Formation Of Hollow Fibers Research Articles

Appealing sheath‐core spun high‐performance composite carbon molecular sieve membranes

AbstractCarbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a “Pseudo Wheel+Hub & Spoke” asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO2 removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof‐of‐concept membrane for this application. Facile formation of the sheath‐core spun precursor with a 6FDA‐DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO2/CH4 (50 : 50) mixed gas feed with an attractive CO2/CH4 selectivity of 64.3 and CO2 permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA‐DAM in a composite sheath‐core CMS membrane with the “Pseudo Wheel+Hub & Spoke” structure.

Appealing sheath-core spun high-performance composite carbon molecular sieve membranes.

Carbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a "Pseudo Wheel+Hub & Spoke" asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO2 removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof-of-concept membrane for this application. Facile formation of the sheath-core spun precursor with a 6FDA-DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO2 /CH4 (50 : 50) mixed gas feed with an attractive CO2 /CH4 selectivity of 64.3 and CO2 permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA-DAM in a composite sheath-core CMS membrane with the "Pseudo Wheel+Hub & Spoke" structure.

Self-assembled poly(2-ethyl-2-oxazoline)/malonic acid hollow fibers in aqueous solutions

Well-defined poly(2-ethyl-2-oxazoline) (PEOX)/Malonic Acid (MA) fibers having hollow tubular morphology were shown to form in aqueous solutions at 25 °C by complexation induced self-assembly between PEOX and MA. The fibers had diameter of ~1–3 μm and a wall thickness of ~40 nm. Different interactions between PEOX and MA were identified for complexation as a function of pH. At pH2, when both ends of MA were protonated, H-bonded complexation was the driving interaction in the fiber formation. IR data showed both PEOX -CO band and MA -COOH band in dried fibers formed at pH2. The downshift in the CO stretching of PEOX by as much as 15 cm−1 confirmed the H-bonded complexation. The interaction enthalpy of PEOX and MA was determined by isothermal titration calorimetry (ITC) as −49.39 kJ/mol which is consistent with H-bonding. Thermogravimetric analysis (TGA) of the fibers showed two distinct decomposition temperatures one between 100 and 150 °C corresponding to MA and the other one at 350–450 °C corresponding to PEOX which also indicated the presence of both components in the fibers. At pH4, when one end of MA was protonated and the other end was ionized, electrostatic complexation between carboxylate (–COO−) group of MA and the amide group of PEOX was the driving interaction in the fiber formation. At pH7, when both ends of MA were ionized, fiber formation was significantly hindered. The results are important in understanding the role of different interactions in the hollow fiber formation mechanism as a function of pH. pH-responsive hollow fibers have great potential to be used in biomedical applications for drug delivery and release purposes.

Preparation of Cell-Paved and -Incorporated Polysaccharide Hollow Fibers Using a Microfluidic Device.

Cellular constructs having hollow tubular structures are expected to be used as artificial blood vessels. We have recently demonstrated that water-insoluble polyion complexes (PICs) were formed from water-soluble polysaccharides with opposite charges at the interface of coaxial flows, which resulted in the formation of hollow fibers. In this study, both inside- and outside-cell-laden chondroitin sulfate C (CS)/chitosan (CHI) hollow fibers were prepared by utilizing a microfluidic device and modification with cell adhesive molecules. Loading of type I collagen (COL) and surface modification with fibronectin and gelatin using layer-by-layer assembly techniques improved the adhesion and spreading of fibroblast cells to/on the surface of CS/CHI hollow fibers. On the other hand, by suspending mesenchymal stem cells (MSCs) in the core flow solution, cells were successfully loaded in the walls of the hollow fibers. As the culture time extended, cells trapped in the PIC structures constituting the wall of the hollow fibers migrated to the interface between the hollow fibers and the medium: cells adhered to and stretched "on" the lumen surfaces in the COL-loaded fibers. In contrast, for the case of unmodified hollow fibers, it was difficult for cells to adhere to the lumen surfaces. Therefore, cell aggregates were formed "in" the lumen. Results of the live/dead assay and MTT assay clearly demonstrated that MSCs possessed certain levels of cell viability and proliferated for up to 10 days, especially for the cases of COL-loaded hollow fibers. On the basis of these results, the utility of the present hollow fibers in the formation of cellular constructs corresponding to blood vessels is also discussed.

