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Selective modification of block copolymers as proton exchange membranes
Electrochimica Acta 50 (2004) 617–620
Selective modification of block copolymers as proton exchange membranes Jung-Eun Yang, Jae-Suk Lee∗ Department of Materials Science and Engineering, Center for Frontier Materials (BK21), Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea Received 2 June 2003; received in revised form 31 March 2004; accepted 31 March 2004 Available online 13 August 2004
Abstract A series of selectively sulfonated polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene copolymers (SSEBS) with varying degree of sulfonation were synthesized. The morphology of SSEBS characterized by AFM and TEM showed the microphase separated nano-structures. The SSEBS formed nano-sized ionic channels by association of the sulfonate groups in the styrene blocks of SEBS. The well-ordered ionic channels are continuously linked, which resulted in enhanced proton conductivity. Proton conductivity values of SSEBS determined by membrane electrical resistance (MER) measurement was more than that of the commercially available proton exchange membranes. © 2004 Elsevier Ltd. All rights reserved. Keywords: Proton exchange membranes; Electrolyte; Sulfonation; Block copolymer; Ionic channels
1. Introduction Polymer electrolytes bearing strong acid groups (e.g. sulfonic acid) have been developed and received great attention as proton exchange membranes (PEM) for fuel cells due to their high energy densities and low emission of CO, VOCs [1,2]. For use in fuel cells the essential properties of PEM, such as high proton conductivity, good mechanical strength, and chemical stability need to be optimized [3]. Most of the PEM studied so far are polymer electrolytes containing randomly distributed regions of ionic groups in a hydrophobic polymer matrix [4–6]. These ionic groups associate to form isolated ionic clusters leading to a random distribution of ionic channels in the polymer matrix. As a result, ions or protons can only be transported through the interfacial region of ionic channels in the presence of a small amount of water [7,8]. If the ionic channels are continuously arranged in order, protons can be better transferred through these regularly ∗
Corresponding author. Tel.: +82 62 970 2306; fax: +82 62 970 2304. E-mail address: [email protected] (J.-S. Lee).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.03.064
distributed channels. Moreover, the conductivity of PEM will be improved. Recently, sulfonated styrene–ethylene/butylenes–styrene [9] and styrene–butadiene–styrene [10] block copolymers have been developed, in which proton conductivity has been suggested to take place through continuous ionic channels. In this work, we present the AFM and TEM studies on the selectively sulfonated A–B–A triblock copolymer, polystyrene-bpoly(ethylene-r-butylene)-b-polystyrene (SEBS). The effect of degree of sulfonation on proton conductivity was also investigated. 2. Experimental 2.1. Materials The polymer used in this work was polystyrene-bpoly(ethylene-r-butylene)-b-polystyrene block copolymer (MW = 50,000 and 30 wt.% styrene units) purchased from Kumho Chemicals, Korea. Sulfonating reagent was acetyl sulfate prepared by the reaction of acetic anhydride and concentrated sulfuric acid (96%).
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J.-E. Yang, J.-S. Lee / Electrochimica Acta 50 (2004) 617–620
done by the complex impedance method using a 4192A LF Impedance Analyzer (Hewlett-Packard). Membrane electrical resistance (MER) of SSEBS films cast from THF and MeOH (4/1 v/v) solution was carried out by impedance measurement using a LCZ meter (NF electronic instruments) running at a frequency of 100 kHz. The magnitude of impedance (|Z|), and the phase angle of impedance (θ) of the membranes were measured and converted into MER ( cm2 ) using the following equation: MER = (|Z|sample · cos θsample −|Z|blank · cos θblank ) × area.
3. Results and discussion
Fig. 1. Sulfonation of polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene block copolymers.
