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Preparation and properties of conducting polypyrrole-sulfonated polycarbonate composites
Synthetic Metals 123 (2001) 327±333
Preparation and properties of conducting polypyrrole-sulfonated polycarbonate composites Wan-Jin Lee*, Yong-Ju Kim, Mi-Ok Jung, Do-Heyoung Kim, Dong Lyun Cho, Shinyoung Kaang Faculty of Applied Chemistry, College of Engineering, Chonnam National University, 300 Yongbong-dong, Puk-gu, Kwangju 500-757, South Korea Received 9 August 2000; received in revised form 31 October 2000; accepted 17 January 2001
Abstract The electrically conducting composites are prepared by chemical oxidative polymerization using polypyrrole (PPy) and polycarbonate (PC) or sulfonated polycarbonate (SPC) in chloroform. The pyrrole was protonated and polymerized using iron(III) chloride (FeCl3). The sulfonic group was introduced into the structure of PC in order to enhance the coulombic interaction between each phase of composites. The electrical conductivity and morphology were observed as a function of the amount of PPy. The electrical conductivity was increased up to 0.82 S/cm with the amount of PPy. The PPy/SPC composites were stable in atmosphere. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; Polycarbonate; Sulfonation; Chemical oxidative polymerization; Composites; Electrical conductivity
1. Introduction Conducting polymers such as polyaniline, polypyrrole (PPy) and polythiophene have received lots of attention due to their potential applications in the area of rechargeable batteries, gas separation membranes and electroluminescent diodes [1±3]. However, their poor mechanical properties and processability pointed out as major disadvantages in their extensive applications [4]. Several methods have been used to improve mechanical properties and processability including the introduction of long alkyl groups into the main chain [5], the synthesis of soluble precursors [6] and the preparation of conducting polymer composites by chemical oxidative polymerization [7±13]. Recently, the preparation of conductive composite by chemical oxidative polymerization have been developed, since this method is the easier and more effective. In preparation of PPy composite, pyrrole monomer is polymerized by chemical oxidative polymerization using oxidant such as FeCl3 within host polymer solution. In the case of this method, the distribution between PPy and host polymer is good, and then electrical conductivities and mechanical
*
Corresponding author. Tel.: 82-62-530-1895; fax: 82-62-530-1889. E-mail address: [email protected] (W.-J. Lee).
properties might be increased. But, two phases are basically immiscible, and then phase separation is still existed between each phase. If the miscibility of these blends is enhanced between two phases, the electrical properties will be enhanced due to the decrease of interfacial tension between two phases [14]. The methods to enhance electrical and mechanical properties in immiscible blend are to use precursor or compatibilizers such as block copolymer and to introduce ionic polymer [15]. By introducing ionic group such as sulfonic group to insulating polymer such as polystyrene and polycarbonate (PC), it is possible to enhance miscibility between two phases [16,17]. Therefore, it will be minimized the phase separation between conducting polymer and insulating polymer. This leads to increase of the mechanical and electrical properties due to the increase of the compatibility and induce of electrostatic interaction. We have focused on the preparation of conductive, ¯exible composites from PPy formed by chemical oxidative polymerization and sulfonated polycarbonate (SPC). The effect of the introduction of ionic group on compatibility and on electrical and mechanical properties was studied. Also the results of variable range hopping expression is treated. A similar work was already reported as a title of ``Electrical properties of polyaniline-sulfonated polycarbonate blends'' [18]. In this system, the polyaniline/SPC composites (polyaniline was doped by camphor sulfonic acid or
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 3 0 2 - 2
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dodecylbenzene sulfonic acid) were synthesized and electrical conductivity was increased to 7.5 S/cm due to the effects of sulfonation and miscibility [18]. 2. Experimental 2.1. Materials Polycarbonate (PC) obtained from Sumitomo Co. (type 201-15 resin, MW 22000, density 1.20 g/cm3, Tg 1508C). The pyrrole purchased from Acros Chemical Co., was used with distillation. The iron(III) chloride (FeCl3) was purchased from Aldrich Co. The chloroform (Junsei Chemical Co.) as a solvent and chlorosulfuric acid (Kanto Chemical Co.) as a sulfonating agent were used without puri®cation. The sulfonated polycarbonate (SPC) was made in the laboratory. The preparation of SPC was reported in detail in previous work [18]. This report was included the data of ion exchange capacity, thermal property, FT-IR analysis and NMR analysis for SPC.
