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Structural changes associated with thermal and chemical treatment of polythiophenes doped with perchlorate
Polymer Degradation and Stability 31 (1991) 37~9
Structural Changes Associated with Thermal and Chemical Treatment of Polythiophenes Doped with Perchlorate K. G. Neoh," E. T. Kang a & K. L. Tan b Department of Chemical Engineering, h Department of Physics, National University of Singapore, Kent Ridge, Singapore 0511 (Received l0 January 1990; accepted 25 January 1990)
ABSTRACT Electrically conductive polymers have been synthesized by the reaction o.t" 3-methylthiophene or 2,2'-bithiophene with ferric perchlorate. With copper perchlorate, only 2,2'-bithiophene undergoes simultaneous polymerization and oxidation. Neither ferric perchlorate nor copper perchlorate reacts with thiophene to yield a conducting polymer. X-ray photoelectron spectroscopy of" the perchlorate doped polythiophenes indicates that, depending on the perchlorate used, a significant amount of the chlorine may be covalentlv bonded to the polymer. Treatment of the doped samples with methanol results in the ClOg /S ratio decreasing to one-third of the pristine value. Treatment with NaOH results in almost complete removal of the Cl04 anions in poly( bithiophene ) synthesized with Fe( Cl0 4 ) 3 . 9 H20. Concurrent with the loss of ClOg anions is the decrease in electrical conductivity. The undoped polymer has a high degree of thermal stability while the decomposition of the doped polymer appears to be initiated by the decomposition Of the Cl04 anions. Prolonged heating of the doped samples at 150°C results in the conversion of ClOg anions to Cl covalent O, bonded to the poO,mer and to volatile species which are lost .from the polymer.
INTRODUCTION Polythiophene belongs to the family of electrically conducting polymers which have attracted a great deal o f attention in recent years because of their 37 Polymer Degradation and Stability 0141-3910/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
38
K. G. Neoh, E. T. Kang, K. L. Tan
unusual electronic properties. Highly conductive polythiophene can be prepared electrochemically by coupling of the monomers in the 2- and 5positions in an electrolytic medium such as acetonitrile with N(Bu)4C104.1 Polythiophene and its derivatives can also be synthesized chemically by Grignard coupling reactions, 2'3 or by dehydrohalogenation of 2halogenothiophene (or its 3-alkyl derivatives) or by dehydrogenation of thiophene (or its 3-alkyl derivatives) in the presence of metal halogenide.4 The polymer can then be doped with oxidants such as I2. Recently, a onestep simultaneous polymerization and doping of bithiophene with copper perchlorate has been reported. 5 In general, the polyheterocycles such as polypyrrole and polythiophene exhibit higher environmental stability than polyacetylenes. Degradation studies of electrochemically synthesized polythiophene-BF4 and polythiophene-C104 in air have shown that the conductivity decreases substantially upon heating to above 70°C and the rate of decrease is dependent on the counter-ion. 6n Electrochemically synthesized polythiophene doped with FeC13 has been reported to show higher thermal stability than polythiophene-BF4 and polythiophene-C104, a However, FeCla-doped poly(3-alkylthiophene) with long side chains may undergo rapid thermal undoping due to the conversion of the anions to FeC12.9 The chemical method for the simultaneous synthesis and doping of polythiophene by the use of perchlorate as oxidant 5 has the advantage of being a simple one-step process. In the present work, the stability of the products from such a process is assessed. The structural changes in the doped polymer brought about by heating and treatment with base, acid and organic solvents were studied using elemental analysis, infrared (IR) absorption spectroscopy and X-ray photoelectron spectroscopy (XPS).
EXPERIMENTAL
Synthesis and treatment of polymers Polymer samples were prepared by adding the monomer to a solution of metal perchlorate in acetonitrile. The metal perchlorates used were Fe(C104) 3 . 9H20 and Cu(C104) 2 . 6H20, and the perchlorate to monomer ratio was kept at 3:1. The monomers tested were thiophene, 3methylthiophene and 2,2'-bithiophene. Since bithiophene is a solid at room temperature, it was dissolved in acetonitrile prior to reaction with the metal perchlorate. Depending on the monomer used, the reaction time was 1 h, 24 h or 48 h. The reaction products were washed with copious amounts of acetonitrile and then dried by pumping under reduced pressure.
Treatment of polythiophenes doped with perchlorate
39
The stability of the polymer samples at elevated temperatures and in organic solvents, H z S O 4 and N a O H was investigated. A Netzsch STA409 simultaneous thermogravimetric (TG)-differential thermal analyzer (DTA) unit was used to monitor the degradation of the polymer samples. A heating rate of 10°C/rain and a N 2 flowrate of 100 ml/min were used. The effect of elevated temperatures on the electrical conductivity (a) of the polymer samples was assessed by slowly heating (,,~0-5°C/min) the samples in air to 150°C and simultaneously monitoring a. Some samples were also kept at 150°C for 24h. To assess the stability of the polymer samples in common organic solvents, NaOH and H2SO 4 solutions, the samples were mixed thoroughly with the respective solvent or solution and left for 18-20 h. The acid and base concentrations were 1"0 and 0"5M, respectively. After the treatment, the sample was washed either with deionized water (in the case of acid or base treatment) or with the respective solvent and dried under reduced pressure.
Polymer characterization After thermal or chemical treatment, the electrical conductivity, structure and composition of the samples were compared with those of the pristine sample. The electrical conductivity was measured using the standard collinear four-probe or two-probe technique on compressed pellets. The carbon and hydrogen contents of the samples were determined with a Perkin-Elmer 240C Elemental Analyzer. The sulfur content was determined using the combustion method? ° IR absorption spectroscopy was performed using a Perkin-Elmer model 682 spectrophotometer with the samples dispersed in KBr pellets, and X-ray photoelectron spectroscopy (XPS) measurements were made on a VG Scientific ESCALAB Mk II spectrometer with a MgK~ X-ray source (1253.6eV photons). In the XPS experiment, the polymer samples in powder form were mounted on the standard sample studs by using double-sided Scotch tape. All core-level spectra were referenced to the Cls neutral carbon peak at 284.6 eV. The peak area ratios for the various elements were corrected using experimentally determined instrumental sensitivity factors.
RESULTS A N D DISCUSSION
Pristine polymer samples With Cu(C104)2.6H20 as oxidant, only bithiophene undergoes simultaneous polymerization and doping. No significant amount of solid product
TABLE 1
Thiophene 3-Methylthiophene Bithiophene Bithiophene Bithiophene
Monomer
Fe(C104)3.9HzO, Fe(CIO4)3.9H20, Fe(CIOa)3.9H20, Cu(CIOa)2.6H20, Cu(CIOJ/.6HEO,
48 h 18 h 18 h 18 h 1h
Synthesis conditions
a Based on corrected S2p and C12p component spectral area ratios.
