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Ion exchange membrane based on block copolymers. Part III: preparation of cation exchange membrane
Journal of Membrane Science 156 (1999) 61±65
Ion exchange membrane based on block copolymers. Part III: preparation of cation exchange membrane Gab-Jin Hwanga, Haruhiko Ohyab,*, Toshiyuki Nagaib a
Japan Atomic Energy Research Institute, Shirakata Shirane 2-4, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan b Department of Material Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-gu, Yokohama 240-8501, Japan Received 11 May 1998; received in revised form 8 October 1998; accepted 8 October 1998
Abstract A new type of ion exchange membrane was investigated for the purpose of applying electrodialysis process. The cation exchange membrane was prepared by sulfonation of the home-made block copolymers of polysulfone (PSf) and polyphenylenesul®desulfone (PPSS). The lowest measured membrane area resistance, equilibrated in 2 M (mol/dm3) KCl aqueous solution, was 2.5 cm2. The cation exchange capacity reached up to 1.9 meq/(g-dry-resin), and the transport number of K was in the 0.77±0.87 range. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Polysulfone; Block copolymers; Ion exchange membrane; Ion exchange capacity; Cation selective membrane
1. Introduction The ion exchange membrane is now widely used in the electrodialysis (ED) for the desalination of brackish water, the production of table salt, recovery of valuable metals from the ef¯uents of metal-plating industry, and for many other purposes. For the purpose of applying electrodialysis process under severe conditions such as high temperatures and strongly oxidizing conditions, a more stable ion exchange membrane should be developed and successfully applied under those conditions. However, it has found only a few major industrial applications, other than in the chloralkali industry, primarily *Corresponding author. Tel.: +81-45-339-3989; fax: +81-45339-4012; e-mail: [email protected]
because of its high cost. It was, therefore, the objective of this research to develop new types of ion exchange membrane which would be cheap but also have good electrochemical properties and excellent resistance to degradation by heat and chemical attack. Recently, engineering plastics such as polysulfone and polyethersulfone have been widely used as a base polymer for ultra®ltration and gas separation because of their excellent workability and mechanical strength [1±5]. In particular, a polysulfone membrane having excellent chemical resistance has been studied for its application as an ion exchange membrane by improving the permeability for ulfra®ltration and reverse osmosis, or imparting ion permselectivity by introducing ion exchange groups into the membrane [6±8]. Several workers [9±11] had earlier prepared ion exchange membranes using polysulfone as a base
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00331-7
62
G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
polymer of the membrane. In previous paper the authors [12,16] reported that an anion exchange membrane prepared by the amination of the homemade block copolymers of polysulfone (PSf) and polypheylenesu®desulfone (PPSS) had a good electrical resistance, ion permselectivity, and chemical stability.
2.1.2. Chloromethylation In the same manner as the synthesis in a previous paper [12], an aromatic PSfPPSS copolymer was dissolved in 1,1,2,2-tetrachloroethane; thereafter, SnCl4 and chloromethylmethylether were reacted to obtain a chloromethylated copolymer of the following structure:
The aims of this paper are to prepare a cation exchange membrane using block copolymers and to measure membrane properties.
