concentration. The filtration potential data were obtained in situ, while the ζ-potentials were determined from electrophoretic measurements of ground membrane material. The membranes are positively charged at low pH and negatively charged at high pH – the isoelectric points being pH 3.1 and 3.8 from electrophoretic and filtration potential measurements, respectively (the difference being attributed to differences in the surface characteristics). The minimum rejection occurs at around pH 4, which corresponds to the isoelectric point, found using filtration potentials, showing that the electrostatic interactions between ions and the membrane surface are an important factor in salt rejection. P. Narong and A.E. James: Separation and Purification Technology 49(2) 122–129 (15 April 2006). DOI: 10.1016/j.seppur.2005.09.005
Thermo-osmosis of mixtures of water and methanol through Nafion membrane Mass transport of mixtures of water and methanol through a Nafion membrane as a function of the temperature difference between the two sides of the membrane was measured under different experimental conditions. The results show that the composition of the solutions, the temperature difference across the membrane, and the mean temperature in the membrane cell are governing factors in the thermo-osmotic transport process. In all cases studied, the thermo-osmotic flux through the membrane goes from the low temperature side to the high temperature side, and it increases linearly with the temperature difference. Moreover, the flux also increases with the mean temperature in the membrane cell. The thermo-osmotic coefficient of methanol in the membrane is higher than that of water.
For water/methanol mixtures, the total flux through the Nafion membrane can be larger than the fluxes of pure solvents, and it increases with the methanol content for water/methanol mixtures. J.P.G. Villaluenga, B. Seoane, V.M. Barragán and C. Ruiz-Bauzá: J. Membrane Science 274(1–2) 116–122 (5 April 2006). DOI: 10.1016/j.memsci.2005.08.010
Preparation of asymmetric or microporous hollow-fiber membranes In this study poly(4-methyl-1-pentene) (PMP) hollow fibers were prepared and fabricated into gas separation or microporous membranes using the melt-spun and cold-stretched method. PMP resin was melt-extruded into hollow fibers using cold air as the cooling medium. The effects of take-up speed and thermotreatment on the mechanical behavior and morphology of the fibers were investigated. Scanning electronic microscope (SEM) images were used to reveal the geometric structure of sections and the surface of the fibers. It was found that the original fiber had an asymmetric structure. A sandwich mode was used to describe the formation of this special fine structure. A series of PMP hollow-fiber membranes were prepared by subsequent drawing, and it was found that there was a skin-core structure on the cross-section of these hollowfiber membranes. Asymmetric or microporous PMP hollow-fiber membranes could be obtained by controlling post-treatment conditions. The morphology of these membranes was characterized by SEM, and the gas (oxygen, nitrogen and carbon dioxide) permeation properties of the membranes were also measured. The results indicate that the annealing time of the original fiber and the stretching ratio were the key factors influencing the structure of the resulting membrane. J. Wang, Z. Xu and Y. Xu: J. Applied Polymer Science 100(3) 2131–2141 (5 May 2006). DOI: 10.1002/app.23597
Simulation of membrane made by phase inversion method In a steady fabricating process, a two-dimensional hollow-fiber membrane, near the spinneret, was numerically simulated using the finite-element method (FEM). The unknown positions of free surface and moving interface were calculated simultaneously using the velocity and pressure fields. The effects of seven relevant parameters on the velocity and diameter profile were studied. These are the inertia term, gravity term, dope flow-rate, bore flow-rate, dope viscosity, tensile force and end velocity (and non-Newtonian). On the
Membrane Technology November 2006
RESEARCH TRENDS
basis of the simulated results, the inertia term in the fiber-spinning process could be safely neglected at low speed. However, the effect of gravity could not be overlooked. Furthermore, the outer diameter of the fiber grows larger with an increase in the dope flow-rate and the bore flow-rate. A large tensile force or high end velocity can cause a large deformation in the air gap, while a dope solution with high viscosity tended to reduce the deformation in the air gap. It was found that an increase in the dope flow-rate, at a small dope flow-rate, resulted in an increase in the inner diameter, while at a large dope flow-rate this tended to decrease. X.-T. Yang, Z.-L. Xu and Y.-M. Wei: J. Applied Polymer Science 100(3) 2067–2074 (5 May 2006). DOI: 10.1002/app.23020
Electro-spinning fibrous membranes from acrylonitrile-based polymers
