- Email: [email protected]
Novel hollow fiber membranes with defined unit-step morphological change
Journal of Membrane Science 193 (2001) 123–128
Novel hollow fiber membranes with defined unit-step morphological change Tai-Shung Chung a,b,∗ , Chan Mya Tun b , K.P. Pramoda c , Rong Wang d a Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Department of Material Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore d Environmental Technology Institute, Innovation Centre (NTU), Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723, Singapore b
c
Received 1 February 2001; received in revised form 21 May 2001; accepted 21 May 2001
Abstract We have found that sharply defined unit-step morphological changes can be created in the middle of the cross-section of hollow fiber membranes wet-spun from highly concentrated immiscible poly(2,2 -m-phenylene)-5,5 -polybenzimidazole (PBI) and polysulfone (PSf) blend solutions. A halo is observed with yellowish color having wavelength in the range 580–595 nm. The abrupt change in transmittance light implies distinct morphological changes at the interface because the halo region possesses significantly different morphology and pore sizes from the inner and outer regions. The width of the halo ring is approximately 18–20% of the hollow fiber wall thickness. The mapping image and spectra obtained from field emission scanning electron microscopy coupled with energy dispersive X-ray analysis (FESEM–EDX) suggest that the distribution of the elements present in hollow fiber is homogeneous. X-ray photoelectron spectroscopic (XPS) results indicate that the halo formation is not caused by the phase separation of PBI and PSf, but by a physical phenomenon. © 2001 Elsevier Science B.V. All rights reserved. Keywords: PBI/polysulfone membranes; PBI; PBI blend fibers; PBI/polysulfone fiber
1. Introduction Membranes have been employed for the provision of pure drinking water systems from seawater to the public and manufacturing of enriched oxygen, artificial kidney and liver for patients, high purity syngas and natural gas as energy sources for power generation and high purity nitrogen for the electronic industry. In addition, membranes provide the basic technology for the development of bioseparation [1] and controlled release devices [2,3] in the field of biomedical sciences. ∗ Corresponding author. E-mail address: [email protected] (T.-S. Chung).
Membrane morphology plays the most important role in determining membrane applications and separation performance. Depending on morphology and pore size, membranes have been chosen for reverse osmosis, ultra-filtration (kidney dialysis) and gas separation. The phase inversion process was developed by Loeb and Sourirajan 40 years ago [4,5] and is still one of the most important means of preparing asymmetric membranes [6–9]. The morphological change during the membrane formation is due to a combination of nucleation growth and spinodal decomposition. The basic principle of demixing mechanisms during the phase inversion may be described by the Markoffian and Onsager’s thermodynamic systems [10–12], while the diffusion kinetics of solvent exchange
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 5 2 9 - 4
124
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
during the membrane formation may be expressed with the aid of the Flory–Huggins theory to describe the Gibbs free energy for the states of solutions [13–15]. The resultant membranes have a skin layer, which are integrally bonded in series with a thick porous substructure. The skin and the substructure are composed of the same material. The skin layer, which contains the effective separating layer, is one of the key elements in determining the membrane permeability and selectivity. In almost all the cases, the dense selective skin of asymmetric membranes is located in or near either inner, outer or in both skin surfaces. The changes in porosity and morphology from the dense selective skin to the substructure are always reported to be progressive. To the best of our knowledge, membranes with defined unit-step morphological changes were first reported by one of us when researchers observed a peculiar yellowish halo ring in the cross-section of polybenzimidazole (PBI) and polyetherimide hollow fibers [16]. In other words, uniform porosity was created in the middle of hollow fiber cross-section area, which performed as a filter for light transmission. Since PBI and polyetherimide formed fully miscible blends, the ‘halo’ is not chemically different from the matrix and is a physical phenomenon of unique pore morphology.
