Morphology Of Hollow Fiber Membranes Research Articles

Macrovoid-free high performance polybenzimidazole hollow fiber membranes for elevated temperature H2/CO2 separations

Thermally robust membranes are required for H2 production and carbon capture from hydrocarbon fuel derived synthesis (syn) gas. Polybenzimidaole (PBI) materials have exceptional thermal, chemical and mechanical characteristics and high H2 perm-selectivity for efficient syngas separations at process relevant conditions. The large gas volumes processed mandate the use of a high-throughput, small footprint hollow fiber membrane (HFM) platform. In this work, an industrially attractive spinning protocol is developed to fabricate PBI HFMs with unprecedented H2/CO2 separation performance. A unique dope composition incorporating an acetonitrile diluent is discovered enabling asymmetric macro-void free PBI HFM fabrication using a water coagulant. The influences of dope viscosity, coagulant chemistry, and air gap on HFM morphology are evaluated. Elevated temperature (up to 350 °C) H2 permeances of 400 GPU with H2/CO2 selectivities > 20 are achieved. This unprecedented separation performance is a ground breaking achievement at temperatures traditionally considered out-of-reach for polymeric membranes.

Preparation and characterization of polyvinylidenedifluoride-co-chlorotrifluoroethylene hollow fiber membranes with high alkaline resistance

In the present work, polyvinylidenedifluoride-co-chlorotrifluoroethylene (PVDF-co-CTFE) hollow fiber membranes were prepared via non-induced phase separation (NIPS) using 1-methyl-2-pyrrolidone (NMP) or triethyl phosphate (TEP) as the solvent. The thermodynamic characterization of both systems PVDF-co-CTFE/NMP/water and PVDF-co-CTFE/TEP/water) and the rheological properties of dope solutions were first investigated. Subsequently, the effect of dope concentration and spinning conditions such as air gap on the membrane permeability and mechanical strength was discussed in detail. The rejection of polystyrene with a size of 50 nm was 85, 100 and 100% for PVDF-co-CTFE hollow fiber membranes prepared with PVDF-co-CTFE concentrations of 14,17 and 20 wt%, while the permeability was 119, 39 and 2.5 L/m2 h bar, respectively. The morphologies of hollow fiber membranes were observed using scanning electron microscopy to evaluate the effect of solvent and spinning conditions on the membrane structure. The resultant PVDF and PVDF-co-CTFE hollow fiber membranes were immersed in a strong alkaline solution (NaOH (1 M)+NaClO (4000 ppm)) to evaluate the chemical resistance of PVDF-co-CTFE hollow fiber membranes in comparison with that of PVDF membranes. The mechanical properties of PVDF and PVDF-co-CTFE hollow fiber membranes immersed in the alkaline solution for 180 days were declined 90% and 10% as compared to the original ones, respectively. Moreover, the hollow fiber membranes were characterized using laser Raman spectra, Fourier-transform infrared spectra, differential scanning calorimetry, and X-ray photoelectron spectroscopy, before and after the immersion, to hypothesize the degradation mechanism of PVDF-co-CTFE and PVDF hollow fiber membranes in the alkaline solution.

Study on the evolution of reinforced PVDF hollow fiber membrane morphology and strong hydrophobicity

Study on the evolution of reinforced PVDF hollow fiber membrane morphology and strong hydrophobicity

Mechanism of formation of hollow fiber membranes for membrane distillation: 1. Inner coagulation power effect on morphological characteristics

Hollow fiber membrane morphology and inner surface structure were analyzed based on the thermodynamic and kinetics of the phase inversion process. Different nonsolvent mixtures formed by N,N-dimethyl acetamide (DMAC) and distilled water were considered. By means of Hansen solubility parameters, lower interaction between the nonsolvent and the mixed solvent (DMAC/trimethyl phosphate, TMP) was observed when greater amount of DMAC was added in the nonsolvent. The spinning solution became thermodynamically more stable and kinetically showed slower coagulation rates, being both related to the resultant membrane formation. In this first study poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes were prepared by the dry/wet spinning technique employing the above cited nonsolvent mixtures as an internal coagulant. Their effect on the internal structure of the membrane evolved towards a more open-porous inner surface when increasing the solvent content in the bore liquid. Various characterization techniques were used in order to analyze the adequacy of these membranes for desalination by direct contact membrane distillation (DCMD). It was found that all the properties together with the permeability of the hollow fibers improved when the coagulation power of the nonsolvent was reduced.

