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Carbonization and graphitization of polyamidine films
Synthetic Metals 125 (2002) 197±200
Carbonization and graphitization of polyamidine ®lms Fujio Okinoa,*, Shinji Kawasakia, Hidekazu Touharaa, Hai Linb, Mutsumi Kimurab, Hirofusa Shiraib a
b
Faculty of Textile Science and Technology, Department of Chemistry, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan Faculty of Textile Science and Technology, Department of Functional Polymer Science, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan Received 14 May 2001; received in revised form 5 June 2001; accepted 2 July 2001
Abstract Carbonization and graphitization of polyamidine ®lms, a copolymer of acrylonitrile and N-vinylformamide, were carried out at 1000, 1500, 2000, 2500, 28008C with and without oxidation-treatment in advance at 2808C to stabilize the ®lms. The stabilization raised the weight yield of carbonization by ca. 5%. The highest degree of graphitization was attained by the sample heat treated at 28008C after stabilization, the c-axis interlayer distance being 0.3376 nm. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyamidine; Stabilization; Graphitization; Carbonization; N-vinylformamide; Polyvinylamine
1. Introduction Polyamidine with ®ve membered amidine rings is obtained by acid-treating the copolymer of N-vinylformamide (NVF) and acrylonitrile. The pendant formamide moiety of the copolymer is readily hydrolyzed to the corresponding amine. The processes for the formation of polyamidine is shown in Fig. 1. NVF is a precursor of polyvinylamine [1±4]. Polyvinylamine cannot be obtained from the hypothetical vinyl monomer; the relationship of poly-NVF (PNVF) with polyvinylamine (PVAm) is analogous to that of poly(vinyl acetate) with poly(vinyl alcohol). PNVF undergoes acid- or base-catalyzed hydrolysis to form PVAm. Poly(vinyl alcohol) and PVAm are the simplest organofunctinal polymers. PVAm has a cationic polymer with the highest density of amino groups, and its applications include water treatment, papermaking, textiles, adhesives and coatings, rheology modi®ers, and oil ®eld chemicals, because of the strong nulceophilicity, hydrophilic character, and ionization ability. Many studies have been carried out on carbonization of organic polymers to form carbon ®bers and ®lms [5±8]. These carbon materials possess high strength, high elasticity, high electrical and thermal conductivities, adsorptive properties, etc., and ®nd wide application. Carbon ®bers obtained from polyamidine can be regarded as PAN-based carbon ®bers, since one of the copolymerizing units is acrylonitrile [9]. * Corresponding author. Tel.: 81-268-21-5393; fax: 81-268-21-5391. E-mail address: [email protected] (F. Okino).
PVAm can form chelating complexes with various metal ions such as Co2, Ni2, Cu2, Zn2, and Cd2 [10]. Polyamidine is also expected to chelate metal ions that are known to catalyze the formation of graphite [11] and carbon nanotubes [12]. Catalytic effects of chelated metal ions as well as the ®ve membered-ring with a double bond are expected to promote carbonization and graphitization of polyamidine-based materials. Another advantage of polyamidine is its water solubility; polyamidine ®lms can be formed without using organic solvents, making the materials eco-friendly. Microspaces in carbon materials can be created by controlling the precursors. To our knowledge, this paper describes carbonization of polyamidine for the ®rst time, and the usage of polyamidine as a new precursor can lead to the formation of novel materials with microspaces and new properties. In this study, although ®bers of polyamidine can be obtained using Na2CO3, Na2SO4 or (NH4)2C2O4, carbonization and graphitization behavior of polyamidine ®lms were investigated to obtain fundamental data, because ®lms of polyamidine are more readily made. 2. Experimental Polyamidine powder was supplied by Mitsubishi Chemical (Diaclear MK64, MW > 100 000). The pristine polyamidine is acidic because of the acid (HCl) used in the hydrolysis process, some amino groups taking the form of NH3 Cl . Polyamidine powder was dissolved in water and neutralized with NaOH. Polyamidine ®lms were made
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 5 3 3 - 1
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F. Okino et al. / Synthetic Metals 125 (2002) 197±200
Fig. 2. TG±DTA curves of pristine polyamidine powder.
