Properties of polypyrrole prepared by chemical polymerization using aqueous solution containing Fe2(SO4)3 and anionic surfactant

ELSEVIER

Synthetic Metals 79 (1996) 17-22

Properties of polypyrrole prepared by chemical polymerization using aqueous solution containing Fe, ( S04) 3 and anionic surfactant Yasuo Kudoh Advanced

Materials

Research

Laboratory,

Matsushita

Research

Institute

Tokyo, Inc., 3-10-l

Higashirnita,

Tama-ku,

Kawasaki

214, Japan

Received 18 July 1995; revised 7 September 1995; accepted 14 November 1995

Abstract

This study deals with polypyrrole (PPy) prepared with an aqueous solution. The combinations of Fe(S0J3 as an oxidant and anionic surfactants,such as sodium dodecylbenzenesulfonate,sodium alkylnaphthalenesulfonate.and sodium alkylsulfonate, brought about enhanced conductivity as well as increasedyield of the resultant PPy. These phenomena resulted from the large-sized surfactantanions being predominantly incorporated into the PPy backbone. Furthermore, the addition of surfactantsacceleratedthe polymerization reactions. This seemed to derive from an insoluble product temporarily being formed from Fe2(SO4)3 and the surfactantfunctions being innumerable initial points of the polymerization reactions. PPy prepared with surfactantsalso showedhigh thermal and moisture stabilities in air at 12.5“C, and at 85 “C and 85% RH. Keywords:

Polypyrrole; Chemical polymerization; Anionic surfactant; Conductivity

1. Introduction

Intrinsic conducting polymers with conjugated double bonds have been attracting much attention as advanced materials. Especially, polypyrrole (PPy) is one of the most promising conducting polymers, because it has higher conductivity and environmental stability in the conductive (oxidative) state than many other conducting polymers. PPy can be easily prepared by chemical [ 11 and electrochemical polymerization [ 21: the former gives powdered PPy and the latter gives filmy PPy. To prepare a large quantity of PPy, the chemical polymerization is the better method, for it is free from the restriction of an electrode shape. In chemical polymerization, the generally used oxidants are as follows: (NH4)&08, H202 and many kinds of salts containing transition metal ions, for example, Fe3+, Cu2+, Cr6+, Ce4+, Ru3+ and Mn”. For the wide range application of chemically prepared PPy, processability, conductivity and environmental stability are the main issues to be improved. To improve the processability, many researchers have been engaged in the development of soluble or swollen PPy [ 3-71, and dispersible fine-powdered PPy [ 8-121. The electric properties and/or stability of chemically prepared PPy have also been researched at many laboratories [ 13-221. Nevertheless, there are few reports that mention the specific data for the conductivity decay under severe conditions, except those of Kuhn et al. [ 211 and ThiBblemont et al. [ 221. 0379-6779/96/$15.00

0 1996 Elsevier Science S.A. All rights reserved

We have previously shown that electropolymerized PPy doped with a large-sized anion, for example, alkylnaphthalenesulfonate, has excellent thermal and moisture stabilities [ 23,241. Furthermore, the author has recently found that PPy effectively doped with a large-sized surfactant anion, such as alkylnaphthalenesulfonate, dodecylbenzenesulfonate or alkylsulfonate, can be prepared by chemical polymerization using an aqueous solution containing Fe,( S04) 3 and a surfactant with the corresponding anion. This paper details the PPy prepared by using combinations of Fe,( S04)3 and anionic surfactants. 2. Experimental

PPy was prepared by chemical polymerization in a deionized water solution. The polymerization began with adding 100 dm3 of a solution containing an oxidant into 100 dm3 of a stirred solution containing a pyrrole monomer and a surfactant. After the prescribed polymerization time, synthesized PPy was filtered from the solution with a Kiriyama funnel and a Kiriyama No. 5C filter paper, and thoroughly washed with deionized water until the filtrate indicated neutral. After being further washed with ethanol several times, PPy was dried in vacuum at about 40 “C. Fe,( S04) 3 and (NH4)&Os were studied as oxidants. The investigated surfactants were as follows: 40 wt.% aqueous

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Y. Kudoh /Synthetic

solution of sodium alkylnaphthalenesufonate (NaANS, mean molecular weight 338)) sodium dodecylbenzenesulfonate (NaDBS, molecular weight 349) and 40 wt.% aqueous solution of sodium alkylsufonate (NaAS, mean molecular weight 328). All substances including the pyrrole monomer were purchased as refined grades and used without any purification. The conductivity of PPy was measured using a Mitsubishi Kagaku Loresta AP resistivity meter in the atmosphere at room temperature. For the measurement, a disc-shaped PPy sample with a diameter of 13 mm was prepared under a pressure of 30 MPa. To study environmental stability, while aging the discshaped PPy samples in air at 125 “C, and at 85 “C and 85% RI-I, the conductivity was intermittently measured at room temperature.