PT1 PMT1 PBT1 PBT2 PBT3
Sample
0"08 0-65 1'12 1'11 0"53
wt. Product wt. Monomer
0"54 0'90 0"50 0-56 0.65
H/C
0'24 0-20 0.25 0'25 0-25
SIC
-0'16 0"12 0-18 0'16
ClOg/S a
Elemental composition
< 10 7 0"05 0'13 0"2 1
~rS/cm
Conductivity
Yield, Chemical Composition and Electrical Conductivity (tr) of Products from Reaction of Thiophenes and Metal Perchlorates
Treatment of polythiophenes doped with perchlorate
41
was obtained with either thiophene or 3-methylthiophene over a reaction time of 24 h to 48 h. However, with Fe(C104)3.9H20, both 3-methylthiophene and bithiophene undergo simultaneous polymerization and doping although the yield from the latter monomer was substantially higher. With thiophene, only a very small amount of dark brown solid was obtained. The yield, chemical composition and conductivity of the various reaction products are summarized in Table 1. With the exception of PBT3 which was polymerized in a N 2 atmosphere, the other polymerization experiments were carried out in air. The H/C ratio of the products is generally higher than that expected for a chain of thiophene (or 3-methylthiophene) rings coupled in the ~,d positions. This may be due to the presence of water associated with the CIO~ anions (see below). The S/C ratio is very close to the ideal value. The IR absorption spectra of the solid products from reaction of thiophene, 3-methylthiophene and bithiophene with Fe(C104)3.9H20 (Samples PT1, PMT1 and PBT1) are shown in Fig. 1. The spectra of the two latter samples show a long absorption tail from 4000 to about 1500cm 1 This absorption tail obscures the C-H stretching band at around 3000 cm 1 The IR absorption spectrum of PBT1 (Fig. l(c)) is similar to that of electrochemically synthesized polythiophene with four strong bands at WICRONS 2.5
3
G
i
i
6
B
12
20
jJ,i "/
I
6000
Fig. 1.
3000
I
i
I
1600 1200 WAVELENGTH (cm q ) 2000
I
BOO
400
IR absorption spectra of products from reaction of (a) thiophene, (b) 3methylthiophene and (c) bithiophene with Fe(C104)3.9H20.
K. G. Neoh, E. T. Kang, K. L. Tan
42
1340, 1200, 1115 and 1030cm -1 which are characteristic of doped polythiophene. 11 The band at 785cm-1 is attributed to the out-of-plane vibration of C-H at the fl-position of 2,5-disubstituted thiophene. 11 The bands due to the perchlorate ion are at ll00 and 625 cm-1 although the former band is masked by the doped thiophene band. The IR absorption spectrum of PMT1 (Fig. l(b)) shows similarities with that of electrochemically prepared poly(3-alkylthiophene) with strong bands at 1390, 1300, 1140-1070cm- 1 and 830cm-1.9 The last band has been assigned to the C-H out-of-plane vibration of the 2,3,5-trisubstituted thiophene.12 The C10£ ion band at 625 cm-1 is also prominent in this spectrum. The IR absorption spectra provide evidence that the product from the reaction between 3-methylthiophene and Fe(C104)3.9H20 is a doped polymer. This is consistent with the fact that Fe(CIO4) 3 . 9H20 is a stronger oxidant than Cu(C104)2.6H20.13 However, with thiophene monomer, no significant amount of doped polymer can be obtained even with Fe(C104) 3 . 9H20 over a period of 48 h. The IR absorption spectra of PT1 (Fig. l(a)) showed no strong characteristic bands of doped polythiophene and the 785 c m - 1 and 625 cm - x bands are also small. A prominent band at 1680 c m - 1 attributable to the C~---O group suggests reaction with 02. It has been reported that polythiophene films generated in non-degassed electrolytic medium are susceptible to attack by oxygen. 1 In the IR absorption spectra of the poly(3methylthiophene) and poly(bithiophene) samples synthesized in air, a peak at 1680cm-x is barely discernible indicating some C~-O structures. The XPS Cl2p core-level spectra of the doped polymers (i.e. all the samples in Table 1 with the exception of PT1) indicate the presence of at least two chlorine species. The C12p core-level spectrum of the poly(bithiophene) sample synthesized with Fe(C104) 3 . 9H20 (Sample PBT1) is shown in Fig. 2. The C12P3/2 component at the binding energy of 207"3 eV corresponds to
196
199
205
214
BINDING ENERGY(eV)
Fig. 2.
XPS
Cl2p
core-level spectrum of poly(bithiophene) Fe(ClO4) a . 9H20 (Sample PBT1).
