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Preparation of anion exchange membrane based on block copolymers. Part II: the effect of the formation of macroreticular structure on the membrane properties
Journal of Membrane Science 149 (1998) 163±169
Preparation of anion exchange membrane based on block copolymers. Part II: the effect of the formation of macroreticular structure on the membrane properties Gab-Jin Hwanga, Haruhiko Ohyab,* a
b
Japan Atomic Energy Research Institute, Shirakata Shirane 2-4, Tokai-mura,naka-gun, Ibaraki-ken, 319-1195, Japan Department of Material Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan Received 12 September 1997; received in revised form 12 September 1997; accepted 15 June 1998
Abstract The anion exchange membrane was prepared by conducting the formation of macroreticular structure and the introduction of ion exchange groups to the chloromethylated copolymers simultaneously. The membrane properties are summarized below; 0.9±1.4 cm2 for area resistivity in 2 M KCl aqueous solution, 1.46±3.7 meq/g-dry-resin for the ion exchange capacity and 0.68±0.87 for the static transport number. The membrane properties of the membrane prepared by forming the macroreticular structure and the introduction of the ion exchange groups to the chloromethylated copolymers simultaneously (B membrane) had higher performance than that of the membrane prepared by the amination of the chloromethylated copolymers (A membrane). It would be expected that the formation of macroreticular structure in the membrane has an effect on the membrane properties. The chemical stability of B membrane which was determined by the oxidative degradation in 2 M nitric acid, had long lifetime with almost the same value of the area resistivity and ion exchange capacity as before durability test. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Polysulfone; Block copolymers; Ion exchange membrane; Ion exchange capacity; Anion 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. Recently, engineering plastics such as polysulfone and polyethersulfone have been widely used as a base *Corresponding author. Tel.: +81-45-339-3989; fax: +81-45339-4012; e-mail: [email protected]
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 ultra®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 membrane using polysulfone for a base polymer of the membrane. In a previous paper [12] the authors reported that an anion exchange membrane
0376-7388/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00194-X
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G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
prepared by the amination of the home made block copolymers of polysufone (PSf) and polyphenylenesu®desulfone (PPSS) had a minimum value of 3.30 cm2 in 2 M KCl aqueous solution for the electrical resistance and a maximum value of 0.92 (meq/gdry-resin) for the ion exchange capacity. The aims of this paper are to prepare an anion exchange membrane using block copolymers by the formation of macroreticular structure and the amination simultaneously to improve membrane properties, and, to study its chemical stability in nitric acid of 2 M (mol/dm3).
2.2. Membrane preparation 2.2.1. A membrane A membrane was prepared by the amination of the chloromethylated copolymers. The detailed preparation method of A membrane was described in a previous publication [12].
2.1.1. Block copolymer In the same manner as the synthesis in a previous paper [12] and other literature [13,14], bisphenol A disodium and 4,40 -dichlorodiphenylsulfone (DCDPS) were reacted to obtain a precursor of comprising aromatic polysulfone units. Then, the precursor, DCDPS and sodium sul®de were reacted to obtain an aromatic polysulfonepolyphenylenesul®desulfone (PSfPPSS) copolymer of the following structure:
2.2.2. B membrane B membrane was prepared by conducting the formation of macroreticular structure and introduction of ion exchange groups to the chloromethylated copolymers simultaneously. The chloromethylated copolymer was dissolved in the solution of the volume ratio 3:1 of N,N-dimethylformamide (DMF) and tert-amylalcohol. Then, 0.002 g of tri-methylamine against 1 g of the chloromethylated copolymer was added, and then tertamylalcohol was allowed to evaporate for a speci®c period of time at 1208C with stirring to conduct macroreticular structure formation and amination of the chloromethylated copolymers simultaneously. This solution was cast on the silicon rubber plate, and heated and dried at 1108C for 30 min in a constant-temperature drying oven to obtain an anion exchange layer. On the other hand, the chloromethylated copolymer
