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     

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.     

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.     

Simultaneously tuning dense skin and porous substrate of asymmetric hollow fiber membranes for efficient purification of aggressive natural gas

Highly permeable, selective, and stable asymmetric membranes are required to replace the traditional separation approaches for natural gas purification with higher energy efficiency and smaller footprints. Herein, we report on the design and engineering of defect-free asymmetric hollow fiber membranes with a thin dense skin and highly porous substrate to effectively deal with aggressive natural gas. A crosslinkable polymer with rigid molecular structure and high molecular weight was synthesized for developing spinning dope with desirable solution properties. Phase separation behavior of the polymer was carefully controlled by systematic formulation of the dope composition and optimizing spinning conditions, thereby realizing simultaneously tuning dense skins and porous substrates of the spun asymmetric hollow fiber membranes. The crosslinked hollow fiber membrane, with well-preserved delicate asymmetric nanostructures, exhibited unprecedentedly high and stable separation performance for long-term processing extremely aggressive CO₂/CH₄ mixtures (with pressure up to 820 psi containing C6⁺ hydrocarbons), thereby showing great potential for practical application of natural gas purification. This work offers a new platform to create hollow fiber membranes with both high permeance and plasticization resistance in natural gas service. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1269–1280, 2019     

Effect of polyethylene glycol molecular weights and concentrations on polyethersulfone hollow fiber ultrafiltration membranes

Polyethersulfone (PES) hollow‐fiber membranes were fabricated using poly(ethyleneglycol) (PEG) with different molecular weights (MW = PEG200, PEG600, PEG2000, PEG6000, and PEG10000) and poly(vinyl pyrrolidone) PVP40000 as additives and N‐methyl‐2‐pyrrolidone (NMP) as a solvent. Asymmetric hollow‐fiber membranes were spun by a wet phase‐inversion method from 25 wt % solids of 20 : 5 : 75 (weight ratio) PES/PEG/NMP or 18 : 7 : 75 of PES/(PEG600 + PVP40000)/NMP solutions, whereas both the bore fluid and the external coagulant were water. Effects of PEG molecular weights and PEG600 concentrations in the dope solution on separation properties, morphology, and mechanical properties of PES hollow‐fiber membranes were investigated. The membrane structures of PES hollow‐fiber membranes including cross section, external surface, and internal surface were characterized by scanning electron microscopy and the mechanical properties of PES hollow‐fiber membranes were discussed. Bovine serum albumin (BSA, MW 67,000), chicken egg albumin (CEA, MW 45,000), and lysozyme (MW 14,400) were used for the measurement of rejection. It was found that with an increase of PEG molecular weights from 200 to 10,000 in the dope solution, membrane structures were changed from double‐layer fingerlike structure to voids in the shape of spheres or ellipsoids; moreover, there were crack phenomena on the internal surfaces and external surfaces of PES hollow‐fiber membranes, pure water permeation fluxes increased from 22.0 to 64.0 L m^?2 h^?1 bar^?1, rejections of three protein for PES/PEG hollow‐fiber membranes were not significant, and changes in mechanical properties were decreased. Besides, with a decrease of PEG600 concentrations in the dope solution, permeation flux and elongation at break decreased, whereas the addition of PVP40000 in the dope solution resulted in more smooth surfaces (internal or external) of PES/(PEG600 + PVP40000) hollow‐fiber membranes than those of PES/PEG hollow‐fiber membranes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3398–3407, 2004     

Advanced porous polyphenylsulfone membrane with ultrahigh chemical stability and selectivity for vanadium flow batteries

Porous polyphenylsulfone (PPSU) membranes are facilely prepared via the nonsolvent-induced phase separation method. The typical asymmetric structure of such prepared porous membranes can be controlled by optimizing the sulfonation degree of the sulfonated poly(ether ether ketone) to 84.7% in the casting solution. Scanning electron microscopy images show that the porous membrane comprises a thin dense top skin layer, a sublayer structure with distinct long finger-like pores and the large pores in the substructure. The porous PPSU membrane was then used in vanadium flow battery (VFB). The optimized porous membrane yields an admirable performance, including excellent selectivity, chemical stability, and high columbic efficiency. Furthermore, the low cost of porous PPSU membranes indicates the promise of this technology for use in VFB applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47752.     