3. Results and discussion 3.1. FT-IR analysis and UV±VIS spectra of polypyrrole Fig. 1 shows the results of FT-IR for PC and SPC. The typical peaks of SO3H are absorption at 1250±1150 and 1060±1030 cm 1. The strong band of the frequency, 1250± 1150 cm 1, can be ascribed to stretch vibration for S=O, and the absorption band at the 1060±1030 cm 1 is assigned to the symmetric stretching band. However, the stretching band of C=O is 1765±1720 cm 1 in the ester group for PC, while the C±O±C for asymmetric stretching band is 1290± 1180 cm 1, and the O±C±O peak is 645±575 cm 1. The spectra were so complex that they could not easily be distinguished, due to an overlap of the absorption of the sulfonic group and ester group of PC. So, the NMR analysis is shown in Ref. [18] in detail. Fig. 2 shows UV±VIS spectrum of protonated PPy solution in chloroform. Two distinct transitions at 410, 850 nm can be seen [21].
2.2. Preparation of composite film PC or SPC of a 10 wt.% was dissolved in chloroform and then pyrrole was added. FeCl3 was dissolved in methanol and inserted dropwise under vigorous stirring. The concentration of pyrrole was varied as a 5, 10, 15, 20, 25 and 30 wt.%, respectively. The molar ratio of FeCl3 to pyrrole was 2.2 [19]. All compounds were stirred for 1 day at room temperature [20]. The product was ®ltered and washed with distilled water and methanol. The concentration of PPy polymerized in the conducting composite is based on the amount of monomer (pyrrole). The prepared composites were dried in a vacuum oven at 508C for 6 h, and were carried out compression molding at 1808C for 20 min under 4500 psi. The thickness of the samples was about 100 mm. 2.3. Measurements of electrically conductive polymer composites The conductivity of the sample in the plane direction was determined by a standard four-probe method. Infrared studies and UV visible absorption spectra of composite ®lms were carried out on a Mattson 1000 FT-IR spectrometer and Hitachi U-3501 UV±VIS absorption spectrometer, respectively. The sample was cut to the size required by the ASTM D638. The tensile test was performed by an Instron testing instrument (Uniframe TC-55, Satec System) at room temperature with load cell of 200 lbs and cross-head speed of 3.0 mm/min. Scanning electron micrographs (SEM) and energy dispersive spectrum (EDS) analyzes of the electrical composite ®lm were obtained with a Jeol Co., JSM-5400 instrument. The elemental analysis was performed by Elementar Analysensysteme GmbH Vario EL.
Fig. 1. FT-IR spectra of PC and SPC.
Fig. 2. UV±VIS spectrum of doped-PPy in chloroform.
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3.2. Reaction mechanism of SPC/PPy composite Fig. 3 represents the reaction scheme of PPy/PC and PPy/ SPC composite. In Fig. 3(a), the FeCl3 is an oxidant as well as a dopant. In Fig. 3(b), the FeCl3 plays roles an oxidant and a dopant, and SPC acts as a dopant. This gives a considerable speculation as shown in Fig. 3(b) that PPy makes electrostatic interaction with SPC as well as Cl under chemical oxidative polymerization. Therefore, the compatibility and miscibility will be enhanced between SPC and PPy. These phenomena might be minimized the phase separation between SPC and PPy. This will be leaded the increases of electrical properties. 3.3. EDS investigation and elemental analysis Fig. 4(a) and (b) represent EDS investigations. In Fig. 4(a), the amount of Cl in PPy/PC composite is much higher than that of Fe because most Fe washed out by water and methanol. The elemental analysis for the PPy/PC composite with a feeding weight ratio of pyrrole/PC, 1/5 was that the contents of N, Cl, and Fe are 1.28, 1.15 and 0.35 wt.%, respectively.
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This number represents that the molar ratios of N (oxidized pyrrole)/Cl/Fe for the composite were 1/0.35/0.06. During the chemical oxidative polymerization, FeCl3 acts as an oxidant and a dopant. A large amount of Cl anion is included in the composite as the counter-ion of the doped PPy (35% of PPy for PC is doped according to the elemental analysis) [22]. In Fig. 4(b), the amount of S and Cl in PPy/SPC composite is much higher than that of Fe. Also, the amount of S is higher than that of Cl. This means that the amount of PPy doped by SPC is larger than that of PPy doped by Cl. Both SPC and Cl act as counter anions. The elemental analysis for the PPy/SPC composite with a feeding weight ratio of pyrrole=SPC 1=5 was that the contents of N, S, Cl, and Fe are 2.77, 1.63, 0.76 and 0.32 wt.%, respectively. This number indicates that the molar ratios of N (oxidized pyrrole)/S (SPC as a dopant)/Cl/Fe were 1/0.26/0.1/0.03. During the chemical oxidative polymerization, FeCl3 acts as an oxidant and a dopant, and SPC acts as a dopant. According to the elemental analysis, the amount of SPC anions and Cl anions is included 26 and 10%, respectively, in the composite as the counter-ions doped to PPy. This means that 36% of PPy is doped [22].