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:
was dissolved in DMF. Then, 0.002 g of tri-methylamine (1.2 M (mol/dm3)) against 1 g of the chloromethylated copolymer were added, and then methanol was added to this solution in the ratio of 4:1 to obtain the casting solution. The casting solution was cast on the anion exchange layer. Then, a polypropylene nonwoven fabric of
2. Experimental 2.1. Chemicals
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
165
0.14 mm thick (PO30FW NB, ROKI) was overlapped and dried at 1108C for 30 min in a constant-temperature drying oven. Following this, the product was released from the silicon plate to obtain a laminated anion exchange membrane. A and B anion exchange membrane were as the following structure:
resin is prepared by the introduction of the ion exchange groups to the copolymers. In this study, ion exchange membrane was prepared by conducting the formation of macroreticular structure and the introduction of ion exchange groups to the copolymers simultaneously. From this behavior, the ion exchange membrane would have the surface of the macropore
2.3. Membrane properties (area resistivity in 2 M KCl aqueous solution, ion exchange capacity (IEC), and static transport number)
distribution and high ion exchange capacity. Fig. 1 shows the photograph of the surface of B membrane obtained with a scanning electron microscope (SEM). The surface of the B membrane shows the macropore distribution. It is con®rmed from Fig. 1 that B membrane has the macroreticular structure.
The measurement methods and experimental apparatus for area resistance, ion exchange capacity and static transport number were same as reported earlier by Hwang and Ohya [12,15]. 2.4. Chemical stability in nitric acid The chemical stability was determined by the change of the surface state and the membrane properties of the prepared membrane induced by the oxidative degradation during the storage in 2 M nitric acid. 3. Results and discussion 3.1. Macroreticular structure In general, the formation of macroreticular structure is prepared by copolymerizations with solvents for the monomers in the presence of nonpolymerizing solvents [16±18]. When nonpolymerizing solvents were added in the reaction mixture, those solvents would locally concentrate on the internal copolymers. As the solvents were removed from the copolymers, the macropores would remain at the location of the removed solvents. The macroreticular ion exchange
3.2. Area resistivity in 2 M KCl aqueous solution Fig. 2 illustrates the area resistivities in 2 M KCl aqueous solution of A and B membranes. Area resistivities of the membranes were between 5.2 and 3.3 cm2 for A membrane, between 0.9 and 1.4 cm2 for B membrane. Compared with A membrane, B membrane had much lower area resistivity. This suggests that B membrane had the capability of lower speci®c conductivity than A membrane. This lower area resistivity is related to the ion exchange layer which has macroreticular structure formed to obtain better membrane properties and consequently has more porosity and lower area resistivity. In general, the thickness of the ion exchange membrane is one of the important properties of the membrane. The thickness of membranes were between 0.3 and 0.64 mm for A membrane, between 0.170 and 2.2 mm for B membrane. From the data of Fig. 2, it would be expected that macroreticular structure formation in the membrane have an effect on the electrical resistance.
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Fig. 1. SEM photograph of the surface of B membrane.
Fig. 2. Area resistivities in 2 M KCl aqueous solution of A and B membranes.
3.3. IEC and static transport number Fig. 3 illustrates the ion exchange capacities of A and B membranes.
Fig. 3. Ion exchange capacities of A and B membranes.
Ion exchange capacities of the membranes were 0.5±0.9 meq/g-dry-resin for A membrane, 1.46± 3.7 meq/g-dry-resin) for B membrane. Compared with A membrane, B membrane had much higher ion exchange capacity. This suggests
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
167
is interesting to note that the membrane having higher ®xed ion concentration than 3.7 meq/g-H2O, had lower static transport number than that having the ®xed ion concentration of 2.2±3.7 meq/g-H2O. From these results, it can be seen that the optimal regions for the best static transport number occur at relatively high ®xed ion concentration in the range 2.2±3.7 meq/ g-H2O. 3.4. Chemical stability in nitric acid
Fig. 4. The relationship between the fixed ion concentration and static transport number of chlorine anion of A and B membranes.