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     

Preparation and performances of poly(phthalazinone ether sulfone ketone) hollow fiber membranes with excellent thermotolerance

The cloud points of PPESK/NMP/H₂O ternary system at different temperatures were measured by titrimetric method. The binodal lines in the ternary phase diagram of the poly(phthalazinone ether sulfone ketone (PPESK) dope system was determined, on the basis of the cloud point experimental data being linearly fitted with the semiempirical linear cloud point correlation. Furthermore, phase separation behavior during the phase inversion of PPESK membrane‐forming system was discussed in terms of the phase diagram. Then, dry–wet spinning technique was employed in manufacturing PPESK hollow fiber membranes by immersion precipitation method. The cross‐section morphologies of hollow fibers were observed by scanning electronic microscopy. Also, the effects of dope solution composition and spinning parameters, including the coagulant composition and the spinning temperature on the separation performances of fibers, were evaluated by permeability measurements. The thermotolerance of the PPESK hollow fiber membranes prepared in the work was examined for the permeation operation at different temperatures and pressure differences. The experimental results showed that pure water flux increases several fold along with the temperature increases from 20 to 80°C at different operation pressures, while the solute rejection only decreases slightly. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 878–884, 2006     

Hollow Fiber Ultrafiltration Membranes from Poly(Vinyl Chloride): Preparation,Morphologies, and Properties

Hollow fiber poly(vinyl chloride) membranes were prepared by using the dry/wet spinning method. Cross-section, internal, and external surfaces of the hollow fibers structure were studied by SEM. The pore size and pore size distribution of the hollow fibers were measured by a PMI capillary flow porometer. UF experiments of pure water and aqueous solution of PVP K-90 were carried out. The effect of the PVC concentration on the hollow fibers mechanical properties was also investigated. It was found that the PVC fibers cross-sectional structure was affected by the polymer concentration in the dope solution. In particular, reduction of macrovoids size was observed when increasing PVC concentration from 15 to 19 wt%. The pore size distribution of the PVC hollow fibers was controlled by adjusting the PVC concentration. Indeed, an increase of PVC concentration up to 19 wt% leads to fibers with sharp pore size distribution (the 99% of pores is about 0.15 µm).The pure water permeation flux decreased from 162 to 128 (l/m² · h · bar), while the solute separation performance increased from 82 to 97.5%, when increasing the PVC concentration. The elongation at break, the tensile strength, and the Young's modulus of the PVC hollow fibers were improved with PVC concentration in dope solution.     

纺丝液喷出流量的数学模型

纺丝液喷出流量对中空纤维膜的形态和性能具有重要影响,而纺丝液的喷出流量与其黏度和压强关系密切。试验首先以二甲基乙酰胺为溶剂,聚砜为聚合物,聚乙烯基吡咯烷酮和丙酮为添加剂配制纺丝液,研究纺丝液喷出流量分别与其黏度和压力的关系;然后,研究纺丝液喷出流量数学模型;最后,以二甲基乙酰胺为溶剂,聚醚砜为聚合物,聚乙烯基吡咯烷酮和丙酮为添加剂配制纺丝液,对模型进行验证。研究表明:纺丝液喷出流量与黏度关系为Q∝μ-1.08,与压强关系为Q∝p1.18,对试验所用设备,纺丝液喷出流量Q=426μ-1.08p1.18。验证结果表明,该模型具有较普遍适用性,对中空纤维膜的制作和研究具有重要的指导意义。     

Long‐term study of CO2 absorption by PVDF/ZSM‐5 hollow fiber mixed matrix membrane in gas–liquid contacting process

In order to fabricate hollow fiber mixed matrix membrane (HFMMM) for long‐term CO₂ absorption process, ZSM‐5 (Zeolite Socony Mobil–5) zeolite was modified using hexadecyltrichlorosilane for increasing hydrophobicity and then added to the polyvinylidene fluoride (PVDF) spinning dope. The in‐house made HFMMMs were characterized in terms of gas permeance, overall porosity, average pore size, effective surface porosity, surface roughness, mechanical stability, and wetting resistance. The morphology of the HFMMMs was studied using SEM. The cross‐sectional SEM images indicated that the membrane structure has changed from sponge‐like to finger‐like by ZSM‐5 loading. The surface roughness increased by increasing ZSM‐5 concentration in the spinning dope. The HFMMM spun from the spinning dope with 0.5 wt % of ZSM‐5 zeolite showed that the CO₂ absorption flux decreased 18.9% in the initial 115 h of the operation and then the absorption flux remained constant until the end of the operation. For plain PVDF HFM the absorption flux decreased 36% from the initial value in the first 15 h of the experiment. Thus it could be concluded that the long term stability of HFM was improved by the incorporation of ZSM‐5. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44606.     