Fig. 3. Reaction schemes of (a) PPy/PC and (b) PPy/SPC preparation.
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dispersion of PPy grains in the sulfonated PC matrix was considerably enhanced by the presence of the coulombic interaction. As the results, the sulfonation effect is induced the electrical conductivity due to the increase of miscibility between conducting polymer and sulfonated PC. The percolation threshold in both cases is observed in the vicinity of 15 wt.%. This results are consistent with a three dimensional network of conducting polymer aggregates in an insulating matrix. As the results, when the content of PPy was 30 wt.%, the electrical conductivity of SPC composite was 0.82 S/cm, and that of PC composite was 0.23 S/cm. 3.5. Temperature dependence of electrical conductivity in PPy composite
Fig. 4. EDS investigations of (a) PPy/PC and (b) PPy/SPC composites. The feeding weight ratio of pyrrole/host polymer is 1:5.
3.4. The effect of sulfonation for electrical conductivity Fig. 5 represents the changes of electrical conductivity with the sulfonation effect of the PC. The charge carriers promote to induce coulombic interactions between positive charge and negative one as shown in Fig. 3 [7]. Also, the
Fig. 6(a) shows the variations of electrical conductivity of the PPy/SPC composite ®lms with temperature in the range of room temperature to 1608C. The electrical conductivity can be explained as the following equation, s nem equation. Where n is the number of electrons per unit volume, e the magnitude of the electrical charge on an electron, and m is called the electron mobility. Thus, the electrical conductivity is proportional to both the number of electrons and the electron mobility. The electrical conductivity is almost constant in the range of room temperature to about 1208C. The reason is that the increase of electron number is almost equal to the decrease of electron mobility by thermal vibration of SPC matrix. On the other hand, the electrical conductivity decreases very slightly in the range of about 120 to 1608C. These phenomena can be explained that the resistance is increased to expand the space of between PPy particles due to thermal vibration of SPC matrix [23]. Thermal vibration brings out the scattering of electrons and results in the decrease of electrical conductivity. Fig. 6(b) shows temperature dependence of the electrical conductivity in the range 1968C to room temperature, and the increase of electrical conductivity is observed with increasing temperature. It can be explained by variable range hopping expression [24], which is charge transfer model of the semiconductor region. The temperature dependence of electrical conductivity is represented by power-law relationship. The reason is that the mechanism of charge transfer is followed a three dimensional network of conducting polymer aggregates in an insulating matrix. This equation can be explained as the following equation: s T / exp T0 =Tg , where T0 is a characteristic temperature, and g a temperature index. The experimental result is linearlized when g is 1/2. This result might be explained due to coulombic interaction [25]. 3.6. Ageing effect in ambient conditions of composite film
Fig. 5. Electrical conductivity of the conducting polymer composites as a function of sulfonation.
Fig. 7 shows the variations of electrical conductivity of the composite ®lms in the atmosphere, and the decrease of electrical conductivity was observed with a period of 30 days. These phenomena can be explained that the PPy
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Fig. 7. Electrical conductivity vs. time of ambient air exposure for PPy/ SPC composites. (&: PPy (5 wt.%), &: PPy (10 wt.%), ~: PPy (15 wt.%), ~: PPy (20 wt.%), *: PPy (25 wt.%), *: PPy (30 wt.%)).
Fig. 6. Electrical conductivity changes of PPy/SPC composites as a function of temperature: 20±1608C; (b) 196±208C. (&: PPy (5 wt.%), &: PPy (10 wt.%), ~: PPy (15 wt.%), ~: PPy (20 wt.%), *: PPy (25 wt.%), *: PPy (30 wt.%)).
the tensile strength of the two cases are almost the same in the vicinity of 10 wt.% PPy. The mechanical property mainly depends on the content of PPy. The tensile strength decreases with the content of PPy in the composites. When the content of the PPy is low, the mechanical property is relatively high, and decreases steeply in the vicinity of percolation threshold. In other words, when the amount of PPy increase, mechanical property decreases due to the interruption of PC or SPC phase by the large amount of rigid PPy. As shown in Fig. 9, below the percolation threshold, i.e. the PC or SPC matrix is a continuous phase and the PPy is dispersed as a disperse phase, and above the percolation threshold, i.e. the 3-D network of small conducting granular aggregates in the PC or SPC matrix is well formed.