that B membrane has the capability of higher ion selectivity than A membrane. When the ion exchange membrane is prepared by conducting the macroreticular structure formation and introduction of the ion exchange groups to the copolymers simultaneously, ion exchange groups would readily distribute to the chloromethylated copolymers, and then the highly introduced ion exchange groups would lead to the increase of the ion exchange capacity. From the data of Fig. 3, it is expected that the preparation method which formed the macroreticular structure and introduced ion exchange groups to the chloromethylated copolymers simultaneously, that have an effect on the ion selectivity in comparison to the preparation method was amination of the chloromethylated copolymers. Fig. 4 illustrates the relationship between the ®xed ion concentration and the transport number of the chlorine anion of A and B membranes. Static transport numbers of the chloride anion of the membranes were 0.61±0.64 for A membrane, 0.68± 0.87 for B membrane. B membrane had higher ion permselectivity than A membrane. In general, the ion permselectivity (static transport number) is higher as the ®xed ion concentration in the membrane (amount of ion exchange groups per the water content in the membrane). As shown in Fig. 4, it
Fig. 5 illustrates the comparison of area resistivities and ion exchange capacities of B membranes before and after durability test which is carried out by soaking it in 2 M nitric acid. There is a small increase of the area resistivity of B2 membrane after a soaking time of 1080 h, and after 2350 h the increase in it is bigger than it is after 1080 h, then after 3340 h it has almost the same value as after 2350 h. The area resistivities of the other membranes after each soaking time had almost the same value as that before durability test. As for B-1 membrane, the ion exchange capacity after a soaking time of 1080 h had almost the same value as that before the durability test, and after 2350 h decreased compared with that after 1080 h, then after 3340 h had almost the same value as that after 2350 h. After each soaking time the ion exchange capacities of the other membranes had almost the same value as that before durability test. As shown in Fig. 5(a) and (b), the area resistivities and the ion exchange capacities of B membranes after 3340 h of durability test have almost the same value as that before durability test. It would be expected that the ion exchange membrane made of PSfPPSS used as a base polymer has a good chemical stability with largely unchanged area resistivities in 2 M KCl aqueous solution and ion exchange capacities. From the results presented, it can be concluded that B membrane has very good membrane properties with a low area resistivity and a high ion exchange capacity, and have a good chemical stability with long lifetime in 2 M nitric acid. 4. Conclusion 1. The anion exchange membrane was prepared by conducting the formation of macroreticular
168
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
Fig. 5. The comparison of area resistivities and ion exchange capacities of B membrane before and after durability test which is carried out by soaking it in 2 M nitric acid.
2. 3. 4. 5.
6.
structure and introduction of the ion exchange groups to the chloromethylated copolymers simultaneously (B membrane). Area resistivities in 2 M KCl aqueous solution of B membranes were 0.9±1.4 cm2. Ion exchange capacities of B membranes were 1.46±3.7 meq/g-dry-resin. Static transport numbers of chloride anion of B membranes were 0.68±0.87. The properties of the membrane prepared by forming the macroreticular structure and introducing the ion exchange groups to the chloromethylated copolymers simultaneously (B membrane) had higher performance than that of the membrane prepared by the amination of the chloromethylated copolymers (A membrane). The chemical stability of B membrane was determined by the oxidative degradation in 2 M nitric acid, had long lifetime with almost the same value of area resistivity and ion exchange capacity as that before durability test.
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.
[2] C.M. Tam, M. Dal-cin, M.D. Guiver, Polysulfone membraneIV. 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, Memoirs 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.
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169 [13] Japan Patent 61-168629, 30 July 1986. [14] US Patent 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] J.R. Millar, D.G. Smith, W.E. Marr, T.R.E. Kressman, Solvent-modified polymer networks. Part I. The preparation and characterisation of expanded-network and macroporous styrene±divinylbenzene copolymers and their sulphonates, J. Chem. Soc. (1963) 218±225.