2.2. Selective sulfonation of the block copolymer A series of sulfonated SEBS were prepared following the procedure reported in the literature [11]. SEBS (3 g) dissolved in 50 ml 1,2-dichloroethane (DCE) was stirred at 55 ◦ C under nitrogen for 1 h. In order to control the degree of sulfonation, the required amount of acetyl sulfate was added (Fig. 1). The solution was stirred for 5 h and then methanol was added to stop the reaction. Evaporating DCE and washing with deionized water several times led to the selectively sulfonated SEBS (SSEBS). The sulfonation of the polystyrene blocks in SEBS was confirmed from FT-IR measurements [12]. 2.3. Membrane preparation A solution of SSEBS in THF/methanol (4/1 v/v) mixture was prepared, and then poured into a glass dish and placed at room temperature until evaporation of the solvent. The membrane was put under nitrogen and residual solvent was removed under vacuum. The membrane was immersed in deionized water before the tests are carried out. 2.4. Characterization The morphology of microphase separated SSEBS was investigated using AFM operated in tapping mode (Digital Instruments). The Si substrate was dipped into SSEBS solution (THF/MeOH), and then was evaporated. To obtain TEM image (EF-TEM, EM 912 OMEGA ZEISS, Germany), SSBES film was cast on carbon coated copper specimen grid and then was annealed for 24 h at 60 ◦ C under nitrogen. For proton conductivity measurement, the samples were equilibrated in 0.5N NaCl at 25 ◦ C for more than a day. Measurement was
Block copolymers consisting of hydrophilic and hydrophobic blocks associate uniformly on microscopic scales and their morphology, i.e. spherical, cylindrical, or lamellar shape depends on the relative volume fractions of the constituent components [13–15]. Partially and selectively sulfonated SEBS block copolymers consisting of hydrophobic and hydrophilic blocks are known to demonstrate selfassembly phenomena leading to separation of the microphase domains [9,10]. Such microphase separation can lead to formation of continuous ionic channels, and hence membranes cast out of these copolymers can be used for efficient proton transport in fuel cells. The AFM image of SSEBS on Si substrate is shown in Fig. 2a. The topography of AMF image shows the mircophase separation of SSEBS. However, the morphology seems rather disordered. Hence, we examined the morphology of the sample stained with RuO4 by TEM (Fig. 2b). The hydrophilic blocks (dark area from staining) in TEM image clearly show the formation of the continuously connected nano-channels of about 10 nm average width. Furthermore, nano-structures of the hydrophobic domains (light regions) appear as lamellar/cylindrical with average width of about 50 nm. This is a direct evidence of well-ordered and continuous ionic channels formed by the microphase separation between hydrophilic domains containing sulfonate groups and hydrophobic domains constituted by the polymer backbone, corroborating an earlier computational modeling study [9a]. Protons transfer through these nano-channels of SSEBS containing the sulfonated hydrophilic domains surrounded by the hydrophobic polymer as backbone is schematically shown in Fig. 2c. The SSEBS sample contained 30% PS blocks. The dominant portion being ethylene–butylenes blocks, it may be argued that the PS regions should constitute an isolated rather than continuous phase. However, a microphase-separated structure does arise because of a delicate balance between minimizing the unfavorable interaction energy between the incompatible blocks and maximizing the conformational entropy of the system [10]. The drive for this balance leads to formation of narrow hydrophilic nano-channels in a continuous network embedded in the hydrophobic polymer matrix, even if the PS content is low. Furthermore, the formation of
J.-E. Yang, J.-S. Lee / Electrochimica Acta 50 (2004) 617–620
619
Fig. 2. (a) Tapping mode AFM image of SSEBS on Si wafer substrate, (b) TEM image of SSEBS film after staining, and (c) schematic illustration of continuous ionic channels as nano-domains for proton.
