Preparation and characterisation of SrCe0.95Yb0.05O2.975 hollow fibre membranes

A mixed proton and electronic conducting hollow fibre membrane, SrCe_0.95Yb_0.05O_2.975 (SCYb), has been prepared by spinning a polymer solution containing suspended SCYb particles to a hollow fibre precursor, which is then sintered at elevated temperatures. The SCYb powders having a sub-micron size, i.e. an essential size for fabrication of the hollow fibre with good mechanical strength, were synthesised through a polymerised water-soluble complex method. By controlling weight ratio of the SCYb ceramic powder to the polymer binder and sintering temperatures, the SCYb ceramic hollow fibres with gas-tight properties can be prepared. Some primary factors affecting microstructures, gas tightness and mechanical strength of the mixed conducting hollow fibre membranes were studied in details.     

Fabrication of high performance Matrimid/polysulfone dual-layer hollow fiber membranes for O2/N2 separation

Matrimid/polysulfone (PSf) dual-layer hollow fiber membranes were fabricated by using co-extrusion and dry-jet wet-spinning phase-inversion techniques. The effects of the spinning dope composition, spinneret dimension, spinneret temperature and the air gap distance on the hollow fiber membranes separation performance were studied. Aging phenomenon was also studied. After coated by 3 wt% silicon solution, the hollow fiber membranes have an O₂/N₂ selectivity of 7.55 at 25 °C, 506.625 kPa which exceeds the intrinsic value of Matrimid. The membranes have an O₂ permeance of 9.36 GPU with an apparent dense-layer thickness of 1421 Å calculated from the O₂ permeability. SEM images show the high porosity underneath the dense skin. It indicates that non-solvent addition is not necessary in the inner spinning dope to induce the macroviod formation. The binodals of the Matrimid/solvent/H₂O and PSf/solvent/H₂O indicate that the composition of the spinning dope plays an important role in the structure and the gas separation performance of the dual-layer hollow fiber membranes. The delayed demixing of the inner spinning dope may fabricate low resistance support layers in the dual-layer hollow fiber membranes.     

聚全氟乙丙烯中空纤维膜研究

以聚全氟乙丙烯(FEP)为成膜聚合物,复合无机粒子为成孔剂,邻苯二甲酸二辛酯(DOP)为稀释剂,采用熔融纺丝工艺制备得到FEP中空纤维膜.分析和讨论了不同成膜体系对FEP中空纤维膜热性能、动态力学性能和力学性能的影响,并对膜的纯水通量和孔径分布进行表征.用扫描电子显微镜(SEM)观察了膜的横断面和表面形貌.结果表明,所得FEP中空纤维膜为由溶出微孔和界面微孔组成的海绵状孔结构.随着成孔剂含量的增加,成孔剂在成膜体系中分散程度变差,容易发生团聚,最终导致膜孔径变大,孔径分布变宽.成孔剂和稀释剂对FEP中空纤维膜的热性能和动态力学性能影响较小.当FEP含量增加到70 wt％时,膜表面容易形成一层致密层,降低了膜的通透性.     

Effect of the preparation conditions on the formation of asymmetric poly(vinylidene fluoride) hollow fibre membranes with a dense skin

Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes were prepared and characterized. The effects of phase inversion methods (dry-wet or wet) and spinning conditions, such as the type of solvent (NMP, DMAc), the concentration of polymer in dope solution, temperature of the external coagulation bath and the composition of the inner coagulant on the morphology and on the formation of a dense skin layer were investigated. The structure of the membranes was analyzed by scanning electron microscopy and the gas permeation properties with six different gases (He, H₂, N₂, O₂, CH₄ and CO₂) were measured at 25 °C to confirm the integrity of the selective skin layer. Under the proper conditions highly selective and permeable PVDF hollow fibre membranes were thus obtained by dry-wet spinning of a 30 wt.% PVDF solution in DMAc, using hot water (50 °C) as the external coagulant and a bore fluid of pure water as the internal coagulant. The best membrane had a selective outer skin with an effective thickness of approximately 0.2 μm. The ideal selectivity of the hollow fibres approached or even exceeded the intrinsic ideal selectivity of a dense PVDF film, for instance the selectivity for He over N₂ was 86.2 for the hollow fibre, whereas it was 83.5 for a dense PVDF reference film. DSC and FT-IR/ATR analysis indicated a higher fraction of the β-crystal phase in the selective skin and a high overall crystallinity than in the melt-processed film. The latter explains the relatively high selectivity and low permeability of the membranes. Intrinsic polymer properties make the membranes also suitable for vapour transport than for gas separation.     

