Fabrication and characterization of dual-layer hollow-fiber ultrafiltration membranes

In this study, poly(vinylidene fluoride) (PVDF) dual-layer hollow-fiber UF membranes were prepared via phase inversion in one step. Laboratory-synthesized amphiphilic poly(vinylidene fluoride)-g-poly(ethylene glycol) methyl ether methacrylate (PVDF-g-POEM) was incorporated as a hydrophilic modifier of the outer layer by blending. The effects of the dope formulation and membrane formation conditions on membrane structure and UF performance were investigated. The parameters investigated included the PVDF-g-POEM loading in the outer layer, the PEG additive content in the outer layer, the external coagulant composition, and the polymer concentration in the inner layer. The effects of adding PVDF-g-POEM and PEG were found to depend on the external coagulant composition; when a water/ethanol mixture was used as coagulant, the fibers formed in the presence of PEG exhibited larger pores, as confirmed by both SEM characterization and a solute rejection method. The porosity of the inner layer was observed to increase with decreasing inner-layer dope concentration and upon weakening the external coagulant. A more porous inner layer led to a higher transmembrane pure water flux. An antifouling test confirmed that both membrane hydrophobicity and surface pore size affected the membrane fouling pattern and the final FRR. The highest FRR of 83.3% was obtained with the hollow fiber M5A, which was characterized by a compact surface and contained PVDF-g-POEM in its polymer matrix.     

Interfacial resistance of dual-layer asymmetric hollow fiber pervaporation membranes formed by co-extrusion

Interface is critical for dual-layer membranes fabricated by co-extrusion and dry-jet wet spinning. In this work, for the first time, the importance of interface structure in overall membrane transport property was revealed by using Polysulfone (outer layer)/Matrimid (inner) dual-layer hollow fibers as a demonstration system. Due to the dope formulations of the two layers, dense skins came into formation at the shell side of Matrimid inner layer facing the interface. The Matrimid inner layer obtained from the dual-layer hollow fiber with the thinnest Polysulfone outer layer exhibited a flux around 1.0 × 10⁻³ kg/m²-s and a separation factor of ~800 in tert-butanol dehydration (feed flow rate 30 L/h, temperature 80 °C, permeate pressure 2 mbar, the same for the other tests). An estimation based on resistance model clearly indicated the dominance of Matrimid inner layer in the overall mass transfer of corresponding dual-layer membrane. As for hollow fibers with the thickest Polysulfone outer layer, the bulk substrate comprising the interfacial dense skin of Matrimid inner layer also displayed significant resistance and appreciable selectivity. Conclusively, the function of interface should not be ignored. The rule for the evolution of interface structure requires further exploration for fully understanding and utilizing the composite membrane by co-extrusion and phase inversion approach.     

Dual-layer hollow fiber of polyaniline–cellulose acetate prepared with simple wet technique of chemical polymerization of aniline

Polyaniline–cellulose acetate hollow fibers prepared from cellulose acetate (CA) solution of aniline (ANI) were founded to show dual layers structure, when wet technique was applied for using syringe injection of the solution to acidic water for ANI polymerization. In this technique, the polymerization of ANI occurred simultaneously by the monomer injection in coagulated CA fibers. Then, the PANI–CA composite fibers were obtained. The formation of the PANI–CA composite fiber was dependent upon HCl concentration and initiator of ammonium persulfate. Especially, when the coagulation time was 1 min, the obtained PANI–CA fibers showed hollowed dual-layer structure having outer layer of PANI and inner layer of CA. Evidence was presented that the dual structure fiber had 40 μm outer layer and 60 μm inner porous layer in their thicknesses. Cyclic voltammograms of the PANI–CA fiber were indicated that the outer layer was composed of PANI layer showing electrochemical properties with electrical capacity of about 0.003 C.     

