Simple synthesis of water-soluble conducting polyaniline

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Synthetic Metals 96 (1998) 161-163

Short Communication Simple synthesis of water-soluble conducting polyaniline Shoji Ito a, Kazuhiko Murata a,,, Seiichi Teshima a, Ryuji Aizawa a, Yoshinobu Asako ", Kohshin Takahashi b, Brian M. Hoffman c aTsukuba Research Laboratory, Nippon Shokubai Co., Ltd., Kannondai, Tsukuba 305, Japan bDepartment of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kodatsuno, Kanazawa 920, Japan Department of Chemistry and Materials Research Center, Northwestern University, Evanston, IL 60208-3113, USA Received 13 February 1998; received in revised form 7 May 1998; accepted 8 May 1998

Abstract A water-soluble externally (HC1)-doped conducting polyaniline (ED-SPAN) is prepared by such a simple synthetic method that emeraldine salts are sulfonated by chlorosulfonic acid in dichloroethane at 80°C and subsequently hydrated in water at 100°C. Sulfonating any emeraldine salts (counter anion X- = CI-, SO42-, and BF4- ) or emeraldine base results in the production of HCl-doped sulfonated polyaniline, where HC1 dopant from hydrolysis of chlorosulfonic group exchanges with the original dopant. The degree of sulfonation, namely, sulfur-to-nitrogen (S/N) ratio, can be controlled by adjusting the amount of chlorosulfonic acid. With increasing S/N ratio from 0.65 to 1.3, the solubility in neutral water increases from 22 to 88 g/l and the four-probe conductivity for a compressed pellet decreases from 0.023 to 1.7 × 10-5 S/cm, showing sulfonation-induced undoping. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Synthesis

1. Introduction Polyaniline [ 1-3 ] has superior stability in air than that of other conducting polymers. From the practical point of view, solubilizing it in common solvents [4-6] especially in water is expected. Epstein and co-workers [7,8] reported the sulfonic-acid self-doped polyaniline (SD-SPAN) by reacting emeraldine base with fuming sulfuric acid. Although SDSPAN shows excellent resistance against undoping, it is insoluble in water. Dissolving SD-SPAN in aqueous alkali solution is possible, but it leads to undoping and becomes an insulator. Therefore, to obtain a conducting SD-SPAN film, redoping of the cast film prepared from the alkali solution is necessary. Several water-soluble polyanilines in the doped conducting form, for example, poly(aniline-co-N-propanesulfonic acid-aniline) (PAPSAH) [9,10], poly(o-aminobenzylphosphonic acid) (PABPA) [ 11 ], and poly(2-methoxyaniline-5-sulfonic acid) (PMAS) [ 12,13] have been reported. However, dissolving the PAPSAH solid directly into water is impossible; redoping the aqueous alkali solution by ion-exchange resin is the only way to obtain the doped * Corresponding author. Tel.: +81 298 38 2562; fax: +81 298 38 2566; e-mail: [email protected]

PAPSAH solution. PABPA and PMAS are prepared from the corresponding substituted aniline monomer, but elaborate synthesis or high cost is required for obtaining the monomers and, in general, it is difficult to polymerize such substituted anilines. Hence, no simple method for producing water-soluble polyaniline in the doped conducting form has been reported so far [ 14,15 ]. We report a water-soluble conducting polyaniline, an externally (HC1) -doped sulfonated polyaniline (ED-SPAN) which is prepared by a direct sulfonation of emeraldine salts with chlorosulfonic acid [ 16] in an inert solvent. Because all row chemicals are cheap and the preparation is simple, the ED-SPAN may be the first low-cost water-soluble conducting polymer suitable for an industrial-scale production.

2. Experimental Emeraldine salts were synthesized by a chemical oxidation of aniline with (NIL) 2S2Os in various protonic acid solutions [ 1-3]. Sulfonation of the emeraldine salts was performed similarly as follows. Emeraldine hydrochloride powder (9.0 g) was dispersed in 270 ml of 1,2-dichloroethane (DCE) being heated at 80°C (Scheme 1). The chlorosulfonic acid

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S. /to et al. / Synthetic Metals 96 (1998) 161-163 Table 1 Physical properties of ED-SPAN with different S/N ratio

X"

= cr, SO2", BF4",or none

oo°c [ HSO~I

ED-SPAN

CISO3H (equiv.)

S/N a

Solubility (g/l)

Conductivity ~ (S/cm)

Sample a ~ Sample b Sample c

2.5 2.0 1.5

1.3 0.80 0.65

88 51 22

1.66× 10 -5 5.72× 10 -3 0.028

a From elemental analysis. b Four-probe d.c. conductivity at room temperature for a compressed pellet. c Sulfonated in 1,1,2,2-tetrachloroethane.

100°C I 1"120 1,5

SO~H

SO#

SO#

.SO3H]

= of - 1.0 ¢0

Scheme 1. Synthesis of externally ( HCl)-doped sulfonated polyaniline ( EDSPAN).