Fig. 1. Synthesis scheme of polyamidine.
by casting an aqueous solution of the powder on a Te¯on plate and drying it under vacuum in a desiccator. After removing ®lms from the plate they were cut into ribbons of 1±2 mm wide and 20±30 mm long. The thickness of the ®lm was ca. 0.1 mm. A Seiko TG/DTA220 was used for TG± DTA measurements. The temperature was raised at the rate of 15 K/min from 25 to 5008C under a nitrogen ¯ow. Polyamidine ribbons were carbonized and graphitized with or without prior oxidation to stabilize the ®lms. For the stabilization of the ®lms, a sample of ribbons in a Ni-mesh box was placed at the center of a ceramic tube (600 mm long, inner diameter 42 mm) and, under an air ¯ow of 50 ml/ min, the temperature was raised from room temperature to 2808C at the rate of 0.32 K/min and kept at 2808C for 1000 min. Carbonization and graphitization of stabilized and unstabilized ®lms were carried out using an ultra-high temperature furnace with graphite heaters (Kurata Giken, SCC-100/150) at 1000, 1500, 2000, 2500 and 28008C. The temperature was raised to 12008C under dynamic vacuum and above 12008C Ar gas (purity > 99:999%) was introduced to the furnace. Samples were heat treated for 15 min at 1000, 1500, 2000 or 25008C, and for 10 min at 28008C. Chemical analyses were done using a PERKIN-ELMER MODEL 2400 CHN elemental analyzer. For X-ray diffraction measurements samples were pulverized and loaded into 0.7 mm diameter thin-walled quartz capillaries. X-ray diffraction patterns were obtained by a Rigaku Rint 2200 using Cu Ka radiation in the Debye±Scherrer geometry. Pyrolytic graphite was used as the counter monochromator. 3. Results and discussion Fig. 2 shows TG±DTA curves of pristine polyamidine powder. As the temperature is raised, a slow and steady
weight loss is observed till 3008C, and a sudden large loss occurs above 3008C. The initial slow loss is mainly attributed to the loss of water from the polymer. The large weight loss above 3008C is caused by the thermal decomposition of the polymer and an endothermic DTA peak corresponds to this decomposition. The weight decreases down to 25% at 5008C. From the TG±DTA results the stabilization temperature was set to 2808C. Table 1 shows the results of CHN analysis for the pristine, heat treated ®lms without stabilization, stabilized ®lms, and heat treated ®lms with stabilization. Samples are named according to the heat-treatment temperatures (divided by 100) and a pre®x `S-' is used to denote stabilization. For example, the sample heat treated at 20008C without stabilization is named 20, and the sample heat treated at 10008C that had been stabilized in advance is named S-10. The stabilized sample without further heattreatment is denoted simply as S. As the pristine ®lm is rather hygroscopic and contains sodium, chlorine and oxygen con®rmed by XPS measurements, the analytical data of the pristine ®lm is grossly different from the polyamidine composition C5H8N2. The table shows that the stabilization Table 1 Elemental analysis data of pristine, heat treated films without stabilization, stabilized films, and heat treated films with stabilization (in %) Sample ID
C
H
N
Pristine 10 15 20 25 28 S S-10 S-15 S-20 S-25 S-28
34.5 79.2 91.6 98.3 99.1 99.1 41.0 83.0 93.3 99.0 98.2 ca. 100a
5.8 ± ± ± ± ± 3.5 ± ± ± ±
18.4 7.1 0.9 0.6 0.6 ± 17.9 9.3 1.9 0.6 0.6
a Carbon content value obtained was 101.2% owing to uncertain experimental errors.
F. Okino et al. / Synthetic Metals 125 (2002) 197±200
Fig. 3. Weight yields of polyamidine films upon heat treatment without and with stabilization in advance.