continuous ionic channel is dependant on the structure of the copolymer. No specific structural domains may be detected in the TEM micrographs of certain copolymer samples with small variations in the structure from other copolymers that show continuous proton channel [10]. Fig. 3 shows the results of membrane electrical resistance and proton conductivity as a function of the degree of sulfonation. As expected, the value of MER decreased with increasing the concentration of sulfonate groups, while proton conductivity increased. Moreover, the lowest value of MER from SSEBS is 2.04 cm2 , while the value of MER in commercial membranes Neosepta® CMX from Tokuyama Corp. (Japan) is 2.98 cm2 . The commercial Nafion 117 has a pro-
ton conductivity of 10−3 S/cm, whereas for SSEBS a value of ∼10−2 S/cm could be reached. For MER and proton conductivity measurements, the samples were equilibrated with 0.5N NaCl. This would lead to exchange of –SO3 H protons with Na+ , and hence the MER/conductivity values may be representative of membranes rich with –SO3 Na groups. Nevertheless, the purpose in this preliminary study was a comparison of samples with varying degree of sulfonation. Incidentally, SSEBS membranes with only –SO3 H groups showed high conductivity. At full hydration, conductivities as high as 0.1 S/cm are routinely observed for SSEBS [9a]. SSEBS is easily processable and a superior material than the known commercial product as regards the proton conductivity, which again could be manipulated through a control over the degree of sulfonation. 4. Conclusions The TEM studies on selectively sulfonated polystyreneb-poly(ethylene-r-butylene)-b-polystyrene copolymers provided direct evidence of well-ordered, nano-sized, and continuous ionic channels. A control over the extent of sulfonation is easily achieved leading to proton exchange membranes of varying degree of proton conductivity exceeding those of the commercially available membranes. Acknowledgements
Fig. 3. Effect of sulfonation degree on membrane electrical resistance and proton conductivity for SSEBS.
This work was partially supported by a grant from the Basic Research Program (No. R01-2001-000-00424-0),
620
J.-E. Yang, J.-S. Lee / Electrochimica Acta 50 (2004) 617–620
Korea–Japan Joint Research Project (No. F01-2001-00020032-0) of the Korea Science & Engineering Foundation, National R&D Project for Nano Science and Technology of the Ministry of Science & Technology (M1021400016902B1500-02710).
[7] [8] [9]
References [1] A.J. Appleby, F.R. Foulkes, Fuel Cell Handbook, Van Norstrand Reinhold, New York, 1989. [2] M.S. Wilson, S.J. Gottesfeld, Electrochem. Soc. 139 (1992) 28. [3] J. Larmine, A. Dicks, Fuel Cell Systems, Plenum, New York, 1993. [4] M. Rikukawa, K. Sanui, Prog. Polym. Sci. 25 (2000) 1463. [5] S.G. Ehrenberg, J.M. Serpico, G.E. Wnek, J.N. Rider, U.S. Patent 5,468,574 (1995). [6] (a) K.A. Mauritz, I.D. Stefanithis, R.W. Davis, R.K. Scheetz, S.V. Pope, G.L. Wilkes, H.H. Huang, J. Appl. Polym. Sci. 55 (1995) 181; (b) K.A. Mauritz, P.L. Shao, R.B. Moore, Chem. Mater. 7 (1995) 192;
[10] [11] [12] [13] [14] [15]
(c) K.A. Mauritz, R. Ju, Chem. Mater. 6 (1994) 2269; (d) K.A. Mauritz, S.F. Su, Macromolecules 27 (1994) 2079. T.A. Zawodzinski, J. Davey, J. Valerio, S. Gottesfeld, Electrochim. Acta 40 (1995) 297. 8.G. Cho, K. Park, J. Jang, S. Jung, J. Moon, T. Kim, Electrochem. Commun. 4 (2002) 336. (a) J.M. Serpico, S.G. Ehrenberg, J.J. Fontanella, X. Jiao, D. Perahia, K.A. McGrady, E.H. Sanders, G.E. Kellogg, G.E. Wnek, Macromolecules 35 (2002) 5916; (b) C.A. Edmondson, J.J. Fontanella, S.H. Chung, S.G. Greenbaum, G.E. Wnek, Electrochim. Acta 46 (2001) 1623; (c) M.C. Wintersgill, J.J. Fontanella, Electrochim. Acta 43 (1998) 1533. J. Won, H.H. Park, Y.J. Kim, S.W. Choi, H.Y. Ha, I.-H. Oh, H.S. Kim, Y.S. Kang, K.J. Ihn, Macromolecules 36 (2003) 3228. R.A. Weiss, A. Sen, C.L. Willis, L.A. Pottick, Polymer 32 (1991) 1867. A. Mokrini, J.L. Acosta, Polymer 42 (2001) 9. X. Lu, W.P. Steckel, R.A. Weiss, Macromolecules 26 (1993) 5876. J.H. Laurer, R. Bukovnik, R.J. Spontak, Macromolecules 29 (1996) 5760. R.F. Storey, D.W. Baugh, Polymer 41 (2000) 3205.