3. Results and discussion Table 1 shows the yields and conductivities of PPy samples prepared under various polymerization conditions. In a solution containing only the pyrrole monomer and the oxidant, both Fe,( SOJa and (NHJ2S208 give PPy samples having rather low conductivities, i.e., less than 5 S cm-‘. On the other hand, PPy samples prepared with the added solution of anionic surfactants showed not only increased yields but also greatly enhanced conductivities. When NaAS was added the maximal effect was obtained, and the conductivity reached 40 S cm-‘. Even in the case of NaANS whose effect was minimal, the conductivity of the resultant PPy showed more than a ten-fold increase. When the solution containing Fe,( SOJ s was added into the solution containing the pyrrole monomer and the surfactant, a white precipitate was immediately formed and then the color gradually changed into black. The precipitate seemed to be a salt composed of Fe3+ and surfactant anions with the hydrophobic atomic group. A similar phenomenon

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79 (1996)

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was seldom observed, with the combinations of ( NHJ2S208 and sulfonic surfactants or Fez( SOJ s and other sulfonic acid salts that do not function as the surfactant, for example, sodium 2-naphthalenesulfonate, disodium 2,6-naphthalenedisulfonate and sodiump-toluenesulfonate. To study the reason for the increase in yield of PPy prepared with the surfactant, elemental analysis was carried out. Table 2 summarizes elemental analysis data for PPy samples prepared under various conditions. The increase in content of Fe was not observed in the PPy samples prepared with NaDBS. The increase in yield, therefore, is not ascribed to the formation of a composite of PPy and the salt of the Fe3’surfactant anion. However, it is notable that S/N ratios drastically increase in the PPy samples prepared with the surfactant. If the doping ratio of PPy is not affected by the polymerization conditions, the increase in the S/N ratio shows that the monovalent sulfonate anions are also incorporated into PPy as the dopant. The doping ratio of each dopant can be calculated by the following simultaneous equations: x+y=SIN

ratio

2xfy = total doping ratio (constant) where x and y are the doping ratios of sulfate and sulfonate, respectively. Table 3 shows the calculated doping ratios and theoretical yields. The theoretical yields were estimated from the known quantity of oxidant on the supposition that the resulting polymer has the formula Py (sulfate), (sulfonate) Y, for the excess pyrrole monomer is used in the polymerization reaction. Since the experimental values agree well with the theoretical yields, the supposition seems to be appropriate. Furthermore, more than a 100% yield, which was observed when 0.03 mol dms3 of NaDBS was added, is caused by the excess DBS being adsorbed on the PPy surface. Although the concentration of sulfate is more than an order of magnitude higher than that of sulfonate in the polymerization solution, the doping ratio of sulfate to whole dopant is reversed. This result shows that the sulfate is more difficult

Table 1 Yield and conductivity of PPy samples prepared under various conditions n Oxidant (mol dmm3)

(NW&& (0.1) WL&Ws (0.1) WLAW, (0.1) Fe2(SW3 (0.1)

Additive (mol dm-“)

Yield (g)

Conductivity (S cm-‘)

1.36

4.42

NaDBS (0.0225)

2.01

0.570

NaANS (0.024)

1.91

0.221

1.28

1.33

Fe2(S04)3 (0.05) (N%)~S,O, (0.05)

NaDBS (0.0225)

2.46

20.4

Fe,(SW,

NaDBS (0.0225)

2.44

26,l

Fe2(S0.d3 (0.1)

NaANS (0.024)

2.65

15,7

b(SO&

NaAS (0.022)

2.24

40.7

(0.1) (0.1)

a Polymerization time, 60 mm; polymerization temperature, 25 ‘C; pyrrole monomer concentration, 0.375 mol dm-a; solvent, 200 dm3 of deionized water; NaDBS, sodium dodecylbenzenesulfonate; NaANS, sodium alkylnaphthalenesulfonate; NaAS, sodium alkylsulfonate.

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19

Table 2 Data for elemental analysis of PPy samples prepared under various conditions a No.

