Long-term conductivity decrease in polyacetylene samples

Synthetic Metals, 10 (1985) 273 - 280

273

LONG-TERM CONDUCTIVITY DECREASE IN POLYACETYLENE SAMPLES H. K. MOLLER and J. HOCKER

Bayer AG, D-5090 Leverkusen, Bayerwerk (F.R.G.) K. MENKE*, K. EHINGER and S. ROTH

Max-Planck-lnstitut fiir Festk6rperforschung, D-7000 Stuttgart 80 (F.R.G.) (Received and accepted October 8, 1984)

Abstract Investigations of the conductivity decrease of polyacetylene samples doped with iodine, AsFs and FeCls, respectively, and stored under various conditions (laboratory atmosphere, dry and humid air, vacuum, argon, argon at --30 °C) are reported. In laboratory atmosphere the conductivity drops by several orders of magnitude within a few weeks in samples prepared by the conventional Shirakawa technique. In argon atmosphere at --30 °C, after an initial drop by a factor of about two within a few days, the samples are very stable. With the exception of iodine doping, highlydoped samples are less stable in laboratory atmosphere or humid air than moderately-doped ones. Samples prepared by a new method from a polyacetylene suspension turn out to be much more stable than Shirakawa samples.

1. Introduction Polyacetylene [1 ] has attracted considerable attention as a prototype of polymers with metal-like electrical conductivity. From the point of view of basic research, the possibility of soliton-like excitations in polyene chains has led to extensive discussions. Many applications of this novel material have been proposed, including solar cells, batteries, electrochemical electrodes and even molecular computers. It is hoped that finally doped polyacetylene and other conjugated polymers might combine the mechanical and chemical properties of polymers with the electrical conductivity of metals. At present, however, one of the major drawbacks of these materials is their instability against environmental influences. *Present address: Battelle-Institut e.V., Am RSmerhof 35, D-6000 Frankfurt/Main 90, F.R.G. 0379-6779/85/$3.30

© Elsevier Sequoia/Printed in The Netherlands

274 This paper presents a systematic investigation of the long-term degradation of doped polyacetylene stored under various conditions. Our results will be useful for the technical application of conductive polymers. Furthermore, they are intended to help experimentalists, who plan to measure various properties of the same sample and have to decide how much time they may allow to elapse between two measurements. In addition, polyacetylene films produced by a new method, a suspension technique, are investigated and turn out to be more stable than conventional Shirakawa-type samples.

2. Experimental Polyacetylene has been synthesized by two different methods: (a) Employing the conventional Shirakawa technique [2], keeping the catalyst solution at --80 °C and using the films grown on the liquid-gas interface, which are predominantly in the cis isomeric state. In some cases conversion into trans-polyacetylene has been carried out by thermal annealing for 45 min at 190 °C ('Shirakawa samples'). (b) Using Al(alk)3/VOCl(neopentyl)2 or Al(alk)3/Ti(OC4H9) 4 as a Ziegler-Natta catalyst system at --60 °C, stirring the catalyst solution and adjusting the concentrations in such a way that a suspension of particles from 0.01 to 5 mm size is obtained, filtering the suspension, washing and peeling off the polymer film [3]. All samples of this type were doped witho u t thermal annealing beforehand ('Suspension samples'). Iodine and AsFs doping were carried out by exposing the samples to iodine or AsFs vapour, respectively, at room temperature and subsequent pumping for several hours. In the case of AsFs doping the 'slow doping procedure' has been applied using gas bursts interspersed with pumping cycles to ensure a fairly homogeneous distribution of the dopant. For FeC13 doping a solution of FeC13 in acetonitrile or nitromethane was used [4]. The doping level was controlled via the FeC13 concentration in the solution. The achieved doping concentration was determined by weightuptake and in some cases checked by elemental analysis. Every doped film was cut into four strips, 2 cm long, 0.5 cm wide and 0.1 mm thick. Four-probe contacts (gold plated stainless steel or painted graphite) were applied to the samples to enable conductivity measurements to be carried out. One set of samples was kept under ordinary laboratory conditions (designated 'air'). The other three sets were sealed in individual glass tubes, which were evacuated ('vacuum') or filled with argon. One argon set was kept at room temperature ('At'), the other at --30 °C in a refrigerator ('At --30 °C'). Additional samples were kept in a room-temperature dry box ('air dry') or in a room-temperature box with air of 50% constant humidity ('air hum.'). D.c. conductivity readings were taken every day, the 'At --30 °C' samples being allowed to warm up to room temperature before reading.

275

3. Results

Figure 1 shows the room temperature conductivity o0 of freshly prepared samples as a function of AsFs, iodine and FeC13 concentrations, respectively [5]. The concentrations are given in molar percentages referred to a CH-unit. 10 3

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Fig. 1. Initial conductivity vs. doping concentration for freshly prepared Shirakawa samples (cis isomeric state before doping). All measurements were carried out at room temperature.

