Processible and environmentally stable conducting polymers

Synthetic Metals, 15 (1986) 169 - 174

169

PROCESSIBLE AND ENVIRONMENTALLY STABLE CONDUCTING POLYMERS* R. L. ELSENBAUMER, K. Y. JEN and R. OBOODI

Polymer Laboratory, Corporate Technology, Allied Corporation, Morristown, NJ 07960 (U.S.A.)

Aus~ract Highly conducting polythiophene is rendered environmentally stable and solution processible in both its neutral and conductive forms in c o m m o n organic solvents by appropriate alkyl substitution on the thiophene rings. Solubility increases with increasing chain length of the substituent in the order n-butyl > ethyl >> methyl. Neither the size of the substituent, nor the nature of the dopant has much influence on the conductivity of the doped complexes--typical conductivities are 1 - 5 ohm -1 cm -1. Copolymers of thiophenes containing different alkyl substituents were also prepared and were found to exhibit properties similar to the homopolymers. Dialkyl substitution on the thiophene rings gives polymers with less extended conjugation and lower conductivities on doping.

Introduction Interest in electrically conductin~ organic polymers ensues from the large number of potential applications [1 - 3] for these materials. As electronic materials, the most promising synthetic semiconductors and metals for practical large scale electronic applications are those that can combine convenient solution or melt processibility with high environmental stability, good mechanical properties and controllable conductivity. As yet, none of the widely known polymers possesses the desired combination of these properties [4, 5]. Recently, Frommer e t al. [6] and Jenekhe e t al. [7] reported the discovery of conducting polymer processibility in liquid AsF3 and I2, respectively, from which stable conducting polymers can be cast. But these procedures suffer from industrial impracticability due to the high environmental reactivity and toxicity of the solvents used. We now report the synthesis and characterization of a series of poly(3-alkylthiophenes) that form highly conductive, environmentally stable complexes with electron acceptors and are solution processible from c o m m o n organic solvents in both their conductive and neutral forms. *A preliminary account of this work was presented at the ACS Fall Meeting, Chicago, IL, 1985. (K.Y. Jen, R. Oboodi and R.L. Elsenbaumer, Polym. Mater. Sci. Eng., 53 (1985) 79.) 0379-6779/86/$3.50

© Elsevier Sequoia/Printed in The Netherlands

170 Experimental Preparation o f m o n o m e r s and p o l y m e r s The starting materials, 3-methyl-, 3-ethyl-, 3-n-butyl- and 3-n-octylthiophene and 3,4-dimethylthiophene, were prepared according to the procedure of Tamao et al. [8]. 3-methylthiothiophene was prepared according to Gronowitz et al. [9]. The alkylthiophenes were iodinated by the method of Barker et al. [10] to give substituted-2,5-diiodothiophenes. These were polymerized by a nickel-catalyzed Grignard coupling method similar to the one used by Kobayashi et al. [11] except that THF was used as the reaction solvent. Copolymers were prepared by polymerizing different ratios of 2,5diiodo-3-methylthiophene and either 3-n-butyl-2,5
Results and discussion

In contrast to conductive poly(thiophene-2,5-diyl), PT, the doped forms of poly(3-methylthiophene-2,5-diyl), P3MT, are reported to be environmentally stable and highly conductive [12]. Neither the electrochemically nor chemically prepared conductive forms of PT or P3MT are solution or melt processible. Since electron-donating substituents on the thiophene ring improve the environmental stability of doped polythiophene, presumably by lowering the chemical potential of the ionized complexes, suitable choice of long-chain hydrocarbon- or heteroatom-containing substituents on PT may also improve solubility and processibility of both the

171

undoped and doped polymers. Recently, Blankespoor and Miller [13] reported that electrochemically polymerized 3-methoxythiophene gave a soluble polymer but with undeterminable conductivity. As the bulk of the substituents gets large, the conductivity of the doped complexes is expected to diminish due to impaired electron transport between polymer chains. Determining just how severe this penalty might be was one purpose of our study. We found that alkyl substituents equal to or greater in size than butyl greatly improve the solubility of the undoped polymers and also render the doped, conductive forms soluble in many c o m m o n organic solvents (THF, nitromethane, nitropropane, DMF, etc.). We were surprised to find that neither the size of the substituent nor the nature of the dopant much affects the conductivity of the doped complexes (Table 1). TABLE 1 Conductivities of various doped poly(alkylthiophenes) Polymer

Dopant

Oa00K (ohm -1 cm -1)

Poly(3-methylt hiophene) P3MT

I2 NOBF4 NOPF 6 NOSbF 6

3 4 4 3

Poly(3~ethylthiophene) P3ET

12

3

Poly(3-n-butylthiophene) P3BT

12 NOSbF 6

4 0.1

Poly(3-thiomethylthiophene) P3MST

12

0.5

Poly(3,4-dimethylthiophene) PDMT

NOSbF 6 I2

0.5 --

Poly(3-methylthiophene~o3'-n-butylthiophene) (50:50) P3MTBT

NOSbF 6 I2

6 4

Poly(3-methylthiophene~co3t-n-octylthiophene) (60:40) P3MTOT

NOSbF 6

5

The soluble polymers were readily characterized by i.r. (Fig. 1) and n.m.r. (Fig. 2), which indicate that they have a predominantly regular linear structure. The solubility of the homopolymers, poly(3-alkylthiophenes), increases with increasing chain length of the substituent in the order n-butyl > ethyl >~ methyl. The molecular weights, determined by end group analysis, were generally 2500. We found that higher molecular weight polymers were produced by random copolymerization of diiodinated derivatives of 3methylthiophene and 3-butylthiophene (50: 50), giving polymers with much