synthesized
with
Treatment of polythiophenes doped with perchlorate
43
that of C104 anion. TM The lower binding energy peak with the C12p3/2 component at 200"3eV is assigned to covalently bonded chlorine. 15 The component at the lowest binding energy (C12p3/2 peak at 198-6 eV) suggests the presence of an environment which is more electron rich relative to the neutral covalent chlorine. Since the binding energy difference between this component and covalent chlorine is 1"7 eV rather than 3 eV that is expected for ionic chlorine (C1-), 16 this component may be considered partially ionic. The C104 anion shows a high reduction potential and may undergo reactions to produce C1 - ions. 13 These C1 - ions may then, in turn, react with the polymer resulting in partially ionic and covalent chlorine. The distribution of the chlorine as partially ionic, covalent or as C10,~ species in the polymer depends on the m o n o m e r as well as the perchlorate used. With Fe(C104)a.9H20 , the ratio of partially ionic chlorine:covalent chlorine:C10,~ is about 0.12:0.19:1 in poly(bithiophene) and about 0"08:0"08:1 in poly(3-methylthiophene). With Cu(C104)2.6H20, the covalent chlorine c o m p o n e n t is very small and the partially ionic chlorine: C10£ ratio is about 0" 12:1. The higher fraction of chlorine that has formed covalent bonds with the polymer in the PBT 1 sample as compared to the PBT2 sample gives further support to the fact that lZe(C104)3 . 9H20 is a stronger oxidant for the polymerization and doping of thiophenes than the corresponding Cu salt. This is consistent with the observation that 3methylthiophene is not polymerized and doped to any significant degree with Cu(C104)2.6H20 but does so readily with Fe(CIO4) 3 . 9H20. The presence of chlorine as covalent chlorine or partially ionic chlorine species indicates that the ratio of total C1 to S will not accurately reflect the degree of oxidation of polythiophenes chemically synthesized with perchlorates. Therefore, in the present work, the C104/S ratio is calculated from the corrected XPS spectral area ratio. The C10£/S mole ratio for the doped polymers ranges from 0"12 to 0.18 (Table 1). These values are comparable to the anion to ring ratio of 1:7 reported for oxidized poly(bithiophene) films grown electrochemically from solutions containing bithiophene and a perchlorate electrolyte. 14 The electrical conductivity of the electrochemically synthesized poly(bithiophene)-perchlorate is in the range of 10 -2 to 10 -1S/cm. 14 Similar values were obtained for samples PMT1, PBT1 and PBT2, while the PBT3 sample has a higher conductivity. Conductivity values as high as 8S/cm have been reported for poly(bithiophene) synthesized with C u ( C 1 0 4 ) 2 . 6H20 under N 2 and short reaction time. 5 It has been reported that the conductivity ofpolythiophenes is more sensitive to non-a, ~' bonding than is the case of polypyrroles with the same degree of oxidation. 14 Hence the absence of 02 and the shorter reaction time during the synthesis of PBT3 may help to minimize such reactions. However, the yield after 1 h is only half of the long reaction time
44
K. G. Neoh, E. 7". Kang, K. L. Tan
(18 h) yield. In addition, the H/C ratio of sample PBT3 is substantially higher than that in the other samples. This may indicate a higher water content and/or the excess H may arise from the terminal positions of shorter polymer chains. XPS analysis of all the pristine doped samples indicates no Cu or Fe species are present. Thus, all unreacted metal perchlorate has been washed out of the samples. Treatment with solvents
The electrical conductivity of the doped poly(bithiophene) is affected by organic solvents. The treatment of the doped polymers with CHC13 and THF results in a factor of 3 decrease in a while treatment with methanol results in o"decreasing by a factor of 30 to 100. This decrease in cr is related to the decrease in C10,~/S ratio (Table 2). In Table 2, the subscript 'o' denotes the pristine sample. The C10~-/S ratio of the methanol treated samples is only about one-third of the pristine value. The H/C ratio of the treated samples also shows a slight decrease to about 0-48 and this is probably due to a decrease in the amount of water associated with the polymer (see later section). In addition to the decrease in C10,~ anions, the amount of partially ionic and covalent chlorine also decreases. The ratio of these chlorine species to S in PBT1 after methanol treatment decreases by about 25% while the partially ionic chlorine in PBT2 is almost completely removed by methanol. The IR absorption spectrum of the methanol treated PBT1 sample is shown in Fig. 3(a). Comparing this spectrum with that of the pristine sample in Fig. l(c), the treated sample still possesses the characteristic bands of doped polythiophene and the band due to C10~- anion at 625cm-1 However, in the treated sample, the intensities of these bands have decreased relative to that of the 785 c m - 1 band and the intensity of the absorption tail has also decreased. The treatment of the doped poly(bithiophene) with base resulted in a drastic loss of conductivity. From Table 2 it can be seen that NaOH has a TABLE 2 Effects of Solvents on Poly(bithiophene) Pristine sample
Solvent
(CI04/S)/(CIO•/S)o
PBT1 PBT1 PBT1 PBT2 PBT2
Methanol NaOH H2SO4 Methanol NaOH
0"33 ~0 ~ 0' 1 0"29 0"16
(CI/S)/(CI/S)o ~
0.75 0.95 0-67 ~0 ~0
a/cr°
0"01 10- ~ 10 3 0-03 10 4
The (CI/S) ratio is the ratio of covalent and partially ionic chlorine to sulfur.
-
Treatment of polythiophenes doped with perchlorate
45
HICRONS 2.5
3
6
5
B
12
20
l
l
e
e
//
J I
ooo
36oo
2 oo
,600
A
, oo
WAVENUHBER(cm-]) Fig. 3.
1R absorption spectra of(a) methanol treated PBT1, (b) NaOH treated PBTI and (c) PBT2 after heating at 150C for 24 h.
more adverse effect on the poly(bithiophene) sample synthesized with Fe(C104)3.9H20 (sample PBT1). In this sample, the drastic decrease in o- is consistent with the almost complete removal of C10£ anions. However, the total amount of partially ionic and covalent chlorine in PBT1 is not significantly affected by NaOH treatment. The partially ionic chlorine in PBT2 is completely removed by NaOH. The IR absorption spectrum of the NaOH treated PBT1 sample is shown in Fig. 3(b). The characteristic bands of doped polythiophenes between 1400 and 1000 c m - ~ are very weak in this spectrum but the 785 cm-1 band is very pronounced indicating that the polymer still consists basically of 2,5-disubstituted thiophene rings. This is confirmed by the C : H : S ratio of 1:0-49:0.24 in the NaOH treated samples. An experiment was also carried out to explore whether poly(bithiophene) undoped by N a O H can be reoxidized by mixing such a sample with either Fe(C104)3.9H20 o r C u ( C 1 0 4 ) 2 . H 2 0 in acetonitrile for 18 h. The resulting product shows no significant weight gain indicating no substantial uptake of C10£ anions and the conductivity also does not show substantial improvement.
46
K. G. Neoh, E. T. Kang, K. L. Tan
The treatment of pristine poly(bithiophene)-perchlorate with H 2 S O 4 also results in a decrease in o- (Table 2). The weight of the sample and the amount of C10£ anions and partially ionic C1 decrease also. There is no significant change in the amount of covalent Cl. The XPS S2p core-level spectrum of the H 2 S O 4 treated sample shows only a peak at 163-6 eV due to S atoms in the thiophene rings. The S2p core-level spectrum of poly(bithiophene)sulfate would show a peak at 168.4eV arising from the sulfate anions in addition to the 163"6 eV peak.14 Thus, even though poly(bithiophene) films can be electrochemically grown from a sulfuric acid-acetonitrile solution, the C10,~ anions of chemically synthesized poly(bithiophene)-perchlorate are removed in H 2 S O 4 and not replaced by SO£ anions.
Thermal degradation The TG scans of pristine poly(bithiophene)-perchlorate in N 2 generally show a small initial weight loss immediately upon heating followed by the major weight loss commencing at about 150°C. The DTA scans show that the onset of the weight loss at 150°C is an exothermic process and may be related to the decomposition of the ClOg anions. The small initial weight loss is attributed to the loss of water associated with the polymer. The TG scans of the pristine and methanol and NaOH treated samples are shown in Fig. 4. The N a O H treated sample shows the highest thermal stability followed by the methanol treated sample and the pristine sample shows the 1.0
~
-
-
-
:.~.- - - - . . . . . . . . . . . . . . . . .
q
4
0.8
,r.,,,
.F---... Q6
0.~ --
0.2
PBT 2
----
PBT 2 treated with MeOH
....