2.1.3. Sulfonation
2.1.1. Block copolymer In the same manner as the synthesis in a previous
An aromatic PSfPPSS copolymer was dissolved in 1,1,2,2-tetrachloroethane; thereafter, triethyl phosphate and sulfuric acid of the molar ratio 1:2 were added in the solution. Then, the mixture was reacted at 1108 for 4 h to obtain a product. Next, the product was washed with the water and methanol, then dried to obtain the sulfonated copolymer of the following structure:
paper [12] and other literature [13,14], bisphenol A disodium and 4,40 -dichlorodiphenylsulfone (DCDPS) were reacted to obtain a precursor comprising aromatic polysulfone units. Then, the precursor, DCDPS and sodiumsul®de were reacted to obtain an aromatic polysulfonepolyphenylenesul®desulfone (PSfPPSS) copolymer of the following structure:
2.1.4. Membrane preparation The chloromethylated copolymer and the sulfonated copolymer of the weight ratio 3:1 were dissolved in N-methylpyrilidone (NMP) to obtain a casting solution of 40 wt%. The casting solution was cast on the glass plate, and heated and dried at 180± 2008C with a nitrogen gas in gauge pressure of 1 atm to obtain the cation exchange layer. Then, the
2. Experimental 2.1. Chemicals
G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
solution of a casting solution of 5 wt% was cast on cation exchange layer. Next, a polypropylene nonwoven fabric of 0.14 mm (PO30FW NB, ROKI) was overlapped and dried at 1008C with a nitrogen gas in gauge pressure of 1 atm. Following this, the product was released from the glass plate to obtain a laminated cation exchange membrane. 2.2. Membrane properties 2.2.1. Membrane area resistance in 2 M KCl aq solution and static transport number The measurement methods and experimental apparatus for membrane area resistance and static transport number were same as reported earlier by Hwang and Ohya [12,15]. 2.2.2. Ion exchange capacity (IEC) After immersion in pure water for one day, a membrane sample was immersed for one day in a large volume of 1 M (mol/dm3) HCl aq solution to give the membrane in the H-form. The membrane was then washed free of excess HCl with distilled water and for a further 4 h equilibrated with distilled water, with frequent changes in the distilled water to remove the last traces of acid. The membrane was then equilibrated with exact 50 ml of 0.01 M NaOH aq solution for 24 h and the cation exchange capacity was determined from the reduction in alkalinity determined by back titration. The ion exchange capacity of the cation exchange membrane was calculated from the following equation: IEC MO;NaOH ÿ ME;NaOH =W:
The ®xed ion concentration (Af) can be calculated from the following equation: Af IEC=WC :
(3)
3. Results and discussion 3.1. Membrane area resistance in 2 M KCl aq solution Fig. 1 illustrates the membrane thickness and the membrane area resistance in 2 M KCl aq solution. In the prepared cation exchange membrane, the membrane thickness had in the range of 0.15± 0.2 mm, and the membrane area resistance had in the range of 2±17 cm2. The lowest measured membrane area resistance, equilibrated in 2 M KCl aqueous solution, reached 2.5 cm2 at the thickness of 0.158 mm. Membrane area resistances increased with an increase of membrane thickness. From the data of Fig. 1, it was con®rmed that the prepared cation exchange membrane having below the thickness of 0.17 mm have the lower membrane area resistance. This suggests that the membrane thickness required at least 0.17 mm and thinner than it in order to decrease the membrane area resistance. 3.2. Ion exchange capacity (IEC) Fig. 2 illustrates the relationship between the ®xed ion concentration and the cation exchange capacity.
(1)
2.2.3. Water content The membrane was immersed in distilled water for 24 h, then its surface moisture was wiped and the wet resin weighed. Then, this weighed wet membrane was dried at a ®xed temperature (608C) until constant weight as dry-resin was achieved. The water content can be calculated from the following equation: WC WW ÿ W=WW :
63
(2)
Fig. 1. The relationship between the membrane thickness and the membrane area resistance in 2 M KCl aq solution.
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G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
Fig. 2. The relationship between the fixed ion concentration and the ion exchange capacity.
Fig. 3. The relationship between the fixed ion concentration and the transport number of K.
With an increase of the ®xed ion concentration in the membrane, the ion exchange capacity increased. In general, increasing the ®xed ion concentration leads to increase in ion exchange capacity. From the data of the Fig. 2, it seems that in order to obtain a higher ion exchange capacity, the membrane should have a high ®xed ion concentration. The prepared cation exchange membranes possessed the ion exchange capacity in the range 1.5± 1.9 meq/g-dry-resin. 3.3. Static transport number Fig. 3 illustrates the relationship between the ®xed ion concentration and the static transport number of the potassium cation of the prepared cation exchange membrane, estimated from the membrane potential of a potassium chloride concentration cell. With an increase of the ®xed ion concentration in the membrane, the static transport number increased, and then decreased over the ®xed ion concentration of 6 meq/g-H2O. In general, the ion permselectivity (static transport number) is higher as the ®xed ion concentration in the membrane (amount of ion exchange groups per water content in the membrane). From the data of the Fig. 3, it seems that in order to obtain a higher static transport number, the membrane should have at least 4±6 meq/g-H2O.