2. Results and discussion Surprisingly, the unique defined unit-step morphological changes can also be created in hollow fibers made from immiscible PBI and polysulfone (PSf) blend solutions and spun from similar spinning apparatus and conditions published elsewhere [16]. Fig. 1 shows microscopic pictures of the cross-sections of hollow fibers spun from a 10:90 and a 15:85 (weight ratio) PBI:PSf solutions with a total polymer content of 25.6 wt.%, while Fig. 2 displays a transmittance microscopic picture of the cross-section of hollow fibers spun from a 10:90 PBI:PSf solution with a polymer content of 25.6%. The halo region is yellowish in color having wavelength of 580–595 nm. The sharp difference between the middle section and the inner and outer sections arises from the fact that they have different morphologies and porosities. The abrupt change in transmittance light implies distinct morphological changes at the interface because the inner and outer regions have almost the same morphology, which is significantly different from the middle region. As illustrated in Fig. 3, the inner and outer regions have average pore sizes of 0.41–0.42 m, whereas for the inner region they are 0.18–19 m. The width of the halo ring is approximately 18–20% of hollow fiber wall thickness. A step change in the
Fig. 1. The microscopic picture of the cross-section of hollow fibers spun from: (a) 25.6 wt.% 10:90 PBI:PSf solution; (b) 25.6 wt.% 15:85 PBI:PSf solution (magnification: 100×).
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
125
Fig. 2. The transmittance microscopic pictures of the cross-section of hollow fibers spun from a 25.6 wt.% 10:90 PBI:PSf solution: (a) magnification: 200×; (b) magnification: 500×.
cell structure can be observed as indicated by arrow lines in the Fig. 3(b) and (c). Two possible causes for the formation of this unique halo ring structure have been hypothesized: one is phase separation of PBI and PSf during precipitation, the other is a physical phenomenon because the halo region has a defined morphology with a relatively uniform pore size, which differs from the inner and outer regions. The former hypothesis is raised due to the fact that PBI and PSf are not miscible [17,18] and the layer structure is a result of phase separation and molecular rearrangement during extrusion. The high viscosity component (i.e. PBI-rich solution) tends to move to the low shear rate region that is located in the middle of the cross-section, while the low viscosity component (i.e. PSf-rich solution) tends to migrate to the edge [19]. This hypothesis is abandoned because data from X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy coupled with energy dispersive X-ray (FESEM–EDX) strongly suggest that no chemical difference exists across the hollow fiber thickness. To ascertain the nature of the halo in hollow fibers, XPS spectra were recorded using a monochromated Mg K␣ X-ray source focused on a cross-section spot of 50 m. Wide scan spectra were taken from the halo ring region and also from the surrounding matrix as shown in Fig. 4 spectrums (a) and (b). The spectra show peaks corresponding to S and N apart
from the C and O peaks for both the halo ring region and the surrounding matrix. There is no significant difference between these spectra, indicating that halo formation within the hollow fibers is a physical phenomenon of phase morphology. FESEM–EDX mappings of nitrogen (only PBI has it) and sulfur (only PSf has it) also indicate that both elements are uniformly distributed across the hollow fiber wall as illustrated in Fig. 5. Then, what causes the halo ring formation. This is due to the fact that a uniform nucleation growth occurs in the ring region during the early stage of phase separation because of the high solution viscosity and the diffusion controlled demixing process, while spinodal decompositions occur in the inner and outer substructure regions because they contact directly with internal and external coagulants, respectively. The spinodal decompositions grow from a mechanism of small amplitude composition fluctuations to statistically induce a two- or three-dimensionally sinusoidal composition modulation with a certain wavelength [20–22]. Since the state of the solution at the halo ring region is in a stable metastable condition with minimum diffusion from its surroundings because of the high viscosity, and since the precipitated outermost and innermost layers further retard the diffusion and demixing process, the vitrification of the halo ring region is so isolated that it develops a sharp and unique porous structure from its surroundings.
126
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
127
Fig. 4. XPS analysis of hollow fiber spun from a 25.6 wt.% 10:90 PBI:PSf solution: (a) halo ring region; (b) surrounding region.
Fig. 5. FESEM-EDX mapping of the cross-section of hollow fiber spun from a 27% 10:90 PBI:PSf solution: (a) matrix mapping; (b) nitrogen mapping; (c) sulfur mapping.
128
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
The other possibility is viscosity and shear related effects on phase separation. Since the shear rate and stress vary across the annular flow region, the conformation of the polymer molecules in the nascent may differ across the membrane wall. Thus, it may induce different types of phase separation [23,24]. Further, experiments are underway to elucidate the cause of the interesting phenomenon.
Acknowledgements Chan Mya Tun would like to thank National University of Singapore for ASEAN scholarship. The authors also appreciate one of the reviewers’ valuable comments.