Selective modification of block copolymers as proton exchange membranes Jung-Eun Yang, Jae-Suk Lee∗ Department of Materials Science and Engineering, Center for Frontier Materials (BK21), Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea Received 2 June 2003; received in revised form 31 March 2004; accepted 31 March 2004 Available online 13 August 2004
Abstract A series of selectively sulfonated polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene copolymers (SSEBS) with varying degree of sulfonation were synthesized. The morphology of SSEBS characterized by AFM and TEM showed the microphase separated nano-structures. The SSEBS formed nano-sized ionic channels by association of the sulfonate groups in the styrene blocks of SEBS. The well-ordered ionic channels are continuously linked, which resulted in enhanced proton conductivity. Proton conductivity values of SSEBS determined by membrane electrical resistance (MER) measurement was more than that of the commercially available proton exchange membranes. © 2004 Elsevier Ltd. All rights reserved. Keywords: Proton exchange membranes; Electrolyte; Sulfonation; Block copolymer; Ionic channels
1. Introduction Polymer electrolytes bearing strong acid groups (e.g. sulfonic acid) have been developed and received great attention as proton exchange membranes (PEM) for fuel cells due to their high energy densities and low emission of CO, VOCs [1,2]. For use in fuel cells the essential properties of PEM, such as high proton conductivity, good mechanical strength, and chemical stability need to be optimized [3]. Most of the PEM studied so far are polymer electrolytes containing randomly distributed regions of ionic groups in a hydrophobic polymer matrix [4–6]. These ionic groups associate to form isolated ionic clusters leading to a random distribution of ionic channels in the polymer matrix. As a result, ions or protons can only be transported through the interfacial region of ionic channels in the presence of a small amount of water [7,8]. If the ionic channels are continuously arranged in order, protons can be better transferred through these regularly ∗
Corresponding author. Tel.: +82 62 970 2306; fax: +82 62 970 2304. E-mail address: [email protected] (J.-S. Lee).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.03.064
distributed channels. Moreover, the conductivity of PEM will be improved. Recently, sulfonated styrene–ethylene/butylenes–styrene [9] and styrene–butadiene–styrene [10] block copolymers have been developed, in which proton conductivity has been suggested to take place through continuous ionic channels. In this work, we present the AFM and TEM studies on the selectively sulfonated A–B–A triblock copolymer, polystyrene-bpoly(ethylene-r-butylene)-b-polystyrene (SEBS). The effect of degree of sulfonation on proton conductivity was also investigated. 2. Experimental 2.1. Materials The polymer used in this work was polystyrene-bpoly(ethylene-r-butylene)-b-polystyrene block copolymer (MW = 50,000 and 30 wt.% styrene units) purchased from Kumho Chemicals, Korea. Sulfonating reagent was acetyl sulfate prepared by the reaction of acetic anhydride and concentrated sulfuric acid (96%).
618
J.-E. Yang, J.-S. Lee / Electrochimica Acta 50 (2004) 617–620
done by the complex impedance method using a 4192A LF Impedance Analyzer (Hewlett-Packard). Membrane electrical resistance (MER) of SSEBS films cast from THF and MeOH (4/1 v/v) solution was carried out by impedance measurement using a LCZ meter (NF electronic instruments) running at a frequency of 100 kHz. The magnitude of impedance (|Z|), and the phase angle of impedance (θ) of the membranes were measured and converted into MER ( cm2 ) using the following equation: MER = (|Z|sample · cos θsample −|Z|blank · cos θblank ) × area.
3. Results and discussion
Fig. 1. Sulfonation of polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene block copolymers.