(21.8 g, 2 equiv.) diluted with 15 ml of DCE was added dropwise during 30 min into the dispersion liquid, and then the reaction mixture was held for 5 h. The produced chlorosulfonated polyaniline was separated by filtration, immersed in 400 ml of water, and heated for 4 h at 100°C to promote its hydrolysis. After concentrating the resulting greenish solution until almost dried by evaporation, the ED-SPAN was precipitated and washed by acetone, then collected by filtration and dried at 60°C under vacuum. Anal. Found: C, 39.8; H, 3.20; N, 8.02; O, 28.3; S, 14.5; CI, 6.21. (Oxygen by difference.) Calc. for C 2 4 H z o N 4 ( S O 3 ) 3 . 3 C l l . 3 ( H 2 0 ) 3 . o : C , 39.6; H, 3.60; N, 7.69; O, 28.3; S, 14.5; C1, 6.30%1 The result of elemental analysis suggests that using any emeraldine salts (counter anion X - = CI-, SO2 - , and BF4-) or emeraldine base as.starting material for sulfonation gives the HCl-doped sulfonated polyaniline. This HCI dopant was generated during hydrolysis of chlorosulfonic polyaniline and exchanged with the original dopant in the emeraldine salt. The exchange of the initial dopant with HC1 is clearly seen in the case of HBFa-doped polyaniline. After sulfonation, F atoms in the polyaniline completely disappeared.

3. Results and discussion

One of the advantages of this sulfonation method reacting in a solvent is that the degree of sulfonation, namely, sulfurto-nitrogen ( S / N ) ratio, can be controlled easily by adjusting the amount of chlorosulfonic acid added. Physical properties of the ED-SPAN with different S/N ratios are summarized in Table 1 and the UV-Vis absorption spectra are shown in Fig, 1. With increasing amount of chlorosulfonic acid from 1.5 to 2.5 equiv, against an aniline unit, the S/N ratio increases from 0.65 to 1.3, solubility increases from 22 to 88 g/l, and the conductivity decreases from 0.023 to 1.7 × 10 -5 S/cm. Samples b and c showed pure electronic conduction because the conductivities were the same at ambient pressure

e~ 0 . 5

0.0

I

",

I

I

(a) (b) ........ (c) .....

\ ~

¼ ~

I

400

/

,

"

I

,

o.."""

I

600 800 Wavelength (nrn)

,

1000

Fig. 1. UV-VisabsorptionspectraforED-SPANin waterwiththe S/N ratio of (a) 1.3, (b) 0.80, and (c) 0.65. and in vacuum without showing time dependence, while sample a showed a contribution from ionic conduction with time dependence. The electric conduction in sample a should be low. In the UV-Vis absorption spectra, the peaks characteristic for polyaniline, around 320 nm (3.9 eV) for the aT-at* transition and around 440 nm (2.8 eV) for the polaron band transition, are seen [7,8]. As the S/N ratio increases, the hypsochromic shift of the aT--or*transition from 325 nm (3.82 eV) to 311 nm (3.99 eV) and the bathochromic shift of the polaron band transition from 436 nm (2.85 eV) to 442 nm (2.81 eV) are observed due to the reduced extent of "trconjugation in the polyaniline backbone, which is caused by the electron-withdrawing property of--SO3H groups and steric repulsion among them. For a highly sulfonated sample ( S / N = 1.3), the intensity of the polaron band transition decreases while the intensity of the peak around 650 n m ( 1.9 eV) characteristic of the undoped quinoid unit increases, suggesting that sulfonation-induced undoping occurs. Due to the reduced aT-conjugation by sulfonation, cation radicals created by doping turn out to be unstable, resulting in less doping. The observed moderate conductivity ( 1.66 × 10 -5) is from ionic conduction. Thus, introducing too many sulfonic groups into polyaniline brings about an electrically insulating polymer. From the FT-IR spectra of ED-SPAN ( S / N = 0 . 8 0 ) , absorption peaks at 1172 and 1074 cm-1 are assigned to asymmetric and symmetric O=S--O stretching vibrations, respectively, and peaks at 703 and 615 c m - I are assigned to S-O and C-S stretching vibrations, respectively [9,10]. The absorption peak at 818 c m - ~ is due to out-of-plane bending