PBT 2 treated with NaOH
0.0 TEMPERATURE (°C)
Fig. 4. TG scans of pristine PBT2 sample and PBT2 samples treated with methanol and NaOH. WRT is the weight at room temperature and W is the weight at any temperature.
Treatment of polythiophenes doped with perchlorate
47
largest weight loss. Since the amount of C10~- anions is least in the NaOH treated sample and most in the pristine sample, the weight loss behavior in Fig. 4 suggests that the undoped polymer chain of 2,5-disubstituted thiophene rings is thermally quite stable up to at least 400°C and the decomposition of the doped polymer is initiated by the decomposition of the CIO£ anions. The initial weight loss ( T < 100°C) attributable to water is much reduced in the methanol and NaOH treated samples. This decrease in water is consistent with the decrease in the H/C ratio of these samples and suggests that the water molecules are associated with the C10£ anions. The effect of heat on the o- of the polymer is shown in Fig. 5. When heated from room temperature to 150°C, the pristine PBT2 sample shows a continuous increase in o- while the a of pristine PBT1 reaches a maximum at about 135°C. Thus, the conduction process in the poly(bithiophene) samples is thermally activated. When the sample is cooled to room temperature in a desiccator, it still retains about 70% to 85% of the initial conductivity. In contrast, o- of electrochemically synthesized polythiophene-perchlorate decreases by two orders of magnitude upon heating to 150°C in argon and cooling to room temperature. 8 However, this comparison has to be interpreted with caution since the heating and cooling rates of the two experiments may not be identical. From the TG scan (Fig. 4) it can be expected that the o- of the polymer will be drastically affected if it is heated above 150°C or maintained at 150°C for extended periods of the time since the polymer starts to decompose at this temperature. When the polymer is maintained at 150°C for 24 h, its o- upon cooling to room temperature is only 10- 5 S/cm. The IR absorption spectrum of the heat treated sample (Fig. 3(c)) indicates a decrease in the intensity of the characteristic bands of doped
///
2
%o &
After f PBT1-,,.o cooling to 30oc
PBT2/e
TEMPERATURE(°C)
Fig. 5.
Effect of temperature on electrical conductivity, a o is the initial conductivity.
K. G. Neoh, E. T. Kang, K. L. Tan
48
196
199
202
205
206
211
2B
BINDINGENERGY(eVl Fig. 6. XPS CI2p core-level spectrum of heat-treated PBT2.
polythiophene and the 625 cm-1 band due to C10,~ anion. The 785 c m - 1 band remains strong indicating that the polymer backbone is intact. In the IR absorption spectra of the NaOH treated and heat treated samples (Fig. 3(b), (c)), the absorption band in the 1600-1700 c m - 1 region can be seen more clearly than in the spectrum of the pristine sample. This is due to the reduction in the intensity of the absorption tail and/or an increase in C = O type structures in the treated samples. The XPS data show that upon heat treatment, the C10,~ is partially converted to covalent chlorine. The C12p core-level spectrum of the heat treated PBT2 sample is shown in Fig. 6. The covalent chlorine component (C12p3/2 signal at 200.3eV) is larger than the CIO,~ anion component (C12p3/2 signal at 207.4 eV). In pristine PBT2, there is no ,significant amount of covalent chlorine (binding energy of 200"3 eV) but the partially ionic chlorine (binding energy of 199 eV) species accounts for about 10% of the total chlorine. The total CI/S ratio in the heated sample is about 0.08 compared to 0.20 in the pristine sample. However, about 60% of the total C1 in the heated sample is now covalently bonded to the polymer. Hence prolonged heat treatment of poly(bithiophene)-perchlorate at 150°C has resulted in the loss of C10~- anions through conversion to C1 which is covalently bonded to the polymer and to volatile species which are lost from the polymer. This is consistent with the TG scan which shows the onset of significant weight loss at 150°C.
CONCLUSION XPS analysis of the polythiophenes synthesized with perchlorate indicates that, depending on the perchlorate used, a significant amount of the chlorine may be covalently bonded to the polymer. Therefore, the total C1/S ratio does not give an accurate indication of the degree of oxidation of the
Treatment of polythiophenes doped with perchlorate
49
polymer. A substantial portion of the C102 anions can be removed by methanol, acid and base. The undoped polymer cannot be reoxidized in an acetonitrile-perchlorate solution. Thermogravimetric analysis indicates that although the undoped polymer has a high degree of thermal stability, the doped polymer starts to decompose at about 150°C. Prolonged heating of the doped polymer at 150°C results in the conversion of the C102 anions to C1 covalently bonded to the polymer and to volatile species which are lost from the polymer. REFERENCES 1. Tourillon, G. &Garnier, F., J. Electroanal. Chem., 135 (1982) 173. 2. Yamamoto, T., Sanechika, K. & Yamamoto, A., J. Polym. Sci. Polvm. Chem. Ed., 18 (1980) 9. 3. Kobayashi, M., Chen, J., Chung, T. C., Moraes, F., Heeger, A. J. & Wudl, F., Synth. Met., 9 (1984) 77. 4. Hotta, S., Soga, M. & Sonoda, N., Synth. Met., 26 (1988) 267. 5. Inoue, M. B., Velazquez, E. F. & Inoue, M., Synth. Met., 24 (1988) 223. 6. Munstedt, H., Polymer, 29 (1988) 296. 7. Billingham, N. C., Calvert, P. D., Foot, P. J. S. & Mohammad, F., Poly. Deg. and Stab., 19 (1987) 323. 8. Osterholm, J. E. & Passiniemi, Synth. Met., 18 (1987) 213. 9. Gustafsson, G., Inganas, O., Nilsson, J. O. & Liedberg, B., Synth. Met., 26 (1988) 297. 10. Milton, R. F. & Waters, W. A. (eds), Methods of Quantitative Micro-Analysis, Edward Arnold & Co., 1949, London. 11. Dong, S. & Zhang, W., Synth. Met., 30 (1989) 359. 12. Yamamoto, T., Sanechika. & Yamamoto, A., Bull. Chem. Soc. Jpn, 56 (1983) 1497. 13. Weast, R. C. & Astle, M. J. (eds). CRC Handbook of Chemistry andPhysics (59th edn), CRC Press, 1979, Florida. 14. Pfluger, P. & Street, G. B., J. Chem. Phys., 80 (1984) 544. 15. Brundle, C. R. & Baker, A. D. (eds), Electron Spectroscopy: Theory, Techniques and Applications, Academic Press, 1981, London. 16. Tan, K. L., Tan, B. T. G., Kang, E. T. & Neoh, K. G., Phys. Rev. B, 39 (1989) 8070.