The preparation and characterization of silver‐loading cellulose acetate hollow fiber membrane for water treatment

Silver‐loading asymmetric cellulose acetate (CA) hollow fiber membrane was spun via the dry jet‐wet spinning technique. The spinning solution was prepared by dissolving AgNO₃ and CA in N,N‐dimethylformamide (DMF). The silver ions were reduced in the spinning dope into silver nano‐particles. The morphology of the resulting hollow fibers was examined using a scanning electron microscope and the silver content in the fiber was measured using an inductively coupled plasma atomic emission spectrometer. The antibacterial activities were evaluated. These hollow fibers had a sponge‐like structure and dense inner and outer surfaces. At a 50 k magnification, the pore on the skin layer was not observable, while the nodule size was smaller than 10 nm. The residual silver content of as‐spun hollow fiber was about 60% of the original silver added in the polymer solution. After immersing in water bath for 180 days, the silver content in the bulk of the hollow fibers decreased to 60% and the silver content on the surface reduced to 10%, yet still showed antibacterial activity against Escherichia coli and Staphylococcus aureus. After permeating with water for 5 days, the silver content in the hollow fibers decreased, and did not show antibacterial activity against E. coli and S. aureus. Thus, silver content must be periodically replenished after permeation. The proper range of AgNO₃ in the spinning solution for CA hollow fiber should be about 100–1000 ppm. Copyright © 2005 John Wiley & Sons, Ltd.     

Effect of solution extrusion rate on morphology and performance of polyvinylidene fluoride hollow fiber membranes using polyvinyl pyrrolidone as an additive

<正>Polyvinylidene fluoride(PVDF) hollow fiber membranes prepared from spinning solutions with different polyvinyl pyrrolidone(PVP) contents(1%and 5%) at different extrusion rates were obtained by wet/dry phase process keeping all other spinning parameters constant.In spinning these PVDF hollow fibers,dimethylacetamide(DMAc) and PVP were used as a solvent and an additive,respectively.Water was used as the inner coagulant.Dimethylformamide(DMF) and water(30/70) were used as the external coagulant.The performances of membranes were characterized in terms of water flux,solute rejection for the wet membranes.The structure and morphology of PVDF hollow fiber were examined by BET adsorption,dry/wet weight method and scanning electron microscopy(SEM).It is found that the increase in PVP content and extrusion rate of spinning solution can result in the increase of water flux and decrease of solute rejection.The improvements of interconnected porous structure and pore size are induced by shear-thinning behavior of spinning solution at high extrusion rates,which could result in the increase of water flux of hollow fiber membranes.The increase of extrusion rate also leads to the increase of membrane thickness due to the recovery effect of elastic property of polymer chains.     

Visualization of the effect of die shear rate on the outer surface morphology of ultrafiltration membranes by AFM

We have demonstrated the effect of shear rate on the outer surface morphology of polyethersulfone (PES) hollow fiber ultrafiltration (UF) membranes by an atomic force microscope (AFM). A digital instrument (DI) AFM was used to reveal the surface morphology of hollow fiber membranes prepared with varying shear rates from 1305 to 11,066 s⁻¹. A tapping mode was operated for studying the polymeric membranes when AFM was applied to image the surface of a fiber in air. AFM images of the outer surface have revealed that the nodules in the outer skin appeared to be randomly arranged at low shear rates but formed bands that were aligned in the direction of dope extrusion when the shear rate increased. Both nodule sizes in the fiber spinning and transversal directions decreased with increasing shear rate possibly because of chain disentanglement and thermodynamically favored. This result has not been reported so far. The analysis of AFM images showed that the roughness of the outer surface of hollow fiber UF membranes in terms of R_ms, R_a and R_z decreased with an increase in shear rate. The pure water flux of the membranes was nearly proportional to the mean roughness and higher mean roughness resulted in lower separation of membranes. AFM data also imply that there was a certain critical value of shear rate around 3585 s⁻¹, the roughness decreased significantly with an increase in shear rate below 3585 s⁻¹ and almost leveled off or in a much slower pace above this shear rate.     