Dynamic pressure‐driven covalent assembly of inner skin hollow fiber multilayer membrane

A covalent assembly was accomplished onto hollow fibers via a dynamic pressure‐driven layer‐by‐layer (LbL) technique. The covalent crosslinking multilayers were successfully formed onto the inner surfaces of hollow fiber porous substrates during the alternatively filtration of polyethyleneimine (PEI) and glutaraldehyde (GA) solutions. The formation of covalent bond between PEI and GA was confirmed using fourier transform infrared (FTIR) spectra. The thickness increment on a quartz slide clearly suggested the stepwise growth of multilayer at nanometer scale. The regular alternation of zeta potentials demonstrated that the successful formation of GA‐crosslinked PEI multilayers on the hollow fibers. The multilayer membranes showed excellent pervaporation performances for the dehydration of different solvent–water mixtures. The selectivity and permeability can be controlled by varying the PEI layer number. More importantly, the covalent assembled multilayer membrane rendered much higher stabilities compared with those from electrostatically LbL assembly, which offers much opportunity for practical applications. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Fabrication of polysulfone asymmetric hollow‐fiber membranes by coextrusion through a triple‐orifice spinneret

Polysulfone (PSf) asymmetric hollow‐fiber membranes, which have a dense outer layer but a loose inner layer, were tentatively fabricated by coextrusion through a triple‐orifice spinneret and a dry/wet‐phase inversion process. Two simple polymer dopes were tailored, respectively, for the dense outer layer and the porous inner layer according to the principles of the phase‐inversion process. By adjusting the ratio of the inner/outer extrusion rate, the hollow‐fiber membranes with various thicknesses of outer layers were achieved. The morphology of the hollow‐fiber membranes was exhibited and the processing conditions and the water permeability of the membrane were investigated. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 259–266, 2004     

The effects of spinning conditions on the structure formation and the dimension of the hollow-fiber membranes and their relationship with the permeability in dry–wet spinning technology

Hollow-fiber membranes were prepared by dry–wet spinning under different spinning conditions. The used spinneret was tube-in-orfice type and the membrane material was polysulfone (Udel P-3500). N-methyl-2-pyrrolidinone (NMP) and water were used as solvent and coagulant, respectively. The concentration of the dope solution was 22 wt %. The effects of the three spinning factors—spinning height; extrusion rates of dope solution and inner coagulant; dimensions (inner diameter, outer diameter, and thickness) and permeation properties of the hollow-fiber membranes—were studied. The results were as follows: With changes in the spinning factors, spinning velocity and falling time before the membrane entered the water (coagulant) were changed; consequently, the structures and the dimensions of the hollow-fiber membranes varied with a certain tendency. The permeation properties of the hollow fibers were related very closely to the changes in the fibers' structures and dimensions. © 1995 John Wiley & Sons, Inc.     

Two‐dimensional simulation of hollow fiber membrane fabricated by phase inversion method

In the steady fabricating process, 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 by the velocity and pressure fields. The effects of seven relevant parameters, i.e., inertia term, gravity term, dope flow rate, bore flow rate, dope viscosity, tensile force, end velocity and non‐Newtonian on the velocity and diameter profile were studied. On the basis of the simulated results, the inertia term in hollow fiber‐spinning process was safely neglected in low speed, while the effect of gravity was not be neglected. Besides, the outer diameter of the fibers increased with an increase of dope flow rate and bore flow rate; Large tensile force or large end velocity could cause large deformation in the air gap; larger viscous dope solution tended to make less deformation in the air gap. It was found that an increase of the dope flow rate at small dope flow rate resulted in an increase of the inner diameter, while at large dope flow rate, it decreased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2067–2074, 2006     

Investigation on the effect of spinning conditions on the properties of hollow fiber membrane for hemodialysis application