Preparation and characterization of ECTFE hollow fiber membranes via thermally induced phase separation (TIPS)

Abstract A porous poly (ethylene-chlorotrifluoroethylene) (ECTFE) membrane was prepared and evaluated in this study. Initially, solvent screening revealed diethyl phthalate (DEP) and glycerol triacetate (GTA) as proper solvents for the preparation of homogenous ECTFE solutions with the required properties for fabricating a hollow fiber membrane. The thermodynamic phase diagrams for both systems (ECTFE/DEP and ECTFE/GTA) showed a liquid–liquid (L-L) phase separation region, which is needed for a bicontinuous structure, prior to solid–liquid (S-L) phase separation (polymer crystallization). The ECTFE hollow fiber membranes were prepared by a thermally induced phase separation (TIPS) method using a unary solvent (DEP or GTA). The morphologies of hollow fiber membranes were observed by scanning electron microscopy (SEM) and it was observed that a bicontinuous structure was formed through L-L phase separation. The effect of the solvent types on properties of the prepared membrane such as water permeability and mechanical strength was discussed in detail. Moreover, the spherulite growth rate and crystallization degree were measured to assess the differences in the mechanical properties of the membranes prepared using DEP and GTA. In this study, with a polymer concentration of 20 wt%, the membrane showed a superior water flux and comparable mechanical properties with the ECTFE membranes already reported in published papers. Furthermore, the chemical resistance properties of ECTFE hollow fiber membranes were evaluated using the harsh treatments.

Effects of coagulation bath temperature on polyurethane-based hollow fiber membrane morphology

Polyurethane-based hollow fiber membranes were prepared via melt-spinning method. Effects of coagulation bath temperature (CBT) on membrane outer surface and inner surface morphology were studied in detail by image analysis software. Based on the SEM images, membrane surface pore diameter and porosity were calculated, whereas the interconnected pore size and its distribution were determined using capillary flow porometer. Results show that increasing CBT could improve the membrane outer surface porosity at least twice. Thinner hollow fiber membrane with larger inward water flux could be prepared at higher CB-I (the first coagulation bath) temperature. Higher CB-II temperature (the second coagulation bath) could cause pore contraction and reduce the average diameter of outer surface pore. CBT in the early stage controlled the obtained membrane pore size and its distribution.

Characterization and pervaporation dehydration of heat-treatment PAN hollow fiber membranes

In this study, heat-treated polyacrylonitrile (PAN) hollow fiber membranes have been used to investigate the pervaporation performances of aqueous ethanol solution. The heat-treated PAN hollow fiber membranes were prepared by heat treating the as-spun PAN precursor hollow fiber membranes at 120, 160 and 210°C for 12h under air atmosphere, respectively. Scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), simultaneous thermal analysis/mass spectrometry (TA/MS) and thermogravimetric analysis (TGA) are used to characterize the properties of hollow fiber membranes. The 90wt.% aqueous ethanol solution at 25°C has been used to investigate the pervaporation performances through hollow fiber membrane. Furthermore, in this investigation, we also use the positron annihilation spectroscopy (PAS) to identify the effect of heat-treatment on free volume variation of PAN hollow fiber membranes. To our knowledge, it is the pioneering work for PAS analysis technique used in hollow fiber membranes. The results have shown that the pervaporation performances of PAN hollow fiber membrane can be improved by heat-treatment. The permeation rate decreases while water content in permeate increases with increasing heat-treatment temperature. These phenomena might be due to that after heat-treating, the PAN hollow fiber membrane morphology becomes denser and the free volume of PAN polymer chain is reduced. Finally, heat-treated PAN/polyethylene glycol (PEG)10K blended hollow fiber membranes are also fabricated to investigate the pervaporation performance of aqueous ethanol solution. It has shown that the permeation rate as well as water content in permeate increases with increasing heat-treatment temperature. These phenomena can be interpreted that some of the PEG10K in hollow fiber membrane matrix has burnt away by heat-treating, and the occupied space of PEG10K would lead water molecules more easy to permeate. The long-term pervaporation stability of 210°C heat-treated PAN hollow fiber membranes was also investigated in this study. It has shown that the long-term pervaporation stability can last at least 250 days.