Polymerization conditions Oxidant (mol dmm3)

1

(NHd&Os

2 3 4

WQh FedSO& F%(SQ.h

Elemental analysis (wt.%) Additive (mol dme3)

C

H

N

s

4.06

15.95

4.0

3.91

15.12

NaDBS (0.015) NaDBS (0.030)

55.13 55.01 64.20 68.28

6.28 7.17

10.83 8.93

(0.1) (0.1) (0.1) (0.1)

Molar ratio Fe

S/N

4.2

0.11

0.110 0.117

5.1 4.1

0.031 0.0016

0.205 0.229

Doping ratio

0.220 0.234

a Polymerization time, 60 min; polymerization temperature, 25 “C; Py monomer concentration, 0.375 mol dm-“; solvent, 200 dm3 of deionized water; NaDBS, sodium dodecylbenzenesulfonate. Table 3 Molecular weight per residue of the PPy composition and yield ’ No.

PPy composition

MW per residue

Theor. yield (g)

Exp. yield (g)

Yield (%)

1

ppY(swoJ,o ppY(Q)o.117 PPY(SO,)O,O,,(DBS)O,,,, PPY(SO,)O.~~S(DBS)O,~~~

75.6 16.2 125.1 138.4

1.36 1.36 2.24 2.48

1.36 1.28 2.15 2.57

100.0

2 3 4

94.1 96.0 103.6

a Polymerization time, 60 min; polymerization temperature, 25 “C; Py monomer concentration, 0.375 mol dme3; oxidant concentration, 0.1 mol dmW3;solvent, 200 dm3 of deionized water; DBS, dodecylbenzenesulfonate.

30,

,3.0

0.1.0 0.00

0.01

0.02

0.03

NaDBS

Concentration

/ moldmm3

0.04 NaANS

Concentration

/ moldmm3

Fig. 1. Conductivity and yield of PPy vs. concentration of surfactantNaDBS. Polymerization temperature, 25 “C; polymerization time, 60 mm; solvent, deionized water 200 dm3; pyrrole monomer concentration, 0.375 mol dmT3; Fe,(SO,), concentration, 0.1 mol dme3; NaDBS, sodium dodecylbenzenesulfonate.

Fig. 2. Conductivity and yield of PPy vs. concentration of surfactantNaANS. Polymerization temperature, 25 “C; polymerization time, 60 mm; solvent, deionized water 200 dm3; pynole monomer concentration, 0.375 mol dmm3; F%(SO,), concentration, 0.1 mol dmm3; NaANS, sodium alkylnaphthalenesulfonate.

to incorporate into the PPy backbone than sulfonate. This can be accounted for as follows: (1) in order for each divalent sulfate to be incorporated into PPy as the dopant, two close cation sites heve to be prepared in the intrachains or interchains of the PPy backbone; (2) the incorporation of divalent and the small-sized sulfate probably produce a distorted PPy backbone. The latter also seems to be the reason why PPy doped with sulfate shows such low conductivity. When only (NH,) .&08 is used as the oxidant, the addition of anionic surfactants caused moderate increases in the yields and great decreases in the conductivities (Table 1). In addition, in those cases, it took a very long time to filter the resultant polymers for the formation of fine colloidal particles. These phenomena can also be explained by interaction between ( NH4) 2S208 and the surfactant: ( 1) PPy doped with

sulfonate is hardly yielded, since the dissociation of the anionic surfactant is prevented due to the presence of the highly concentrated and strongly electrolytic oxidant; (2) the undissociated molecules of the surfactant appear to be thickly adsorbed on the PPy surface in the polymerization process so that it may function as the steric stabilizer. The presence of the steric stabilizer led to the increase in yield, and the decrease in particle size and conductivity of PPy, as Aldissi and Armes [ 121 mentioned. Figs. 1 and 2 show the conductivities and yields of PPy samples prepared under various concentrations of surfactants. If the doping ratio is 0.23 as shown in Table 1, the maximal concentration of the surfactant anion consumed as the dopant is estimated at 0.021 mol dmp3 when 0.1 mol dmm3 of Fep( S04) 3 is used as the oxidant, because the stoichiometric