Polyethersulfone (PES) hollow fiber membranes were fabricated via the dry‐wet phase inversion spinning technique, aiming to produce an asymmetric, micro porous ultrafiltration hollow‐fiber specifically for hemodialysis membrane. The objective of this study is to investigate the effect of spinning conditions on the morphological and permeation properties of the fabricated membrane. Among the parameters that were studied in this work are air gap distance, dope extrusion rate, bore fluid flow rate, and the take‐up speed. The contact angle was measured to determine the hydrophilicity of the fibers. Membrane with sufficient hydrophilicity properties is desired for hemodialysis application to avoid fouling and increase its biocompatibility. The influences of the hollow fiber's morphology (i.e., diameter and wall thickness) on the performance of the membranes were evaluated by pure water flux and BSA rejection. The experimental results showed that the dope extrusion rate to bore fluid flow rate ratio should be maintained at 1:1 ratio to produce a perfectly rounded asymmetric hollow fiber membrane. Moreover, the flux of the hollow fiber spun at higher air gap distance had better flux than the one spun at lower air gap distance. Furthermore, spinning asymmetric hollow fiber membranes at high air gap distance helps to produce a thin and porous skin layer, leading to a better flux but a relatively low percentage of rejection for BSA separation. Findings from this study would serve as primary data which will be a useful guide for fabricating a high performance hemodialysis hollow fiber membrane. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43633.     

Preparation and characterization of poly(vinyl chloride)/polystyrene/poly(ethylene glycol) hollow‐fiber ultrafiltration membranes

Hollow‐fiber ultrafiltration (UF) membranes were prepared from blends of poly(vinyl chloride) (PVC) and polystyrene (PS) with a dry/wet phase inversion method. Poly(ethylene glycol) (PEG) and N,N‐dimethylacetamide were used as the additive and solvent, respectively. The effects of the PEG concentration in the dope solution as an additive on the cross sections and inner and outer surface morphologies, permeability, and separation performance of the hollow fibers were examined. The mean pore size, pore size distribution, and mean roughness of both the inner and outer surfaces of the produced hollow fibers were determined by atomic force microscopy. Also, the mechanical properties of the hollow‐fiber membranes were investigated. UF experiments were conducted with aqueous solutions of poly(vinyl pyrrolidone) (PVP; K‐90, M_w = 360 kDa). From the results, we found that the PVC/PS hollow‐fiber membranes had two layers with a fingerlike structure. These two layers were changed from a wide and long to a thin and short morphology with increasing PEG concentration. A novel and until now undescribed shape of the nodules in the outer surfaces, which was denoted as a sea‐waves shape, was observed. The outer and inner pore sizes both increased with increasing PEG concentration. The water permeation flux of the hollow fibers increased from 104 to 367 L m^?2 h^?1 bar^?1) at higher PEG concentrations. The PVP rejection reached the highest value at a PEG concentration of 4 wt %, whereas at higher values (from 4 to 9 wt %), the rejection decreased. The same trend was found also for the tensile stress at break, Young's modulus, and elongation at break of the hollow fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 989‐1004, 2013     

Composite hollow fiber membranes

Composite polysulfone hollow fibers consisting of a polysulfone porous substrate coated with crosslinked polyethyleneimine (PEI) or furan resin are reported. These composite hollow fibers are analogous to the flat-sheet composite membranes known as NS-100 and NS-200. The surface structure of the porous substrate was rigorously studied before and after coating. Scanning electron microscope observations and reverse osmosis transport studies showed that the support fiber must have surface pore diameters of less than 0.2 μm to obtain a durable composite hollow fiber membrane. The curing process would normally follow in situ condensation of the PEI or the cationic polymerization of the furfuryl alcohol. However, since both the dense layer and surface of the porous substrate contract when exposed to the curing temperature (110–150°C), it was found to be profitable to cure the hollow fiber before applying the coating. When tested in a reverse osmosis rig, PEI/TDI-coated polysulfone hollow fiber bundles displayed 98% salt rejection and a flux of 5–7 gfd for a feed solution of 10,000 ppm NaCl at a hydraulic pressure of 400 psi. A new method of depositing furan resin on the polysulfone hollow fiber is described. The furfuryl alcohol is instantaneously polymerized by exposing the alcohol-soaked fiber to a 60% solution of concentrated sulfuric acid. It has been demonstrated that in such a polymerization procedure a dense, semipermeable layer is formed on top of the porous substrate; the resulting composite hollow fiber membrane yields salt rejections higher than 98% when tested under the above reverse osmosis conditions.     

