In situ doping polymerization of pyrrole with sulfonic acid as a dopant

SMflTHETI[ gTRLS ELSEVIER

Synthetic Metals 96 (1998) 127-132

In situ doping polymerization of pyrrole with sulfonic acid as a dopant Youqing Shen, Meixiang Wan * Institute of Chemistry, The ChineseAcademy of Sciences, Beijing 100080, China Received 4 September 1997; received in revised form 1 May 1998; accepted 4 May 1998

Abstract

Effects of sulfonic acid on solubility, electrical and thermal properties as well as morphology of polypyrrole (PPy) prepared by in situ doping polymerization of pyrrole in the presence of sulfonic acid as a dopant have been investigated. It was found that sulfonic acids which have a good solvating ability, e.g. 5-butylnaphthalene sulfonic acid (BNSA), 13-naphthalene sulfonic acid (NSA) and p-dodecylbenzene sulfonic acid (DBSA), render PPy soluble in m-cresol, while sulfonic acids, e.g. camphor sulfonic acid (CSA) and p-methylbenzene sulfonic acid (MBSA), cannot. Therefore, it was proposed that a good solvating ability of sulfonic acid induces the solubility of PPy. The UV-Vis and ESR spectra indicated that the charge carriers in PPy doped with NSA, BNSA, CSA, DBSA, p-hydroxylbenzene sulfonic acid (HBSA) and 5-sulfo-isophthalic acid (SIA) are polarons and bipolarons, while only bipolarons serve as charge carriers in PPy doped with alizarin red acid (ARA) and 8-hydroxy-7-iodo-5-quinoline sulfonic acid (QSA). The nature of sulfonic acid also has influence on morphology and thermostability of the resulting PPy. It was found that the polymerization medium significantly influences the solubility and conductivity of PPy. The in situ doping polymerization of pyrrole in organic solvent such as CHC13, CH3NO2 or THF gave soluble but nonconductive PPy, compared with insoluble but conductive (o'= 18 S/cm) PPy prepared in water. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Polypyrrole; Doping; Sulfonic acid

1. Introduction

Of conducting polymers, polypyrrole (PPy) stands out as an excellent one due to its high conductivity and good environmental stability and hitherto a large variety of application potential [ 1-10]. However, poor processibility of PPy due to its insolubility and infusibility has retarded further investigation on the structure and structure-physical properties. Therefore, several kinds of soluble PPy have been synthesized. For example, poly ( 3-alkyl pyrrole) with an alkyl group equal to or greater than a butyl group is easily soluble in common solvents [ 11-13] or soluble in water when the substituents bear hydrophilic groups, such as - S O 3 H [ 14], giving conductivity ranging from 10 -2 to 10 j S/cm depending on the bulkiness of the alkyl group. However, the main problem of this method is the complicated synthesis of 3-substituted pyrrole monomer [ 11-14]. Poly(N-substituted pyrroles), however, have much lower conductivity due to greatly suppressed conjugation along the polymer chains by the substituents on nitrogen [ 15,16]. At the same time, they are only partly soluble in some organic solvents even with long alkyl groups on the nitrogen of pyrrole rings. * Corresponding author. Tel.: +86 010 6256 5821; fax: +86 010 6256 9564; e-mail: [email protected]

It has been found that some dopants, for example, camphor sulfonic acid (CSA) and p-dodecylbenzene sulfonic acid (DB SA), render polyaniline soluble in organic solvents with high conductivity [17,18]. Especially, polyaniline doped with CSA not only dissolves in m-cresol and chloroform, but also has a conductivity as high as 2 0 0 4 0 0 S/cm [ 17,18], compared with 10° S/cm for insoluble HCl-doped polyaniline. Lee et al. [ 19] found that PPy doped by DBSA with a conductivity of 2 S/cm also dissolves in m-cresol or chloroform in the presence of an extra amount of DBSA. Compared to the substitution method to prepare soluble poly(3-substituted pyrrole) and poly(N-substituted pyrrole), doping is simple for PPy which may give comparable conductivity to poly ( 3-substituted pyrrole ), but much higher than poly (Nsubstituted pyrrole) [ 19,20]. We reported that PPy prepared by in situ doping polymerization in the presence of 13-naphthalene sulfonic acid (NSA) can be dissolved in m-cresol and shows high conductivity [20]. We found in this work that, when using the in situ doping polymerization method to prepare soluble PPy, a sulfonic acid in the polymerization strongly affected the solubility, conductivity, morphology and thermostability of PPy. We discuss the effects of sulfonic acids in the in situ doping polymerization on the physical properties and molecular structure of PPy.