Fig. 4. The prepared cation exchange membrane: transport number of K and ion exchange capacity, as a function of water content.
G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
The prepared cation exchange membranes possessed the static transport number in the range 0.77±0.87 and the ion exchange capacity in the range 1.5±1.9 meq/g-dry-resin. The relationship between their water contents, static transport number, and the ion exchange capacities is shown in Fig. 4. It can be seen that the optimal regions for the best static transport number±ion exchange capacity combination occur at relatively low water content in the range 0.35±0.45 g-H2O/g-dry-resin. 4. Conclusion 1. The cation exchange membrane was prepared by sulfonation of the home-made block copolymers of polysulfone (PSf) and polyphenylenesul®desulfone (PPSS). 2. The lowest measured membrane area resistance, equilibrated in 2 M KCl aqueous solution, was 2.5 cm2. 3. The cation exchange capacity reached up to 1.9 meq/(g-dry-resin), and the transport number of K was in the 0.77±0.87 range. 5. Nomenclature MO,NaOH ME,NaOH W WC WW
moles of NaOH in the flask at the start of titration moles of NaOH after equilibration weight of dry-resin (g) water content (g-H2O/g-dry-resin) weight of wet resin (g)
References [1] M. Sara, G. Wolf, U.B. Sleytr, Structure and characteristics of ultrafiltration membranes from crystalline bacterial envelope layers, Sen-I Gakkaishi 44(1) (1988) 4±10 (in Japanese).
65
[2] C.M. Tam, M. Dal-cin, M.D. Guiver, Polysulfone membrane IV. Performance evaluation of radel A/PVP membranes, J. Membr. Sci. 78 (1993) 123±124. [3] G. Popescu, G. Nechifor, B. Albu, N. Luca, Microporous polyethersulphone membrane, Revue Roumaine de Chemie 34(2) (1989) 577±580. [4] W. Pusch, Synthetic membranes for separation processes, Sen-I Gakkaishi 44(1) (1988) 20±35 (in Japanese). [5] B.T. Swinyard, D.S. Sagoo, J.A. Barrie, R. Ash, The transport and sorption of water in polyethersulfone, polysulphone and polyethersulphone/phenxyl blends, J. Appl. Polym. Sci. 41 (1990) 2479±2485. [6] H. Inoue, K. Kaibara, N. Ohta, S. Nakabo, A. Fujita, H. Kimizuka, Ion transport across charged ultrafiltration membrane, Memories of the faculty of science, Kyushu University Ser. C 16(2) (1988) 137±148. [7] L. Breitbach, E. Hinke, E. Staude, Heterogeneous functionalizing of polysulfone membranes, Die Angewandte Makromolekulare Chemie 184 (1991) 183±196. [8] K.E. Kinzer, D.R. Lloyd, J.P. Wightman, J.E. Mcgrath, Asymmetric membrane preparation from nonsolvent casting systems, Desalination 46 (1988) 327±334. [9] P. Zschocke, D. Guellmalze, Novel ion exchange membrane based on an aromatic polyethersulfone, J. Membr. Sci. 22 (1985) 325±332. [10] O. Kedem, A. Warshawsky, Composite ion exchange membranes, Sen-I Gakkaishi 44(1) (1988) 11±19 (in Japanese). [11] I. Terada, H. Horie, Y. Sugaya, H. Miyake, Sankaishyuyouno sinkina ionkoukanmaku, Kobunshi Kakou 40(8) (1991) 38±41 (in Japanese). [12] G.-J. Hwang, H. Ohya, Preparation of anion exchange membrane based on block copolymers part I: Amination of the chloromethylated copolymers, J. Membr. Sci. 140 (1998) 195±203. [13] Japan Patent no. 61 168 629, 30 July 1986. [14] US Patent no. 5 180 750, 19 January 1993. [15] G.-J. Hwang, H. Ohya, Preparation of cation exchange membrane as a separator for the all-vanadium redox flow battery, J. Membr. Sci. 120(1) (1996) 55±67. [16] G.-J. Hwang, H. Ohya, Preparation of anion exchange membrane based on block copolymers part II. The effect of the formation of macroreticular structure on the membrane properties, J. Membr. Sci. 149 (1998) 163±169.