Dynamic layer‐by‐layer self‐assembly of organic–inorganic composite hollow fiber membranes

Multilayer membranes constructed layer‐by‐layer (LbL) is finding increasing importance in many separation applications. The efficient construction of LbL multilayer on to hollow fiber substrates may offer many new opportunities for industrial applications. An organic–inorganic composite hollow fiber membrane has been developed using a dynamic LbL self‐assembly. This poly(acrylic acid)/poly(ethyleneimine) multilayer was dynamically assembled onto the inner surfaces of ceramic hollow fiber porous substrates pretreated by Dynasylan Ameo silane coupling agents. The hollow fibers were subsequently heat crosslinked to obtain stable permselective membranes. The formation of multilayers on the hollow fibers was characterized with a SEM, EDX, an electrokinetic analyzer and IR spectra. The effects of layer number, feed temperature and water content in the feed on the pervaporation performance have been investigated. To the best of our knowledge, this is the first report of LbL assembly of polymer building blocks onto ceramic hollow fiber porous substrates. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3176–3182, 2012     

Improved microstructure of asymmetric niobium pentoxide hollow fiber membranes

Asymmetric niobium pentoxide (Nb₂O₅) hollow fiber membranes were prepared by the phase inversion and sintering process at temperatures ranging from 1000 to 1350°C. The effects of extrusion parameters on the morphology and properties of the produced membranes were systematically explored. Asymmetric hollow fibers with regular inner contour were obtained at extrusion flow rates of 15 and 25 ml min⁻¹ of ceramic suspension and internal coagulant, respectively. Hollow fibers sintered at temperatures greater than 1200°C presented modifications in the morphology of Nb₂O₅ grains, which were also evidenced by X-ray diffraction and Raman analyses. Hollow fibers produced with an air gap of 50 mm presented a dense outer sponge-like layer and micro-voids formed from the inner surface. These hollow fibers sintered at 1200°C presented suitable bending resistance and water permeability (24.2 ± 0.60 MPa and 3.00 ± 0.01 L h^-1 m^-2 kPa^-1, respectively). The outer sponge like layer was mitigated when the fibers were produced without air-gap.     

Construction of multilayer coatings for flame retardancy of ramie fabric using layer‐by‐layer assembly

Complex multilayer coatings composed of α‐zirconium phosphate (ZrP), polyethylenimine (PEI), and ammonium polyphosphate (APP) were constructed via layer‐by‐layer assembly method for flame retardant ramie fabric. Bicomponent PEI/ZrP layers served as insulating barrier coating, and bicomponent PEI/APP layers served as intumescent coating. The flame retardancy of the coated ramie fabric was strongly dependent on the nature of the coatings and the layer‐by‐layer assembly patterns. The coated ramie fabric with inside PEI/ZrP layers and outside PEI/APP layers possessed the most uniform and consistent coating surface morphology, as well as the highest content of N and P elements, resulting in an excellent improvement in flame retardancy of ramie fabrics. When this kind of coated ramie fabric was heated, the inner PEI/ZrP layer effectively prevent oxygen and heat from penetrating into the substrate, and the outer PEI/APP layer exposed to air with good expansion during combustion. The synergistic effect was formed during the combustion process and could impart ramie fabrics with high flame retardancy. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45556.     

Optimization and modification of PVDF dual-layer hollow fiber membrane for direct contact membrane distillation; application of response surface methodology and morphology study

RSM methodology was applied to present mathematical models for the fabrication of polyvinylidene fluoride (PVDF) dual-layer hollow fibers in membrane distillation process. The design of experiments was used to investigate three main parameters in terms of polymer concentration in both outer and inner layers and the flow rate of dope solutions by the Box-Behnken method. According to obtained results, the optimization was done to present the proper membrane with desirable properties. The characteristics of the optimized membrane (named HF-O) suggested by the Box-Behnken (at the predicted point) showed that the proposed models are strongly valid. Then, a morphology study was done to modify the fiber by a combination of three types of a structure such as macro-void, sponge-like and sharp finger-like. It also improved the hydrophobicity of outer surface from 87 to 113° and the mean pore size of the inner surface from 108.12 to 560.14 nm. The DCMD flux of modified fiber (named HF-M) enhanced 62% more than HF-O when it was fabricated by considering both of RSM and morphology study results. Finally, HF-M was conducted for long-term desalination process up to 100 hr and showed stable flux and wetting resistance during the test. These stepwise approaches are proposed to easily predict the main properties of PVDF dual-layer hollow fibers by valid models and to effectively modify its structure.     