172

4000

3000

2000

1000

400

WAVENUMBERS(CM-~)

Fig. 1. Infrared spectra of pristine poly (3-butylthiophene-2,5-diyl) and copolymer with 3-methylthiophene.

i 10

I 9

i

I 8

i

I 7

I

I 6

i

I 5

~

I 4

I

A 3

I

I 2

~

I I

I 0

ppm

Fig. 2. Proton n.m.r, spectrum of P3BT in CDC13 (60 MHz).

improved film-forming properties. Similar results were found for copolymers of 3-methylthiophene and 3'-octylthiophene. Why copolymer formation gives higher molecular weight polymers may be related to the relief of steric

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interactions during polymerization by providing alternating monomers with very different steric requirements. Polymerization of 2,5-diiodo-3,4 ethyl >> methyl. The butyl derivative lost only a small fraction of its weight with almost no change in its conductivity, while the methyl derivative lost nearly all of its iodine dopant and conductivity (< 10 -4 o h m - 1 cm- 1). Thermogravimetric analyses were used to determined thermal stabilities of doped complexes as a function of dopant species. With poly(3-methylthiophene) the doped complexes were found to decompose at widely different temperatures (Fig. 3). Among the dopants investigated, complexes containing SbF 6- showed the best thermal stability, while those containing I3or BF4- showed the worst. 100 90 80

NOPF6

70 60 50 NOSBF6

40 30 20 10 i 1 O0

I 200

I 300

I 400

1 500

I 600

700

Temperature°C

Fig. 3. Thermogravimetric analysis of P3MT doped with various species (under argon: 5 °C/rain).

174 The solution processibility o f the neutral pol y(3-n-but yl t hi ophene) or its c o p o l y m e r with 3 - m e t h y l t h i o p h e n e is excellent. The pristine polymers can be dissolved in c o m m o n organic solvents (toluene, t e t r a h y d r o f u r a n , m e t h y l e n e chloride etc.) at r o o m t e m p e r a t u r e , and t hen cast into films or coatings o n a variety of substrates. The films or the coated substrates after exposure t o iodine vapor or immersion in a solution of N O S b F j C H 3 N O 2 remained flexible, showed high conduct i vi t y and good environmental and thermal stability. H o m o g e n e o u s blue solutions o f d o p e d p o l y m e r can be f o r m e d by dissolving d o p e d p o l y ( 3 - n - b u t y l t h i o p h e n e ) or the d o p e d c o p o l y m e r s in T H F , DMF or CH3NO2. D ope d p o l y m e r can be cast by evaporating the solvent u n d e r vacuum. However, cast films were brittle and their conduct i vi t y was an o r d e r o f magnitude lower t han t h a t obtained on films doped by the inverse process. This m a y be explained by the presence of impurities, m ost likely in the f o r m of excess d o p a n t remaining in the cast polymer. This class o f conduct i ng p o l y m e r s promises t o com bi ne convenient processibflity and high environmental and thermal stability with the mechanical properties n eed e d f or a variety of applications. The quality of cast films, especially f r o m solutions o f the d o p e d p o l y m e r , clearly needs t o be improved. F u r t h e r studies aimed at preparing pol ym ers with m u c h higher molecular weights and b e t t e r film forming properties are in progress.

Acknowledgement The authors thank Jane Frommer for helpful discussions and suggestions. References 1 P. J. Nigrey, D. MacInnes Jr., D. P. Nairns, A. G. MacDiarmind and A. J. Heeger, J. Electrochem. Soc., 128 (1981) 1651. 2 G. Tourillon and F. Gamier, J. Electroanal. Chem. Interfacial Electrochem., 135 (1982) 173. 3 K. Kaneto, Y. Kohno, K. Yoshino and Y. Inuishi, J. Chem. Soc., Chem. Commun., (1983) 382. 4 K.J. Wynne and G. B. Street, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 23. 5 R.H. Baughman, J. L. Br6das, R. R. Chance, R. L. Elsenbaumer and L. W. Shacklette, Chem. Rev., 82 (1982) 209. 6 J. E. Frommer, R. L. Elsenbaumer and R. R. Chance, in T. Davidson (ed.), ACS S y m p o s i u m Series 242, Polymers in Electronics, Am. Chem. Soc., Washington, DC, 1984, p. 447. 7 S. A. Jenekhe, S. T. Wellinghoff and J. F. Reed, Mol. Cryst., 105 (1984) 175. 8 K. Tamao, S. Kodama, I. Nakajima, M. Kumada, A. Minato and K. Suzuki, Tetrahedron, 38 (1982) 3347. 9 S. Gronowitz, P. Moses and R. Hakansson, Ark. Kemi, 16 (1960) 624. 10 J. Barker, P. R. Huddleston and M. L. Wood, Syn. Commun., 5(1) (1975) 59. 11 M. Kobayashi, J. Chen, T.-C. Moraes, A. J. Heeger and F. Wudl, Synth. Metals, 9 (1984) 77. 12 G. Tourillon and F. Gamier, J. Electrochem. Soc., Electrochem. Sci. Technol., 130 (1983) 2043. 13 R. L. Blankespoor and L. L. Miller, J. Chem. Soc., Chem. Commun., (1985) 90. 14 G. Tourillon, D. Gourier, P. Gamier and D. Vivien, J. Phys. Chem., 88 (1984) 1049.