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Electrical and structural properties of a new conducting polymer: Pyrolytic poly (p-Phenylene-1,3,4-Oxadiazole)
Solid State Communications, Vol. 45, No. 12, pp. 1085-1088, 1983. Printed in G~eat Britain.
0038-1098/83/121085-04503.00/0 Pergamon Press Ltd.
ELECTRICAL AND STRUCTURAL PROPERTIES OF A NEW CONDUCTING POLYMER: PYROLYTIC POLY (p-PHENYLENE- 1,3,4-OXADIAZOLE) M. Murakami, H. Yasujima, Y. Yumoto, S. Mizogami and S. Yoshimura The Research Development Corporation of Japan, c/o Matsushita Research Institute Tokyo, Inc., Higashimita 3-10-1, Tama-ku, Kawasaki 214, Japan
(Received 11 December 1982 by W. Sasaki) Poly(p-phenylene-1,3,4-oxadiazole) (POD) films were pyrolyzed at various temperatures and their electrical and structural properties were investigated. A room temperature conductivity of about 510 S cm- 1 was obtained for pyrolysis temperature of 1400°C. The pyrolyzed films were composed of both extended electronic states which contribute to a temperatureindependent metallic conductivity and localized states giving rise to a variable range hopping motion of electrons. The structure was thought to be a graphite-like crystal having condensed aromatic layers with nitrogencontaining heterocyclic rings. 1. INTRODUCTION THE DEVELOPMENT of electrically conducting polymers has been a challenging subject of physics and chemistry. Linear conjugated polymers, such as polyacetylene, (CH)x, can be rendered highly conductive by chemical doping, and various kinds of novel polymers and dopants have been systematically investigated [1]. On the contrary, there has been little attention given to planar polymers with an extended graphite-like structure. Polymer pyrolysis or high temperature polycondensation is a simple procedure which presents the latter structure of relatively high electrical conductivity [2]. The pyrolytic polymers, however, have not been well appreciated as a synthetic metal, partly because their chemical and electronic structures cannot be accessed very easily. A basic question unsolved may be whether the structure and properties of the pyrolytic polymers bear any relationship to the molecular structure of the starting polymers. The fact that there have been quite a few numbers of highly conducting pyrolytic polymers [3-8] may arouse interest in exploring the possibility of designing a molecular structure of the starting polymers in favor of unique physical properties. Such an interest seems to be growing with recent studies on pyrolytic polyimide in connection with their pyrolysis mechanisms [6] and the nature of the conduction process [5, 7, 8]. In this paper we will report the electrical properties of a new highly conducting pyrolytic polymer based on poly(p-phenylene-1,3,4-oxadiazole) (POD)which is
obtained in the form of a thin cast film with high crystallinity [9]. We will report the realization of electrical conductivity as high as 510 S cm -1 at room temperature and of metallic conductivity at low temperatures, showing the structural aspects of the new conductor as a function of the pyrolysis temperature. 2. EXPERIMENTAL POD films of 25 or 50/~m thickness were received from Furukawa Electric Co. [10]. They were cut into small pieces and sandwiched between two alumina plates. These supported POD films were heat-treated in an argon atmosphere at various heating rates and temperatures, Tp, (400-1400°C). The pyrolyzed POD was obtained as a black film with metallic luster and with reduced flexibility as in the case of polyimide films. The electrical conductivity was measured using the ordinary four probe method, and the temperature dependence of the conductivity down to 10 K were determined using Cryosystems LTS-21. In order to elucidate the pyrolysis mechanism and the structure of the product, we carried out powder X-ray diffraction (using Philips PW 1051), X-ray photoelectron spectroscopy (XPS, using Shimadzu ESCA 750), and infrared spectroscopy (FT-IR, Nicolet 7199c) measurements for the pyrolyzed films. 3. RESULTS AND DISCUSSION Figure 1 shows the dependence of the room temperature conductivity, ORT, on Tp, where each POD film was kept at Tp for 60 min after being heated at a rate of 10°C min -1 . The figure also includes data for polyimide, Kapton R H (du Pont), pyrolyzed under the
1085
PROPERTIES OF A NEW CONDUCTING POLYMER
1086 103
, ,
i
i
,
,
,
,
,
Vol. 45, No. 12
tO°
,
r~
~,0
102
O
& 03
10-2
I0' i o -3
I I 01
I
I
I
I
I
I
I
I
Q2
I
600 800 I000 1200 1400 T (°C)
Fig. 1. Electrical conductivity at room temperature vs pyrolysis temperature for POD ( I ) and polyimide (o).