formed carbonium ion by chemical polymerization. The decrease of the continuity for the conjugated double bond of conducting polymer brought out the decrease of electrical conductivity [26] because carbonium ion was highly reacted on oxygen or moisture. But, the decrease of electrical conductivity does not really matter because conventional polymers have large scale of electrical conductivity. Thus, the PPy/SPC composites were very stable in the atmosphere. It can be explained that the SPC obstructed reaction of conducting polymer and oxygen (or moisture). 3.7. Mechanical properties of composite film Fig. 8 shows the tensile strength curve of PPy/PC or PPy/ SPC composite ®lms with the content of PPy. The values of
Fig. 8. Tensile strength of PPy/PC and PPy/SPC composite films.
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Fig. 9. SEM images of conducting polymer composites: (a)±(c); PPy/PC composites and (d)±(f); PPy/SPC composites. ((a): 10 wt.% PPy, (b): 15 wt.% PPy, (c): 25 wt.% PPy, (d): 10 wt.% PPy, (e): 15 wt.% PPy, (f): 25 wt.% PPy).
3.8. Characteristics of morphology Fig. 9 represents the views of scanning electron microscopy (SEM) for the surfaces of PPy/PC composites and PPy/SPC composites in the 10, 15, 25 wt.% PPy. In Fig. 9(a)±(c), the PPy in PPy/PC composite is showed to distribute as the types of small spherical granules within matrix. In this case, the phase separation still remains due to the interfacial tension between PPy and PC matrix, and
then this phase separation results in the decline of the mechanical properties and the electrical conductivity. Fig. 9(d)±(f) represents the morphology of surfaces for PPy/SPC composites. In spite of low content of PPy, the PPy within the SPC matrix is well distributed due to the effect of sulfonation. In other words, this phenomena might be leaded the increase of the electrical conductivity by inducing electrostatic interaction between PPy and SPC matrix.
W.-J. Lee et al. / Synthetic Metals 123 (2001) 327±333
4. Conclusions The electrical conductivity of PPy/SPC composite was higher than that of PPy/PC composite. This phenomena might be explained due to the inducement of electrostatic interaction and miscibility between PPy and SPC matrix. The temperature-dependence of electrical conductivity in the composites in the range of 196±1608C was explained by variable range hopping model and thermal vibration of the matrix. The mechanical properties of PPy/SPC composites were similar to PPy/PC composites. In PPy/SPC conducting composites, the electrical conductivity was increased up to 0.82 S/cm with the amount of PPy. The PPy/SPC composites were very stable in the atmosphere. Acknowledgements The authors wish to acknowledge the ®nancial support of the Korea Science and Engineering Foundation (KOSEF) under contract KOSEF 981-1109-045-2. References [1] H. Tsutsumi, M. Araki, K. Onimura, T. Oishi, Synth. Met. 97 (1998) 53. [2] M.R. Anderson, B.R. Matters, H. Reiss, R.B. Kaner, Synth. Met. 41±43 (1991) 1151.