Mesporous 3C-SiC Hollow Fibers

In the present work, for the first time, we reported the exploration of mesoporous 3C-SiC hollow fibers via single-spinneret electrospinning of polyureasilazane (PSN) and polyvinylpyrrolidone (PVP) solution followed by high-temperature pyrolysis treatment. The resultant products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and N2 adsorption. The as-prepared hollow fibers with totally mesoporous walls were uniformly sized in diameter and high purity in morphology. They were composed of single-crystalline 3C-SiC nanoparticles with a surface area of 21.75 m2/g and average pore diameter of ~34 nm. The PSN concentration played a determined role on the formation of hollow fibers rather than the conventional solid counterparts, enabling their growth in a tunable manner. A possible mechanism was proposed for the formation of mesoporous SiC hollow fiber.

PVDF hollow fiber formation via modified NIPS method: Evolution elucidation of phase separation mechanism, structure and properties of membrane with coagulation strength varied

In this work, hollow fiber membrane was prepared from poly(vinylidene fluoride) (PVDF)/triethyl phosphate (TEP)/water systems by modified non-solvent induced phase separation technique (mNIPS), which was characterized in the process with relative high polymer content of 30 wt% in spinning dope. The ternary phase diagram and precipitation rate were determined to elucidate the various membrane formation mechanisms, and the membranes morphologies were observed by field emission scanning electronic microscopy (FESEM). It was indicated that the phase separation mechanism was found to heavily depend on the bath strength. Although the solid-liquid demixing process initiated the precipitation process, by changing the bath gradually from pure water to 40% TEP, the liquid-liquid demixing occurred earlier and earlier. Accordingly the top surface morphologies evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). In addition, the permeability, maximum stress at break, medium pore size and porosity of hollow fiber membranes were also investigated. In conclusion, the successful application of modified non-solvent induced phase separation (mNIPS) may provide an effective route to improve the membrane structure modulation.

Formation of defect-free 6FDA-DAM asymmetric hollow fiber membranes for gas separations

This paper reports the formation of defect-free 6FDA-DAM asymmetric hollow fiber membranes. 6FDA-polyimides are of great interest for advanced gas separation membranes, and 6FDA-DAM polyimide is a representative polymer in this family with attractive dense film properties for several potential applications. The work reported here for the 6FDA-DAM polyimide provides insight for the challenging fabrication of defect-free asymmetric hollow fiber membranes for this class of 6FDA-polyimides, which behave rather different from lower free volume polymers. Specifically, the 6FDA based materials show relatively slow phase separation rate in water quench baths, which presents a challenge for fiber spinning. For convenience, we refer to the behavior as more “non-solvent resistant” in comparison to other lower free volume polymers, since the binodal phase boundary is displaced further from the conventional position near the pure polymer-solvent axis on a ternary phase diagram in conventional polymers like Matrimid® and Ultem®. The addition of lithium nitrate to promote phase separation has a useful impact on 6FDA-DAM asymmetric hollow fiber formation. 6FDA-DAM phase diagrams using ethanol and water as non-solvent are reported, and it was found that water is less desirable as a non-solvent dope additive for defect-free fiber spinning. Phase diagrams are also reported for 6FDA-DAM dope formulation with and without the addition of lithium nitrate, and defect-free asymmetric hollow fiber membranes are reported for both cases. The effect of polymer molecular weight on defect-free fiber spinning was also investigated. Gas transport properties and morphology of hollow fibers were characterized. With several thorough case studies, this work provides a systematic guideline for defect-free fiber formation from 6FDA-polymers.

Giant hollow fiber formation through self-assembly of oppositely charged polyelectrolyte brushes and gold nanoparticles

We report on the use of binary mixtures of oppositely charged gold nanoparticles (AuNPs) and spherical polyelectrolyte brushes (SPBs), consisting of a polystyrene core onto which long polystyrene sulfonate chains are grafted, as a simple model system to investigate the influence of directional interactions on self-assembly. We demonstrate that the mixing ratio, i.e., the number of AuNPs per SPB, has a profound influence on self-assembly. In particular we report on the formation of giant hollow fibers, and present a thorough characterization of these nanostructures. We speculate that the adsorption of a few AuNPs on the SPBs appears to direct the tubular self-assembly, and discuss the analogy to the case of modified proteins such as tubulin under the action of nucleotides.