S. Ito et al. / Synthetic Metals 96 (1998) 161-163

of 1,2,4-trisubstituted aromatic rings [ 7,8 ]. All above FT-IR absorption peaks are also observed in the spectra for SDSPAN, indicating the -SO3- groups are directly attached to the aromatic rings. A broad peak around 250(03700 c m - 1 is strong for ED-SPAN compared to SD-SPAN containing the same amount of water. Therefore, the peak assigned to O-H stretching vibration is not all from H20 and at least from -SO3H in ED-SPAN. This result suggests that sulfonic groups exist in the form of -SO3H and are not used for selfdoping. The temperature dependence of four-probe d.c. conductivity for the compressed pellet of ED-SPAN (S/N = 0.8) was semiconducting over the entire temperature range from 300 to 125 K with a well-defined activation energy of AE = 0.059 eV. The result indicates that carrier conduction in ED-SPAN is limited by an intermolecular thermal hopping of conduction electrons and large AE compared to unsubstituted polyaniline (AE = 0.020 eV) is due to the intermolecular steric hindrance of-SO3H groups. When the ED-SPAN ( S / N = 0 . 8 0 ) aqueous solution ( p H = 2 . 8 ) was titrated with NaOH,q solution, two endpoints were detected by a potentiometer. Until the first endpoint, a neutralization of free protons (Hf ÷ ) from the - S O 3 H group occurred. Then, neutralization of protons (Hi ÷) from imine nitrogens, namely, undoping, took place with accompanying color change of the solution until the second endpoint. The amounts of substituted - S O 3 H ( S / N ratio) obtained from [Hf+ ] / [aniline unit] and doping level (D) obtained from [Hi + ] / [ a n i l i n e unit] were S / N = 0 . 7 0 and D = 0.39, respectively. The molar amount of aniline unit was estimated from its weight and the composition from the elemental analysis. These results are consistent with the result of elemental analysis; S / N = 0 . 8 0 and D = 0 . 3 3 from the C1-/N ratio. A rather small S/N ratio from titration suggests the possibility that intermolecular -SO2- linkage exists, SO42- is included as a dopant, or partial self-doping by the - S O 3 H group occurs. These results clearly show that protons in -SO3H are free and not used for self-doping; thus, ED-SPAN is externally doped with HCI. Epstein et al. also reported a highly sulfonated self-

163

doped polyaniline ( S / N = 0.75) which did not seem to show good solubility in water [ 17]. Therefore, not only the S/N ratio in sulfonated polyaniline limits the solubility but also the amount of free protons. When protons in the-SO3H group are used for self-doping, it is no longer useful for dissociating in water. The concept of external doping in ED-SPAN is effective for increasing free protons and solubilizing polyaniline into water. The present study may open the way for obtaining a water-soluble conducting polyaniline with higher conductivity. References [ 1] A.G. MacDiarmid, J.C. Chiang, M. Halpern, W.S. Huang, S.L. Mu, N.L.D. Somasiri, W. Wu, S.I. Yaniger, Mol. Cryst. Liq. Cryst. 121 (1985) 173. [2] A.G. MacDiarmid, J.C. Chiang, Synth. Met. 13 (1986) 193. [3] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer 30 (1989) 2305. [4] Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91. [5] J.W. Chevalier, J.Y. Bergeron, L.H. Dao, Macromolecules 25 (1992) 3325. [6] M. Abe, A. Ohtani, Y. Umemoto, S. Akizuki, M. Ezoe, H. Higuchi, K. Nakamoto, A. Okuno, Y. Noda, J. Chem. Soc., Chem. Commun. (1989) 1736. [7] J. Yue, A.J. Epstein, J. Am. Chem. Soc. 112 (1990) 2800. [8] J. Yue, Z.H. Wang, K.R. Cromack, A.J. Epstein, A.G. MacDiarmid, J. Am. Chem. Soc. 113 (1991) 2665. [9] S.A. Chen, G.W. Hwang, J. Am. Chem. Soc. 116 (1994) 7939. [ 10] S.A. Chen, G.W. Hwang, J. Am. Chem. Soc. 117 (1995) 10055. [ 11 ] H.S.O. Chan, P.K.H. Ho, S.C. Ng, B.T.G. Tan, K.L. Tan, J. Am. Chem. Soc. 117 (1995) 8517. [ 12] S. Shimizu, T. Saitoh, M. Uzawa, M. Yuasa, K. Yano, T. Maruyama, K. Watanabe, Synth. Met. 84-86 (1997) 1337. [13] W. Lee, G. Du, S.M. Long, A.J. Epstein, S. Shimizu, T. Saitoh, M. Uzawa, Synth. Met. 84-86 (1997) 807. [ 14] Excluding the composition such as polyaniline/polyacid which is soluble in water due to the polyacid: L. Sun, S.C. Yang, J.M. Liu, Polym. Prepr. 33 (1992) 379. [ 15] M. Angelopoulos, N. Patel, J.M. Shaw, N.C. Labinca, S.A. Rishton, J. Vac. Sci. Technol. B 11 (1993) 2794. [ 16] Sulfonation of emeraldine base in chlorosulfonic acid has been tried in the following literature, but the product was insoluble in water: J. Yue, G. Gordon, A.J. Epstein, Polymer 33 (1992) 4410. [ 17] X.-L. Wei, Y.Z. Wang, S.M. Long, C. Bobeczko, A.J. Epstein, J. Am. Chem. Soc. 118 (1996) 2545.