Pushing the limits of high performance dual‐layer hollow fiber fabricated via I2PS process in dehydration of ethanol

The immiscibility induced phase separation (I²PS) process was introduced as a novel method to fabricate hollow fibers with exceptionally high water permeance and reasonably high water/ethanol selectivity in dehydration of ethanol by pervaporation. As a continuation of the previous work, this study discloses the mechanisms to enhance the performance of hollow fibers spun via I²PS by elucidating the material selection at the inner‐layer. Moreover, it revealed the methods to reduce mass‐transport resistance by enhancing surface porosity for both inner and outer surfaces to further improve the permeation flux of the membranes. The continuous performance test demonstrates that the fibers spun from the I²PS possess a stable dehydration performance throughout the monitored period of 300 h. A comparison with pervaporation membranes in the literatures verifies the superiority of the membranes spun via I²PS process with the highest water permeation flux of 9.5 kg/m² h and the permeate water purity of 95.8 wt % at 80°C. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3006–3018, 2013     

Modeling study of oxygen permeation through an electronically short‐circuited YSZ‐based asymmetric hollow fiber membrane

Here, oxygen fluxes through an electronically short‐circuited asymmetric Ag‐YSZ|YSZ|LSM‐YSZ hollow fiber prepared via a combined spinning and sintering route were tested and correlated to an explicit oxygen permeation model. The average oxygen permeation through such asymmetric hollow fiber with a 27 μm‐thick YSZ dense layer reached 0.52 mL (STP) cm^?2 min^?1 at 1173 K. From the model results, we can obtain the characteristic thickness, the effects of the temperature, and the effect of He sweep gas flow rate to the individual step contribution. The oxygen partial pressure variation in the permeate side, the local oxygen flux, and the three‐different resistance distribution along the axial direction of the asymmetric hollow fiber are theoretically studied; providing guidelines to further improve the membrane performance for oxygen separation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3491–3500, 2017     

湿纺工艺纺制PAN中空纤维成形机理研究

利用C型喷丝板进行挤出凝固,采用湿法纺丝工艺制备聚丙烯腈(PAN)中空纤维,从PAN/二甲基亚砜(DMSO)纺丝原液的流变性能和凝固过程的相分离两个方面探讨了PAN中空纤维的成形机理。结果表明:纺丝液随剪切速率(γ)的增加逐渐发生由粘性向弹性的转变是挤出胀大的主要原因,其粘弹转变点随着温度的升高而向高γ移动,在60℃下的纺丝液弧片接触成孔的理论临界γ为212 s~(-1);纺丝液在凝固浴中表层成膜是PAN-DMSO-H_2O三元体系相分离的结果,纺丝液细流表面成膜速度是影响孔结构闭合的重要因素,可以通过凝固浴浓度和凝固浴调节剂来控制。     

Integrally skinned polysulfone hollow fiber membranes for pervaporation

From polysulfone as polymer, integrally skinned hollow fiber membranes with a defect-free top layer have been spun. The spinning process described here differs from the traditional dry-wet spinning process where the fiber enters the coagulation bath after passing a certain air gap. In the present process, a specially designed tripple orifice spinneret has been used that allows spinning without contact with the air. This spinneret makes it possible to use two different nonsolvents subsequently. During the contact time with the first nonsolvent, the polymer concentration in the top layer is enhanced, after which the second coagulation bath causes further phase separation and solidification of the ultimate hollow fiber membrane. Top layers of ± 1 μm have been obtained, supported by a porous sublayer. The effect of spinning parameters that might influence the membrane structure and, therefore, the membrane properties, are studied by scanning electron microscopy and pervaporation experiments, using a mixture of 80 wt % acetic acid and 20 wt % water at a temperature of 70°C. Higher fluxes as a result of a lower resistance in the substructure could be obtained by adding glycerol to the spinning dope, by decreasing the polymer concentration, and by adding a certain amount of solvent to the bore liquid. Other parameters studied are the type of the solvent in the spinning dope and the type of the first nonsolvent. © 1994 John Wiley & Sons, Inc.     

Pervaporation separation of water/isopropanol mixtures through crosslinked carboxymethyl chitosan/polysulfone hollow‐fiber composite membranes

Carboxymethyl chitosan (CMCS)/polysulfone (PS) hollow‐fiber composite membranes were prepared through glutaraldehyde (GA) as the crosslinking agent and PS hollow‐fiber ultrafiltration membrane as the support. The permeation and separation characteristics for dehydration of isopropanol were investigated by the pervaporation method. Pure chitosan, carboxymethyl chitosan, and crosslinked carboxymethyl chitosan membranes were characterized by Fourier transform infrared (FT‐IR) spectroscopy and X‐ray diffraction (XRD) to study the crosslinking reaction mechanism and degree of crystallinity, respectively. The effects of feed composition, crosslinking agent, membrane thickness, and feed temperature on membrane performance were investigated. The results show that the crosslinked CMCS/PS hollow‐fiber composite membranes possess high selectivity and promising permeability. The permeation flux and separation factor for isopropanol/water is 38.6 g/m²h and 3238.5, using 87.5 wt % isopropanol concentration at 45°C, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1959–1965, 2007     