Preparation and characterization of semi-crystalline poly(ether ether ketone) hollow fiber membranes

Semi-crystalline poly(ether ether ketone) (PEEK) hollow fiber membranes were successfully prepared utilizing a two step membrane preparation process. In the first step, precursory hollow fibers were formed from blends of PEEK and poly(ether imide) (PEI) by melt extrusion. In the second step, porous PEEK hollow fiber membranes were formed by selective decomposition and removal of the polyimide phase utilizing a primary amine reagent monoethanolamine (MEA). Quantitative removal of the PEI phase was confirmed by FT-IR spectra and gravity analysis. Porous PEEK hollow fiber membranes were successfully prepared from PEEK/PEI blends with PEEK/PEI blend ratios of 50/50, 40/60 and 30/70, respectively. The hollow fiber membrane morphology, thermal stability characteristics, gas and organic solvent transport properties were studied in detail for the hollow fiber membrane prepared from the PEEK/PEI blend with PEEK/PEI blend ratio of 50/50. The average pore size of the porous PEEK hollow fiber membranes prepared from the 50/50 blend is 11 nm. The permeance for pure oxygen is 2.26 × 10 −5 m 3 (STP) m −2 s −1 kPa −1 (3000 GPU) and the permeance for nitrogen is 2.41 × 10 −5 m 3 (STP) m −2 s −1 kPa −1 (3200 GPU). The ideal pure gas selectivity for oxygen/nitrogen is 0.938. Temperature and organic solvent permeation studies confirmed that the PEEK hollow fiber membranes prepared by the novel process can be operated at high temperatures and exhibit solvent resistance superior to most conventional commercial polymeric membranes.

Effects of PVDF-HFP concentration on membrane distillation performance and structural morphology of hollow fiber membranes

Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by the dry/wet spinning technique using different copolymer concentrations in the dope solutions ranging from 17 to 24 wt.%. All the spinning parameters were maintained constant except the copolymer concentration. The morphological properties of the hollow fiber membranes were studied in terms of scanning electron microscopy (SEM), atomic force microscopy (AFM) and void volume fraction. The effects of PVDF-HFP content in the spinning solutions were also studied by measuring the water entry pressure and direct contact membrane distillation (DCMD) permeate flux of the hollow fiber membranes. An increase in the copolymer concentration of the spinning solution resulted in a decrease in the precipitation rate and a transition of the cross-section structure from a finger-type structure to a sponge-type structure. Pore size, nodule size and roughness parameters of both the internal and external hollow fiber surfaces were determined by AFM. It was observed that the pore size decreased in both the internal and external surfaces of the hollow fiber membranes with increasing the copolymer concentration and reached a minimum value at the outer surface for PVDF-HFP concentrations greater than 20 wt.%. Water entry pressure values were decreased whereas both the void volume fraction and the DCMD permeate flux increased with decreasing the copolymer concentration.

The investigation of irregular inner skin morphology of hollow fiber membranes at high-speed spinning and the solutions to overcome it

In the fabrication of hollow fiber membranes, a high-speed spinning is important to maximize the productivity and minimize the production cost. However, the high-speed spinning process has been shown to influence the morphology of the resultant fibers, especially inner layer of the fibers. For the first time, we report that the increase in the take-up speed results in the deformation of inner shapes of the fibers. This phenomenon is not desirable as it results in the formation of low mechanical strength fibers. It is hypothesized that the phenomenon occurs due to the formation of vacuum condition in the lumen side of the fiber, which may be due to rapid formation of dense skin inner layer. With increasing take-up speed the inner surface becomes denser leading to insufficient bore fluid supply which promotes more vacuum condition in the lumen side. Four possible solutions were proposed to eliminate the inner layer deformation phenomena, i.e. (1) by increasing the bore fluid flow rate, (2) by chemical modification of the bore fluid, (3) by simultaneously increasing dope and bore flow rates while maintaining a constant ratio between them and (4) by simultaneously increasing bore and dope flow rates in the same order with the increase in take-up speed. The increase in bore fluid flow rate can effectively eliminate the deformation. The addition of the solvent to the bore fluid can also eliminate the deformation when 70 wt.% NMP/water was used as bore fluid. No inner layer deformation was observed when dope and bore flow rates were increased up to 4 and 3 ml/min, respectively. However, the first three approaches may have negative side effects. The fourth approach gives us the most promising results. Even after increasing take-up speed up to 470% from its original take-up speed, the resultant fibers show no irregular formation at inner layer morphology and no changes in their inner and outer diameter. Thus, it can be said that by utilizing this approach, more flexible and easy control over fibers can be achieved.