Modified poly(vinylidene fluoride) hollow fiber composite membranes reinforced by hydroxyapatite nanocrystal whiskers

The modified poly(vinylidene fluoride) (PVDF) hollow fiber composite membranes reinforced by hydroxyapatite (HAP) nanocrystal whiskers were fabricated with wet‐spinning method. The PVDF/HAP/N‐methyl‐2‐pyrrolidone dope solutions experienced delayed demixing mechanism, and the precipitation rate slightly increased as the HAP whisker content increased. The cross sections of PVDF‐HAP and neat PVDF hollow fiber composite membranes were composed of five distinct layers: two skin layers, two finger‐like sublayers, and a sponge‐like layer. The Young's modulus of and tensile strength of the PVDF‐HAP hollow fiber membranes gradually increased with the addition of nano‐HAP whiskers. The elongation ratio was also improved, which was different from the polymeric membranes modified by other inorganic nanofillers. The permeation flux of the PVDF‐HAP hollow fiber membranes slightly increased with the increase of HAP content in the composite membranes as its hydrophilicity was improved. The crystallization behaviors of PVDF in the composite membranes were also investigated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Engineering design of outer‐selective tribore hollow fiber membranes for forward osmosis and oil‐water separation

Outer‐selective thin‐film composite (TFC) hollow fiber membranes offer advantages like less fiber blockage in the feed stream and high packing density for industrial applications. However, outer‐selective TFC hollow fiber membranes are rarely commercially available due to the lack of effective ways to remove residual reactants from fiber's outer surface during interfacial polymerization and form a defect‐free polyamide film. A new simplified method to fabricate outer‐selective TFC membranes on tribore hollow fiber substrates is reported. Mechanically robust tribore hollow fiber substrates containing three circular‐sector channels were first prepared by spinning a P84/ethylene glycol mixed dope solution with delayed demixing at the fiber lumen. The thin wall tribore hollow fibers have a large pure water permeability up to 300 L m^?2 h^?1 bar^?1. Outer‐selective TFC tribore hollow fiber membranes were then fabricated by interfacial polymerization with the aid of vacuum sucking to ensure the TFC layer well‐attached to the substrate. Under forward osmosis studies, the TFC tribore hollow fiber membrane exhibits a good water flux and a small flux difference between active‐to‐draw (i.e., the active layer facing the draw solution) and active‐to‐feed (i.e., the active layer facing the feed solution) modes due to the small internal concentration polarization. A hyperbranched polyglycerol was further grafted on top of the newly developed TFC tribore hollow fiber membranes for oily wastewater treatment. The membrane displays low fouling propensity and can fully recover its water flux after a simple 20‐min water wash at 0.5 bar from its lumen side, which makes the membrane preferentially suitable for oil‐water separation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4491–4501, 2015     

NI/NI‐YSZ Current Collector/Anode Dual Layer Hollow Fibers for Micro‐Tubular Solid Oxide Fuel Cells

A co‐extrusion technique was employed to fabricate a novel dual layer NiO/NiO‐YSZ hollow fiber (HF) precursor which was then co‐sintered at 1,400 °C and reduced at 700 °C to form, respectively, a meshed porous inner Ni current collector and outer Ni‐YSZ anode layers for SOFC applications. The inner thin and highly porous “mesh‐like” pure Ni layer of approximately 50 μm in thickness functions as a current collector in micro‐tubular solid oxide fuel cell (SOFC), aiming at highly efficient current collection with low fuel diffusion resistance, while the thicker outer Ni‐YSZ layer of 260 μm acts as an anode, providing also major mechanical strength to the dual‐layer HF. Achieved morphology consisted of short finger‐like voids originating from the inner lumen of the HF, and a sponge‐like structure filling most of the Ni‐YSZ anode layer, which is considered to be suitable macrostructure for anode SOFC system. The electrical conductivity of the meshed porous inner Ni layer is measured to be 77.5 × 10⁵ S m^–1. This result is significantly higher than previous reported results on single layer Ni‐YSZ HFs, which performs not only as a catalyst for the oxidation reaction, but also as a current collector. These results highlight the advantages of this novel dual‐layer HF design as a new and highly efficient way of collecting current from the lumen of micro‐tubular SOFC.     