Investigation of unique interactions between cellulose acetate and ionic liquid [EMIM]SCN, and their influences on hollow fiber ultrafiltration membranes

This study investigates the molecular interactions between ionic liquid, 1-ethyl-3-methylimidazolium thiocyanate ([EMIM]SCN) and cellulose acetate (CA), employing not only experimental characterizations including FTIR and rheological tests, but also molecular dynamics simulations. Due to the electrostatic nature of ionic liquids, [EMIM]SCN interacts intensely with CA molecules through pronounced hydrogen bonding, Coulombic forces and van der Waals interactions, which play an important role in dissolving CA and also greatly contribute to the unique rheological characteristics of CA/[EMIM]SCN solutions. The competition between the charge-ordered structure and polymer chain entanglement in the CA/[EMIM]SCN solutions leads to a three-region flow curve under shear stress. The charge-ordered network in CA/[EMIM]SCN solutions as well as the affinity and unique solvent exchange characteristics between non-solvents and [EMIM]SCN are found to greatly influence the phase inversion paths of membranes. In addition, the effects of dope flow rate, dope temperature and air-gap distance on hollow fiber formation have been elucidated and correlated to the interactions between CA and [EMIM]SCN and the phase inversion mechanisms. By tuning the spinning conditions, CA hollow fiber membranes are successfully fabricated for ultrafiltration with a PWP value of 90.10 (L/m 2 bar h) and a mean effective pore diameter of 16.68 nm.

Preparation of Solid and Hollow Asphaltene Fibers by Single Step Electrospinning

Electrospinning has been used to produce micrometer size fibers, both solid and hollow from non-covalently associating small molecules of a low grade hydrocarbon reject of crude oil, asphaltene. Asphaltene defined as a solubility class of crude oil is by no means a polymer but relatively large organic molecule of molecular weight ranging from 500 to 1000 Dalton. The production of asphaltene fibers is feasible by electrospinning because of its unique molecular structure which allows molecular aggregation. Fibers with diameter ranging from 2–20 μm were successfully spun from an asphaltene in toluene solution. Adding 2% hydrogen peroxide enhances electrospinning, leading to formation of hollow fibers, whereas adding 2% water inhibits electrospinning. The structure and morphology of the electrospun fibers were investigated with optical microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and time-of-flight secondary ion mass spectroscopy. The electrospun asphaltene fibers offer the potential for direct fabrication of membranes without use of multiple synthetic steps, complex electrospinning designs, or post processing surface treatments.

High temperature transformation of electrospun BaZrO3 nanotubes into nanoparticle chains

BaZrO3 nanofibers were electrospun from a 0.25 M metal cation solution containing barium acetate and zirconium 2,4-pentadionate dissolved in glacial acetic acid. Polyvinylpyrrolidone was added to increase the solution viscosity and allow formation of hollow fibers during electrospinning. Microstructural evolution during thermal annealing at temperatures between 600 °C and 1400 °C was studied revealing crystal growth evolution. Strong grain coarsening is observed above 1200 °C, leading to linear and branched nanoparticle chains originating from the finer initial hollow nanofibers. Faceting of the nanoparticles occurs in the low-energy surfaces. On the other hand, grain boundaries are planar and follow the same facet planes as the nanoparticles. High resolution transmission electron microscopy reveals no general epitaxial relationship at the interface with the exception of a minor amount of low angle grain boundaries.

The role of additives on dope rheology and membrane formation of defect-free Torlon ® hollow fibers for gas separation