References [1] B.B. Lakshmi, C.R. Martin, Enantioseparation using apoenzymes immobilized in a porous polymeric membrane, Nature 388 (1997) 758–760. [2] K.J. Pekarek, J.S. Jacob, E. Mathiowitz, Double-walled polymer microspheres for controlled drug release, Nature 367 (1994) 258–260. [3] J.T. Santini, M.J. Cima, R. Langer, A controlled release microchip, Nature 397 (1999) 335–338. [4] S. Loeb, S. Sourirajan, Seawater demineralization by means of an osmotic membrane, Adv. Chem. Ser. ACS 38 (1963) 117–132. [5] S. Sourirajan, Separation of hydrocarbon liquids by flow under pressure through porous membranes, Nature 203 (1964) 1348–1349. [6] W.J. Koros, I. Pinnau, Membrane formation for gas separation process, in: D.R. Paul, Y.P. Yampol’skii (Eds.), Polymeric Gas Separation Membranes, CRC press, Boca Raton, 1994 (Chapter 5). [7] R.E. Kesting, Synthetic Polymeric Membranes, Wiley, New York, 1985. [8] D.R. Paul, Y.P. Yampol’skii (Eds.), Polymeric Gas Separation Membranes, CRC press, Boca Raton, 1994.
[9] X.J. Yang, C. Pin, A.G. Fane, Hollow fiber membrane chromatography: a novel analytical system for trace metal separation, Anal. Chim. Acta 369 (1998) 17–20. [10] L.P. Cheng, C.C. Gryte, Limitations of composite change during isothermal mass-transfer processes in systems with limited miscibility, Macromolecules 25 (1992) 3293–3294. [11] T.S. Chung, S.K. Teoh, Breaking the limitation of composition change during isothermal mass-transfer processes at the spinodal, J. Membr. Sci. 130 (1997) 141–147. [12] D. Jou, J. Casas-Vazquez, G. Lebon, Extended Irreversible Thermodynamics, Verlag, New York, 1993. [13] H. Strathmann, K. Kock, P. Amar, R.W. Baker, The formation mechanism of asymmetric membranes, Desalination 16 (1975) 179–203. [14] A.J. Reuvers, J.W.A. van den Berg, C.A. Smolders, Formation of membranes by means of immersion precipitation. Part I. A model to describe mass-transfer during immersion precipitation, J. Membr. Sci. 34 (1987) 45–65. [15] T.S. Chung, The limitations of using Flory–Huggins equation for the states of solutions during asymmetric hollow fiber formation, J. Membr. Sci. 126 (1997) 19–34. [16] T.S. Chung, Z.L. Xu, C.H. Huan, A halo formation in asymmetric PEI and PBI blend hollow fiber membranes, J. Polym. Sci., Polym. Phys. 37 (1999) 1575–1585. [17] T.S. Chung, M. Glick, E.J. Powers, Polybenzimidazole and polysulfone blends, Polym. Eng. Sci. 33 (1993) 1042–1048. [18] V. Deimede, G.A. Voyiatzis, J.K. Kallitsis, L. Qingfeng, N.J. Bjerrum, Miscibility behavior of polybenzimidazole/sulfonated polysulfone blends for use in fuel cell applications, Macromolecules 33 (2000) 7609–7617. [19] C.D. Han, Rheology in Polymer Processing, Academic Press, New York, 1976. [20] O. Olabisi, L.M. Robenson, M.T. Shaw, Polymer–Polymer Miscibility, Academic Press, New York, 1979. [21] J.W. Cahn, Phase separation by spinodal decomposition in isotropic systems, J. Chem. Phys. 42 (1965) 93–99. [22] P.F. James, Liquid-phase separation in glass-forming systems, J. Mater. Sci. 10 (1975) 1802–1825. [23] C. Krause, B.A. Wolf, Shear effects on the phase diagrams of solutions of highly incompatible polymers in a common solvent. Part 1. Equilibrium behavior and rheological properties, Macromolecules 30 (1997) 885. [24] C. Krause, R. Horst, B.A. Wolf, Shear effects on the phase diagrams of solutions of highly incompatible polymers in a common solvent. Part 2. Experiment and theory, Macromolecules 30 (1997) 890.