199
treatment at 2808C tends to raise slightly the carbon yields of the heat treated samples. Fig. 3 shows weight yields of the heat treated ®lms with and without stabilization at 2808C. The yields of the stabilized ®lms are higher than that of the unstabilized ®lms. The yields at 28008C are 19.2 and 12.2% for the stabilized and unstabilized samples. Fig. 4(a) and (b) shows the XRD patterns of heat treated samples without and with stabilization, respectively. The peaks marked with an asterisk for sample 10 and observed also for 15, 20, S-10 and S-15 are attributed to NaCl crystallized in the heat-treatment process. The salt vaporizes at higher temperatures. The salt forming ions can be removed by dialysis and the investigation using purer polyamidine ®lms is in progress. As the heat-treatment temperature goes higher, the 0 0 2 re¯ection of carbon becomes sharper and shifts towards higher angles, re¯ecting the progress of graphitization. At the same HTTs the stabilized samples clearly shows higher graphitization degree than the unstabilized samples. The 10 band of sample 28, e.g., is typical of turbostratic carbon. In the case of S-28 the 10 band is split into 1 0 0 and 1 0 1 re¯ections, indicating a threedimensional graphitic structural development. From the interlayer distance of this sample, 0.3376 nm, the graphitization degree or the probability of nearest neighboring pairs ordering in the graphite relation, P1 [13], is estimated to be 0.77 by the following equation: d0 0 2 0:335P1 0:344 1
P1
although the value should be taken with reservations [14]. The graphitization degree of this film is higher than PANbased carbon fibers in general but lower than the films obtained from polyimide [5] and POD (poly-p-phenylene1,3,4-oxadiazole) [6]. 4. Conclusions
Fig. 4. (a) X-ray diffraction patterns of pristine and heat treated polyamidine films without stabilization in advance. (b) X-ray diffraction patterns of pristine, stabilized and heat treated polyamidine films with stabilization in advance.
In the present study carbonization and graphitization of polyamidine ®lms were carried out for the ®rst time. The stabilization of the ®lms in advance at 2808C enhanced the graphitization and increased the weight yield of the products. The stabilization was carried out mainly because it was thought to be a prerequisite for the carbonization of polyamidine ®lms and ®bers. The present results showed that the stabilization has favorable effects in terms of the weight yield and graphitization. However it is worthwhile to mention that polyamidine carbon ®lms can be formed without stabilization: pristine polyamidine ®lms did not melt in the process of carbonization even without stabilization. Furthermore the effect on graphitization of polyamidine carbon ®lms was opposite to the stabilization or oxidation treatment of PAN-based carbon ®bers, where the oxidation has adverse effect on graphitization because of the cross-linking caused by oxidation. Nonetheless our ongoing investigation using
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F. Okino et al. / Synthetic Metals 125 (2002) 197±200
pure polyamidine ®lms obtained by dialysis will con®rm the favorable effects of stabilization on graphitization. Acknowledgements This work was supported by the Research for the Future Program of JSPS-RFTF96R11701 and COE research (10CE2003). References [1] S. Spange, A. Madl, U. Eismann, J. Utecht, Macromol. Rapid Commun. 18 (1997) 1075. [2] A. El Achari, X. Coqueret, A. Lablache-Combier, Makromol. Chem. 194 (1993) 1879. [3] E.E. Kathmann, L.A. White, C.L. McCormick, Macromolecules 29 (1996) 5268.
[4] A. El Achari, X. Coqueret, J. Polym. Sci. Polym. Chem. 35 (1997) 2513. [5] Y. Hishiyama, A. Yoshida, Y. Kaburagi, M. Inagaki, Carbon 30 (1992) 333. [6] M. Murakami, S. Yoshimura, Synthetic Met. 18 (1987) 509. [7] H. Konno, H. Oka, K. Shiba, H. Tachikawa, M. Inagaki, Carbon 37 (1999) 887. [8] A.C. Pastor, F. Rodriguez-Reinoso, H. Marsh, M.A. Martinez, Carbon 37 (1999) 1275. [9] O.P. Bahl, Z. Shen, J.G. Lavin, R.A. Ross, in: J.-B. Donnet, T.K. Wang, J.C.M. Peng, S. Rebouillat (Eds.), Carbon Fibers, Marcel Dekker, New York, 1998, pp. 4±5. [10] S. Kobayashi, K.-D. Suh, Y. Shirokura, Macromolecules 22 (1989) 2363. [11] A. Oya, TANSO, No. 102, 1980, p. 124. [12] A. Thess, R. Lee, O. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [13] C.R. Houska, B.E. Warren, J. Appl. Phys. 25 (1954) 1503. [14] N. Iwashita, M. Inagaki, Carbon 31 (1993) 1107.