500 ~
Tp ,2oo*c
400 k~,.~-. . . . . . .__:
:
_
,ooo*c
:
u 80 I-
800"C
40
,
0
I
20 IO00/T
,
I
40 (K -~)
,
60
Fig. 2. Temperature dependence of conductivity for POD pyrolyzed at 800, 1000 and 1200°C. same condition. The pyrolyzed POD exhibits ORT of 68,350, 410 and 510Scm -1 for Tp = 800, 1000, 1200 and 1400°C, respectively. Conductivity data of polyimide films reported by Brom et al. [5] were based on samples pyrolyzed for 48 h, and their ORT tends to saturate to a value of about 100 S cm -1 at 800°C. Considering the difference in the experimental conditions between ours and Brom's, we conclude that the pyrolyzed POD has ORT more than twice as high as that of pyro-
I
Q3 0.4 T -~/4 [ K-t/'']
Q5
Fig. 3. Temperature-dependent part of the conductivity vs T-1/4 for pyrolyzed POD. Zero-temperature conductivity o(0) obtained from extrapolation of o(T) vs T curves. Solid lines indicate the best fit line (equation (1)). lyzed polyimide. Figure 2 shows the temperature dependence of the electrical conductivity of the POD pyrolyzed at Tp = 800, 1000 and 1200°C. Another characteristic feature of POD is a temperature independent conductivity at low temperatures, and the a vs T curves as a whole are expressed with an equation, a ( T ) = a(O) + al(T), where o(0) is the zero-temperature limit conductivity. This dependence differs markedly from that of pyrolyzed polyimides [8]. The existence of a(0) has been observed with inorganic metals, such as N b - C u [11] and Si-Au [12], which has attracted much attention in connection with the metal-insulator transition in a disordered system. In such disordered metals, el(T) is given by T n (where n is 1/2 or 1/3) or In T according to the sample morphology and alloy content. If we want to fit the o v s T curve to the T n form, a value of n between 1.6 and 1.8 is obtained, which we cannot rationalize with an ordinary theory. After all, the best fit was obtained to a function of the form o ( T ) = o(0) + o(°°) exp {-- (To~T)1'4},
as shown in Fig. 3. The el(T) or the T-1/4-dependence can be interpreted on the basis of a three-dimensional variable range hopping model [13] which describes the behavior of a disordered system with an electron just below the Fermi level hopping to a distant state for which the required energy is as small as possible. The
(1)
Vol 45, No. 12
PROPERTIES OF A NEW CONDUCTING POLYMER
1087
I
1--200*C
1000 *C
(c) Tp= 1200"C
(a) Tp = 800°C
40
55
50
2e
25
20
15
(°)
Fig. 4. Powder X-ray diffraction patterns for POD pyrolyzed at 800, 1000 and 1200°C. slope To is obtained from Fig. 3 as 1.21 x l0 s, 4.95 x 10 s, and 5.86 x l0 s K for Tp = 800, 1000 and 1200°C, respectively. These To values are much smaller than that observed with ordinary amorphous carbon films [ 14]. In the variable range hopping model, To is expressed as:
To = 16ota/kN(EF),
1
Tp--IO00°C
Tp=i400°C
Fig. 5. Scanning electron microscope pictures of cross sections of POD pyrolyzed at 800 (a), 1000 (b), 1200 (c) and 1400°C (d).