Structural Changes Associated with Thermal and Chemical Treatment of Polythiophenes Doped with Perchlorate K. G. Neoh," E. T. Kang a & K. L. Tan b Department of Chemical Engineering, h Department of Physics, National University of Singapore, Kent Ridge, Singapore 0511 (Received l0 January 1990; accepted 25 January 1990)
ABSTRACT Electrically conductive polymers have been synthesized by the reaction o.t" 3-methylthiophene or 2,2'-bithiophene with ferric perchlorate. With copper perchlorate, only 2,2'-bithiophene undergoes simultaneous polymerization and oxidation. Neither ferric perchlorate nor copper perchlorate reacts with thiophene to yield a conducting polymer. X-ray photoelectron spectroscopy of" the perchlorate doped polythiophenes indicates that, depending on the perchlorate used, a significant amount of the chlorine may be covalentlv bonded to the polymer. Treatment of the doped samples with methanol results in the ClOg /S ratio decreasing to one-third of the pristine value. Treatment with NaOH results in almost complete removal of the Cl04 anions in poly( bithiophene ) synthesized with Fe( Cl0 4 ) 3 . 9 H20. Concurrent with the loss of ClOg anions is the decrease in electrical conductivity. The undoped polymer has a high degree of thermal stability while the decomposition of the doped polymer appears to be initiated by the decomposition Of the Cl04 anions. Prolonged heating of the doped samples at 150°C results in the conversion of ClOg anions to Cl covalent O, bonded to the poO,mer and to volatile species which are lost .from the polymer.
INTRODUCTION Polythiophene belongs to the family of electrically conducting polymers which have attracted a great deal o f attention in recent years because of their 37 Polymer Degradation and Stability 0141-3910/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
38
K. G. Neoh, E. T. Kang, K. L. Tan
unusual electronic properties. Highly conductive polythiophene can be prepared electrochemically by coupling of the monomers in the 2- and 5positions in an electrolytic medium such as acetonitrile with N(Bu)4C104.1 Polythiophene and its derivatives can also be synthesized chemically by Grignard coupling reactions, 2'3 or by dehydrohalogenation of 2halogenothiophene (or its 3-alkyl derivatives) or by dehydrogenation of thiophene (or its 3-alkyl derivatives) in the presence of metal halogenide.4 The polymer can then be doped with oxidants such as I2. Recently, a onestep simultaneous polymerization and doping of bithiophene with copper perchlorate has been reported. 5 In general, the polyheterocycles such as polypyrrole and polythiophene exhibit higher environmental stability than polyacetylenes. Degradation studies of electrochemically synthesized polythiophene-BF4 and polythiophene-C104 in air have shown that the conductivity decreases substantially upon heating to above 70°C and the rate of decrease is dependent on the counter-ion. 6n Electrochemically synthesized polythiophene doped with FeC13 has been reported to show higher thermal stability than polythiophene-BF4 and polythiophene-C104, a However, FeCla-doped poly(3-alkylthiophene) with long side chains may undergo rapid thermal undoping due to the conversion of the anions to FeC12.9 The chemical method for the simultaneous synthesis and doping of polythiophene by the use of perchlorate as oxidant 5 has the advantage of being a simple one-step process. In the present work, the stability of the products from such a process is assessed. The structural changes in the doped polymer brought about by heating and treatment with base, acid and organic solvents were studied using elemental analysis, infrared (IR) absorption spectroscopy and X-ray photoelectron spectroscopy (XPS).
EXPERIMENTAL
Synthesis and treatment of polymers Polymer samples were prepared by adding the monomer to a solution of metal perchlorate in acetonitrile. The metal perchlorates used were Fe(C104) 3 . 9H20 and Cu(C104) 2 . 6H20, and the perchlorate to monomer ratio was kept at 3:1. The monomers tested were thiophene, 3methylthiophene and 2,2'-bithiophene. Since bithiophene is a solid at room temperature, it was dissolved in acetonitrile prior to reaction with the metal perchlorate. Depending on the monomer used, the reaction time was 1 h, 24 h or 48 h. The reaction products were washed with copious amounts of acetonitrile and then dried by pumping under reduced pressure.
Treatment of polythiophenes doped with perchlorate
39
The stability of the polymer samples at elevated temperatures and in organic solvents, H z S O 4 and N a O H was investigated. A Netzsch STA409 simultaneous thermogravimetric (TG)-differential thermal analyzer (DTA) unit was used to monitor the degradation of the polymer samples. A heating rate of 10°C/rain and a N 2 flowrate of 100 ml/min were used. The effect of elevated temperatures on the electrical conductivity (a) of the polymer samples was assessed by slowly heating (,,~0-5°C/min) the samples in air to 150°C and simultaneously monitoring a. Some samples were also kept at 150°C for 24h. To assess the stability of the polymer samples in common organic solvents, NaOH and H2SO 4 solutions, the samples were mixed thoroughly with the respective solvent or solution and left for 18-20 h. The acid and base concentrations were 1"0 and 0"5M, respectively. After the treatment, the sample was washed either with deionized water (in the case of acid or base treatment) or with the respective solvent and dried under reduced pressure.
Polymer characterization After thermal or chemical treatment, the electrical conductivity, structure and composition of the samples were compared with those of the pristine sample. The electrical conductivity was measured using the standard collinear four-probe or two-probe technique on compressed pellets. The carbon and hydrogen contents of the samples were determined with a Perkin-Elmer 240C Elemental Analyzer. The sulfur content was determined using the combustion method? ° IR absorption spectroscopy was performed using a Perkin-Elmer model 682 spectrophotometer with the samples dispersed in KBr pellets, and X-ray photoelectron spectroscopy (XPS) measurements were made on a VG Scientific ESCALAB Mk II spectrometer with a MgK~ X-ray source (1253.6eV photons). In the XPS experiment, the polymer samples in powder form were mounted on the standard sample studs by using double-sided Scotch tape. All core-level spectra were referenced to the Cls neutral carbon peak at 284.6 eV. The peak area ratios for the various elements were corrected using experimentally determined instrumental sensitivity factors.
RESULTS A N D DISCUSSION
Pristine polymer samples With Cu(C104)2.6H20 as oxidant, only bithiophene undergoes simultaneous polymerization and doping. No significant amount of solid product
TABLE 1
Thiophene 3-Methylthiophene Bithiophene Bithiophene Bithiophene
Monomer
Fe(C104)3.9HzO, Fe(CIO4)3.9H20, Fe(CIOa)3.9H20, Cu(CIOa)2.6H20, Cu(CIOJ/.6HEO,
48 h 18 h 18 h 18 h 1h
Synthesis conditions
a Based on corrected S2p and C12p component spectral area ratios.