169
[17] J.R. Millar, D.G. Smith, T.R.E. Kressman, Solvent-modified polymer networks. Part IV. Styrene±divinylbenzene copolymers made in the presence of non-solvating diluents, J. Chem. Soc. (1965) 304±310. [18] K.A. Kun, R Kumin, Macroreticular resins. III. Formation of macroreticular styrene±divinylbenzene copolymers, J. Polym. Sci., Part A-1 6 (1968) 2689±2701.
Preparation of anion exchange membrane based on block copolymers. Part II: the effect of the formation of macroreticular structure on the membrane properties Gab-Jin Hwanga, Haruhiko Ohyab,* a
b
Japan Atomic Energy Research Institute, Shirakata Shirane 2-4, Tokai-mura,naka-gun, Ibaraki-ken, 319-1195, Japan Department of Material Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan Received 12 September 1997; received in revised form 12 September 1997; accepted 15 June 1998
Abstract The anion exchange membrane was prepared by conducting the formation of macroreticular structure and the introduction of ion exchange groups to the chloromethylated copolymers simultaneously. The membrane properties are summarized below; 0.9±1.4 cm2 for area resistivity in 2 M KCl aqueous solution, 1.46±3.7 meq/g-dry-resin for the ion exchange capacity and 0.68±0.87 for the static transport number. The membrane properties of the membrane prepared by forming the macroreticular structure and the introduction of the ion exchange groups to the chloromethylated copolymers simultaneously (B membrane) had higher performance than that of the membrane prepared by the amination of the chloromethylated copolymers (A membrane). It would be expected that the formation of macroreticular structure in the membrane has an effect on the membrane properties. The chemical stability of B membrane which was determined by the oxidative degradation in 2 M nitric acid, had long lifetime with almost the same value of the area resistivity and ion exchange capacity as before durability test. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Polysulfone; Block copolymers; Ion exchange membrane; Ion exchange capacity; Anion 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. Recently, engineering plastics such as polysulfone and polyethersulfone have been widely used as a base *Corresponding author. Tel.: +81-45-339-3989; fax: +81-45339-4012; e-mail: [email protected]
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 ultra®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 membrane using polysulfone for a base polymer of the membrane. In a previous paper [12] the authors reported that an anion exchange membrane
0376-7388/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00194-X
164
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
prepared by the amination of the home made block copolymers of polysufone (PSf) and polyphenylenesu®desulfone (PPSS) had a minimum value of 3.30 cm2 in 2 M KCl aqueous solution for the electrical resistance and a maximum value of 0.92 (meq/gdry-resin) for the ion exchange capacity. The aims of this paper are to prepare an anion exchange membrane using block copolymers by the formation of macroreticular structure and the amination simultaneously to improve membrane properties, and, to study its chemical stability in nitric acid of 2 M (mol/dm3).
2.2. Membrane preparation 2.2.1. A membrane A membrane was prepared by the amination of the chloromethylated copolymers. The detailed preparation method of A membrane was described in a previous publication [12].