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[3] S.K. Dhawan, D.C. Trivedi, J. Appl. Electrochem. 22 (1992) 563. [4] J.M. Machado, F.E. Karasz, R.W. Lenz, Polymer 29 (1988) 1412. [5] M. Sato, S. Tanaka, K. Kaeriyama, J. Chem. Soc., Chem. Commun. (1986) 873. [6] I. Murase, T. Chinishi, T. Naguchi, M. Hirooka, Polym. Commun. 25 (1984) 327. [7] T.J. Skotheim, Handbook of Conducting Polymer, Marcel Dekker, New York, 1986. [8] K.X. Yoshino, H. Yin, S. Morita, Y. Nakanishi, S. Nakagawa, H. Yamamoto, T. Watanuki, I. Isa, Jpn. J. Appl. Phys. 32 (1993) 979. [9] M.E. Galvin, G.E. Wnek, Polym. Commun. 23 (1982) 795. [10] M.E. Galvin, G.E. Wnek, J. Polym. Sci. A: Polym. Chem. 21 (1983) 2727. [11] M.F. Rubner, S.K. Tripathy, J. Georger, P. Cholewa, Macromolecules 16 (1983) 870. [12] Y.A. Dubistky, B.A. Zhubanou, Synth. Met. 53 (1983) 303. [13] H.V. Dijk, O. Aagard, R. Schellekens, Synth. Met. 55 (1993) 1085. [14] D.R. Paul, J.W. Barlow, J. Macromol. Sci. C-1 (1980) 109. [15] J.A. Brydson, Polymeric Materials, Butterworths, London, 1989. [16] J.-M. Liu, L. Sun, S.C. Yang, US Patent 5,489,400 (1996). [17] J.-M. Liu, L. Sun, J.-H. Hwang, S.C. Yang, Mater. Res. Soc. Symp. Proc. 247 (1992) 601. [18] W.-J. Lee, Y.-J. Kim, S. Kaang, Synth. Met. 113 (2000) 237. [19] S.S. Im, S.W. Byun, J. Appl. Polym. Sci. 51 (1994) 1221. [20] K. Ishizu, H. Tanaka, R. Saito, Polymer 37 (1996) 863. [21] W.S. Huang, A.G. MacDiarmid, Polymer 34 (1993) 1833. [22] Y. Sun, E. Ruckenstein, Synth. Met. 72 (1995) 261. [23] W.D. Callister Jr., Materials Science and Engineering, 4th Edition, Wiley, New York, 1994. [24] M. Reghu, Y. Cao, A.J. Heeger, Synth. Met. 65 (1994) 167. [25] Z.H. Wang, C. Li, E.M. Scherr, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. Lett. 66 (1991) 745. [26] H. Munsted, Polymer 29 (1988) 296.
Preparation and properties of conducting polypyrrole-sulfonated polycarbonate composites Wan-Jin Lee*, Yong-Ju Kim, Mi-Ok Jung, Do-Heyoung Kim, Dong Lyun Cho, Shinyoung Kaang Faculty of Applied Chemistry, College of Engineering, Chonnam National University, 300 Yongbong-dong, Puk-gu, Kwangju 500-757, South Korea Received 9 August 2000; received in revised form 31 October 2000; accepted 17 January 2001
Abstract The electrically conducting composites are prepared by chemical oxidative polymerization using polypyrrole (PPy) and polycarbonate (PC) or sulfonated polycarbonate (SPC) in chloroform. The pyrrole was protonated and polymerized using iron(III) chloride (FeCl3). The sulfonic group was introduced into the structure of PC in order to enhance the coulombic interaction between each phase of composites. The electrical conductivity and morphology were observed as a function of the amount of PPy. The electrical conductivity was increased up to 0.82 S/cm with the amount of PPy. The PPy/SPC composites were stable in atmosphere. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; Polycarbonate; Sulfonation; Chemical oxidative polymerization; Composites; Electrical conductivity
1. Introduction Conducting polymers such as polyaniline, polypyrrole (PPy) and polythiophene have received lots of attention due to their potential applications in the area of rechargeable batteries, gas separation membranes and electroluminescent diodes [1±3]. However, their poor mechanical properties and processability pointed out as major disadvantages in their extensive applications [4]. Several methods have been used to improve mechanical properties and processability including the introduction of long alkyl groups into the main chain [5], the synthesis of soluble precursors [6] and the preparation of conducting polymer composites by chemical oxidative polymerization [7±13]. Recently, the preparation of conductive composite by chemical oxidative polymerization have been developed, since this method is the easier and more effective. In preparation of PPy composite, pyrrole monomer is polymerized by chemical oxidative polymerization using oxidant such as FeCl3 within host polymer solution. In the case of this method, the distribution between PPy and host polymer is good, and then electrical conductivities and mechanical
*
Corresponding author. Tel.: 82-62-530-1895; fax: 82-62-530-1889. E-mail address: [email protected] (W.-J. Lee).