PT1 PMT1 PBT1 PBT2 PBT3
Sample
0"08 0-65 1'12 1'11 0"53
wt. Product wt. Monomer
0"54 0'90 0"50 0-56 0.65
H/C
0'24 0-20 0.25 0'25 0-25
SIC
-0'16 0"12 0-18 0'16
ClOg/S a
Elemental composition
< 10 7 0"05 0'13 0"2 1
~rS/cm
Conductivity
Yield, Chemical Composition and Electrical Conductivity (tr) of Products from Reaction of Thiophenes and Metal Perchlorates
Treatment of polythiophenes doped with perchlorate
41
was obtained with either thiophene or 3-methylthiophene over a reaction time of 24 h to 48 h. However, with Fe(C104)3.9H20, both 3-methylthiophene and bithiophene undergo simultaneous polymerization and doping although the yield from the latter monomer was substantially higher. With thiophene, only a very small amount of dark brown solid was obtained. The yield, chemical composition and conductivity of the various reaction products are summarized in Table 1. With the exception of PBT3 which was polymerized in a N 2 atmosphere, the other polymerization experiments were carried out in air. The H/C ratio of the products is generally higher than that expected for a chain of thiophene (or 3-methylthiophene) rings coupled in the ~,d positions. This may be due to the presence of water associated with the CIO~ anions (see below). The S/C ratio is very close to the ideal value. The IR absorption spectra of the solid products from reaction of thiophene, 3-methylthiophene and bithiophene with Fe(C104)3.9H20 (Samples PT1, PMT1 and PBT1) are shown in Fig. 1. The spectra of the two latter samples show a long absorption tail from 4000 to about 1500cm 1 This absorption tail obscures the C-H stretching band at around 3000 cm 1 The IR absorption spectrum of PBT1 (Fig. l(c)) is similar to that of electrochemically synthesized polythiophene with four strong bands at WICRONS 2.5
3
G
i
i
6
B
12
20
jJ,i "/
I
6000
Fig. 1.
3000
I
i
I
1600 1200 WAVELENGTH (cm q ) 2000
I
BOO
400
IR absorption spectra of products from reaction of (a) thiophene, (b) 3methylthiophene and (c) bithiophene with Fe(C104)3.9H20.
K. G. Neoh, E. T. Kang, K. L. Tan
42
1340, 1200, 1115 and 1030cm -1 which are characteristic of doped polythiophene. 11 The band at 785cm-1 is attributed to the out-of-plane vibration of C-H at the fl-position of 2,5-disubstituted thiophene. 11 The bands due to the perchlorate ion are at ll00 and 625 cm-1 although the former band is masked by the doped thiophene band. The IR absorption spectrum of PMT1 (Fig. l(b)) shows similarities with that of electrochemically prepared poly(3-alkylthiophene) with strong bands at 1390, 1300, 1140-1070cm- 1 and 830cm-1.9 The last band has been assigned to the C-H out-of-plane vibration of the 2,3,5-trisubstituted thiophene.12 The C10£ ion band at 625 cm-1 is also prominent in this spectrum. The IR absorption spectra provide evidence that the product from the reaction between 3-methylthiophene and Fe(C104)3.9H20 is a doped polymer. This is consistent with the fact that Fe(CIO4) 3 . 9H20 is a stronger oxidant than Cu(C104)2.6H20.13 However, with thiophene monomer, no significant amount of doped polymer can be obtained even with Fe(C104) 3 . 9H20 over a period of 48 h. The IR absorption spectra of PT1 (Fig. l(a)) showed no strong characteristic bands of doped polythiophene and the 785 c m - 1 and 625 cm - x bands are also small. A prominent band at 1680 c m - 1 attributable to the C~---O group suggests reaction with 02. It has been reported that polythiophene films generated in non-degassed electrolytic medium are susceptible to attack by oxygen. 1 In the IR absorption spectra of the poly(3methylthiophene) and poly(bithiophene) samples synthesized in air, a peak at 1680cm-x is barely discernible indicating some C~-O structures. The XPS Cl2p core-level spectra of the doped polymers (i.e. all the samples in Table 1 with the exception of PT1) indicate the presence of at least two chlorine species. The C12p core-level spectrum of the poly(bithiophene) sample synthesized with Fe(C104) 3 . 9H20 (Sample PBT1) is shown in Fig. 2. The C12P3/2 component at the binding energy of 207"3 eV corresponds to
196
199
205
214
BINDING ENERGY(eV)
Fig. 2.
XPS
Cl2p
core-level spectrum of poly(bithiophene) Fe(ClO4) a . 9H20 (Sample PBT1).