Novel hollow fiber membranes with defined unit-step morphological change Tai-Shung Chung a,b,∗ , Chan Mya Tun b , K.P. Pramoda c , Rong Wang d a Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Department of Material Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore d Environmental Technology Institute, Innovation Centre (NTU), Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723, Singapore b
c
Received 1 February 2001; received in revised form 21 May 2001; accepted 21 May 2001
Abstract We have found that sharply defined unit-step morphological changes can be created in the middle of the cross-section of hollow fiber membranes wet-spun from highly concentrated immiscible poly(2,2 -m-phenylene)-5,5 -polybenzimidazole (PBI) and polysulfone (PSf) blend solutions. A halo is observed with yellowish color having wavelength in the range 580–595 nm. The abrupt change in transmittance light implies distinct morphological changes at the interface because the halo region possesses significantly different morphology and pore sizes from the inner and outer regions. The width of the halo ring is approximately 18–20% of the hollow fiber wall thickness. The mapping image and spectra obtained from field emission scanning electron microscopy coupled with energy dispersive X-ray analysis (FESEM–EDX) suggest that the distribution of the elements present in hollow fiber is homogeneous. X-ray photoelectron spectroscopic (XPS) results indicate that the halo formation is not caused by the phase separation of PBI and PSf, but by a physical phenomenon. © 2001 Elsevier Science B.V. All rights reserved. Keywords: PBI/polysulfone membranes; PBI; PBI blend fibers; PBI/polysulfone fiber
1. Introduction Membranes have been employed for the provision of pure drinking water systems from seawater to the public and manufacturing of enriched oxygen, artificial kidney and liver for patients, high purity syngas and natural gas as energy sources for power generation and high purity nitrogen for the electronic industry. In addition, membranes provide the basic technology for the development of bioseparation [1] and controlled release devices [2,3] in the field of biomedical sciences. ∗ Corresponding author. E-mail address: [email protected] (T.-S. Chung).
Membrane morphology plays the most important role in determining membrane applications and separation performance. Depending on morphology and pore size, membranes have been chosen for reverse osmosis, ultra-filtration (kidney dialysis) and gas separation. The phase inversion process was developed by Loeb and Sourirajan 40 years ago [4,5] and is still one of the most important means of preparing asymmetric membranes [6–9]. The morphological change during the membrane formation is due to a combination of nucleation growth and spinodal decomposition. The basic principle of demixing mechanisms during the phase inversion may be described by the Markoffian and Onsager’s thermodynamic systems [10–12], while the diffusion kinetics of solvent exchange
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 5 2 9 - 4
124
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
during the membrane formation may be expressed with the aid of the Flory–Huggins theory to describe the Gibbs free energy for the states of solutions [13–15]. The resultant membranes have a skin layer, which are integrally bonded in series with a thick porous substructure. The skin and the substructure are composed of the same material. The skin layer, which contains the effective separating layer, is one of the key elements in determining the membrane permeability and selectivity. In almost all the cases, the dense selective skin of asymmetric membranes is located in or near either inner, outer or in both skin surfaces. The changes in porosity and morphology from the dense selective skin to the substructure are always reported to be progressive. To the best of our knowledge, membranes with defined unit-step morphological changes were first reported by one of us when researchers observed a peculiar yellowish halo ring in the cross-section of polybenzimidazole (PBI) and polyetherimide hollow fibers [16]. In other words, uniform porosity was created in the middle of hollow fiber cross-section area, which performed as a filter for light transmission. Since PBI and polyetherimide formed fully miscible blends, the ‘halo’ is not chemically different from the matrix and is a physical phenomenon of unique pore morphology.