Influence of Coagulant Temperature and On-Line Drawing on the Mechanical Properties and Permeation Performance of Cellulose Acetate Hollow Fibers

Cellulose acetate (CA) ultrafiltration hollow fibers were prepared via the dry–wet spinning technique. In the spinning process, the coagulant temperature was varied, and hence the on-line draw ratio was affected. The results revealed that the maximal draw ratio increased with the increase of coagulant temperature up to 55 ^○C and then leveled off. The inner diameters, outer diameters and thickness of the hollow fiber decreased with the increase of the draw ratio. The tensile properties of the resulting hollow fibers were measured, and the breaking tensile stress increased with the increase of draw ratio. When the coagulant temperature was increased from 25 to 70 ^○C, the porosity increased, the pore size was slightly enlarged in the outer skin, the hydraulic permeability increased, and the percentage of retention R decreased. In summary, by increasing the coagulant temperature, the maximal draw ratio can be increased, and hence the mass transfer properties and the other properties of drawn CA hollow fiber can be varied.     

Structure control of asymmetric poly(vinyl butyral)‐TiO2 composite membrane prepared by nonsolvent induced phase separation

Poly(vinyl butyral) (PVB)‐TiO₂ composite hollow fiber membranes were prepared via nonsolvent induced phase separation (NIPS). The membrane had a skin layer on both the outer and inner surface at the initial stage after membrane preparation. However, the outer surface became porous with the passage of time, as the polymer in the membrane's outer surface was decomposed by the photocatalysis of TiO₂. The initial water permeability increased with the increase of TiO₂ content. Furthermore, for all the membranes, as time elapsed the water permeabilities increased and became constant after about 15 days, which was in accordance with the alteration on the membrane's outer surface. Despite decomposition of the polymer on the outer surface, particle rejection hardly changed because the inner surface kept the original structure. Thus, addition of TiO₂ to the membrane is a useful way to improve water permeability while maintaining particle rejection. The clear asymmetric structure with both porous structure at the outer surface and skin layer at the inner surface was achieved by the addition of TiO₂. Therefore, the addition of TiO₂ is a new method for achieving the high porosity at the outer surface of the hollow fiber membrane. In addition, tensile strength and elasticity kept constant over time and were higher than those of original PVB membranes. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of hypochlorite concentration on properties of posttreated outer‐skin ultrafiltration membranes spun from cellulose acetate/poly(vinyl pyrrolidone) blends

An outer‐skin hollow‐fiber ultrafiltration (UF) membrane was spun from a new dope solution containing cellulose acetate (CA)/poly(vinyl pyrrolidone) (PVP 360K)/N‐methyl‐2‐pyrrolidone (NMP)/water using a wet‐spinning technique. The as‐spun fibers were posttreated with a hypochlorite solution over a range of concentrations for a fixed period of 24 h. The experimental results showed that the pure water flux of the treated membrane increased with increasing hypochlorite concentration. The treated membrane experienced an increased fouling tendency with increasing hypochlorite concentration because the hydrophilicity of the treated membrane decreased as a result of the removal of PVP contents in the membrane matrix after hypochlorite treatment. SEM images revealed that the membrane had an outer dense skin, a porous inner surface, and a spongelike structure, which confirmed that addition of PVP favored the suppression of macrovoids in the membrane. The membrane pore size could be significantly increased when the hypochlorite concentration reached 200 mg/L. It was concluded that hypochlorite treatment provided an additional option to easily alter the pore size of UF membranes. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 227–231, 2005