properties might be increased. But, two phases are basically immiscible, and then phase separation is still existed between each phase. If the miscibility of these blends is enhanced between two phases, the electrical properties will be enhanced due to the decrease of interfacial tension between two phases [14]. The methods to enhance electrical and mechanical properties in immiscible blend are to use precursor or compatibilizers such as block copolymer and to introduce ionic polymer [15]. By introducing ionic group such as sulfonic group to insulating polymer such as polystyrene and polycarbonate (PC), it is possible to enhance miscibility between two phases [16,17]. Therefore, it will be minimized the phase separation between conducting polymer and insulating polymer. This leads to increase of the mechanical and electrical properties due to the increase of the compatibility and induce of electrostatic interaction. We have focused on the preparation of conductive, ¯exible composites from PPy formed by chemical oxidative polymerization and sulfonated polycarbonate (SPC). The effect of the introduction of ionic group on compatibility and on electrical and mechanical properties was studied. Also the results of variable range hopping expression is treated. A similar work was already reported as a title of ``Electrical properties of polyaniline-sulfonated polycarbonate blends'' [18]. In this system, the polyaniline/SPC composites (polyaniline was doped by camphor sulfonic acid or
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 3 0 2 - 2
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dodecylbenzene sulfonic acid) were synthesized and electrical conductivity was increased to 7.5 S/cm due to the effects of sulfonation and miscibility [18]. 2. Experimental 2.1. Materials Polycarbonate (PC) obtained from Sumitomo Co. (type 201-15 resin, MW 22000, density 1.20 g/cm3, Tg 1508C). The pyrrole purchased from Acros Chemical Co., was used with distillation. The iron(III) chloride (FeCl3) was purchased from Aldrich Co. The chloroform (Junsei Chemical Co.) as a solvent and chlorosulfuric acid (Kanto Chemical Co.) as a sulfonating agent were used without puri®cation. The sulfonated polycarbonate (SPC) was made in the laboratory. The preparation of SPC was reported in detail in previous work [18]. This report was included the data of ion exchange capacity, thermal property, FT-IR analysis and NMR analysis for SPC.
3. Results and discussion 3.1. FT-IR analysis and UV±VIS spectra of polypyrrole Fig. 1 shows the results of FT-IR for PC and SPC. The typical peaks of SO3H are absorption at 1250±1150 and 1060±1030 cm 1. The strong band of the frequency, 1250± 1150 cm 1, can be ascribed to stretch vibration for S=O, and the absorption band at the 1060±1030 cm 1 is assigned to the symmetric stretching band. However, the stretching band of C=O is 1765±1720 cm 1 in the ester group for PC, while the C±O±C for asymmetric stretching band is 1290± 1180 cm 1, and the O±C±O peak is 645±575 cm 1. The spectra were so complex that they could not easily be distinguished, due to an overlap of the absorption of the sulfonic group and ester group of PC. So, the NMR analysis is shown in Ref. [18] in detail. Fig. 2 shows UV±VIS spectrum of protonated PPy solution in chloroform. Two distinct transitions at 410, 850 nm can be seen [21].
2.2. Preparation of composite film PC or SPC of a 10 wt.% was dissolved in chloroform and then pyrrole was added. FeCl3 was dissolved in methanol and inserted dropwise under vigorous stirring. The concentration of pyrrole was varied as a 5, 10, 15, 20, 25 and 30 wt.%, respectively. The molar ratio of FeCl3 to pyrrole was 2.2 [19]. All compounds were stirred for 1 day at room temperature [20]. The product was ®ltered and washed with distilled water and methanol. The concentration of PPy polymerized in the conducting composite is based on the amount of monomer (pyrrole). The prepared composites were dried in a vacuum oven at 508C for 6 h, and were carried out compression molding at 1808C for 20 min under 4500 psi. The thickness of the samples was about 100 mm. 2.3. Measurements of electrically conductive polymer composites The conductivity of the sample in the plane direction was determined by a standard four-probe method. Infrared studies and UV visible absorption spectra of composite ®lms were carried out on a Mattson 1000 FT-IR spectrometer and Hitachi U-3501 UV±VIS absorption spectrometer, respectively. The sample was cut to the size required by the ASTM D638. The tensile test was performed by an Instron testing instrument (Uniframe TC-55, Satec System) at room temperature with load cell of 200 lbs and cross-head speed of 3.0 mm/min. Scanning electron micrographs (SEM) and energy dispersive spectrum (EDS) analyzes of the electrical composite ®lm were obtained with a Jeol Co., JSM-5400 instrument. The elemental analysis was performed by Elementar Analysensysteme GmbH Vario EL.
Fig. 1. FT-IR spectra of PC and SPC.
Fig. 2. UV±VIS spectrum of doped-PPy in chloroform.
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3.2. Reaction mechanism of SPC/PPy composite Fig. 3 represents the reaction scheme of PPy/PC and PPy/ SPC composite. In Fig. 3(a), the FeCl3 is an oxidant as well as a dopant. In Fig. 3(b), the FeCl3 plays roles an oxidant and a dopant, and SPC acts as a dopant. This gives a considerable speculation as shown in Fig. 3(b) that PPy makes electrostatic interaction with SPC as well as Cl under chemical oxidative polymerization. Therefore, the compatibility and miscibility will be enhanced between SPC and PPy. These phenomena might be minimized the phase separation between SPC and PPy. This will be leaded the increases of electrical properties. 3.3. EDS investigation and elemental analysis Fig. 4(a) and (b) represent EDS investigations. In Fig. 4(a), the amount of Cl in PPy/PC composite is much higher than that of Fe because most Fe washed out by water and methanol. The elemental analysis for the PPy/PC composite with a feeding weight ratio of pyrrole/PC, 1/5 was that the contents of N, Cl, and Fe are 1.28, 1.15 and 0.35 wt.%, respectively.