Preparation of poly(ether sulfone) hollow fiber UF membrane for removal of NOM

In this study, a high performance poly(ether sulfone) (PES) hollow fiber ultrafiltration (UF) membrane has been prepared for removal of natural organic matter (NOM). The membrane was spun from a dope solution containing PES/poly (vinyl pyrrolidone) (PVP 40K)/N‐methyl‐2‐pyrrolidone (NMP) by using a wet‐spinning process. Characterization of the membrane in terms of pure water flux, molecule weight cut‐off (MWCO), and retention for a model humic acid (HA) were conducted, and the fouling resistance was analyzed. The experimental results showed that the membrane had a pure water permeability of 20 × 10^?5 L m^?2 h^?1 Pa^?1 and a nominal MWCO of 6000 Da. The results also showed that the membrane retention for humic acid was over 97% and both productivity and selectivity for HA increased with increasing feed velocity. The PES membrane in this study exhibited a much lower fouling tendency than the commercial polysulfone membrane. SEM images revealed that the membrane had an outer dense skin and a porous inner surface. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 430–435, 2006     

Sulfonated polyphenylsulfone asymmetric membranes: Effect of coagulation bath (acetic acid‐NaHCO3/isopropanol) on morphology and antifouling properties

Sulfonated polyphenylsulfone porous asymmetric membranes, S‐PPSU with different sulfonation degrees, 21, 33, 50 wt %, were prepared by phase inversion. Two different coagulation baths were explored for asymmetric membrane preparation: acetone/isopropanol and acetic acid (AA)‐NaHCO₃/isopropanol. The latter bath allows better morphology control for the nucleation and pore formation of the membrane. Scanning electron microscopy of membranes shows that pore interconnectivity is improved, when the mixture of AA‐NaHCO₃/isopropanol was used for asymmetric S‐PPSU ultrafiltration membranes preparation. S‐PPSU asymmetric membranes show an increasing hydrophilicity with increasing sulfonation degree. Asymmetric membrane antifouling properties improve as the concentration of sulfonic groups increases in the membrane showing twice the flux recovery ratio and lower BSA protein absorption in static and dynamic flux tests. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44502.     

Fabrication of efficient pervaporation desalination membrane by reinforcement of poly(vinyl alcohol)–silica film on porous polysulfone hollow fiber

Pervaporation membrane technology is commercially successful in the dehydration of organic solvents, and the technology has potential for seawater desalination with high recovery because of its capability to treat highly saline water. But to make the technology advantageous over the other available membrane desalination technologies in terms of productivity flux without additional energy cost, the selective barrier layer is required to be extremely thin, defect‐free, hydrophilic, and selective to water. In this work, we prepared an efficient membrane by reinforcing a highly water‐permeable but continuous barrier layer of poly(vinyl alcohol)–silica (PVA‐SiO₂) hybrid material on porous polysulfone hollow fibers. The PVA‐SiO₂ in acidified and hydrated ethanol was aged at room temperature for a period to allow solvent evaporation to obtain the solution concentration desired for the reinforcement. The reinforced hollow fiber membrane with optimal PVA‐SiO₂ barrier layer thickness exhibited a performance with a flux of 20.6 L m^?2 h^?1 and 99.9% salt rejection from a saline feed of 2000 ppm NaCl at 333 K. The effects of PVA‐SiO₂, temperature, and feed salinity on the pervaporation performance of the membrane were also studied. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45718.     

Fabrication of 6FDA‐durene polyimide asymmetric hollow fibers for gas separation

We have developed defect‐free asymmetric hexafluoro propane diandydride (6FDA) durene polyimide (6FDA‐durene) hollow fibers with a selectivity of 4.2 for O₂/N₂ and a permeance of 33.1 ×10^?6 cm³ (STP)/cm²‐s‐cmHg for O_2. These fibers were spun from a high viscosity in situ imidization dope consisting of 14.7% 6FDA‐durene in a NMP solvent and the inherent viscosities (IV) of this 6FDA‐durene polymer was 0.84 dL/g. Low IV dopes cannot produce defect‐free hollow fibers, indicating a 6FDA‐durene spinning dope with a viscosity in the region of chain entanglement seems to be essential to yield hollow fibers with minimum defects. The effects of spinning parameters such as shear rates within a spinneret and bore fluids as well as air gap on gas separation performance were investigated. Experimental data demonstrate that hollow fibers spun with NMP/H₂O as the bore liquid have higher permeances and selectivities than those spun with glycerol as the bore liquid because the former has a relatively looser inner skin structure than the latter. In addition, the selectivity of hollow fibers spun with NMP/H₂O as the bore liquid changes moderately with shear rate, while the selectivity of hollow fibers spun with glycerol are less sensitive to the change of shear rate. These distinct behaviors are mainly attributed to the different morphologies generated by different bore fluids. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2166–2173, 2001