NIS
(2)
where k is Boltzmann's constant, cz-1 is the radius of the localized state wave function and N(Ee) is the density of localized states at the Fermi level. So that the change in To going from Tp = 800°C to Tp = 1200°C is understood if we assume a decrease in N(EF). If we use a value of 12 A for e -I as in the case of amorphous carbon, the localized state density of the order of 1020 (eV cm3) -1 is obtained, the value being large almost enough for the localized states to overlap. The temperature-independent conductivity, o(0), shunting the T -1/4-conductance offers evidence of a metallic conduction path with extended electronic states. A simple explanation to the increase in o(0) with increasing Tp is that there are two parallel conduction paths in the pyrolyzed POD with extended states contributing to o(0) and localized states to o1(T), and the latter is converted to the former at higher Tp. This idea is qualitatively supported by the increase in o(0)/o(oo) with increasing Tp, as 0.015, 0.027 and 0.043 for 800, 1000 and 1200°C, respectively. Powder X-ray diffraction patterns (with Cu-K~ radiation) of the pyrolyzed POD films are illustrated in Fig. 4. The lattice spacings are 3.62, 3.57 and 3.49/~ for Tp= 800, 1000 and 1200°C, respectively, and the peak width becomes narrower for higher Tp. These results indicate that the pyrolyzed POD has an increased ordering in the (002) direction of a graphite structure at higher Tp, but it has not completely been converted to graphite with d(002) = 3.35 A. The growth of the graphite structure is also suggested by electron microscopy pictures of cross-sections of POD as shown in Fig. 5. This figure reveals that planar graphitic networks
Tp : 1400"C ~
~
-
-
~
-
1200 1000
8OO 700 620 60O 550 520 500
~
450 Ol
, ,
,
410
Binding
,
I
400
,
~
L
, I
590
Energy (eV)
Fig. 6. X-ray photoelectron spectroscopy peaks in the N18 region for POD pyrolyzed at various temperatures.
(ab plane) develop for higher Tp. Figure 5 along with the smaller line width of the X-ray patterns (Fig. 4) as compared with that of ordinary carbons implies that POD is a polymer in which a graphite-like structure can be formed at relatively low temperatures. The crystalline character of the starting polymer [ 15] with a sharp diffraction line from a plane of d = 3.4 A may account for the result.
1088
PROPERTIES OF A NEW CONDUCTING POLYMER
The next structural feature is that the elemental and XPS analyses confirmed that complete conversion to carbon did not occur even for Tp as high as 1400°C. The relative amount of nitrogen in the pyrolyzed products (for example, N/C = 1/19 by weight for Tp = 1000°C) was much higher than that of pyrolyzed polyimide (N/C = 1/44 for Tp = 1000°C) [16]. Figure 6 shows XPS spectra in the NI, region for POD pyrolyzed at various temperatures, which were measured after being etched with argon ion. For Tp < 500°C, only one type of nitrogen exists with a change in the binding energy starting at 450°C (nearly equal to the decomposition temperature). For Tp higher than 520°C, however, two peaks are clearly observed at about 398.0 and 399.0 eV, the latter shifting to 400.0 eV as Tp goes up to 1000°C. And again only one peak which corresponds to the former remains to exist for Tp > 1200°C. The nitrogen with higher binding energy in the range Tp = 5 2 0 1000°C may be that in an intermediate product of the pyrolysis. The most probable is a product with a - C N end group, because the N - N bond in the oxidiazole ring has the lowest energy (160.7 kJmol -l) and in fact F T IR spectra exhibited a strong - C N absorption at 2225 cm -1 . The peak at the lower binding energy, on the other hand, is likely to come from nitrogen in a final product of polycondensation. Since the lower binding energy suggests a conjugated nitrogen, the final product may contain heterocyclic rings like pyridine, pyrazine, triazine or fused rings of them, which can possibly be formed with the -CN group [17] produced in the intermediate products. The enhancement of the X-ray diffraction peak for Tp = 1200°C is thus interpreted on the basis of the formation of condensed aromatic layers with nitrogen-containing heterocyclic rings.
between the high conductivity and the structure has not been fully explained, but further studies on reaction mechanism and the physical properties will bring forth more information. The chemical aspects of the pyrolysis and the chemical doping made with pyrolysis are being investigated and their results will be published in the near future.
Acknowledgements - We are grateful to Professor N. Ogata for his encouragement and suggestions given in the present study. We are indebted to Matsushita Research Institute Tokyo, Inc. for support with various facilities. We also wish to thank Dr T. Kotani of Japan Synthetic Rubber Co. for providing us with facilities of XPS. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
4. CONCLUSIONS We have shown that the POD films were converted to a high electrical conductor on pyrolysis. The pyrolyzed POD has two conducting channels: one is a metallic channel with extended electronic states for which the conductivity has no temperature dependence, and the other is of a disordered system where the electron motion is described by the three dimensional variable range hopping model. It was suggested from the X-ray diffraction and XPS analyses that the pyrolyzed product has a graphite-like planar structure in which nitrogencontaining heterocyclic rings are included. The relation
Vol. 45, No. 12
12. 13. 14. 15. 16. 17.