Carbonization and graphitization of polyamidine ®lms Fujio Okinoa,*, Shinji Kawasakia, Hidekazu Touharaa, Hai Linb, Mutsumi Kimurab, Hirofusa Shiraib a
b
Faculty of Textile Science and Technology, Department of Chemistry, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan Faculty of Textile Science and Technology, Department of Functional Polymer Science, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan Received 14 May 2001; received in revised form 5 June 2001; accepted 2 July 2001
Abstract Carbonization and graphitization of polyamidine ®lms, a copolymer of acrylonitrile and N-vinylformamide, were carried out at 1000, 1500, 2000, 2500, 28008C with and without oxidation-treatment in advance at 2808C to stabilize the ®lms. The stabilization raised the weight yield of carbonization by ca. 5%. The highest degree of graphitization was attained by the sample heat treated at 28008C after stabilization, the c-axis interlayer distance being 0.3376 nm. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyamidine; Stabilization; Graphitization; Carbonization; N-vinylformamide; Polyvinylamine
1. Introduction Polyamidine with ®ve membered amidine rings is obtained by acid-treating the copolymer of N-vinylformamide (NVF) and acrylonitrile. The pendant formamide moiety of the copolymer is readily hydrolyzed to the corresponding amine. The processes for the formation of polyamidine is shown in Fig. 1. NVF is a precursor of polyvinylamine [1±4]. Polyvinylamine cannot be obtained from the hypothetical vinyl monomer; the relationship of poly-NVF (PNVF) with polyvinylamine (PVAm) is analogous to that of poly(vinyl acetate) with poly(vinyl alcohol). PNVF undergoes acid- or base-catalyzed hydrolysis to form PVAm. Poly(vinyl alcohol) and PVAm are the simplest organofunctinal polymers. PVAm has a cationic polymer with the highest density of amino groups, and its applications include water treatment, papermaking, textiles, adhesives and coatings, rheology modi®ers, and oil ®eld chemicals, because of the strong nulceophilicity, hydrophilic character, and ionization ability. Many studies have been carried out on carbonization of organic polymers to form carbon ®bers and ®lms [5±8]. These carbon materials possess high strength, high elasticity, high electrical and thermal conductivities, adsorptive properties, etc., and ®nd wide application. Carbon ®bers obtained from polyamidine can be regarded as PAN-based carbon ®bers, since one of the copolymerizing units is acrylonitrile [9]. * Corresponding author. Tel.: 81-268-21-5393; fax: 81-268-21-5391. E-mail address: [email protected] (F. Okino).
PVAm can form chelating complexes with various metal ions such as Co2, Ni2, Cu2, Zn2, and Cd2 [10]. Polyamidine is also expected to chelate metal ions that are known to catalyze the formation of graphite [11] and carbon nanotubes [12]. Catalytic effects of chelated metal ions as well as the ®ve membered-ring with a double bond are expected to promote carbonization and graphitization of polyamidine-based materials. Another advantage of polyamidine is its water solubility; polyamidine ®lms can be formed without using organic solvents, making the materials eco-friendly. Microspaces in carbon materials can be created by controlling the precursors. To our knowledge, this paper describes carbonization of polyamidine for the ®rst time, and the usage of polyamidine as a new precursor can lead to the formation of novel materials with microspaces and new properties. In this study, although ®bers of polyamidine can be obtained using Na2CO3, Na2SO4 or (NH4)2C2O4, carbonization and graphitization behavior of polyamidine ®lms were investigated to obtain fundamental data, because ®lms of polyamidine are more readily made. 2. Experimental Polyamidine powder was supplied by Mitsubishi Chemical (Diaclear MK64, MW > 100 000). The pristine polyamidine is acidic because of the acid (HCl) used in the hydrolysis process, some amino groups taking the form of NH3 Cl . Polyamidine powder was dissolved in water and neutralized with NaOH. Polyamidine ®lms were made
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 5 3 3 - 1
198
F. Okino et al. / Synthetic Metals 125 (2002) 197±200
Fig. 2. TG±DTA curves of pristine polyamidine powder.