Preparation and characterization of poly(phthalazinone ether sulfone ketone) hollow fiber ultrafiltration membranes with excellent thermal stability

Hollow fiber ultrafiltration membranes were prepared successfully from poly(phthalazinone ether sulfone ketone) (PPESK) with a dry/wet phase inversion technique. Ethylene glycol methyl ether, diethylene glycol (DegOH) and methyl ethyl ketone were used as non-solvent additives and N-methyl-2-pyrrolidone used as a solvent in membrane preparation. The effects of PPESK concentration, the type of additives and the concentration of DegOH in casting solution on the morphology and performance of hollow fiber ultrafiltration membranes were investigated, respectively. The structures of PPESK hollow fiber membranes including the cross-section, the inner/outer edge (details of the cross-sections at the inner/outer edge of the membrane), and the external surface were characterized by scanning electron microscope (SEM). It was found that the membrane performance is consistent and agrees with the membrane morphology. With the increase of PPESK concentration in the casting solution, the viscosity strongly increases and it becomes shear-rate dependent. The morphologies of hollow fiber membranes changed from fingerlike structure to sponge-shape structure, and the properties of ultrafiltration membrane showed that the pure water flux was about 159 L m −2 h −1, and PEG10,000 rejection was above 95% under the operating pressure of 0.1 MPa. PPESK hollow fiber ultrafiltration membranes prepared in this work was also investigated the thermal stability at different operating temperature. As a result, when the temperature of feed solution was raised from 15 to 100 °C, the permeation flux increased more than three times without significant change of rejection.

Hollow fiber ultrafiltration membranes with enhanced flux for humic acid removal

In this study, polyethersulfone (PES) hollow fiber ultrafiltration membranes with enhanced flux have been developed for removal of humic acid. The membranes were spun from dope solutions containing PES/poly(vinyl pyrrolidone) (PVP)/ N-methyl-2-pyrrolidone (NMP)/additive(s) by using the dry-jet wet-spinning process. Effects of additive such as water, LiNO 3 and 1,2-propanediol (1,2-PD) in dope and air gap length were investigated. Characterization of the membranes in terms of pure water flux and apparent retention for humic acid were conducted at a steady state under almost identical hydrodynamic operating conditions. The experimental results showed that all additives in the dope dramatically enhanced the flux of resultant membranes but reduced the retention depending on the additive. For 1,2-PD as an additive, the fiber with an inner skin spun under an air gap of 600 mm had higher both flux and retention than the wet-spun fiber with an outer skin. However, for LiNO 3 as an additive, 2 mm increase in air gap significantly enhanced both flux and retention. 1,2-PD enhanced the membrane flux but reduced the retention much significantly than LiNO 3. The best membrane had a pure water flux of 110 × 10 −5 L m −2 h −1 Pa −1 and the nominal MWCO 6000 with a retention rate of over 97% for humic acid. The SEM images revealed that the designed morphology of hollow fiber membranes could be obtained by adjusting the relative coagulation rate at each surface of the nascent fiber (adjusting composition of the bore fluid and air gap length in this study), resulting in a single skin at either outside or inside and a porous surface on the other side.

The observation of elongation dependent macrovoid evolution in single- and dual-layer asymmetric hollow fiber membranes

We have reported, for the first time, that macrovoids in asymmetric hollow fiber membranes may be completely eliminated at high elongational draws. The evolution of macrovoids vs. elongational draw was observed for both single- and dual-layer hollow fiber membranes. The number of macrovoids and the number of macrovoid layer decrease with an increase in elongational draw ratio, while the dimension of macrovoids varies with increasing elongational draw ratio until the macrovoids are fully eliminated. This study indicates that the elongational stress may play a much more important role than our original thoughts on hollow fiber membrane morphology.

Morphological aspects and structure control of dual-layer asymmetric hollow fiber membranes formed by a simultaneous co-extrusion approach

We have reviewed the development of dual-layer asymmetric hollow fiber membranes by a simultaneous co-extrusion approach during the past 17 years and conducted a systematic study to investigate the relationship among dual-layer hollow fiber membrane morphology, dope compositions, and spinning conditions. Using Matrimid 5218 as the outer-layer material and polyethersulfone (PES) and its blends as the inner-layer materials, the science and engineering factors to produce dual-layer hollow fiber membranes with high integrity have been investigated and the causes of forming a macrovoid-free structure have been examined. Both the concepts of critical dope viscosity and critical structure–transition thickness have been closely evaluated if they are suitable to explain the formation of a sponge-like structure at certain conditions. Experimental results indicate that the macrovoids in the inner PES dope cannot be easily eliminated by the modification of the inner dope viscosity, but can effectively suppressed by either the addition of polyethylene oxide (PEO) in the PES inner dope or spinning fibers at much higher elongational draw ratios. The effects of different shrinkage percentages of both inner and outer layers on overall membrane morphology were studied and the optimal approach to develop delamination-free interface was proposed.