A.G. MacDiarmid & A.J. Heeger, Synth. Met. 1, 101 (1980). C.B. Duke, Encyclopedia of Chemical Technology (Edited by Kirk-Othmer) Vol. 18, 3rd edn., p.775. John Wiley and Sons, New York (1982). N. Grassie & J.C. McHeill, J. Polym. Sci. 27,707 (1958). S.D. Bruck, Polymer 6, 319 (1965). H.B. Brom, Y. Tomkiewicz, A. Aviram & A. Broors, SolidState Commun. 35,135 (1980). J.W.-P. Lin, A.J. Epstein, L.P. Dudek & H. Rommelman, Org. Coat. Plast. Chem. 43,482 (1980). J.I. Gittleman & E.K. Sichel, J. Electr. Mater. 10, 327 (1981). E.K. Sichel & T. Emma, SolidState Commun. 41, 747 (1982). A.H. Frazer, W. Sweeny & F.T. Wallenberger, J. Polym. Sci. A2, 1157 (1964). H. Sekiguchi & K. Sadamitsu, Japan. Pat. 52-30555 (Aug. 9, 1977). T.R. Werner, I. Banerjee, Q.S. Yang, C.M. Falco & I.K. Schuller, Phys. Rev. B24, 2224 (1982). N. Nishida, M. Yamaguchi, T. Furubayashi, K. Morigaki, H. Ishimoto & K. Ono, Solid State Commun. 44, 305 (1982). F.N. Mott, Metallnsulator Transitions, Taylor & Francis Ltd., London (1974). J.J. Hauser, J. Non-Cryst. Solids 23, 21 (1977). Y. Iwakura, K. Uno & S. Hara, J. Polym. Sci. A3, 45 (1965). A. Biirger, E. Fitzer, M. Heym & B. Terwiesch, Carbon 13,149 (1975). For example: M. Hiirter, G. Kossmehl, G. Manecke, W. Wille, D. W6hrle & D. Zerpner, Angew. Makromol. Chem. 29/30, 307 (1973); B. Wahl & D. WOhrle, Makromol. Chem. 176,849 (1975).
0038-1098/83/121085-04503.00/0 Pergamon Press Ltd.
ELECTRICAL AND STRUCTURAL PROPERTIES OF A NEW CONDUCTING POLYMER: PYROLYTIC POLY (p-PHENYLENE- 1,3,4-OXADIAZOLE) M. Murakami, H. Yasujima, Y. Yumoto, S. Mizogami and S. Yoshimura The Research Development Corporation of Japan, c/o Matsushita Research Institute Tokyo, Inc., Higashimita 3-10-1, Tama-ku, Kawasaki 214, Japan
(Received 11 December 1982 by W. Sasaki) Poly(p-phenylene-1,3,4-oxadiazole) (POD) films were pyrolyzed at various temperatures and their electrical and structural properties were investigated. A room temperature conductivity of about 510 S cm- 1 was obtained for pyrolysis temperature of 1400°C. The pyrolyzed films were composed of both extended electronic states which contribute to a temperatureindependent metallic conductivity and localized states giving rise to a variable range hopping motion of electrons. The structure was thought to be a graphite-like crystal having condensed aromatic layers with nitrogencontaining heterocyclic rings. 1. INTRODUCTION THE DEVELOPMENT of electrically conducting polymers has been a challenging subject of physics and chemistry. Linear conjugated polymers, such as polyacetylene, (CH)x, can be rendered highly conductive by chemical doping, and various kinds of novel polymers and dopants have been systematically investigated [1]. On the contrary, there has been little attention given to planar polymers with an extended graphite-like structure. Polymer pyrolysis or high temperature polycondensation is a simple procedure which presents the latter structure of relatively high electrical conductivity [2]. The pyrolytic polymers, however, have not been well appreciated as a synthetic metal, partly because their chemical and electronic structures cannot be accessed very easily. A basic question unsolved may be whether the structure and properties of the pyrolytic polymers bear any relationship to the molecular structure of the starting polymers. The fact that there have been quite a few numbers of highly conducting pyrolytic polymers [3-8] may arouse interest in exploring the possibility of designing a molecular structure of the starting polymers in favor of unique physical properties. Such an interest seems to be growing with recent studies on pyrolytic polyimide in connection with their pyrolysis mechanisms [6] and the nature of the conduction process [5, 7, 8]. In this paper we will report the electrical properties of a new highly conducting pyrolytic polymer based on poly(p-phenylene-1,3,4-oxadiazole) (POD)which is