Ion exchange membrane based on block copolymers. Part III: preparation of cation exchange membrane Gab-Jin Hwanga, Haruhiko Ohyab,*, Toshiyuki Nagaib a
Japan Atomic Energy Research Institute, Shirakata Shirane 2-4, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan b Department of Material Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-gu, Yokohama 240-8501, Japan Received 11 May 1998; received in revised form 8 October 1998; accepted 8 October 1998
Abstract A new type of ion exchange membrane was investigated for the purpose of applying electrodialysis process. The cation exchange membrane was prepared by sulfonation of the home-made block copolymers of polysulfone (PSf) and polyphenylenesul®desulfone (PPSS). The lowest measured membrane area resistance, equilibrated in 2 M (mol/dm3) KCl aqueous solution, was 2.5 cm2. The cation exchange capacity reached up to 1.9 meq/(g-dry-resin), and the transport number of K was in the 0.77±0.87 range. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Polysulfone; Block copolymers; Ion exchange membrane; Ion exchange capacity; Cation selective membrane
1. Introduction The ion exchange membrane is now widely used in the electrodialysis (ED) for the desalination of brackish water, the production of table salt, recovery of valuable metals from the ef¯uents of metal-plating industry, and for many other purposes. For the purpose of applying electrodialysis process under severe conditions such as high temperatures and strongly oxidizing conditions, a more stable ion exchange membrane should be developed and successfully applied under those conditions. However, it has found only a few major industrial applications, other than in the chloralkali industry, primarily *Corresponding author. Tel.: +81-45-339-3989; fax: +81-45339-4012; e-mail: [email protected]
because of its high cost. It was, therefore, the objective of this research to develop new types of ion exchange membrane which would be cheap but also have good electrochemical properties and excellent resistance to degradation by heat and chemical attack. Recently, engineering plastics such as polysulfone and polyethersulfone have been widely used as a base polymer for ultra®ltration and gas separation because of their excellent workability and mechanical strength [1±5]. In particular, a polysulfone membrane having excellent chemical resistance has been studied for its application as an ion exchange membrane by improving the permeability for ulfra®ltration and reverse osmosis, or imparting ion permselectivity by introducing ion exchange groups into the membrane [6±8]. Several workers [9±11] had earlier prepared ion exchange membranes using polysulfone as a base
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00331-7
62
G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
polymer of the membrane. In previous paper the authors [12,16] reported that an anion exchange membrane prepared by the amination of the homemade block copolymers of polysulfone (PSf) and polypheylenesu®desulfone (PPSS) had a good electrical resistance, ion permselectivity, and chemical stability.
2.1.2. Chloromethylation In the same manner as the synthesis in a previous paper [12], an aromatic PSfPPSS copolymer was dissolved in 1,1,2,2-tetrachloroethane; thereafter, SnCl4 and chloromethylmethylether were reacted to obtain a chloromethylated copolymer of the following structure:
The aims of this paper are to prepare a cation exchange membrane using block copolymers and to measure membrane properties.