Fig. 1. Synthesis scheme of polyamidine.
by casting an aqueous solution of the powder on a Te¯on plate and drying it under vacuum in a desiccator. After removing ®lms from the plate they were cut into ribbons of 1±2 mm wide and 20±30 mm long. The thickness of the ®lm was ca. 0.1 mm. A Seiko TG/DTA220 was used for TG± DTA measurements. The temperature was raised at the rate of 15 K/min from 25 to 5008C under a nitrogen ¯ow. Polyamidine ribbons were carbonized and graphitized with or without prior oxidation to stabilize the ®lms. For the stabilization of the ®lms, a sample of ribbons in a Ni-mesh box was placed at the center of a ceramic tube (600 mm long, inner diameter 42 mm) and, under an air ¯ow of 50 ml/ min, the temperature was raised from room temperature to 2808C at the rate of 0.32 K/min and kept at 2808C for 1000 min. Carbonization and graphitization of stabilized and unstabilized ®lms were carried out using an ultra-high temperature furnace with graphite heaters (Kurata Giken, SCC-100/150) at 1000, 1500, 2000, 2500 and 28008C. The temperature was raised to 12008C under dynamic vacuum and above 12008C Ar gas (purity > 99:999%) was introduced to the furnace. Samples were heat treated for 15 min at 1000, 1500, 2000 or 25008C, and for 10 min at 28008C. Chemical analyses were done using a PERKIN-ELMER MODEL 2400 CHN elemental analyzer. For X-ray diffraction measurements samples were pulverized and loaded into 0.7 mm diameter thin-walled quartz capillaries. X-ray diffraction patterns were obtained by a Rigaku Rint 2200 using Cu Ka radiation in the Debye±Scherrer geometry. Pyrolytic graphite was used as the counter monochromator. 3. Results and discussion Fig. 2 shows TG±DTA curves of pristine polyamidine powder. As the temperature is raised, a slow and steady
weight loss is observed till 3008C, and a sudden large loss occurs above 3008C. The initial slow loss is mainly attributed to the loss of water from the polymer. The large weight loss above 3008C is caused by the thermal decomposition of the polymer and an endothermic DTA peak corresponds to this decomposition. The weight decreases down to 25% at 5008C. From the TG±DTA results the stabilization temperature was set to 2808C. Table 1 shows the results of CHN analysis for the pristine, heat treated ®lms without stabilization, stabilized ®lms, and heat treated ®lms with stabilization. Samples are named according to the heat-treatment temperatures (divided by 100) and a pre®x `S-' is used to denote stabilization. For example, the sample heat treated at 20008C without stabilization is named 20, and the sample heat treated at 10008C that had been stabilized in advance is named S-10. The stabilized sample without further heattreatment is denoted simply as S. As the pristine ®lm is rather hygroscopic and contains sodium, chlorine and oxygen con®rmed by XPS measurements, the analytical data of the pristine ®lm is grossly different from the polyamidine composition C5H8N2. The table shows that the stabilization Table 1 Elemental analysis data of pristine, heat treated films without stabilization, stabilized films, and heat treated films with stabilization (in %) Sample ID
C
H
N
Pristine 10 15 20 25 28 S S-10 S-15 S-20 S-25 S-28
34.5 79.2 91.6 98.3 99.1 99.1 41.0 83.0 93.3 99.0 98.2 ca. 100a
5.8 ± ± ± ± ± 3.5 ± ± ± ±
18.4 7.1 0.9 0.6 0.6 ± 17.9 9.3 1.9 0.6 0.6
a Carbon content value obtained was 101.2% owing to uncertain experimental errors.
F. Okino et al. / Synthetic Metals 125 (2002) 197±200
Fig. 3. Weight yields of polyamidine films upon heat treatment without and with stabilization in advance.