The high demands on high performance membranes for energy, water and life science usages provide the impetus for membrane scientists to search for a comprehensive understanding of membrane formation from molecular level to design membranes with desirable configuration and separation performance. This pioneering work is to elaborate the importance of polymer rheology on hollow fiber formation and reveal the integrated science bridging polymer fundamentals such as polymer cluster size, shear and elongational viscosities, molecular orientation, stress relaxation to membrane microstructure and separation performance for gas separation. Torlon ® poly(amide imide) was employed in this study with various solvent/nonsolvent additives. The effects of additives on polymeric cluster size, hydrogen bonding and dope rheology during the phase inversion have been examined. It is found that hydrogen bonding and strain-hardening characteristics play very important roles in dope rheology and membrane separation performance. Torlon ® possesses strong hydrogen bonds with NMP/water mixtures, the addition of a small amount of water enlarges polymer cluster size, strengthen molecular network (i.e., strain hardening) and facilitate macrovoid-free morphology. However, strong hydrogen bonding may retard chain unfolding during spinning, induce faster relaxation for highly oriented dense-selective skin, and thus reduce gas-pair selectivity. By adjusting dope chemistry and spinning conditions with balanced solubility parameters and dope rheology, we have developed defect-free Torlon ® hollow fiber membranes with an O 2/N 2 selectivity of 8.55 and an ultra-thin layer of 488 Å simply using water as the additive. Fibers spun from dopes containing other additives have the optimal O 2/N 2 selectivity varying from 7.69 to 9.97 at 25 ± 2 °C, and the dense layer thickness varying from 500 Å to 2000 Å. Their corresponding mixed-gas O 2/N 2 selectivity for compressed air varies from 7.12 to 9.00.

Interrupted Wet‐Spinning Process for Chitosan Hollow Fiber Elaboration

AbstractSummary:Chitosan hollow fiber formation with a variable internal diameter was performed from a new wet‐spinning method in interrupted coagulation conditions by means of water washings. The main objective of the present study is to provide a better understanding of the physicochemical events involved during the chitosan fiber formation by this new wet‐spinning method.

La 2(Zr 0.7Ce 0.3) 2O 7—A new oxide ceramic material with high sintering-resistance

Oxide ceramics with high sintering-resistance above 1473 K have very important applications in thermal barrier coatings (TBCs), catalytic combustion and high-temperature structural materials. Lanthanum zirconate (La 2Zr 2O 7, LZ) is an attractive TBC material which has higher sintering-resistance than yttria stabilized zirconia (YSZ), and this property could be further improved by the proper addition of ceria. The composite La 2(Zr 0.7Ce 0.3) 2O 7 (LZ7C3) has the highest sintering-resistance, and it is observed that fine particles of the second phase in the composite concentrate at the grain boundary to prevent the grain growth of the main phase. The formation of hollow fibers is helpful to the further improvement of sintering-resistance of LZ7C3, and this technique may be widely applied to protect the catalyst support against thermal sintering.

Hollow fiber membranes obtained by simultaneous spinning of two polymer solutions: a morphological study

The simultaneous spinning of two polymer solutions by using a triple orifice spinneret presents several benefits such as the possibility of forming each membrane layer by different concepts. However, besides the complexity of the phenomena involved in hollow fiber formation, mass transfer between the polymer solutions requires further investigation. In the present work, the simultaneous spinning of two different polymer solutions to form the top and support layers of the membrane, respectively, was investigated. Polyetherimide and polyethersulfone were used as base polymers. A polymer solution composed of Lewis acid:base complex (adipic acid–Lewis acid, and N-methyl-2-pyrrolidone–Lewis base), and another composed of a macromolecular additive (polyvinylpyrrolidone) were selected in order to form the support layer. Different polymer solutions composed of a volatile component (tetrahydrofuran) were used for top layer. Water and N-methyl-2-pyrrolidone were used at different compositions for the bore liquid, whereas pure water was used for the external precipitation bath. The distance between spinneret and precipitation bath was varied. The membranes were characterized by scanning electron microscopy. Cloud-point experiments were also carried out for characterizing the miscibility region of the polymer solutions. The parameters that allow keeping the stability in the region between the polymer solutions before interpenetration of the solutions seemed to be of greater importance for layers adhesion. According to this, the bore liquid composition seemed to provide an important contribution to fulfill such a requirement.