2.1.1. Block copolymer In the same manner as the synthesis in a previous paper [12] and other literature [13,14], bisphenol A disodium and 4,40 -dichlorodiphenylsulfone (DCDPS) were reacted to obtain a precursor of comprising aromatic polysulfone units. Then, the precursor, DCDPS and sodium sul®de were reacted to obtain an aromatic polysulfonepolyphenylenesul®desulfone (PSfPPSS) copolymer of the following structure:
2.2.2. B membrane B membrane was prepared by conducting the formation of macroreticular structure and introduction of ion exchange groups to the chloromethylated copolymers simultaneously. The chloromethylated copolymer was dissolved in the solution of the volume ratio 3:1 of N,N-dimethylformamide (DMF) and tert-amylalcohol. Then, 0.002 g of tri-methylamine against 1 g of the chloromethylated copolymer was added, and then tertamylalcohol was allowed to evaporate for a speci®c period of time at 1208C with stirring to conduct macroreticular structure formation and amination of the chloromethylated copolymers simultaneously. This solution was cast on the silicon rubber plate, and heated and dried at 1108C for 30 min in a constant-temperature drying oven to obtain an anion exchange layer. On the other hand, the chloromethylated copolymer
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:
was dissolved in DMF. Then, 0.002 g of tri-methylamine (1.2 M (mol/dm3)) against 1 g of the chloromethylated copolymer were added, and then methanol was added to this solution in the ratio of 4:1 to obtain the casting solution. The casting solution was cast on the anion exchange layer. Then, a polypropylene nonwoven fabric of
2. Experimental 2.1. Chemicals
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
165
0.14 mm thick (PO30FW NB, ROKI) was overlapped and dried at 1108C for 30 min in a constant-temperature drying oven. Following this, the product was released from the silicon plate to obtain a laminated anion exchange membrane. A and B anion exchange membrane were as the following structure:
resin is prepared by the introduction of the ion exchange groups to the copolymers. In this study, ion exchange membrane was prepared by conducting the formation of macroreticular structure and the introduction of ion exchange groups to the copolymers simultaneously. From this behavior, the ion exchange membrane would have the surface of the macropore
2.3. Membrane properties (area resistivity in 2 M KCl aqueous solution, ion exchange capacity (IEC), and static transport number)
distribution and high ion exchange capacity. Fig. 1 shows the photograph of the surface of B membrane obtained with a scanning electron microscope (SEM). The surface of the B membrane shows the macropore distribution. It is con®rmed from Fig. 1 that B membrane has the macroreticular structure.
The measurement methods and experimental apparatus for area resistance, ion exchange capacity and static transport number were same as reported earlier by Hwang and Ohya [12,15]. 2.4. Chemical stability in nitric acid The chemical stability was determined by the change of the surface state and the membrane properties of the prepared membrane induced by the oxidative degradation during the storage in 2 M nitric acid. 3. Results and discussion 3.1. Macroreticular structure In general, the formation of macroreticular structure is prepared by copolymerizations with solvents for the monomers in the presence of nonpolymerizing solvents [16±18]. When nonpolymerizing solvents were added in the reaction mixture, those solvents would locally concentrate on the internal copolymers. As the solvents were removed from the copolymers, the macropores would remain at the location of the removed solvents. The macroreticular ion exchange
3.2. Area resistivity in 2 M KCl aqueous solution Fig. 2 illustrates the area resistivities in 2 M KCl aqueous solution of A and B membranes. Area resistivities of the membranes were between 5.2 and 3.3 cm2 for A membrane, between 0.9 and 1.4 cm2 for B membrane. Compared with A membrane, B membrane had much lower area resistivity. This suggests that B membrane had the capability of lower speci®c conductivity than A membrane. This lower area resistivity is related to the ion exchange layer which has macroreticular structure formed to obtain better membrane properties and consequently has more porosity and lower area resistivity. In general, the thickness of the ion exchange membrane is one of the important properties of the membrane. The thickness of membranes were between 0.3 and 0.64 mm for A membrane, between 0.170 and 2.2 mm for B membrane. From the data of Fig. 2, it would be expected that macroreticular structure formation in the membrane have an effect on the electrical resistance.
166
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
Fig. 1. SEM photograph of the surface of B membrane.
Fig. 2. Area resistivities in 2 M KCl aqueous solution of A and B membranes.
3.3. IEC and static transport number Fig. 3 illustrates the ion exchange capacities of A and B membranes.
Fig. 3. Ion exchange capacities of A and B membranes.