obtained in the form of a thin cast film with high crystallinity [9]. We will report the realization of electrical conductivity as high as 510 S cm -1 at room temperature and of metallic conductivity at low temperatures, showing the structural aspects of the new conductor as a function of the pyrolysis temperature. 2. EXPERIMENTAL POD films of 25 or 50/~m thickness were received from Furukawa Electric Co. [10]. They were cut into small pieces and sandwiched between two alumina plates. These supported POD films were heat-treated in an argon atmosphere at various heating rates and temperatures, Tp, (400-1400°C). The pyrolyzed POD was obtained as a black film with metallic luster and with reduced flexibility as in the case of polyimide films. The electrical conductivity was measured using the ordinary four probe method, and the temperature dependence of the conductivity down to 10 K were determined using Cryosystems LTS-21. In order to elucidate the pyrolysis mechanism and the structure of the product, we carried out powder X-ray diffraction (using Philips PW 1051), X-ray photoelectron spectroscopy (XPS, using Shimadzu ESCA 750), and infrared spectroscopy (FT-IR, Nicolet 7199c) measurements for the pyrolyzed films. 3. RESULTS AND DISCUSSION Figure 1 shows the dependence of the room temperature conductivity, ORT, on Tp, where each POD film was kept at Tp for 60 min after being heated at a rate of 10°C min -1 . The figure also includes data for polyimide, Kapton R H (du Pont), pyrolyzed under the
1085
PROPERTIES OF A NEW CONDUCTING POLYMER
1086 103
, ,
i
i
,
,
,
,
,
Vol. 45, No. 12
tO°
,
r~
~,0
102
O
& 03
10-2
I0' i o -3
I I 01
I
I
I
I
I
I
I
I
Q2
I
600 800 I000 1200 1400 T (°C)
Fig. 1. Electrical conductivity at room temperature vs pyrolysis temperature for POD ( I ) and polyimide (o).
500 ~
Tp ,2oo*c
400 k~,.~-. . . . . . .__:
:
_
,ooo*c
:
u 80 I-
800"C
40
,
0
I
20 IO00/T
,
I
40 (K -~)
,
60
Fig. 2. Temperature dependence of conductivity for POD pyrolyzed at 800, 1000 and 1200°C. same condition. The pyrolyzed POD exhibits ORT of 68,350, 410 and 510Scm -1 for Tp = 800, 1000, 1200 and 1400°C, respectively. Conductivity data of polyimide films reported by Brom et al. [5] were based on samples pyrolyzed for 48 h, and their ORT tends to saturate to a value of about 100 S cm -1 at 800°C. Considering the difference in the experimental conditions between ours and Brom's, we conclude that the pyrolyzed POD has ORT more than twice as high as that of pyro-
I
Q3 0.4 T -~/4 [ K-t/'']
Q5
Fig. 3. Temperature-dependent part of the conductivity vs T-1/4 for pyrolyzed POD. Zero-temperature conductivity o(0) obtained from extrapolation of o(T) vs T curves. Solid lines indicate the best fit line (equation (1)). lyzed polyimide. Figure 2 shows the temperature dependence of the electrical conductivity of the POD pyrolyzed at Tp = 800, 1000 and 1200°C. Another characteristic feature of POD is a temperature independent conductivity at low temperatures, and the a vs T curves as a whole are expressed with an equation, a ( T ) = a(O) + al(T), where o(0) is the zero-temperature limit conductivity. This dependence differs markedly from that of pyrolyzed polyimides [8]. The existence of a(0) has been observed with inorganic metals, such as N b - C u [11] and Si-Au [12], which has attracted much attention in connection with the metal-insulator transition in a disordered system. In such disordered metals, el(T) is given by T n (where n is 1/2 or 1/3) or In T according to the sample morphology and alloy content. If we want to fit the o v s T curve to the T n form, a value of n between 1.6 and 1.8 is obtained, which we cannot rationalize with an ordinary theory. After all, the best fit was obtained to a function of the form o ( T ) = o(0) + o(°°) exp {-- (To~T)1'4},