2.1.3. Sulfonation
2.1.1. Block copolymer In the same manner as the synthesis in a previous
An aromatic PSfPPSS copolymer was dissolved in 1,1,2,2-tetrachloroethane; thereafter, triethyl phosphate and sulfuric acid of the molar ratio 1:2 were added in the solution. Then, the mixture was reacted at 1108 for 4 h to obtain a product. Next, the product was washed with the water and methanol, then dried to obtain the sulfonated copolymer of the following structure:
paper [12] and other literature [13,14], bisphenol A disodium and 4,40 -dichlorodiphenylsulfone (DCDPS) were reacted to obtain a precursor comprising aromatic polysulfone units. Then, the precursor, DCDPS and sodiumsul®de were reacted to obtain an aromatic polysulfonepolyphenylenesul®desulfone (PSfPPSS) copolymer of the following structure:
2.1.4. Membrane preparation The chloromethylated copolymer and the sulfonated copolymer of the weight ratio 3:1 were dissolved in N-methylpyrilidone (NMP) to obtain a casting solution of 40 wt%. The casting solution was cast on the glass plate, and heated and dried at 180± 2008C with a nitrogen gas in gauge pressure of 1 atm to obtain the cation exchange layer. Then, the
2. Experimental 2.1. Chemicals
G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
solution of a casting solution of 5 wt% was cast on cation exchange layer. Next, a polypropylene nonwoven fabric of 0.14 mm (PO30FW NB, ROKI) was overlapped and dried at 1008C with a nitrogen gas in gauge pressure of 1 atm. Following this, the product was released from the glass plate to obtain a laminated cation exchange membrane. 2.2. Membrane properties 2.2.1. Membrane area resistance in 2 M KCl aq solution and static transport number The measurement methods and experimental apparatus for membrane area resistance and static transport number were same as reported earlier by Hwang and Ohya [12,15]. 2.2.2. Ion exchange capacity (IEC) After immersion in pure water for one day, a membrane sample was immersed for one day in a large volume of 1 M (mol/dm3) HCl aq solution to give the membrane in the H-form. The membrane was then washed free of excess HCl with distilled water and for a further 4 h equilibrated with distilled water, with frequent changes in the distilled water to remove the last traces of acid. The membrane was then equilibrated with exact 50 ml of 0.01 M NaOH aq solution for 24 h and the cation exchange capacity was determined from the reduction in alkalinity determined by back titration. The ion exchange capacity of the cation exchange membrane was calculated from the following equation: IEC MO;NaOH ÿ ME;NaOH =W:
The ®xed ion concentration (Af) can be calculated from the following equation: Af IEC=WC :
(3)
3. Results and discussion 3.1. Membrane area resistance in 2 M KCl aq solution Fig. 1 illustrates the membrane thickness and the membrane area resistance in 2 M KCl aq solution. In the prepared cation exchange membrane, the membrane thickness had in the range of 0.15± 0.2 mm, and the membrane area resistance had in the range of 2±17 cm2. The lowest measured membrane area resistance, equilibrated in 2 M KCl aqueous solution, reached 2.5 cm2 at the thickness of 0.158 mm. Membrane area resistances increased with an increase of membrane thickness. From the data of Fig. 1, it was con®rmed that the prepared cation exchange membrane having below the thickness of 0.17 mm have the lower membrane area resistance. This suggests that the membrane thickness required at least 0.17 mm and thinner than it in order to decrease the membrane area resistance. 3.2. Ion exchange capacity (IEC) Fig. 2 illustrates the relationship between the ®xed ion concentration and the cation exchange capacity.
(1)
2.2.3. Water content The membrane was immersed in distilled water for 24 h, then its surface moisture was wiped and the wet resin weighed. Then, this weighed wet membrane was dried at a ®xed temperature (608C) until constant weight as dry-resin was achieved. The water content can be calculated from the following equation: WC WW ÿ W=WW :
63
(2)
Fig. 1. The relationship between the membrane thickness and the membrane area resistance in 2 M KCl aq solution.
64
G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
Fig. 2. The relationship between the fixed ion concentration and the ion exchange capacity.
Fig. 3. The relationship between the fixed ion concentration and the transport number of K.
With an increase of the ®xed ion concentration in the membrane, the ion exchange capacity increased. In general, increasing the ®xed ion concentration leads to increase in ion exchange capacity. From the data of the Fig. 2, it seems that in order to obtain a higher ion exchange capacity, the membrane should have a high ®xed ion concentration. The prepared cation exchange membranes possessed the ion exchange capacity in the range 1.5± 1.9 meq/g-dry-resin. 3.3. Static transport number Fig. 3 illustrates the relationship between the ®xed ion concentration and the static transport number of the potassium cation of the prepared cation exchange membrane, estimated from the membrane potential of a potassium chloride concentration cell. With an increase of the ®xed ion concentration in the membrane, the static transport number increased, and then decreased over the ®xed ion concentration of 6 meq/g-H2O. In general, the ion permselectivity (static transport number) is higher as the ®xed ion concentration in the membrane (amount of ion exchange groups per water content in the membrane). From the data of the Fig. 3, it seems that in order to obtain a higher static transport number, the membrane should have at least 4±6 meq/g-H2O.
Fig. 4. The prepared cation exchange membrane: transport number of K and ion exchange capacity, as a function of water content.