2.2. Selective sulfonation of the block copolymer A series of sulfonated SEBS were prepared following the procedure reported in the literature [11]. SEBS (3 g) dissolved in 50 ml 1,2-dichloroethane (DCE) was stirred at 55 ◦ C under nitrogen for 1 h. In order to control the degree of sulfonation, the required amount of acetyl sulfate was added (Fig. 1). The solution was stirred for 5 h and then methanol was added to stop the reaction. Evaporating DCE and washing with deionized water several times led to the selectively sulfonated SEBS (SSEBS). The sulfonation of the polystyrene blocks in SEBS was confirmed from FT-IR measurements [12]. 2.3. Membrane preparation A solution of SSEBS in THF/methanol (4/1 v/v) mixture was prepared, and then poured into a glass dish and placed at room temperature until evaporation of the solvent. The membrane was put under nitrogen and residual solvent was removed under vacuum. The membrane was immersed in deionized water before the tests are carried out. 2.4. Characterization The morphology of microphase separated SSEBS was investigated using AFM operated in tapping mode (Digital Instruments). The Si substrate was dipped into SSEBS solution (THF/MeOH), and then was evaporated. To obtain TEM image (EF-TEM, EM 912 OMEGA ZEISS, Germany), SSBES film was cast on carbon coated copper specimen grid and then was annealed for 24 h at 60 ◦ C under nitrogen. For proton conductivity measurement, the samples were equilibrated in 0.5N NaCl at 25 ◦ C for more than a day. Measurement was
Block copolymers consisting of hydrophilic and hydrophobic blocks associate uniformly on microscopic scales and their morphology, i.e. spherical, cylindrical, or lamellar shape depends on the relative volume fractions of the constituent components [13–15]. Partially and selectively sulfonated SEBS block copolymers consisting of hydrophobic and hydrophilic blocks are known to demonstrate selfassembly phenomena leading to separation of the microphase domains [9,10]. Such microphase separation can lead to formation of continuous ionic channels, and hence membranes cast out of these copolymers can be used for efficient proton transport in fuel cells. The AFM image of SSEBS on Si substrate is shown in Fig. 2a. The topography of AMF image shows the mircophase separation of SSEBS. However, the morphology seems rather disordered. Hence, we examined the morphology of the sample stained with RuO4 by TEM (Fig. 2b). The hydrophilic blocks (dark area from staining) in TEM image clearly show the formation of the continuously connected nano-channels of about 10 nm average width. Furthermore, nano-structures of the hydrophobic domains (light regions) appear as lamellar/cylindrical with average width of about 50 nm. This is a direct evidence of well-ordered and continuous ionic channels formed by the microphase separation between hydrophilic domains containing sulfonate groups and hydrophobic domains constituted by the polymer backbone, corroborating an earlier computational modeling study [9a]. Protons transfer through these nano-channels of SSEBS containing the sulfonated hydrophilic domains surrounded by the hydrophobic polymer as backbone is schematically shown in Fig. 2c. The SSEBS sample contained 30% PS blocks. The dominant portion being ethylene–butylenes blocks, it may be argued that the PS regions should constitute an isolated rather than continuous phase. However, a microphase-separated structure does arise because of a delicate balance between minimizing the unfavorable interaction energy between the incompatible blocks and maximizing the conformational entropy of the system [10]. The drive for this balance leads to formation of narrow hydrophilic nano-channels in a continuous network embedded in the hydrophobic polymer matrix, even if the PS content is low. Furthermore, the formation of
J.-E. Yang, J.-S. Lee / Electrochimica Acta 50 (2004) 617–620
619
Fig. 2. (a) Tapping mode AFM image of SSEBS on Si wafer substrate, (b) TEM image of SSEBS film after staining, and (c) schematic illustration of continuous ionic channels as nano-domains for proton.