as shown in Fig. 3. The el(T) or the T-1/4-dependence can be interpreted on the basis of a three-dimensional variable range hopping model [13] which describes the behavior of a disordered system with an electron just below the Fermi level hopping to a distant state for which the required energy is as small as possible. The
(1)
Vol 45, No. 12
PROPERTIES OF A NEW CONDUCTING POLYMER
1087
I
1--200*C
1000 *C
(c) Tp= 1200"C
(a) Tp = 800°C
40
55
50
2e
25
20
15
(°)
Fig. 4. Powder X-ray diffraction patterns for POD pyrolyzed at 800, 1000 and 1200°C. slope To is obtained from Fig. 3 as 1.21 x l0 s, 4.95 x 10 s, and 5.86 x l0 s K for Tp = 800, 1000 and 1200°C, respectively. These To values are much smaller than that observed with ordinary amorphous carbon films [ 14]. In the variable range hopping model, To is expressed as:
To = 16ota/kN(EF),
1
Tp--IO00°C
Tp=i400°C
Fig. 5. Scanning electron microscope pictures of cross sections of POD pyrolyzed at 800 (a), 1000 (b), 1200 (c) and 1400°C (d).
NIS
(2)
where k is Boltzmann's constant, cz-1 is the radius of the localized state wave function and N(Ee) is the density of localized states at the Fermi level. So that the change in To going from Tp = 800°C to Tp = 1200°C is understood if we assume a decrease in N(EF). If we use a value of 12 A for e -I as in the case of amorphous carbon, the localized state density of the order of 1020 (eV cm3) -1 is obtained, the value being large almost enough for the localized states to overlap. The temperature-independent conductivity, o(0), shunting the T -1/4-conductance offers evidence of a metallic conduction path with extended electronic states. A simple explanation to the increase in o(0) with increasing Tp is that there are two parallel conduction paths in the pyrolyzed POD with extended states contributing to o(0) and localized states to o1(T), and the latter is converted to the former at higher Tp. This idea is qualitatively supported by the increase in o(0)/o(oo) with increasing Tp, as 0.015, 0.027 and 0.043 for 800, 1000 and 1200°C, respectively. Powder X-ray diffraction patterns (with Cu-K~ radiation) of the pyrolyzed POD films are illustrated in Fig. 4. The lattice spacings are 3.62, 3.57 and 3.49/~ for Tp= 800, 1000 and 1200°C, respectively, and the peak width becomes narrower for higher Tp. These results indicate that the pyrolyzed POD has an increased ordering in the (002) direction of a graphite structure at higher Tp, but it has not completely been converted to graphite with d(002) = 3.35 A. The growth of the graphite structure is also suggested by electron microscopy pictures of cross-sections of POD as shown in Fig. 5. This figure reveals that planar graphitic networks
Tp : 1400"C ~
~
-
-
~
-
1200 1000
8OO 700 620 60O 550 520 500
~
450 Ol
, ,
,
410
Binding
,
I
400
,
~
L
, I
590
Energy (eV)
Fig. 6. X-ray photoelectron spectroscopy peaks in the N18 region for POD pyrolyzed at various temperatures.
(ab plane) develop for higher Tp. Figure 5 along with the smaller line width of the X-ray patterns (Fig. 4) as compared with that of ordinary carbons implies that POD is a polymer in which a graphite-like structure can be formed at relatively low temperatures. The crystalline character of the starting polymer [ 15] with a sharp diffraction line from a plane of d = 3.4 A may account for the result.