2. Results and discussion Surprisingly, the unique defined unit-step morphological changes can also be created in hollow fibers made from immiscible PBI and polysulfone (PSf) blend solutions and spun from similar spinning apparatus and conditions published elsewhere [16]. Fig. 1 shows microscopic pictures of the cross-sections of hollow fibers spun from a 10:90 and a 15:85 (weight ratio) PBI:PSf solutions with a total polymer content of 25.6 wt.%, while Fig. 2 displays a transmittance microscopic picture of the cross-section of hollow fibers spun from a 10:90 PBI:PSf solution with a polymer content of 25.6%. The halo region is yellowish in color having wavelength of 580–595 nm. The sharp difference between the middle section and the inner and outer sections arises from the fact that they have different morphologies and porosities. The abrupt change in transmittance light implies distinct morphological changes at the interface because the inner and outer regions have almost the same morphology, which is significantly different from the middle region. As illustrated in Fig. 3, the inner and outer regions have average pore sizes of 0.41–0.42 m, whereas for the inner region they are 0.18–19 m. The width of the halo ring is approximately 18–20% of hollow fiber wall thickness. A step change in the
Fig. 1. The microscopic picture of the cross-section of hollow fibers spun from: (a) 25.6 wt.% 10:90 PBI:PSf solution; (b) 25.6 wt.% 15:85 PBI:PSf solution (magnification: 100×).
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
125
Fig. 2. The transmittance microscopic pictures of the cross-section of hollow fibers spun from a 25.6 wt.% 10:90 PBI:PSf solution: (a) magnification: 200×; (b) magnification: 500×.
cell structure can be observed as indicated by arrow lines in the Fig. 3(b) and (c). Two possible causes for the formation of this unique halo ring structure have been hypothesized: one is phase separation of PBI and PSf during precipitation, the other is a physical phenomenon because the halo region has a defined morphology with a relatively uniform pore size, which differs from the inner and outer regions. The former hypothesis is raised due to the fact that PBI and PSf are not miscible [17,18] and the layer structure is a result of phase separation and molecular rearrangement during extrusion. The high viscosity component (i.e. PBI-rich solution) tends to move to the low shear rate region that is located in the middle of the cross-section, while the low viscosity component (i.e. PSf-rich solution) tends to migrate to the edge [19]. This hypothesis is abandoned because data from X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy coupled with energy dispersive X-ray (FESEM–EDX) strongly suggest that no chemical difference exists across the hollow fiber thickness. To ascertain the nature of the halo in hollow fibers, XPS spectra were recorded using a monochromated Mg K␣ X-ray source focused on a cross-section spot of 50 m. Wide scan spectra were taken from the halo ring region and also from the surrounding matrix as shown in Fig. 4 spectrums (a) and (b). The spectra show peaks corresponding to S and N apart
from the C and O peaks for both the halo ring region and the surrounding matrix. There is no significant difference between these spectra, indicating that halo formation within the hollow fibers is a physical phenomenon of phase morphology. FESEM–EDX mappings of nitrogen (only PBI has it) and sulfur (only PSf has it) also indicate that both elements are uniformly distributed across the hollow fiber wall as illustrated in Fig. 5. Then, what causes the halo ring formation. This is due to the fact that a uniform nucleation growth occurs in the ring region during the early stage of phase separation because of the high solution viscosity and the diffusion controlled demixing process, while spinodal decompositions occur in the inner and outer substructure regions because they contact directly with internal and external coagulants, respectively. The spinodal decompositions grow from a mechanism of small amplitude composition fluctuations to statistically induce a two- or three-dimensionally sinusoidal composition modulation with a certain wavelength [20–22]. Since the state of the solution at the halo ring region is in a stable metastable condition with minimum diffusion from its surroundings because of the high viscosity, and since the precipitated outermost and innermost layers further retard the diffusion and demixing process, the vitrification of the halo ring region is so isolated that it develops a sharp and unique porous structure from its surroundings.
126
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
127
Fig. 4. XPS analysis of hollow fiber spun from a 25.6 wt.% 10:90 PBI:PSf solution: (a) halo ring region; (b) surrounding region.
Fig. 5. FESEM-EDX mapping of the cross-section of hollow fiber spun from a 27% 10:90 PBI:PSf solution: (a) matrix mapping; (b) nitrogen mapping; (c) sulfur mapping.
128
T.-S. Chung et al. / Journal of Membrane Science 193 (2001) 123–128
The other possibility is viscosity and shear related effects on phase separation. Since the shear rate and stress vary across the annular flow region, the conformation of the polymer molecules in the nascent may differ across the membrane wall. Thus, it may induce different types of phase separation [23,24]. Further, experiments are underway to elucidate the cause of the interesting phenomenon.
Acknowledgements Chan Mya Tun would like to thank National University of Singapore for ASEAN scholarship. The authors also appreciate one of the reviewers’ valuable comments.