Ion exchange capacities of the membranes were 0.5±0.9 meq/g-dry-resin for A membrane, 1.46± 3.7 meq/g-dry-resin) for B membrane. Compared with A membrane, B membrane had much higher ion exchange capacity. This suggests
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
167
is interesting to note that the membrane having higher ®xed ion concentration than 3.7 meq/g-H2O, had lower static transport number than that having the ®xed ion concentration of 2.2±3.7 meq/g-H2O. From these results, it can be seen that the optimal regions for the best static transport number occur at relatively high ®xed ion concentration in the range 2.2±3.7 meq/ g-H2O. 3.4. Chemical stability in nitric acid
Fig. 4. The relationship between the fixed ion concentration and static transport number of chlorine anion of A and B membranes.
that B membrane has the capability of higher ion selectivity than A membrane. When the ion exchange membrane is prepared by conducting the macroreticular structure formation and introduction of the ion exchange groups to the copolymers simultaneously, ion exchange groups would readily distribute to the chloromethylated copolymers, and then the highly introduced ion exchange groups would lead to the increase of the ion exchange capacity. From the data of Fig. 3, it is expected that the preparation method which formed the macroreticular structure and introduced ion exchange groups to the chloromethylated copolymers simultaneously, that have an effect on the ion selectivity in comparison to the preparation method was amination of the chloromethylated copolymers. Fig. 4 illustrates the relationship between the ®xed ion concentration and the transport number of the chlorine anion of A and B membranes. Static transport numbers of the chloride anion of the membranes were 0.61±0.64 for A membrane, 0.68± 0.87 for B membrane. B membrane had higher ion permselectivity than A membrane. In general, the ion permselectivity (static transport number) is higher as the ®xed ion concentration in the membrane (amount of ion exchange groups per the water content in the membrane). As shown in Fig. 4, it
Fig. 5 illustrates the comparison of area resistivities and ion exchange capacities of B membranes before and after durability test which is carried out by soaking it in 2 M nitric acid. There is a small increase of the area resistivity of B2 membrane after a soaking time of 1080 h, and after 2350 h the increase in it is bigger than it is after 1080 h, then after 3340 h it has almost the same value as after 2350 h. The area resistivities of the other membranes after each soaking time had almost the same value as that before durability test. As for B-1 membrane, the ion exchange capacity after a soaking time of 1080 h had almost the same value as that before the durability test, and after 2350 h decreased compared with that after 1080 h, then after 3340 h had almost the same value as that after 2350 h. After each soaking time the ion exchange capacities of the other membranes had almost the same value as that before durability test. As shown in Fig. 5(a) and (b), the area resistivities and the ion exchange capacities of B membranes after 3340 h of durability test have almost the same value as that before durability test. It would be expected that the ion exchange membrane made of PSfPPSS used as a base polymer has a good chemical stability with largely unchanged area resistivities in 2 M KCl aqueous solution and ion exchange capacities. From the results presented, it can be concluded that B membrane has very good membrane properties with a low area resistivity and a high ion exchange capacity, and have a good chemical stability with long lifetime in 2 M nitric acid. 4. Conclusion 1. The anion exchange membrane was prepared by conducting the formation of macroreticular
168
G.-J. Hwang, H. Ohya / Journal of Membrane Science 149 (1998) 163±169
Fig. 5. The comparison of area resistivities and ion exchange capacities of B membrane before and after durability test which is carried out by soaking it in 2 M nitric acid.
2. 3. 4. 5.
6.
structure and introduction of the ion exchange groups to the chloromethylated copolymers simultaneously (B membrane). Area resistivities in 2 M KCl aqueous solution of B membranes were 0.9±1.4 cm2. Ion exchange capacities of B membranes were 1.46±3.7 meq/g-dry-resin. Static transport numbers of chloride anion of B membranes were 0.68±0.87. The properties of the membrane prepared by forming the macroreticular structure and introducing the ion exchange groups to the chloromethylated copolymers simultaneously (B membrane) had higher performance than that of the membrane prepared by the amination of the chloromethylated copolymers (A membrane). The chemical stability of B membrane was determined by the oxidative degradation in 2 M nitric acid, had long lifetime with almost the same value of area resistivity and ion exchange capacity as that before durability test.
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.
[2] C.M. Tam, M. Dal-cin, M.D. Guiver, Polysulfone membraneIV. 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, Memoirs 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.
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