1088
PROPERTIES OF A NEW CONDUCTING POLYMER
The next structural feature is that the elemental and XPS analyses confirmed that complete conversion to carbon did not occur even for Tp as high as 1400°C. The relative amount of nitrogen in the pyrolyzed products (for example, N/C = 1/19 by weight for Tp = 1000°C) was much higher than that of pyrolyzed polyimide (N/C = 1/44 for Tp = 1000°C) [16]. Figure 6 shows XPS spectra in the NI, region for POD pyrolyzed at various temperatures, which were measured after being etched with argon ion. For Tp < 500°C, only one type of nitrogen exists with a change in the binding energy starting at 450°C (nearly equal to the decomposition temperature). For Tp higher than 520°C, however, two peaks are clearly observed at about 398.0 and 399.0 eV, the latter shifting to 400.0 eV as Tp goes up to 1000°C. And again only one peak which corresponds to the former remains to exist for Tp > 1200°C. The nitrogen with higher binding energy in the range Tp = 5 2 0 1000°C may be that in an intermediate product of the pyrolysis. The most probable is a product with a - C N end group, because the N - N bond in the oxidiazole ring has the lowest energy (160.7 kJmol -l) and in fact F T IR spectra exhibited a strong - C N absorption at 2225 cm -1 . The peak at the lower binding energy, on the other hand, is likely to come from nitrogen in a final product of polycondensation. Since the lower binding energy suggests a conjugated nitrogen, the final product may contain heterocyclic rings like pyridine, pyrazine, triazine or fused rings of them, which can possibly be formed with the -CN group [17] produced in the intermediate products. The enhancement of the X-ray diffraction peak for Tp = 1200°C is thus interpreted on the basis of the formation of condensed aromatic layers with nitrogen-containing heterocyclic rings.
between the high conductivity and the structure has not been fully explained, but further studies on reaction mechanism and the physical properties will bring forth more information. The chemical aspects of the pyrolysis and the chemical doping made with pyrolysis are being investigated and their results will be published in the near future.
Acknowledgements - We are grateful to Professor N. Ogata for his encouragement and suggestions given in the present study. We are indebted to Matsushita Research Institute Tokyo, Inc. for support with various facilities. We also wish to thank Dr T. Kotani of Japan Synthetic Rubber Co. for providing us with facilities of XPS. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
4. CONCLUSIONS We have shown that the POD films were converted to a high electrical conductor on pyrolysis. The pyrolyzed POD has two conducting channels: one is a metallic channel with extended electronic states for which the conductivity has no temperature dependence, and the other is of a disordered system where the electron motion is described by the three dimensional variable range hopping model. It was suggested from the X-ray diffraction and XPS analyses that the pyrolyzed product has a graphite-like planar structure in which nitrogencontaining heterocyclic rings are included. The relation
Vol. 45, No. 12
12. 13. 14. 15. 16. 17.
A.G. MacDiarmid & A.J. Heeger, Synth. Met. 1, 101 (1980). C.B. Duke, Encyclopedia of Chemical Technology (Edited by Kirk-Othmer) Vol. 18, 3rd edn., p.775. John Wiley and Sons, New York (1982). N. Grassie & J.C. McHeill, J. Polym. Sci. 27,707 (1958). S.D. Bruck, Polymer 6, 319 (1965). H.B. Brom, Y. Tomkiewicz, A. Aviram & A. Broors, SolidState Commun. 35,135 (1980). J.W.-P. Lin, A.J. Epstein, L.P. Dudek & H. Rommelman, Org. Coat. Plast. Chem. 43,482 (1980). J.I. Gittleman & E.K. Sichel, J. Electr. Mater. 10, 327 (1981). E.K. Sichel & T. Emma, SolidState Commun. 41, 747 (1982). A.H. Frazer, W. Sweeny & F.T. Wallenberger, J. Polym. Sci. A2, 1157 (1964). H. Sekiguchi & K. Sadamitsu, Japan. Pat. 52-30555 (Aug. 9, 1977). T.R. Werner, I. Banerjee, Q.S. Yang, C.M. Falco & I.K. Schuller, Phys. Rev. B24, 2224 (1982). N. Nishida, M. Yamaguchi, T. Furubayashi, K. Morigaki, H. Ishimoto & K. Ono, Solid State Commun. 44, 305 (1982). F.N. Mott, Metallnsulator Transitions, Taylor & Francis Ltd., London (1974). J.J. Hauser, J. Non-Cryst. Solids 23, 21 (1977). Y. Iwakura, K. Uno & S. Hara, J. Polym. Sci. A3, 45 (1965). A. Biirger, E. Fitzer, M. Heym & B. Terwiesch, Carbon 13,149 (1975). For example: M. Hiirter, G. Kossmehl, G. Manecke, W. Wille, D. W6hrle & D. Zerpner, Angew. Makromol. Chem. 29/30, 307 (1973); B. Wahl & D. WOhrle, Makromol. Chem. 176,849 (1975).