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This number represents that the molar ratios of N (oxidized pyrrole)/Cl/Fe for the composite were 1/0.35/0.06. During the chemical oxidative polymerization, FeCl3 acts as an oxidant and a dopant. A large amount of Cl anion is included in the composite as the counter-ion of the doped PPy (35% of PPy for PC is doped according to the elemental analysis) [22]. In Fig. 4(b), the amount of S and Cl in PPy/SPC composite is much higher than that of Fe. Also, the amount of S is higher than that of Cl. This means that the amount of PPy doped by SPC is larger than that of PPy doped by Cl. Both SPC and Cl act as counter anions. The elemental analysis for the PPy/SPC composite with a feeding weight ratio of pyrrole=SPC 1=5 was that the contents of N, S, Cl, and Fe are 2.77, 1.63, 0.76 and 0.32 wt.%, respectively. This number indicates that the molar ratios of N (oxidized pyrrole)/S (SPC as a dopant)/Cl/Fe were 1/0.26/0.1/0.03. During the chemical oxidative polymerization, FeCl3 acts as an oxidant and a dopant, and SPC acts as a dopant. According to the elemental analysis, the amount of SPC anions and Cl anions is included 26 and 10%, respectively, in the composite as the counter-ions doped to PPy. This means that 36% of PPy is doped [22].
Fig. 3. Reaction schemes of (a) PPy/PC and (b) PPy/SPC preparation.
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dispersion of PPy grains in the sulfonated PC matrix was considerably enhanced by the presence of the coulombic interaction. As the results, the sulfonation effect is induced the electrical conductivity due to the increase of miscibility between conducting polymer and sulfonated PC. The percolation threshold in both cases is observed in the vicinity of 15 wt.%. This results are consistent with a three dimensional network of conducting polymer aggregates in an insulating matrix. As the results, when the content of PPy was 30 wt.%, the electrical conductivity of SPC composite was 0.82 S/cm, and that of PC composite was 0.23 S/cm. 3.5. Temperature dependence of electrical conductivity in PPy composite
Fig. 4. EDS investigations of (a) PPy/PC and (b) PPy/SPC composites. The feeding weight ratio of pyrrole/host polymer is 1:5.
3.4. The effect of sulfonation for electrical conductivity Fig. 5 represents the changes of electrical conductivity with the sulfonation effect of the PC. The charge carriers promote to induce coulombic interactions between positive charge and negative one as shown in Fig. 3 [7]. Also, the
Fig. 6(a) shows the variations of electrical conductivity of the PPy/SPC composite ®lms with temperature in the range of room temperature to 1608C. The electrical conductivity can be explained as the following equation, s nem equation. Where n is the number of electrons per unit volume, e the magnitude of the electrical charge on an electron, and m is called the electron mobility. Thus, the electrical conductivity is proportional to both the number of electrons and the electron mobility. The electrical conductivity is almost constant in the range of room temperature to about 1208C. The reason is that the increase of electron number is almost equal to the decrease of electron mobility by thermal vibration of SPC matrix. On the other hand, the electrical conductivity decreases very slightly in the range of about 120 to 1608C. These phenomena can be explained that the resistance is increased to expand the space of between PPy particles due to thermal vibration of SPC matrix [23]. Thermal vibration brings out the scattering of electrons and results in the decrease of electrical conductivity. Fig. 6(b) shows temperature dependence of the electrical conductivity in the range 1968C to room temperature, and the increase of electrical conductivity is observed with increasing temperature. It can be explained by variable range hopping expression [24], which is charge transfer model of the semiconductor region. The temperature dependence of electrical conductivity is represented by power-law relationship. The reason is that the mechanism of charge transfer is followed a three dimensional network of conducting polymer aggregates in an insulating matrix. This equation can be explained as the following equation: s T / exp T0 =Tg , where T0 is a characteristic temperature, and g a temperature index. The experimental result is linearlized when g is 1/2. This result might be explained due to coulombic interaction [25]. 3.6. Ageing effect in ambient conditions of composite film
Fig. 5. Electrical conductivity of the conducting polymer composites as a function of sulfonation.