Anisotropic membranes with carboxypeptidase G1

Anisotropic polysulfone membranes were prepared with carboxypeptidase G₁ embedded in the polymer structure. The enzymatically active flat and hollow-fiber membranes were obtained by precipitating the polymer from solution in an organic mixture in which an aqueous solution of the enzyme had been dispersed. The process has been found to be particularly suitable for the immobilization of enzymes in anisotropic hollow fibers that exhibited no detectable enzyme leakage upon perfusion. The pH profiles measured with the enzyme in free solution and in the embedded form were similar. Kinetic parameters of multitubular enzyme reactors were investigated by measuring the rate of hydrolysis of glutamate from folic acid or methotrexate at different flow rates and substrate concentrations. The relatively slow mass transfer in such reactors was found to affect strongly the observed kinetics. The results of in vitro experiments with 5000 fiber reactors suggest that hollow fiber cartridges prepared with such membranes have clinical potential for the extracorporeal removal of methotrexate from blood.     

Formation of high-performance 6FDA-2,6-DAT asymmetric composite hollow fiber membranes for CO2/CH4 separation

We report that 6FDA-2,6-DAT polyimide can be used to fabricate hollow fiber membranes with excellent performances for CO₂/CH₄ separation. In order to simplify the hollow fiber fabrication process and verify the feasibility of 6FDA-2,6-DAT hollow fiber membranes for CO₂/CH₄ separation, a new one-polymer and one-solvent spinning system (6FDA-2,6-DAT/N-methyl-pyrrolidone (NMP)) with much simpler processing conditions has been developed and the separation performance of newly developed 6FDA-2,6-DAT hollow fiber membranes has been further studied under the pure and mixed gas systems.Experimental results reveal that 6FDA-2,6-DAT asymmetric composite hollow fiber membranes have a strong tendency to be plasticized by CO₂ and suffer severely physical aging with an initial CO₂ permeance of 300 GPU drifting to 76 GPU at the steady state. However, the 6FDA-2,6-DAT asymmetric composite hollow fibers still present impressive ultimate stabilized performance with a CO₂/CH₄ selectivity of 40 and a CO₂ permeance of 59 GPU under mixed gas tests. These results manifest that 6FDA-2,6-DAT polyimide is one of promising membrane material candidates for CO₂/CH₄ separation application.     

Preparation of composite hollow fiber membranes of poly(ethylene oxide)-containing polyimide and their CO2/N2 separation properties

Composite hollow fiber membranes were prepared by a dry-jet wet spinning process using a double layer spinneret. These membranes were composed of a thin and dense outer-layer of poly(ethylene oxide)-containing polyimide and a sponge-like inner layer of other polyimide. The outer layer was responsible for the separation and fabricated as thin as 1 μm. The permeation flux of CO₂, R_CO2, and the CO₂/N₂ selectivity decreased 40% and 10–20%, respectively, in a month after the membrane preparation. The steady performance was still high; for example, R_CO2=69×10⁻⁶cm³ (STP)/(cm² s cm Hg) and the selectivity of 33 at 323 K.     

Preparation and characterization of polyvinylidene fluoride (PVDF) hollow fiber membranes

Polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared by dry/wet and wet phase inversion methods. In spinning these PVDF hollow fibers, dimethylacetamide (DMAc) and polyvinyl pyrrolidone (PVP) were used as a solvent and an additive, respectively. Water was used as the external coagulant. Water or ethanol was used as the internal coagulants. The membranes were characterized in terms of water flux, molecular weight cut-off for the wet membranes. Gas permeation fluxes and effective surface porosity were determined by a gas permeation method for the dried membranes. The cross-sectional structures were examined by scanning electron microscopy. The effects of polymer concentration, air-gap, PVP molecular weight, PVP content in the polymer dope, and the internal coagulant on the permeation properties and membrane structures were examined. Highly permeable PVDF hollow fiber membranes could be prepared from a polymer dope containing low molecular weight PVP and using ethanol as the internal coagulant.     