0379-6779/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.

P11S0379-6779(98)00076-9

128

Y. Shen, M. Wan/Synthetic Metals 96 (1998) 127-132

2. Experimental

monomer were dissolved in 20 ml deionized water with vigorous stirring at 0°C. 0.90 g ammonium persulfate (APS) as an oxidant dissolved in 15 ml deionized water was added slowly to the above solution. After 12 h, the reaction mixture was poured into a large excess of deionized water and then filtered. The doped PPy was washed with deionized water and methanol several times, and dried in vacuum at 25°C for 2 days. 1 g of PPy was added to 25 ml m-cresol by ultrasonification, and filtered through a 1 txm filter. The solution was transferred to a glass plate, the solvent was dried and thus solubility was determined. PPy powder was molded into a disk by pressing for measurement of the conductivity. The room-temperature conductivity of PPy was measured by using the standard four-probe method with a Keithley 196 SYSTEM DMM digital multimeter and an A D V A N T E S T R6142 programmable d.c. volt-

Pyrrole (Fluka) was distilled under reduced pressure and stored in a refrigerator. Various sulfonic acids and ammonium persulfate as oxidant were used as received. In this paper, nine sulfonic acids, i.e., p-methylbenzene sulfonic acid ( M B S A ) , p-hydroxybenzene sulfonic acid ( H B S A ) , pdodecylbenzene sulfonic acid ( D B S A ) , 13-naphthalene sulfonic acid (NSA), 5-n-butylnaphthalene sulfonic acid ( B N S A ) , 5-sulfo-isophthalic acid (SIA), 8-hydroxy-7-iodo5-quinoline sulfonic acid ( Q S A ) , alizarin red acid ( A R A ) and camphor sulfonic acid ( C S A ) , were used as dopants, the molecular structures of which are given in Table l. Conductive PPy was synthesized by the in situ doping polymerization method [20]. The typical procedure is as follows: 14 mmol sulfonic acid and 1.0 ml ( 14 mmol) pyrrole

Table 1 Solubility in m-cresoland electrical properties of PPy in situ doped with different sulfonic acids Sulfonic acid

Structure

Solubility a in m-cresol

O.RT (S/cm)

Charge carrier

p-Methylbenzene sulfonic acid (MBSA)

~ H3C~SO3H

×

16.0

polaron and bipolaron

p-Hydroxybenzenesulfonic acid (HBSA)

O~-SO3H

O

11.0

polaron and bipolaron

p-Dodecylbenzenesulfonic acid (DBSA)

CH~CH~)~r~_SO3 H

•

2.0

polaron and bipolaron

fl-Naphthalene sulfonic acid (NSA)

~ S O 3 H

~

18.0

polaron and bipolaron

5-n-Butylnaphthalene sulfonic acid (BNSA)

~ S O 3 H

~

0.5

polaron and bipolaron

O

3.0

polaron and bipolaron

O

3.0

bipolaron

O

8

bipolarou

×

18

y v C4H9 5-Sulfo-isophthalic acid (SIA)

SO3H

HOOC~COOH 8-Hydroxy-7-iodo-5-quinoline sulfonic acid ( QSA)

SO 3H

OH Alizarin red acid (ARA)

° ~ s o ~ 11"

O

Camphor sulfonic acid (CSA)

a ×, insoluble; ~, soluble; O, slightly soluble.