continuous ionic channel is dependant on the structure of the copolymer. No specific structural domains may be detected in the TEM micrographs of certain copolymer samples with small variations in the structure from other copolymers that show continuous proton channel [10]. Fig. 3 shows the results of membrane electrical resistance and proton conductivity as a function of the degree of sulfonation. As expected, the value of MER decreased with increasing the concentration of sulfonate groups, while proton conductivity increased. Moreover, the lowest value of MER from SSEBS is 2.04 cm2 , while the value of MER in commercial membranes Neosepta® CMX from Tokuyama Corp. (Japan) is 2.98 cm2 . The commercial Nafion 117 has a pro-
ton conductivity of 10−3 S/cm, whereas for SSEBS a value of ∼10−2 S/cm could be reached. For MER and proton conductivity measurements, the samples were equilibrated with 0.5N NaCl. This would lead to exchange of –SO3 H protons with Na+ , and hence the MER/conductivity values may be representative of membranes rich with –SO3 Na groups. Nevertheless, the purpose in this preliminary study was a comparison of samples with varying degree of sulfonation. Incidentally, SSEBS membranes with only –SO3 H groups showed high conductivity. At full hydration, conductivities as high as 0.1 S/cm are routinely observed for SSEBS [9a]. SSEBS is easily processable and a superior material than the known commercial product as regards the proton conductivity, which again could be manipulated through a control over the degree of sulfonation. 4. Conclusions The TEM studies on selectively sulfonated polystyreneb-poly(ethylene-r-butylene)-b-polystyrene copolymers provided direct evidence of well-ordered, nano-sized, and continuous ionic channels. A control over the extent of sulfonation is easily achieved leading to proton exchange membranes of varying degree of proton conductivity exceeding those of the commercially available membranes. Acknowledgements
Fig. 3. Effect of sulfonation degree on membrane electrical resistance and proton conductivity for SSEBS.
This work was partially supported by a grant from the Basic Research Program (No. R01-2001-000-00424-0),
620
J.-E. Yang, J.-S. Lee / Electrochimica Acta 50 (2004) 617–620
Korea–Japan Joint Research Project (No. F01-2001-00020032-0) of the Korea Science & Engineering Foundation, National R&D Project for Nano Science and Technology of the Ministry of Science & Technology (M1021400016902B1500-02710).
[7] [8] [9]
References [1] A.J. Appleby, F.R. Foulkes, Fuel Cell Handbook, Van Norstrand Reinhold, New York, 1989. [2] M.S. Wilson, S.J. Gottesfeld, Electrochem. Soc. 139 (1992) 28. [3] J. Larmine, A. Dicks, Fuel Cell Systems, Plenum, New York, 1993. [4] M. Rikukawa, K. Sanui, Prog. Polym. Sci. 25 (2000) 1463. [5] S.G. Ehrenberg, J.M. Serpico, G.E. Wnek, J.N. Rider, U.S. Patent 5,468,574 (1995). [6] (a) K.A. Mauritz, I.D. Stefanithis, R.W. Davis, R.K. Scheetz, S.V. Pope, G.L. Wilkes, H.H. Huang, J. Appl. Polym. Sci. 55 (1995) 181; (b) K.A. Mauritz, P.L. Shao, R.B. Moore, Chem. Mater. 7 (1995) 192;
[10] [11] [12] [13] [14] [15]
(c) K.A. Mauritz, R. Ju, Chem. Mater. 6 (1994) 2269; (d) K.A. Mauritz, S.F. Su, Macromolecules 27 (1994) 2079. T.A. Zawodzinski, J. Davey, J. Valerio, S. Gottesfeld, Electrochim. Acta 40 (1995) 297. 8.G. Cho, K. Park, J. Jang, S. Jung, J. Moon, T. Kim, Electrochem. Commun. 4 (2002) 336. (a) J.M. Serpico, S.G. Ehrenberg, J.J. Fontanella, X. Jiao, D. Perahia, K.A. McGrady, E.H. Sanders, G.E. Kellogg, G.E. Wnek, Macromolecules 35 (2002) 5916; (b) C.A. Edmondson, J.J. Fontanella, S.H. Chung, S.G. Greenbaum, G.E. Wnek, Electrochim. Acta 46 (2001) 1623; (c) M.C. Wintersgill, J.J. Fontanella, Electrochim. Acta 43 (1998) 1533. J. Won, H.H. Park, Y.J. Kim, S.W. Choi, H.Y. Ha, I.-H. Oh, H.S. Kim, Y.S. Kang, K.J. Ihn, Macromolecules 36 (2003) 3228. R.A. Weiss, A. Sen, C.L. Willis, L.A. Pottick, Polymer 32 (1991) 1867. A. Mokrini, J.L. Acosta, Polymer 42 (2001) 9. X. Lu, W.P. Steckel, R.A. Weiss, Macromolecules 26 (1993) 5876. J.H. Laurer, R. Bukovnik, R.J. Spontak, Macromolecules 29 (1996) 5760. R.F. Storey, D.W. Baugh, Polymer 41 (2000) 3205.