199
treatment at 2808C tends to raise slightly the carbon yields of the heat treated samples. Fig. 3 shows weight yields of the heat treated ®lms with and without stabilization at 2808C. The yields of the stabilized ®lms are higher than that of the unstabilized ®lms. The yields at 28008C are 19.2 and 12.2% for the stabilized and unstabilized samples. Fig. 4(a) and (b) shows the XRD patterns of heat treated samples without and with stabilization, respectively. The peaks marked with an asterisk for sample 10 and observed also for 15, 20, S-10 and S-15 are attributed to NaCl crystallized in the heat-treatment process. The salt vaporizes at higher temperatures. The salt forming ions can be removed by dialysis and the investigation using purer polyamidine ®lms is in progress. As the heat-treatment temperature goes higher, the 0 0 2 re¯ection of carbon becomes sharper and shifts towards higher angles, re¯ecting the progress of graphitization. At the same HTTs the stabilized samples clearly shows higher graphitization degree than the unstabilized samples. The 10 band of sample 28, e.g., is typical of turbostratic carbon. In the case of S-28 the 10 band is split into 1 0 0 and 1 0 1 re¯ections, indicating a threedimensional graphitic structural development. From the interlayer distance of this sample, 0.3376 nm, the graphitization degree or the probability of nearest neighboring pairs ordering in the graphite relation, P1 [13], is estimated to be 0.77 by the following equation: d0 0 2 0:335P1 0:344 1
P1
although the value should be taken with reservations [14]. The graphitization degree of this film is higher than PANbased carbon fibers in general but lower than the films obtained from polyimide [5] and POD (poly-p-phenylene1,3,4-oxadiazole) [6]. 4. Conclusions
Fig. 4. (a) X-ray diffraction patterns of pristine and heat treated polyamidine films without stabilization in advance. (b) X-ray diffraction patterns of pristine, stabilized and heat treated polyamidine films with stabilization in advance.
In the present study carbonization and graphitization of polyamidine ®lms were carried out for the ®rst time. The stabilization of the ®lms in advance at 2808C enhanced the graphitization and increased the weight yield of the products. The stabilization was carried out mainly because it was thought to be a prerequisite for the carbonization of polyamidine ®lms and ®bers. The present results showed that the stabilization has favorable effects in terms of the weight yield and graphitization. However it is worthwhile to mention that polyamidine carbon ®lms can be formed without stabilization: pristine polyamidine ®lms did not melt in the process of carbonization even without stabilization. Furthermore the effect on graphitization of polyamidine carbon ®lms was opposite to the stabilization or oxidation treatment of PAN-based carbon ®bers, where the oxidation has adverse effect on graphitization because of the cross-linking caused by oxidation. Nonetheless our ongoing investigation using
200
F. Okino et al. / Synthetic Metals 125 (2002) 197±200
pure polyamidine ®lms obtained by dialysis will con®rm the favorable effects of stabilization on graphitization. Acknowledgements This work was supported by the Research for the Future Program of JSPS-RFTF96R11701 and COE research (10CE2003). References [1] S. Spange, A. Madl, U. Eismann, J. Utecht, Macromol. Rapid Commun. 18 (1997) 1075. [2] A. El Achari, X. Coqueret, A. Lablache-Combier, Makromol. Chem. 194 (1993) 1879. [3] E.E. Kathmann, L.A. White, C.L. McCormick, Macromolecules 29 (1996) 5268.
[4] A. El Achari, X. Coqueret, J. Polym. Sci. Polym. Chem. 35 (1997) 2513. [5] Y. Hishiyama, A. Yoshida, Y. Kaburagi, M. Inagaki, Carbon 30 (1992) 333. [6] M. Murakami, S. Yoshimura, Synthetic Met. 18 (1987) 509. [7] H. Konno, H. Oka, K. Shiba, H. Tachikawa, M. Inagaki, Carbon 37 (1999) 887. [8] A.C. Pastor, F. Rodriguez-Reinoso, H. Marsh, M.A. Martinez, Carbon 37 (1999) 1275. [9] O.P. Bahl, Z. Shen, J.G. Lavin, R.A. Ross, in: J.-B. Donnet, T.K. Wang, J.C.M. Peng, S. Rebouillat (Eds.), Carbon Fibers, Marcel Dekker, New York, 1998, pp. 4±5. [10] S. Kobayashi, K.-D. Suh, Y. Shirokura, Macromolecules 22 (1989) 2363. [11] A. Oya, TANSO, No. 102, 1980, p. 124. [12] A. Thess, R. Lee, O. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [13] C.R. Houska, B.E. Warren, J. Appl. Phys. 25 (1954) 1503. [14] N. Iwashita, M. Inagaki, Carbon 31 (1993) 1107.