H3C .CH3

T

OH

OH

polaron and bipolaron

Y. Shen, M. Wan/Synthetic Metals 96 (1998) 127-132

age/current generator as the current source. FT-IR spectra were measured on a Perkin-Elmer system. UV-Vis spectra of PPy in m-cresol were recorded on a UV-3100 spectrometer. The morphology of PPy was examined on a Hitachi-530 scanning electron microscope. ESR experiments were carded out on a Bruker ER-200D. Thermal stability of the PPy was investigated by a thermogravimetric analyzer (Perkin Elmer, TGA7) with nitrogen as pure gas at a flow rate of 40 ml/ min. The heating rate was 10°C/min.

3. Results and discussion

3.1. Solubility and electrical properties Table 1 shows solubility and electrical properties of PPy prepared by in situ doping polymerization in the presence of different organic sulfonic acids as dopants. Solubilities higher than 0.8 g/100 ml and lower than 0.5 g/100 ml are defined 'soluble' and 'slightly soluble', respectively. It is observed that PPy doped with BNSA or DBSA, which has a long alkyl group and thus good solvating ability, is soluble in m-cresol. It is very interesting to find that NSA, which has no long

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Fig. 1. Dependence of the conductivity (a) and solubility (b) on the concentration of D B S A and NSA dopants.

129

alkyl group, also renders PPy soluble. This may be due to the fact that the naphthalene ring may have strong interaction with the phenyl ring of m-cresol. But the solubility of PPy doped with BNSA is higher than that of PPy doped with NSA. For example, solubilities of 1.4 g/100 ml in m-cresol for PPy-NSA and 1.2 g/100 ml for PPy-NSA were observed. Solubility of PPy-NSA in m-cresol increases with increasing concentration of NSA, and reaches a maximum value of about 1.2 g/100 ml at a NSA concentration of 0.57 mol/1 [20], as shown in Fig. 1 (a). On the other hand, the conductivity at room temperature slightly decreases with increase of NSA concentration after the concentration of NSA is higher than 0.1 mol/1 [20]. Moreover, the same dependence of conductivity on the concentration of DBSA was observed, as shown in Fig. 1(b). It is also noted that, compared with the DBSA-induced solubility of PF'y, benzene sulfonic acid bearing a short alkyl chain, MBSA, fails to make PPy soluble. This suggests that sulfonic acid bearing a long alkyl group is favorable for increasing the solubility of the doped PPy. Therefore, it is postulated that, if a chemical group in sulfonic acid has strong interaction with the solvent, e.g. hydrogen bonding, the solubility of PPy doped with this acid may be enhanced. We tested benzene sulfonic acids beating -COOH and -OH, respectively, such as SIA, HBSA, ARA and QSA, which can form hydrogen bonding with m-cresol. Table 1 shows that, although PPy doped with MBSA is insoluble in m-cresol, PPy in situ doped with SIA or HBSA is partly soluble in mcresol, which is consistent with the above assumption. However, it is surprising to find that the solubility of PPy in situ doped with ARA or QSA, which bears three or one hydroxyl group(s) respectively, is not higher, but lower than that of PPy doped by NSA. In addition, even though CSA induces polyaniline to dissolve in m-cresol and other organic solvents with high conductivity [ 17,18 ], the solubility and conductivity of PPy in situ doped by CSA greatly depend on the polymerization medium. When the polymerization is carried out in water with CSA as a dopant, the resulting PPy is completely insoluble in any solvent, but the conductivity of this PPy is about 18 S/cm. Thus, this suggests that the influence of dopant on the solubility of the doped PPy is very complex. It means that the coordination role of molecular size, hydrogen bonding and long alkyl group on the solubility of the doped PPy should be considered. Moreover, when the polymerization is carded out in an organic solvent, such as chloroform or THF, the obtained PPy dissolves in CHC13, CH3NO 2 or THF, but its conductivity is about eight orders of magnitude lower than that of PPy prepared in water. The UV-Vis spectrum (Fig. 2) of PPy prepared in chloroform shows that there is only one absorption band at 475 nm assigned as ~r--ar* absorption, but no absorption band due to charge carriers, i.e. polarons at 685 nm or bipolarons at 978 nm, is present [ 19-23]. This means that PPy was not doped during the polymerization in chloroform or THF even in the presence of sulfonic acid.