References [1] B.B. Lakshmi, C.R. Martin, Enantioseparation using apoenzymes immobilized in a porous polymeric membrane, Nature 388 (1997) 758–760. [2] K.J. Pekarek, J.S. Jacob, E. Mathiowitz, Double-walled polymer microspheres for controlled drug release, Nature 367 (1994) 258–260. [3] J.T. Santini, M.J. Cima, R. Langer, A controlled release microchip, Nature 397 (1999) 335–338. [4] S. Loeb, S. Sourirajan, Seawater demineralization by means of an osmotic membrane, Adv. Chem. Ser. ACS 38 (1963) 117–132. [5] S. Sourirajan, Separation of hydrocarbon liquids by flow under pressure through porous membranes, Nature 203 (1964) 1348–1349. [6] W.J. Koros, I. Pinnau, Membrane formation for gas separation process, in: D.R. Paul, Y.P. Yampol’skii (Eds.), Polymeric Gas Separation Membranes, CRC press, Boca Raton, 1994 (Chapter 5). [7] R.E. Kesting, Synthetic Polymeric Membranes, Wiley, New York, 1985. [8] D.R. Paul, Y.P. Yampol’skii (Eds.), Polymeric Gas Separation Membranes, CRC press, Boca Raton, 1994.
[9] X.J. Yang, C. Pin, A.G. Fane, Hollow fiber membrane chromatography: a novel analytical system for trace metal separation, Anal. Chim. Acta 369 (1998) 17–20. [10] L.P. Cheng, C.C. Gryte, Limitations of composite change during isothermal mass-transfer processes in systems with limited miscibility, Macromolecules 25 (1992) 3293–3294. [11] T.S. Chung, S.K. Teoh, Breaking the limitation of composition change during isothermal mass-transfer processes at the spinodal, J. Membr. Sci. 130 (1997) 141–147. [12] D. Jou, J. Casas-Vazquez, G. Lebon, Extended Irreversible Thermodynamics, Verlag, New York, 1993. [13] H. Strathmann, K. Kock, P. Amar, R.W. Baker, The formation mechanism of asymmetric membranes, Desalination 16 (1975) 179–203. [14] A.J. Reuvers, J.W.A. van den Berg, C.A. Smolders, Formation of membranes by means of immersion precipitation. Part I. A model to describe mass-transfer during immersion precipitation, J. Membr. Sci. 34 (1987) 45–65. [15] T.S. Chung, The limitations of using Flory–Huggins equation for the states of solutions during asymmetric hollow fiber formation, J. Membr. Sci. 126 (1997) 19–34. [16] T.S. Chung, Z.L. Xu, C.H. Huan, A halo formation in asymmetric PEI and PBI blend hollow fiber membranes, J. Polym. Sci., Polym. Phys. 37 (1999) 1575–1585. [17] T.S. Chung, M. Glick, E.J. Powers, Polybenzimidazole and polysulfone blends, Polym. Eng. Sci. 33 (1993) 1042–1048. [18] V. Deimede, G.A. Voyiatzis, J.K. Kallitsis, L. Qingfeng, N.J. Bjerrum, Miscibility behavior of polybenzimidazole/sulfonated polysulfone blends for use in fuel cell applications, Macromolecules 33 (2000) 7609–7617. [19] C.D. Han, Rheology in Polymer Processing, Academic Press, New York, 1976. [20] O. Olabisi, L.M. Robenson, M.T. Shaw, Polymer–Polymer Miscibility, Academic Press, New York, 1979. [21] J.W. Cahn, Phase separation by spinodal decomposition in isotropic systems, J. Chem. Phys. 42 (1965) 93–99. [22] P.F. James, Liquid-phase separation in glass-forming systems, J. Mater. Sci. 10 (1975) 1802–1825. [23] C. Krause, B.A. Wolf, Shear effects on the phase diagrams of solutions of highly incompatible polymers in a common solvent. Part 1. Equilibrium behavior and rheological properties, Macromolecules 30 (1997) 885. [24] C. Krause, R. Horst, B.A. Wolf, Shear effects on the phase diagrams of solutions of highly incompatible polymers in a common solvent. Part 2. Experiment and theory, Macromolecules 30 (1997) 890.