synthesized
with
Treatment of polythiophenes doped with perchlorate
43
that of C104 anion. TM The lower binding energy peak with the C12p3/2 component at 200"3eV is assigned to covalently bonded chlorine. 15 The component at the lowest binding energy (C12p3/2 peak at 198-6 eV) suggests the presence of an environment which is more electron rich relative to the neutral covalent chlorine. Since the binding energy difference between this component and covalent chlorine is 1"7 eV rather than 3 eV that is expected for ionic chlorine (C1-), 16 this component may be considered partially ionic. The C104 anion shows a high reduction potential and may undergo reactions to produce C1 - ions. 13 These C1 - ions may then, in turn, react with the polymer resulting in partially ionic and covalent chlorine. The distribution of the chlorine as partially ionic, covalent or as C10,~ species in the polymer depends on the m o n o m e r as well as the perchlorate used. With Fe(C104)a.9H20 , the ratio of partially ionic chlorine:covalent chlorine:C10,~ is about 0.12:0.19:1 in poly(bithiophene) and about 0"08:0"08:1 in poly(3-methylthiophene). With Cu(C104)2.6H20, the covalent chlorine c o m p o n e n t is very small and the partially ionic chlorine: C10£ ratio is about 0" 12:1. The higher fraction of chlorine that has formed covalent bonds with the polymer in the PBT 1 sample as compared to the PBT2 sample gives further support to the fact that lZe(C104)3 . 9H20 is a stronger oxidant for the polymerization and doping of thiophenes than the corresponding Cu salt. This is consistent with the observation that 3methylthiophene is not polymerized and doped to any significant degree with Cu(C104)2.6H20 but does so readily with Fe(CIO4) 3 . 9H20. The presence of chlorine as covalent chlorine or partially ionic chlorine species indicates that the ratio of total C1 to S will not accurately reflect the degree of oxidation of polythiophenes chemically synthesized with perchlorates. Therefore, in the present work, the C104/S ratio is calculated from the corrected XPS spectral area ratio. The C10£/S mole ratio for the doped polymers ranges from 0"12 to 0.18 (Table 1). These values are comparable to the anion to ring ratio of 1:7 reported for oxidized poly(bithiophene) films grown electrochemically from solutions containing bithiophene and a perchlorate electrolyte. 14 The electrical conductivity of the electrochemically synthesized poly(bithiophene)-perchlorate is in the range of 10 -2 to 10 -1S/cm. 14 Similar values were obtained for samples PMT1, PBT1 and PBT2, while the PBT3 sample has a higher conductivity. Conductivity values as high as 8S/cm have been reported for poly(bithiophene) synthesized with C u ( C 1 0 4 ) 2 . 6H20 under N 2 and short reaction time. 5 It has been reported that the conductivity ofpolythiophenes is more sensitive to non-a, ~' bonding than is the case of polypyrroles with the same degree of oxidation. 14 Hence the absence of 02 and the shorter reaction time during the synthesis of PBT3 may help to minimize such reactions. However, the yield after 1 h is only half of the long reaction time
44
K. G. Neoh, E. 7". Kang, K. L. Tan
(18 h) yield. In addition, the H/C ratio of sample PBT3 is substantially higher than that in the other samples. This may indicate a higher water content and/or the excess H may arise from the terminal positions of shorter polymer chains. XPS analysis of all the pristine doped samples indicates no Cu or Fe species are present. Thus, all unreacted metal perchlorate has been washed out of the samples. Treatment with solvents
The electrical conductivity of the doped poly(bithiophene) is affected by organic solvents. The treatment of the doped polymers with CHC13 and THF results in a factor of 3 decrease in a while treatment with methanol results in o"decreasing by a factor of 30 to 100. This decrease in cr is related to the decrease in C10,~/S ratio (Table 2). In Table 2, the subscript 'o' denotes the pristine sample. The C10~-/S ratio of the methanol treated samples is only about one-third of the pristine value. The H/C ratio of the treated samples also shows a slight decrease to about 0-48 and this is probably due to a decrease in the amount of water associated with the polymer (see later section). In addition to the decrease in C10,~ anions, the amount of partially ionic and covalent chlorine also decreases. The ratio of these chlorine species to S in PBT1 after methanol treatment decreases by about 25% while the partially ionic chlorine in PBT2 is almost completely removed by methanol. The IR absorption spectrum of the methanol treated PBT1 sample is shown in Fig. 3(a). Comparing this spectrum with that of the pristine sample in Fig. l(c), the treated sample still possesses the characteristic bands of doped polythiophene and the band due to C10~- anion at 625cm-1 However, in the treated sample, the intensities of these bands have decreased relative to that of the 785 c m - 1 band and the intensity of the absorption tail has also decreased. The treatment of the doped poly(bithiophene) with base resulted in a drastic loss of conductivity. From Table 2 it can be seen that NaOH has a TABLE 2 Effects of Solvents on Poly(bithiophene) Pristine sample
Solvent
(CI04/S)/(CIO•/S)o
PBT1 PBT1 PBT1 PBT2 PBT2
Methanol NaOH H2SO4 Methanol NaOH
0"33 ~0 ~ 0' 1 0"29 0"16
(CI/S)/(CI/S)o ~
0.75 0.95 0-67 ~0 ~0
a/cr°
0"01 10- ~ 10 3 0-03 10 4
The (CI/S) ratio is the ratio of covalent and partially ionic chlorine to sulfur.
-
Treatment of polythiophenes doped with perchlorate
45
HICRONS 2.5
3
6
5
B
12
20
l
l
e
e
//
J I
ooo
36oo
2 oo
,600
A
, oo
WAVENUHBER(cm-]) Fig. 3.
1R absorption spectra of(a) methanol treated PBT1, (b) NaOH treated PBTI and (c) PBT2 after heating at 150C for 24 h.
more adverse effect on the poly(bithiophene) sample synthesized with Fe(C104)3.9H20 (sample PBT1). In this sample, the drastic decrease in o- is consistent with the almost complete removal of C10£ anions. However, the total amount of partially ionic and covalent chlorine in PBT1 is not significantly affected by NaOH treatment. The partially ionic chlorine in PBT2 is completely removed by NaOH. The IR absorption spectrum of the NaOH treated PBT1 sample is shown in Fig. 3(b). The characteristic bands of doped polythiophenes between 1400 and 1000 c m - ~ are very weak in this spectrum but the 785 cm-1 band is very pronounced indicating that the polymer still consists basically of 2,5-disubstituted thiophene rings. This is confirmed by the C : H : S ratio of 1:0-49:0.24 in the NaOH treated samples. An experiment was also carried out to explore whether poly(bithiophene) undoped by N a O H can be reoxidized by mixing such a sample with either Fe(C104)3.9H20 o r C u ( C 1 0 4 ) 2 . H 2 0 in acetonitrile for 18 h. The resulting product shows no significant weight gain indicating no substantial uptake of C10£ anions and the conductivity also does not show substantial improvement.
46
K. G. Neoh, E. T. Kang, K. L. Tan
The treatment of pristine poly(bithiophene)-perchlorate with H 2 S O 4 also results in a decrease in o- (Table 2). The weight of the sample and the amount of C10£ anions and partially ionic C1 decrease also. There is no significant change in the amount of covalent Cl. The XPS S2p core-level spectrum of the H 2 S O 4 treated sample shows only a peak at 163-6 eV due to S atoms in the thiophene rings. The S2p core-level spectrum of poly(bithiophene)sulfate would show a peak at 168.4eV arising from the sulfate anions in addition to the 163"6 eV peak.14 Thus, even though poly(bithiophene) films can be electrochemically grown from a sulfuric acid-acetonitrile solution, the C10,~ anions of chemically synthesized poly(bithiophene)-perchlorate are removed in H 2 S O 4 and not replaced by SO£ anions.
Thermal degradation The TG scans of pristine poly(bithiophene)-perchlorate in N 2 generally show a small initial weight loss immediately upon heating followed by the major weight loss commencing at about 150°C. The DTA scans show that the onset of the weight loss at 150°C is an exothermic process and may be related to the decomposition of the ClOg anions. The small initial weight loss is attributed to the loss of water associated with the polymer. The TG scans of the pristine and methanol and NaOH treated samples are shown in Fig. 4. The N a O H treated sample shows the highest thermal stability followed by the methanol treated sample and the pristine sample shows the 1.0
~
-
-
-
:.~.- - - - . . . . . . . . . . . . . . . . .
q
4
0.8
,r.,,,
.F---... Q6
0.~ --
0.2
PBT 2
----
PBT 2 treated with MeOH
....