Study on the effect of spinning conditions and surface treatment on the geometry and performance of polymeric hollow-fibre membranes

Hollow-fibre membranes were prepared by the wet-dry spinning technique from polyether sulfone (PES). The effect of spinning conditions such as the flow-rate of the internal coagulant and the flow-rate, composition and temperature of the polymer solution on the geometry and performance of hollow fibres was studied. In particular, five different ratios of pore former/polymer covering the range 0.2–1.0 were investigated while the polymer content was kept constant. Since the viscosity of the spinning dope affects the morphology of the hollow-fibre membrane, hollow fibres were prepared at different temperatures of the spinning dope from 25 to 60°. By scanning electron microscopy (SEM) two layers sandwiching a finger-like cavity structure were observed. Also, the surface on the bore side of the hollow fibre was modified by grafting polyethylene glycol (PEG) with γ-ray irradiation to improve the ultrafiltration performance.     

Poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) based composite electrolytes for lithium batteries

The composite polymer electrolyte (CPE) membranes, comprising of poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), aluminum oxyhydroxide, (AlO[OH]_n) of two different particle sizes 7 μm/14 nm and LiN(CF₃SO₂)₂ as lithium salt were prepared using solution casting technique. The prepared membranes were subjected to XRD, impedance spectroscopy, compatibility and transport number studies. The incorporation of nanofiller greatly enhanced the ionic conductivity and the compatibility of the composite polymer electrolyte. Also LiCr_0.01Mn_1.99O₄/CPE/Li cells were assembled and their charge-discharge profiles have been made at 70 °C. The film which possesses nanosized filler offered better electrochemical properties than those with micron sized filler. The results are discussed based on Lewis acid-base theory.     

Direct measurement of rheologically induced molecular orientation in gas separation hollow fibre membranes and effects on selectivity

Asymmetric polysulfone hollow fibre membranes for gas separation were spun using a dry/wet spinning process. An optimised four component dope solution was used: 22% (w/w) polysulfone, 31.8% (w/w) N,N-dimethylacetamide, 31.8% (w/ w) tetrahydrofuran and 14.4% (w/w) ethanol. Fibres were spun at low- and high-dope extrusion rates and hence at different levels of shear. Molecular orientation in the active layer of the membranes was measured by plane-polarised infrared spectroscopy. Gas permeation properties (permeability and selectivity) were evaluated using pure carbon dioxide and methane. The spectroscopy indicated that increased molecular orientation occurs in the high-shear membranes. The selectivities of these membranes were heightened and even surpassed the recognised intrinsic selectivity of the membrane polymer. The results suggest that increased shear during spinning increases molecular orientation and, in turn, enhances selectivity.     

Correlation between Performances of Hollow Fibers and Flat Membranes for Gas Separation

This work presents an attempt at correlating the available permeability/selectivity literature data for hollow fibers and flat membranes. Therefore, this paper gathers the information pertaining to membrane materials for which membrane properties of flat membranes and hollow fibers have both been reported. An overview of the relations between selectivity and permeance of hollow fiber membranes for various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂, H₂/N₂, H₂/CO₂, H₂/CH₄ and He/N₂) is presented first. The upper bound lines are the ones proposed by Robeson, which were calculated by assuming a one-micron-thick skin layer as proposed by Robeson in 2008. From the results obtained, a relation between the selectivity ratio in both kinds of membranes (α_H/α_f) and skin layer thickness (l) calculated from flat membranes and hollow fibers gas permeation data for these pairs of gases is also presented. The skin layer thicknesses measured using seven different experimental techniques for six commercial membranes are compared. The influences of spinning parameters on the morphology and performance of hollow fiber membrane gas separation are discussed. Finally, an analysis is made of the reasons why the dense skin layer thicknesses of a hollow fiber calculated using permeance and permeability data vary for different gases and also differ from direct experimental measurements.     