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Y. Kudoh/Synthetic

molar ratio of Fe3+ to the pyrrole monomer is (2 + 0.23) : 1 in the polymerization reaction. The yield is proportional to the surfactant concentration until near the maximal dopable concentration. This result shows that the ratio of sulfonate for all the dopants is dependent on the surfactant concentration in the polymerization solution. Furthermore, the yield still increases in the higher concentration region than the maximal dopable concentration, such as between 0.021 and 0.030 mol dmV3. This seems to result from the excess surfactant anions that are adsorbed on the PPy surface. The increase in conductivity with increasing surfactant concentration is also caused by the increase in ratio of sulfonate to the whole dopant. After the conductivity shows a peak near the maximal dopable concentration, it approaches a slightly lower constant value. This small decrease in conductivity at the higher surfactant concentration region is due to the presence of surfactant anion adsorbed on the PPy surface. When NaAS was added, the behavior of the yield and conductivity was also similar to Figs. 1 and 2, expect for the maximal conductivity, as shown in Table 1. Fig. 3 displays the change in the yields with polymerization time for PPy samples prepared with various additives. Without additives it took 60 min to reach the saturation point of the yield. On the contrary, when NaDBS and NaANS were added, the times were drastically reduced: about 10 min for the former and 30 min for the latter. The addition of anionic surfactant has the obvious effect of the acceleration of the polymerization reaction. It should be noted that the phenomenon occurs together with the formation of the precipitate. Kuhn et al. [21] reported that pyrrole monomer tends to be adsorbed on the surface of an immersed textile in a lowconcentration chemical polymerization solution, so that a conducting textile compounded of PPy can be effectively produced [ 211. Therefore, the acceleration of the polymerization reaction seems to be ascribed to the precipitate adsorbing the pyrrole monomer so as to function as innumerable initial points of the polymerization reaction. However, as can

2.0 0)

2-NS

Polymerization

Time / min

Fig.~3. Yield of PPy prepared under various conditions vs. polymerization time: none, without additive; DBS, with 0.0225 mol dme3 of sodium dodecylbenzenesulfonate; ANS, with 0.024 mol dmW3of sodium alkylnaphthalenesulfonate; 2-NS, with 0.0225 mol dmm3 of sodium 2-naphthalenesulfonate. Polymerization temperature, 25 “C; solvent, deionized water 200 dm3; pyrrole monomer concentration, 0.375 mol dmm3;Fe2(S0,), concentration, 0.1 mol dme3.

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YE 30 G . 2.= 20.E 4 P IOs _ 01 0

'

' 10

*

'

20

I

)

30

8

'

40

1

50

Polymerization Temperature/ “C Fig. 4. Room-temperature conductivity of PPy prepared with surfactant vs. polymerization temperature: DBS, with 0.0225 mol dm-s of sodium dodecylbenzenesulfonate; ANS, with 0,024 mol dmm3 of sodium alkylnaphthalenesulfonate. Polymerization time, 60 min; solvent, deionized water 200 dm3; pyrrole monomer concentration, 0.375 mol dmq3; Fes(SO,), concentration, 0.1 mol dmm3.

be seen from Table 2, it was found that the increase in the content of Fe was hardly observed in the final product. This implies that the precipitate formed at the early stage gradually redissolves with the progress of the polymerization reaction under the presence of excess pyrrole monomer, so that Fe3+ is consumed by the polymerization reaction. On the other hand, the addition of sodium 2-naphthalenesulfonate neither formed the precipitate nor accelerated the polymerization reaction. It appears to act as a kind of retarder, since the increase in yield of PPy was still observed after 120 min. A similar tendency was also observed when disodium 2,6-naphthalenedisulfonate was added. However, sodium 2naphthalenesulfonate led to increase in yield of PPy. This is caused because monovalent 2-naphthalenesulfonate is easier to be incorporated into PPy than divalent and small-sized sulfate. Fig. 4 shows the room-temperature conductivity of PPy samples prepared with NaDBS and NaANS under various polymerization temperatures. In the case of NaANS, the effect of the polymerization temperature on conductivity is considered negligibly small. With NaDBS, although the conductivity tended to decrease with increasing polymerization temperature, the slope was not so great. This disagrees with the result of Satoh et al. [20] that was obtained from PPy prepared with Fe3+ dodecylbenzenesulfonate (Fe-DBS) as the oxidant in a methanol solution. The reason why the PPy prepared here has such a small dependence of room-temperature conductivity on polymerization temperature is not clear. However, the formed precipitate performs some role in the polymerization reaction and, consequently, the decline in the regularity of the PPy backbone seems to be restrained, even at a high temperature such as 45 “C. This newly developed polymerization method is believed to be useful for industrial application, because PPy having a small scattering of conductivity values can be easily obtained without the severe control of polymerization temperature. Figs. 5 and 6 show the conductivity decay of PPy samples prepared under various conditions when they were aged in