Nanocrystalline TiO2 photocatalytic membranes fabricated by sol-gel dip-coating technique A novel sol-gel dip-coating process to fabricate nanocrystalline TiO 2 photocatalytic membranes with a robust hierarchical mesoporous multilayer and improved performance has been studied. Various titania sols have been synthesized containing poly(oxyethylenesorbitan monooleate) (Tween 80) surfactant as a poredirecting agent to tailor-design the porous structure of TiO2 materials at different molar ratios of Tween 80/isopropyl alcohol/acetic acid/titanium tetraisopropoxide = R:45:6:1. The sols are dip-coated onto a ‘home-made’ porous alumina substrate to fabricate TiO 2/ Al 2O 3 composite membranes, and are then dried and calcined. This procedure is repeated with varying sols in succession. The resulting asymmetric mesoporous TiO2 membrane, with a thickness of 0.9 µm, exhibits a hierarchical change in pore diameter from 2–6 nm to 3–8 nm to 5–11 nm, from the top layer to the bottom layer. Moreover, the corresponding porosity is incremented from 46.2% to 56.7% to 69.3%. Compared with a repeated-coating process, using a single sol, the hierarchical multilayer process improves water permeability significantly without sacrificing the organic retention and photocatalytic activity of the TiO 2 membranes. The prepared TiO 2 photocatalytic membrane offers great potential for the development of highly efficient water treatment and reuse systems – for example, the decomposition of organic pollutants, inactivation of pathogenic micro-organisms, physical separation of contaminants, and self-antifouling action – because of its multifunctional capability. H. Choi, A.C. Sofranko and D.D. Dionysiou: Advanced Functional Materials 16(8) 1067–1074 (May 2006). DOI: 10.1002/adfm.200500658
In this study, fibrous membranes with a fiber diameter of 80–800 nm were prepared from polyacrylonitrile and poly[acrylonitrile-co(N-vinyl-2-pyrrolidone)] using an electrospinning process. (The parameters can be controlled to fabricate fibrous membranes with similar fiber diameters of between 600 nm and 800 nm for further studies on swelling behavior and water states.) Water swelling experiments indicate that the fibrous membrane has a great capacity for water sorption, which reaches a maximum in a few minutes because of its extremely high porosity. Furthermore, a remarkable ‘overshoot’ occurs because of polymer chain relaxation and the non-compact structure of the fibrous membranes. Unlike a dense membrane, the equilibrium water content in the fibrous membrane decreases with the content of hydrophilic NVP, although the maximum is almost the same. The results from the DSC experiments demonstrate that only non-freezable bound water and free water can be distinguished in the fibrous membrane. On Behavior of Nafion and sulfonated the basis of the results of the water swelling poly(arylene ether sulfone) and DSC experiments, it was concluded that copolymer membranes the specific behaviors of the fibrous mem- The tensile stress–strain behavior of Nafion branes are induced by the non-compact and 117 and sulfonated poly(arylene ether sulpore-fiber discontinuous structure, which fone) copolymer (BPSH35) membranes were is different from either dense membranes or explored in this study with respect to the hydrogels. effects of the strain rate, counter-ion type, L.-S. Wan, Z.-K. Xu and H.-L. Jiang: molecular weight and presence of inorganic Macromolecular Bioscience 6(5) 364–372 (23 fillers. The yielding properties of the two films May 2006). were most affected by the change in the strain METE10.1002/mabi.200600017 strip ad_0206 24/2/06 14:10 Page 1 stress–strain curves of Nafion films DOI: rate. The
Membrane Technology November 2006
in acid and salt forms exhibited larger deviations at strains above the yield strain. As the molecular weight of the BPSH35 samples increased, the elongation at break improved significantly. Enhanced mechanical properties were observed for the composite membrane of BPSH35 and zirconium phenylphosphonate (2% w/w), compared with its matrix BPSH35 film. The stress–relaxation behavior of the Nafion and BPSH35 membranes was measured at different strain levels and different strain rates. Master curves were constructed in terms of plots of the stress–relaxation modulus and time on a double-logarithm scale. A three-dimensional bundle–cluster model was proposed to interpret these observations, combining the concepts of elongated polymer aggregates, proton-conduction channels and the states of water. The rationale here focused on the polymer bundle rotation/interphase chain readjustment before yielding, and the polymer aggregate disentanglements and reorientation after yielding. D. Liu, S. Kyriakides, S.W. Case, J.J. Lesko, Y. Li and J.E. McGrath: J. Polymer Science Part B: Polymer Physics 44(10) 1453–1465 (15 May 2006). DOI: 10.1002/polb.20813
Fundamental studies of homogeneous cation-exchange membranes Strong acid, homogenous cation-exchange membranes were obtained by simultaneously introducing sulfonic and bromine groups into poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). The ion-exchange capacity (IEC), water content, transport number, diffusion coefficient, contact angle and tensile strength of the obtained membranes were studied. The results show that the membrane’s intrinsic properties are largely dependent on the substitution of bromine: the IEC and water content decrease with bromine content, while the area resistance and permselectivity of the membranes increase with this trend. Therefore, by properly balancing them, a series of homogenous cation-exchange membranes that have good electrical properties and physical stability can be obtained which comply with different industrial electro-membrane processes, such as diffusion dialysis, electrodialysis and electro-deionization. H. Yu and T. Xu: J. Applied Polymer Science 100(3) 2238–2243 (5 May 2006). DOI: 10.1002/app.22525
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