The typical degradation behaviour is shown in Fig. 2 (heavy iodine doping) and Fig. 3 (moderate AsFs doping). These results are consistent with the observations of other authors [6]. In laboratory environment (air) the conductivity drops by one order of magnitude within two weeks and by as 100

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Fig. 2. Typical conductivity decrease under various storage conditions of Shirakawa polyacetylene heavily doped with iodine. Fig. 3. Typical conductivity decrease under various storage conditions of Shirakawa polyacetylene moderately doped with AsF s.

276 much as a factor of two within a day. As seen from the Figure, argon gas is a better protection than vacuum, but reasonable stability is only obtained when the samples are cooled in a refrigerator. Technical applications will probably have to be based on the saturation value of the conductivity, which at room temperature storage seems to be achieved after several weeks. In Fig. 4 the degradation of FeCl3
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Fig. 7. Relative change in conductivity, o30/o0, during 30 days of storage under various conditions for iodine-doped Shirakawa polyacetylene as a function of doping concentration (samples were left in the c/s isomeric state before doping).

Figure 8 presents a similar compilation of degradation data for FeCl3doped Shirakawa samples after 50 days. Since the doping concentration has not been determined chemically for each sample, we have plotted the data versus the initial conductivity o0 instead. A rough estimate of the respective FeC13 concentration can be obtained using Fig. 1. In dry air these samples show a better stability at higher doping levels (right-hand side of the Figure). In humid air, however, the stability becomes worse at higher doping levels. Hardly any degradation is observed for samples stored at --30 °C, whereas the data of highly-doped samples stored in vacuum or in argon scatter considerably and in some cases a drop of two orders of magnitude is found. In Fig. 9 we compare the degradation of undoped and FeC13-doped Shirakawa and suspension samples after storage in dry air for 50 days. The higher stability of suspension samples is clearly seen, especially at low doping concentrations (low initial conductivity o0, left-hand side of the Figure).

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The importance of the storage temperature is seen from Fig. 10 for the case of iodine
4. Conclusions The degradation during storage of polyacetylene samples, undoped and doped with iodine, AsF s and FeC13, respectively, has been investigated under

279

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Fig. 10. Conductivity change of iodine-doped Shirakawa polyacetylene at various temperatures during storage.

various conditions. In laboratory atmosphere (or dry air) the conductivity of all samples of the Shirakawa type drops by several orders of magnitude within a few weeks. If stored in an argon atmosphere at --30 °C, a drop of about a factor of two within the first few days is observed, but afterwards the samples are stable. With the exception of iodine doping, highlydoped samples are less stable in laboratory atmosphere (or in humid air) than moderately doped ones. Samples prepared by a new method from polyacetylene suspension are considerably more stable than conventional Shirakawa samples.

Acknowledgements Financial support for this work by the Bundesministerium fiir Forschung und Technologie of the Federal Republic of Germany and by the Stiftung Volkswagenwerk is gratefully acknowledged. We want to thank F. Hoffmann, M. Kaiser, D. Lambert, M. Schmid, A. Stark and J. Stiihler for their assistance during the experiments.

References 1 As review papers, see e.g., S. Roth and K. Menke, Naturwissenschaften, 70 (1983) 550; D. Baeriswyl et al., in J. Mort and G. Pfister (eds.), Electronic Properties o f Polymers, Wiley, New York, 1982, p. 267; S. Etemad et al., Ann. Rev. Phys. Chem., 33 (1982) 443; and the conference proceedings Low Dimensional Conductors, E. J.

280

2 3 4

5 6

Epstein and E. M. Conwell (eds.), Mol. Cryst. Liq. Cryst., 77 (1981), 79, 81, 83, 85 and 86 (1982) and Conducteurs et Supraconducteurs Synth~tiques ~ Basse Dimension, J. Phys. (Paris) Colloq., 44 (1983)C-3(6). H. Shirakawa and S. Ikeda, Synth. Met., 1 (1979/1980) 175. J. Hocker and G. Schneider, J. Phys. (Paris) Colloq., 147 (1983) C-3. J. Hocker et al., DOS 31 2 3 8 0 2 (1981), EP 0 0 4 5 9 0 8 (1980), EP 0 0 4 5 9 0 5 {1980); M. Przybulski, B. R. Bulka, I. Kulszewicz and A. Pron, Solid State Commun., 48 (1983) 893. K. Ehinger, Ph.D. Thesis, Konstanz, 1984. H. Kiess, W. Meyer, D. Baeriswyl and G. Harbeke, J. Electroehem. Materials, 9 (1980) 763; M. RoUand, S. Lefrant, M. Aldissi, P. Bernier, E. Rzepka and F. Schue, J. Electron. Mater., 10 (1981) 619; W. Deits, P. Cukor, M. Rubner and H. Jopson, Synth. Met., 4 (1982) 199; A Guiseppi-Elie and G. Wnek, J. Chem. Soc., Chem. Commun., (1983) 63 and J. Phys. (Paris) Colloq., 193 (1983) C-3.