G.-J. Hwang et al. / Journal of Membrane Science 156 (1999) 61±65
The prepared cation exchange membranes possessed the static transport number in the range 0.77±0.87 and the ion exchange capacity in the range 1.5±1.9 meq/g-dry-resin. The relationship between their water contents, static transport number, and the ion exchange capacities is shown in Fig. 4. It can be seen that the optimal regions for the best static transport number±ion exchange capacity combination occur at relatively low water content in the range 0.35±0.45 g-H2O/g-dry-resin. 4. Conclusion 1. The cation exchange membrane was prepared by sulfonation of the home-made block copolymers of polysulfone (PSf) and polyphenylenesul®desulfone (PPSS). 2. The lowest measured membrane area resistance, equilibrated in 2 M KCl aqueous solution, was 2.5 cm2. 3. The cation exchange capacity reached up to 1.9 meq/(g-dry-resin), and the transport number of K was in the 0.77±0.87 range. 5. Nomenclature MO,NaOH ME,NaOH W WC WW
moles of NaOH in the flask at the start of titration moles of NaOH after equilibration weight of dry-resin (g) water content (g-H2O/g-dry-resin) weight of wet resin (g)
References [1] M. Sara, G. Wolf, U.B. Sleytr, Structure and characteristics of ultrafiltration membranes from crystalline bacterial envelope layers, Sen-I Gakkaishi 44(1) (1988) 4±10 (in Japanese).
65
[2] C.M. Tam, M. Dal-cin, M.D. Guiver, Polysulfone membrane IV. Performance evaluation of radel A/PVP membranes, J. Membr. Sci. 78 (1993) 123±124. [3] G. Popescu, G. Nechifor, B. Albu, N. Luca, Microporous polyethersulphone membrane, Revue Roumaine de Chemie 34(2) (1989) 577±580. [4] W. Pusch, Synthetic membranes for separation processes, Sen-I Gakkaishi 44(1) (1988) 20±35 (in Japanese). [5] B.T. Swinyard, D.S. Sagoo, J.A. Barrie, R. Ash, The transport and sorption of water in polyethersulfone, polysulphone and polyethersulphone/phenxyl blends, J. Appl. Polym. Sci. 41 (1990) 2479±2485. [6] H. Inoue, K. Kaibara, N. Ohta, S. Nakabo, A. Fujita, H. Kimizuka, Ion transport across charged ultrafiltration membrane, Memories of the faculty of science, Kyushu University Ser. C 16(2) (1988) 137±148. [7] L. Breitbach, E. Hinke, E. Staude, Heterogeneous functionalizing of polysulfone membranes, Die Angewandte Makromolekulare Chemie 184 (1991) 183±196. [8] K.E. Kinzer, D.R. Lloyd, J.P. Wightman, J.E. Mcgrath, Asymmetric membrane preparation from nonsolvent casting systems, Desalination 46 (1988) 327±334. [9] P. Zschocke, D. Guellmalze, Novel ion exchange membrane based on an aromatic polyethersulfone, J. Membr. Sci. 22 (1985) 325±332. [10] O. Kedem, A. Warshawsky, Composite ion exchange membranes, Sen-I Gakkaishi 44(1) (1988) 11±19 (in Japanese). [11] I. Terada, H. Horie, Y. Sugaya, H. Miyake, Sankaishyuyouno sinkina ionkoukanmaku, Kobunshi Kakou 40(8) (1991) 38±41 (in Japanese). [12] G.-J. Hwang, H. Ohya, Preparation of anion exchange membrane based on block copolymers part I: Amination of the chloromethylated copolymers, J. Membr. Sci. 140 (1998) 195±203. [13] Japan Patent no. 61 168 629, 30 July 1986. [14] US Patent no. 5 180 750, 19 January 1993. [15] G.-J. Hwang, H. Ohya, Preparation of cation exchange membrane as a separator for the all-vanadium redox flow battery, J. Membr. Sci. 120(1) (1996) 55±67. [16] G.-J. Hwang, H. Ohya, Preparation of anion exchange membrane based on block copolymers part II. The effect of the formation of macroreticular structure on the membrane properties, J. Membr. Sci. 149 (1998) 163±169.