130

Y. Shen, M. Wan/SyntheticMetals 96 (1998) 127-132

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Fig. 4. ESR spectra of PPy doped with CSA (a), BNSA (b), NSA (c), HBSA (d) and QSA or ARA (e).

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ferent sulfonic acids has very similar structure to the main polymer chain. However, the UV-Vis spectra (Fig. 3) of the PPy are very different from one another. According to the suggestion reported [ 19-23 ], we see that PPy doped by NSA and BNSA has a characteristic w--w* absorption band at 435 nm, the polaron band at 646 nm and the bipolaron band at 978 nm. Most interestingly, PPy doped with ARA shows no polaron absorption, but has a long extended absorption tail beginning at 857 nm, which shows an expanded conformation very similar to the UV-Vis spectrum of CSA-doped polyaniline [ 17,18]. ESR experiments (Fig. 4) further confirm the UV-Vis results. It is shown that all PPy doped with sulfonic acids except ARA, SIA and QSA has a ESR signal with a g factor of 2.0030, indicating the presence of polarons in these PPy samples. However, PPy doped with ARA or QSA has no ESR signal (Fig. 4(e) ), which is consistent with the absence of polaron absorption in their UV-Vis spectra. This means that polarons and bipolarons serve as charge carders in PPy doped by NSA, BNSA, HBSA, etc., but in PPy doped by ARA, or

Fig. 2. UV-Vis spectrumof CHC13solution of PPy prepared in CHCI3. [Py] = 0.43 mol/l, [CSA] = 0.23 mol/l, [FeCI3]= 0.08 mol/1,0°C,CHC13 as media. 3.2. Charge carriers of the doped PPy

FT-IR spectra of PPy in situ doped with DBSA, NSA, QSA, CSA, SIA and HBSA are similar to the spectra of PPy synthesized by a conventional method. In general, the two bands located at 1565 and 1619 c m - ', characteristic of the stretching vibration of C=C, and the various bands between 1475 and 1236 c m - 1due to the vibration modes of the pyrrole ring were observed as well as the three bands situated at 1045, 968 and 922 cm-1 related to the in-plane and out-of-plane vibration modes of N - H and C - H [24]. It is seen that all of characteristic absorption bands described above were observed except for the in-plane and out-of-plane vibration modes of PPy doped with ARA in the region of 500-1000 c m - 1 which may be due to side reactions during polymerization or dopants. This means that the PPy doped with dif0.8

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600

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800

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1000

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Wav=l¢n~ (ran) Fig. 3. UV-Vis spectra of PPy doped with BNSA (a), NSA (b) and A R A (c). [ Py ] = 0.53 tool/1, [ H ÷ ] = 0.57 mot / 1, 0°C, APS and water as oxidant and solvent, respectively.

Y. Shen, M. Wan/Synthetic Metals 96 (1998) 127-132 Table 2 Linewidth of ESR signal of PPy doped with various sulfonic acids Acid

CSA

BNSA

DBSA

NSA

SIA

HBSA

Linewidth AH(G) O'R'r (S/cm)

11.8

6

3.8

3.7

3.24

2.87

18

0.5

2.0

18

3.0

11.0

QSA, only bipolarons serve as charge carriers, as shown in Table 1. It has been proposed that polarons of in situ doped PPy are generated by adding H ÷ to the 13-position of the pyrrole ring during the in situ doping polymerization of pyrrole [20]. Therefore, the absence of polarons in ARA- or QSA-doped PPy indicates that, during the polymerization ofpyrrole, these acids dope PPy poorly with protons. The nature of dopants greatly affects the linewidth (AH) of the ESR spectra, as shown in Table 2. The linewidth (AH=2.87-11.8 G) of PPy doped with various sulfonic acids decreases in the order CSA > BNDS > DBSA > NSA > SIA > HBSA. But, no relationship between linewidth and conductivity of doped PPy was observed. For instance, a wide fiat/( 11.8 G) of CSA-doped PPy with room-temperature conductivity of 18 S/cm was observed. On the other hand, a narrowed AH (3.7 G) of NSA-doped PPy with high room-temperature conductivity (18 S/cm) was observed.