ZIF-8 based polysulfone hollow fiber membranes for natural gas purification

Mixed matrix membranes (MMMs) have received worldwide attention for natural gas purification due to their superior performance in terms of permeability and selectivity. The zeolitic imidazole framework-8 (ZIF-8) blended polysulfone (PSf) membranes have been fabricated for natural gas purification. ZIF-8 was selected due to its low cost, remarkable thermal and chemical stabilities, and tunable microporous structure. The neat PSf hollow fiber membrane and mixed matrix hollow fiber membranes incorporated with the various ZIF-8 loadings up to 1.25% were fabricated. The prepared membranes were evaluated using field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and gas separation performance. The low loading of ZIF-8 nanoparticles to the MMM improved thermal stability and glass transition temperature and yielded low surface roughness. MMMs were tested using pure gases with a significant improvement of 36% in CO₂ permeability and 28% in CO₂/CH₄ selectivity compared to the neat membrane. However, the high ZIF-8 loading reduced the separation performances. Moreover, CO₂/CH₄ selectivity decreased at elevated pressure (8 and 10 bar) due to CO₂-induced plasticization. Previously, the incorporation of ZIF-8 particles has primarily been subjected to the fabrication of flat sheet membranes, whereas this work focused on hollow fiber membranes which are rarely investigated. Hence, the promising results obtained at low feed pressure in this study demonstrated the potential of ZIF-8 based hollow fiber membrane for natural gas purification.     

Microscopic behavior of polyvinylpyrrolidone hydrophilizing agents on phase inversion polyethersulfone hollow fiber membranes for hemofiltration

In some biomedical applications, hollow fiber membranes are highly demanded with desirably asymmetric structures, characterized by a dense selective inner skin with which the blood is in contact and supported by porous outer-layer. In this work, such membranes have been successfully prepared by appropriately adjusting membrane manufacturing parameters. Different molecular weights of polyvinylpyrrolidones (PVPs) were used as the hydrophilizing additives for membrane spinning in order to examine their underlying effects on membrane physicochemical properties, morphological structure, solute rejection behavior and hemofiltration performance. Numerous state-of-the-art characterizations on the resultant membranes showed that the hollow fiber membranes spun with the PVP having a molecular weight of 360K as the additive have the most hydrophilic, smooth and highly net negative charged inner surfaces. These membranes also exhibit the best hemofiltration performance in terms of the characteristically least fouling behavior with a normalized flux above 90%, the highest retention of serum albumin for more than 90%, and the best clearance for the simulated β₂-microglobulin toxin in blood waste.     

Defect-free asymmetric hollow fiber membranes from Torlon, a polyamide–imide polymer, for high-pressure CO2 separations

Torlon^®, a polyamide–imide polymer, was used for high-pressure CO₂ separations, as it can form inter- and intra-chain hydrogen bonding that may provide stability against plasticization. Asymmetric hollow fiber membranes with a defect-free selective skin were successfully formed from Torlon^® using a dry–wet spinning process. Dope and spinning parameters were optimized to obtain these fibers, which had CO₂/CH₄ selectivity of 44 and O₂/N₂ selectivity of 7.7. These selectivities are about 85% of the intrinsic (dense film) value of 52 for CO₂/CH₄ and 90% of the intrinsic value of 8.3 for O₂/N₂, respectively. Based on analyses presented, the reduced selectivities are attributed to substructure resistance rather than actual skin layer defects. Macrovoids, which compromise the strength of the fiber, were reduced by increasing the polymer concentration. The resulting fiber could withstand up to 2000 psi of N₂, and a CO₂ permeation study indicates that this fiber can perform selective separations under supercritical conditions of 1100 psi of CO₂ at 35 °C.     

同质编织管增强型聚丙烯腈中空纤维膜研究

利用二维编织技术将聚丙烯腈(PAN)纤维编织成中空编织管,以聚丙烯腈为成膜聚合物,以聚乙二醇为成孔剂,配制铸膜液,采用同心圆纺丝法制备同质编织管增强型聚丙烯腈中空纤维膜.研究结果表明,所得同质编织管增强型聚丙烯腈中空纤维膜的表面分离层具有类似于非对称膜的结构,铸膜液可渗入编织管纤维束中;随着编织管编织节距的增大,同质编织管增强型聚丙烯腈中空纤维膜表面分离层厚度减小,同时膜的平均孔径增大,膜的纯水通量随之增大;铸膜液渗入编织管纤维束的现象未影响膜的通透性能;编织管的断裂强度最大可达100 MPa以上.通过水浴振荡、超声波水浴振荡及等力拉伸3种方法测试了同质编织管增强型中空纤维膜和异质编织管增强型中空纤维膜中编织管与表面分离层之间的界面结合性能,结果表明前者的界面结合性能优于后者.