Formation of hollow fibers in the melt‐spinning process

Formation of hollow fibers in the melt‐spinning process

Formation of hollow fibers in the melt‐spinning process

AbstractThis article reports an investigation of the formation of hollow fibers in a melt‐spinning process. Experimental results indicate that die swelling is largely responsible for a negative effect on hole formation. The factors that positively affect die swelling, including a decrease in temperature, a decrease in capillary length, and an increase in shear rate, are thus not recommended for the spinning of hollow fibers. For vinyl‐type polymers such as polypropylene, in which the apparent elasticity leads to serious die swelling, the formation of hollow fibers is more complex than that of a typical condensation polymer. Our results further demonstrate that when hollow fibers are being made in a variety of shapes (but of the same denier), spinning a polygonal hollow fiber is significantly more unstable than spinning a circular one. Moreover, an asymmetric bridge along the polygonal contour leads to a melt twist and interrupts the entire spinning process. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2896–2902, 2001

Effects of initial structure on the deformation behavior of PP hollow fiber in continuous drawing

AbstractThe effects of the microcrystalline structure of undrawn fibers on the rheological behavior in the process of the polypropylene (PP) hollow fiber formation system were studied by a simple model describing the continuous drawing process. The predicted and observed drawing behaviors were explained in view of a simple approach based on the concept of strain‐rate sensitivity and strain‐hardening parameters. Various parametric studies showing the interactive nature of the strain‐rate sensitivity and strain‐hardening parameters on the drawing behavior were also numerically performed. Strain‐rate sensitivity affects mainly the intensity of the neck, and strain hardening has more effects on the position of the inflection point in continuous drawing. Details of the necking mechanism in the drawing process were studied by observing the deformation behavior of the specimens with different initial microcrystalline structures. The distinctive initial structure of undrawn PP hollow fibers can be formed by controlling the quenching condition in melt spinning. It was shown that the water‐quenched PP fiber exhibited an unstable smectic form, whereas the stable monoclinic phase was observed for the fiber prepared without quenching by forced convection. The experimental results of the drawing behavior indicated that the strain‐rate sensitivity of the fiber with a smectic form was larger than that with the monoclinic form. It was also shown that the hollowness was affected by the quenching condition in melt spinning and the deformation behavior in the drawing process. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2170–2182, 2001

Hollow fiber formation using Lewis acid : Base complex in the polymer solution

AbstractIn the present work, hollow fiber formation was investigated by using Lewis acid : base complexes in the polymer solution. The studied systems consisted of poly(ether sulfone); N‐methyl‐2‐pyrrolidone (NMP) as solvent and Lewis base, and acetic, propionic, n‐caproic, and adipic acids as additives and Lewis acids. Bore liquid was formed by water/NMP solutions, as well as vaseline; whereas water was used as external precipitation bath. The spinneret and precipitation bath distance (i.e., air gap) also varied. The membranes were characterized by scanning electron microscopy and pure gas permeation tests. The influence of the complex dissociation rate was observed mainly when a reduction to water inflow from the bore liquid to the polymer solution occurred, because in this case, dissociation rate was inhibited. Therefore, stability was favored in the sublayer for a longer period, allowing macrovoid formation. These results are also evidenced by the permeability coefficients, which increased as fast as the polymer solution reaches water (i.e., the kinetics of precipitation was accelerated). © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 908–917, 2001

High-quality recycling : Leis, Adalbert in Kunststoffe plast Europe, vol 90, issue 2, 2000

The simultaneous spinning of two polymer solutions by using a triple orifice spinneret presents several benefits such as the possibility of forming each membrane layer by different concepts. However, besides the complexity of the phenomena involved in hollow fiber formation, mass transfer between the polymer solutions requires further investigation. In the present work, the simultaneous spinning of two different polymer solutions to form the top and support layers of the membrane, respectively, was investigated. Polyetherimide and polyethersulfone were used as base polymers. A polymer solution composed of Lewis acid:base complex (adipic acid–Lewis acid, and N-methyl-2-pyrrolidone–Lewis base), and another composed of a macromolecular additive (polyvinylpyrrolidone) were selected in order to form the support layer. Different polymer solutions composed of a volatile component (tetrahydrofuran) were used for top layer. Water and N-methyl-2-pyrrolidone were used at different compositions for the bore liquid, whereas pure water was used for the external precipitation bath. The distance between spinneret and precipitation bath was varied. The membranes were characterized by scanning electron microscopy. Cloud-point experiments were also carried out for characterizing the miscibility region of the polymer solutions. The parameters that allow keeping the stability in the region between the polymer solutions before interpenetration of the solutions seemed to be of greater importance for layers adhesion. According to this, the bore liquid composition seemed to provide an important contribution to fulfill such a requirement.