131

Moreover, the doped PPy-CSA is completely insoluble but the doped PPy-NSA is soluble in m-cresol. In fact, the linewidth AH is affected by spin relaxation time, including spinlattice relaxation time (t~) and spin-spin relaxation time (t2) [25]. Thus, the difference in AH and conductivity of doped PPy with various sulfonic acids may be due to different spin relaxation times.

3.3. Morphology and thermal stability of the doped PPy It has been demonstrated that morphology of PPy depends on the preparation method and conditions [20]. The counterions also have influence on morphology of PPy, as shown in Fig. 5. The morphology of PPy doped with CSA, DBSA and MBSA is typically granular (Fig. 5(a)). However, PPy doped with NSA is fibrillar (Fig. 5(b)) and PPy doped with ARA looks like short thick ropes interlocked by PPy granulates (Fig. 5(c)), while PPy doped with QSA is flaky (Fig. 5(d) ). However, the morphology of PPy prepared by the 'two-step method', in which PPy is firstly prepared by polymerization of pyrrole by a similar method but in the absence of sulfonic acid, and then doped with different sulfonic acids, is granular and independent of the kind of sulfonic acid. This means that the morphology of PPy is induced by sulfonic acid during the in situ doping polymerization of pyrrole. Moreover, it was found that the PPy samples doped by the

Fig. 5. Morphology of PPy doped with CSA (a), NSA (b), ARA (c) and QSA (d). [Py] =0.53 mol/l, [H + ] =0.57 mol/1, 0°C, APS and water as oxidant and media, respectively.

132

Y. Shen, M. Wan/Synthetic Metals 96 (1998) 127-132

Acknowledgements 1.o

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The authors would like to thank the National Natural Science Foundation of China, the National Advance Materials Committee of China, the Foundation of the Chinese Academy of Sciences, the Third World Academy of Sciences and the Chinese Postdoctoral Foundation for financial support.

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References 0.0

i

i 200

=

i 400

=

i 600

=

i 800

= 1000

Temperature(°(3) Fig. 6. Thermogravimetric analysis of PPy doped with SIA (a), DBSA (b), ARA (¢), NSA (d) and CSA (e).

two-step method before and after doping are completely insoluble in m-cresol. Fig. 6 shows the weight loss of PPy doped with different sulfonic acids. It is found that the thermal stabilities of PPy doped by CSA or DBSA are very similar. Both begin to lose weight at about 217°C, but PPy-CSA decomposes more rapidly. However, PPy samples doped with NSA or BNSA are more therrnostable. Weight loss of these PPy samples begins at 317°C. This result is consistent with the results of Kudoh et al. [ 26] showing that PPy doped with naphthalene sulfonic acid possesses a good thermostability.

4. Conclusions Solubility in m-cresol, room-temperature conductivity, morphology and thermal stability of PPy synthesized by in situ doping polymerization in the presence of sulfonic acid were measured as a function of sulfonic acid as dopant. (1) It was found that good solvating ability of sulfonic acid, such as DBSA and BNSA, renders PPy soluble, while sulfonic acids only having large molecular size, such as CSA and MBSA, fail to make PPy soluble. (2) Charge carriers in PPy doped with NSA, BNSA, CSA, DBSA, HBSA and SIA are polarons and bipolarons, while only bipolarons serve as charge carriers in PPy doped with ARA and QSA. (3) The nature of sulfonic acid also has an influence on morphology of the resulting PPy. The images of PPy doped with CSA, DBSA and MBSA have typical granular morphology but PPy doped with NSA is fibrillar. (4) Based on the measurement of weight loss, it was found that PPy doped with NSA and BNSA are more thermostable.

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