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79 (1996)

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The direct synthesis of PPy doped with a large-sized anion, such as ANS or DBS, is also possible through the use of the corresponding Fe3+ salts as the oxidant. However, for their limited solubility in water, it is necessary to use an organic solvent that is often inflammable, toxic and expensive. Thus, since the above-mentioned new polymerization method can use water as the solvent, it has a great advantage for industrial applications. 10.7'

0

200

400

600

800

1000

1200

Time/h

Fig. 5. Logarithm of normalized conductivity of PPy prepared under various conditions vs. time aged in air at 125 “C: none, without additive; DBS, with 0.0225 mol dmv3 of sodium dodecylbenzenesulfonate added; ANS, with 0.024 mol dmm3 of sodiumalkylnaphthalenesulfonateadded; AS, with 0.022 mol dm-” of sodium alkylsulfonate. Polymerization temperature, 25 “C; polymerization time, 60 min; solvent, deionized water 200 dm3; pyrrole monomer concentration, 0.375 mol dme3; Fe(S04)3 concentration, 0.1 mol dmW3.

4. Conclusions Highly conducting and environmentally stable PPy was prepared by chemical polymerization using the aqueous solution containing Fe,(SO,), and an anionic surfactant, for example, NaDBS, NaANS or NaAS. These improved properties of PPy were ascribed to the large-sized surfactant anions that are effectively incorporated into PPy as the dopant. PPy also showed a small dependence of room-temperature conductivity on the polymerization temperature. Furthermore, the addition of the surfactant accelerated the polymerization reaction. It seems that the insoluble product formed from Fe,(SO,), and the anionic surfactant by metathesis performs some important role for these desirable effects.

Acknowledgements

10-5'

0

200

400

600

800

1000

1200

Time/h

Fig. 6. Logarithm of normalized conductivity of PPy prepared under various conditions vs.time aged at 85 “C and 85% RH: none, without additive; DBS, with 0.0225 mol dmW3 of sodium dodecylbenzenesulfonate added, ANS, with 0.024 mol dmW3of sodium alkylnaphthalenesulfonate added; AS, with 0.022 mol dmm3 of sodium alkylsulfonate. Polymerization temperature, 25 “C; polymerization time, 60 min; solvent, deionized water 200 dm3; pyrrole monomer concentration, 0.375 mol dmv3; Fe,(S04)3 concentration, 0.1 mol dms3.

The author gratefully acknowledges Professor T. Yamamoto, Tokyo Technology Institute, for his valuable suggestions and helpful discussion on preparing this paper.

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433.

S.J. Jasne, US PatentNo. 4 731 JO8 (1988). [4] H. Masuda, S. Tanaka and K. Kaeriyama, J. Chetn. SOL, Gem. [3]

air at 125 “C, and at 85 “C and 85% RH, respectively. Although the condition of 125 “C in air is much severer than that of 85 “C and 85% RH, the PPy samples prepared with the surfactants have enhanced thermal and moisture stabilities. The thermal stability of PPy prepared with NaANS is comparable to that of electrochemically prepared PPy doped with ANS [ 231. The result seems to be more evidence that the anion of the surfactant is incorporated into the PPy backbone. Furthermore, aromatic sulfonate has a stronger tendency to increase the moisture and thermal stabilities than alkylsulfonate. Therefore, the high environmental stability of PPy samples prepared with the surfactant is due to the largesized dopant anion that is difficult to dedope in the regions of such high temperature and humidity.

Commun., [5]

(1989)

725.

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[6]

[7] [8] [9] [lo] [ll]

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[21] H.H. Kuhn, WC. Kimbrell, J.E. Fowler and C.N. Barry, Qrrfh. Met., 55-57 (1993) 3707. [22] J.C. Thieblemont, M.F. Planche, C. Petrescu, J.M. Bouvier and G. Bidan, Synth. Met., 59 (1993) 81. [23] Y. Kudoh, S. Tsuchiya, T. Kojima, M. Fukuyama and S. Yoshimura, Synth. Met., 41113 (1991) 1133. [24] Y. Kudoh, M. Fukuyama, T. Kojima, N. Nanai and S. Yoshimura, in M. Aldissi (ed.), Intrinsically Corldwtirg Polymer: An Emerging Technology, Kluwer, Dordrecht, 1993, p, 191.