The effects of air gap length on the internal and external morphology of hollow fiber membranes

A systematic study of the air gap effects on both the internal and the external morphology, permeability and separation performance of polyvinylidene fluoride (PVDF) hollow fiber membranes has been carried out. The hollow fibers were prepared using the dry-jet wet spinning process using a dope solution containing PVDF/ethylene glycol/ N, N-dimethylacetamide with a weight ratio of 23/4/73. Ethanol aqueous solution, 50% by volume, was used as internal and external coagulants. The inner and the outer surfaces of the prepared hollow fibers were analyzed by atomic force microscopy (AFM), while their cross-sectional structure was studied by scanning electron microscopy (SEM). Ultrafiltration experiments were conducted using non-ionic solutes of different molecular weights. The results show that both the pore sizes and nodule sizes have a log-normal distribution. The pore size, nodule size and roughness parameters of the inner and outer surfaces of the hollow fibers were affected by the air gap distance. Alignment of nodules to the spinning direction was observed. Experimental results indicate that an increase in air gap distance, from 1 to 80 cm , results in a hollow fiber with a lower permeation flux and a higher solute separation performance due to the decrease of the pore size. AFM analysis reveals that the air gap introduces an elongational stress because of gravity on the internal or external surfaces of the PVDF hollow fibers. At low air gap distance, the inner surface controls the ultrafiltration performance of the PVDF hollow fiber membranes because of its lower pore size, while at high air gap lengths the inner pore size becomes larger than the outer pore size. The turning point was observed at an air gap distance of 20.3 cm .

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.

Effect of Shear Stress within the Spinneret on Hollow Fiber Membrane Morphology and Separation Performance

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionORIGINAL ARTICLEThis notice is a correctionEffect of Shear Stress within the Spinneret on Hollow Fiber Membrane Morphology and Separation PerformanceT.-S. Chung, S. K. Teoh, W. W. Y. Lau, and M. P. SrinivasanCite this: Ind. Eng. Chem. Res. 1998, 37, 12, 4903Publication Date (Web):November 12, 1998Publication History Published online12 November 1998Published inissue 1 December 1998https://doi.org/10.1021/ie980833mCopyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views337Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (12 KB) Get e-Alerts Get e-Alerts

Effect of Shear Stress within the Spinneret on Hollow Fiber Membrane Morphology and Separation Performance

The effects of shear stress and shear experience within a spinneret during hollow fiber spinning on membrane morphology, gas separation performance, and thermal and mechanical properties have been experimentally determined. We purposely spun the hollow fibers using a wet phase inversion process and water as the external coagulant with the belief that the effect of gravity (elongational stress) on fiber formation can be significantly reduced and the orientation induced by shear stress within the spinneret can be frozen into the wet-spun fibers. In addition, we chose 80/20 NMP/H2O as the bore fluid with a constant bore fluid to dope fluid flow rate ratio in order to minimize the complicated coupling effects of elongational stresses, uneven internal and external solvent exchange rates, and substructure resistance on fiber formation and separation performance. Asymmetric hollow fibers for gas separation were spun from a 37% poly(ether sulfone) (PES)/N-methyl-2-pyrrolidone (NMP) dope solution using a spinneret with a L/ΔD (die length to flow channel gap) ratio of 17.5 that is much higher than the conventional spinneret. Experimental results suggest that hollow fiber membranes spun from this large L/ΔD die with high shear have a tighter molecular packing structure and therefore a higher selectivity that surpasses the intrinsic value but a lower permeance. For example, the selectivity of H2/N2 for fibers spun with high shear rate is 4-fold of the PES intrinsic value (292−307 vs 73.7). Hollow fibers spun from high shear have a lower coefficient of thermal expansion (CTE) and a higher loss modulus. Most surprisingly, we are not able to identify the nodular structure that has been observed previously in the as-cast flat membranes or at the outer skin of the hollow fibers spun from the spinneret with a small L/ΔD ratio. Clearly, the fully developed high shear stress within the spinneret has altered the thermodynamics of nodular formation, and the nodules either might not exist or become too small to be detected or deform into ambiguous elliptical shape. For the first time, we have also observed a threadlike inner skin structure in high-sheared membranes. In addition, the apparent dense layer thickness for the fiber spun with low shear is the thinnest that has ever been reported in the literature for hollow fiber membranes (450 Å).