PBT 2 treated with NaOH
0.0 TEMPERATURE (°C)
Fig. 4. TG scans of pristine PBT2 sample and PBT2 samples treated with methanol and NaOH. WRT is the weight at room temperature and W is the weight at any temperature.
Treatment of polythiophenes doped with perchlorate
47
largest weight loss. Since the amount of C10~- anions is least in the NaOH treated sample and most in the pristine sample, the weight loss behavior in Fig. 4 suggests that the undoped polymer chain of 2,5-disubstituted thiophene rings is thermally quite stable up to at least 400°C and the decomposition of the doped polymer is initiated by the decomposition of the CIO£ anions. The initial weight loss ( T < 100°C) attributable to water is much reduced in the methanol and NaOH treated samples. This decrease in water is consistent with the decrease in the H/C ratio of these samples and suggests that the water molecules are associated with the C10£ anions. The effect of heat on the o- of the polymer is shown in Fig. 5. When heated from room temperature to 150°C, the pristine PBT2 sample shows a continuous increase in o- while the a of pristine PBT1 reaches a maximum at about 135°C. Thus, the conduction process in the poly(bithiophene) samples is thermally activated. When the sample is cooled to room temperature in a desiccator, it still retains about 70% to 85% of the initial conductivity. In contrast, o- of electrochemically synthesized polythiophene-perchlorate decreases by two orders of magnitude upon heating to 150°C in argon and cooling to room temperature. 8 However, this comparison has to be interpreted with caution since the heating and cooling rates of the two experiments may not be identical. From the TG scan (Fig. 4) it can be expected that the o- of the polymer will be drastically affected if it is heated above 150°C or maintained at 150°C for extended periods of the time since the polymer starts to decompose at this temperature. When the polymer is maintained at 150°C for 24 h, its o- upon cooling to room temperature is only 10- 5 S/cm. The IR absorption spectrum of the heat treated sample (Fig. 3(c)) indicates a decrease in the intensity of the characteristic bands of doped
///
2
%o &
After f PBT1-,,.o cooling to 30oc
PBT2/e
TEMPERATURE(°C)
Fig. 5.
Effect of temperature on electrical conductivity, a o is the initial conductivity.
K. G. Neoh, E. T. Kang, K. L. Tan
48
196
199
202
205
206
211
2B
BINDINGENERGY(eVl Fig. 6. XPS CI2p core-level spectrum of heat-treated PBT2.
polythiophene and the 625 cm-1 band due to C10,~ anion. The 785 c m - 1 band remains strong indicating that the polymer backbone is intact. In the IR absorption spectra of the NaOH treated and heat treated samples (Fig. 3(b), (c)), the absorption band in the 1600-1700 c m - 1 region can be seen more clearly than in the spectrum of the pristine sample. This is due to the reduction in the intensity of the absorption tail and/or an increase in C = O type structures in the treated samples. The XPS data show that upon heat treatment, the C10,~ is partially converted to covalent chlorine. The C12p core-level spectrum of the heat treated PBT2 sample is shown in Fig. 6. The covalent chlorine component (C12p3/2 signal at 200.3eV) is larger than the CIO,~ anion component (C12p3/2 signal at 207.4 eV). In pristine PBT2, there is no ,significant amount of covalent chlorine (binding energy of 200"3 eV) but the partially ionic chlorine (binding energy of 199 eV) species accounts for about 10% of the total chlorine. The total CI/S ratio in the heated sample is about 0.08 compared to 0.20 in the pristine sample. However, about 60% of the total C1 in the heated sample is now covalently bonded to the polymer. Hence prolonged heat treatment of poly(bithiophene)-perchlorate at 150°C has resulted in the loss of C10~- anions through conversion to C1 which is covalently bonded to the polymer and to volatile species which are lost from the polymer. This is consistent with the TG scan which shows the onset of significant weight loss at 150°C.
CONCLUSION XPS analysis of the polythiophenes synthesized with perchlorate indicates that, depending on the perchlorate used, a significant amount of the chlorine may be covalently bonded to the polymer. Therefore, the total C1/S ratio does not give an accurate indication of the degree of oxidation of the
Treatment of polythiophenes doped with perchlorate
49
polymer. A substantial portion of the C102 anions can be removed by methanol, acid and base. The undoped polymer cannot be reoxidized in an acetonitrile-perchlorate solution. Thermogravimetric analysis indicates that although the undoped polymer has a high degree of thermal stability, the doped polymer starts to decompose at about 150°C. Prolonged heating of the doped polymer at 150°C results in the conversion of the C102 anions to C1 covalently bonded to the polymer and to volatile species which are lost from the polymer. REFERENCES 1. Tourillon, G. &Garnier, F., J. Electroanal. Chem., 135 (1982) 173. 2. Yamamoto, T., Sanechika, K. & Yamamoto, A., J. Polym. Sci. Polvm. Chem. Ed., 18 (1980) 9. 3. Kobayashi, M., Chen, J., Chung, T. C., Moraes, F., Heeger, A. J. & Wudl, F., Synth. Met., 9 (1984) 77. 4. Hotta, S., Soga, M. & Sonoda, N., Synth. Met., 26 (1988) 267. 5. Inoue, M. B., Velazquez, E. F. & Inoue, M., Synth. Met., 24 (1988) 223. 6. Munstedt, H., Polymer, 29 (1988) 296. 7. Billingham, N. C., Calvert, P. D., Foot, P. J. S. & Mohammad, F., Poly. Deg. and Stab., 19 (1987) 323. 8. Osterholm, J. E. & Passiniemi, Synth. Met., 18 (1987) 213. 9. Gustafsson, G., Inganas, O., Nilsson, J. O. & Liedberg, B., Synth. Met., 26 (1988) 297. 10. Milton, R. F. & Waters, W. A. (eds), Methods of Quantitative Micro-Analysis, Edward Arnold & Co., 1949, London. 11. Dong, S. & Zhang, W., Synth. Met., 30 (1989) 359. 12. Yamamoto, T., Sanechika. & Yamamoto, A., Bull. Chem. Soc. Jpn, 56 (1983) 1497. 13. Weast, R. C. & Astle, M. J. (eds). CRC Handbook of Chemistry andPhysics (59th edn), CRC Press, 1979, Florida. 14. Pfluger, P. & Street, G. B., J. Chem. Phys., 80 (1984) 544. 15. Brundle, C. R. & Baker, A. D. (eds), Electron Spectroscopy: Theory, Techniques and Applications, Academic Press, 1981, London. 16. Tan, K. L., Tan, B. T. G., Kang, E. T. & Neoh, K. G., Phys. Rev. B, 39 (1989) 8070.