Fig. 7 shows the variations of electrical conductivity of the composite ®lms in the atmosphere, and the decrease of electrical conductivity was observed with a period of 30 days. These phenomena can be explained that the PPy
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Fig. 7. Electrical conductivity vs. time of ambient air exposure for PPy/ SPC composites. (&: PPy (5 wt.%), &: PPy (10 wt.%), ~: PPy (15 wt.%), ~: PPy (20 wt.%), *: PPy (25 wt.%), *: PPy (30 wt.%)).
Fig. 6. Electrical conductivity changes of PPy/SPC composites as a function of temperature: 20±1608C; (b) 196±208C. (&: PPy (5 wt.%), &: PPy (10 wt.%), ~: PPy (15 wt.%), ~: PPy (20 wt.%), *: PPy (25 wt.%), *: PPy (30 wt.%)).
the tensile strength of the two cases are almost the same in the vicinity of 10 wt.% PPy. The mechanical property mainly depends on the content of PPy. The tensile strength decreases with the content of PPy in the composites. When the content of the PPy is low, the mechanical property is relatively high, and decreases steeply in the vicinity of percolation threshold. In other words, when the amount of PPy increase, mechanical property decreases due to the interruption of PC or SPC phase by the large amount of rigid PPy. As shown in Fig. 9, below the percolation threshold, i.e. the PC or SPC matrix is a continuous phase and the PPy is dispersed as a disperse phase, and above the percolation threshold, i.e. the 3-D network of small conducting granular aggregates in the PC or SPC matrix is well formed.
formed carbonium ion by chemical polymerization. The decrease of the continuity for the conjugated double bond of conducting polymer brought out the decrease of electrical conductivity [26] because carbonium ion was highly reacted on oxygen or moisture. But, the decrease of electrical conductivity does not really matter because conventional polymers have large scale of electrical conductivity. Thus, the PPy/SPC composites were very stable in the atmosphere. It can be explained that the SPC obstructed reaction of conducting polymer and oxygen (or moisture). 3.7. Mechanical properties of composite film Fig. 8 shows the tensile strength curve of PPy/PC or PPy/ SPC composite ®lms with the content of PPy. The values of
Fig. 8. Tensile strength of PPy/PC and PPy/SPC composite films.
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Fig. 9. SEM images of conducting polymer composites: (a)±(c); PPy/PC composites and (d)±(f); PPy/SPC composites. ((a): 10 wt.% PPy, (b): 15 wt.% PPy, (c): 25 wt.% PPy, (d): 10 wt.% PPy, (e): 15 wt.% PPy, (f): 25 wt.% PPy).
3.8. Characteristics of morphology Fig. 9 represents the views of scanning electron microscopy (SEM) for the surfaces of PPy/PC composites and PPy/SPC composites in the 10, 15, 25 wt.% PPy. In Fig. 9(a)±(c), the PPy in PPy/PC composite is showed to distribute as the types of small spherical granules within matrix. In this case, the phase separation still remains due to the interfacial tension between PPy and PC matrix, and
then this phase separation results in the decline of the mechanical properties and the electrical conductivity. Fig. 9(d)±(f) represents the morphology of surfaces for PPy/SPC composites. In spite of low content of PPy, the PPy within the SPC matrix is well distributed due to the effect of sulfonation. In other words, this phenomena might be leaded the increase of the electrical conductivity by inducing electrostatic interaction between PPy and SPC matrix.
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4. Conclusions The electrical conductivity of PPy/SPC composite was higher than that of PPy/PC composite. This phenomena might be explained due to the inducement of electrostatic interaction and miscibility between PPy and SPC matrix. The temperature-dependence of electrical conductivity in the composites in the range of 196±1608C was explained by variable range hopping model and thermal vibration of the matrix. The mechanical properties of PPy/SPC composites were similar to PPy/PC composites. In PPy/SPC conducting composites, the electrical conductivity was increased up to 0.82 S/cm with the amount of PPy. The PPy/SPC composites were very stable in the atmosphere. Acknowledgements The authors wish to acknowledge the ®nancial support of the Korea Science and Engineering Foundation (KOSEF) under contract KOSEF 981-1109-045-2. References [1] H. Tsutsumi, M. Araki, K. Onimura, T. Oishi, Synth. Met. 97 (1998) 53. [2] M.R. Anderson, B.R. Matters, H. Reiss, R.B. Kaner, Synth. Met. 41±43 (1991) 1151.
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