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The surface polymerization of aniline, which precedes the precipitation mode, then produces thicker polyaniline films. The morphology of the films cha...
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Langmuir 2003, 19, 7413-7416

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Surface Polymerization and Precipitation Polymerization of Aniline in the Presence of Sodium Tungstate Irina Sapurina and Svetlana Fedorova Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia

Jaroslav Stejskal* Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic Received April 18, 2003. In Final Form: June 18, 2003 Conducting polyaniline films are spontaneously produced on surfaces immersed in the aqueous reaction mixture used for the oxidation of aniline. When a small amount of sodium tungstate, 0.5-5 mol % relative to aniline, is added into the medium, the induction period of the reaction becomes several times longer and the precipitation polymerization is delayed. The surface polymerization of aniline, which precedes the precipitation mode, then produces thicker polyaniline films. The morphology of the films changes from granular to coral-like at the same time. The conductivity of the polyaniline is moderately reduced as the concentration of tungstate increases.

Introduction Thin polymer films produced on various substrates in situ during the oxidative polymerization of aniline or pyrrole are important for the processing of conducting polymers.1,2 The diverse applications are illustrated by the surface modification of graphite in supercapacitor electrodes,3 image-producing material for optical recording,4 generation of micropatterns by template growth,5,6 acidity7 or ammonia8 sensors, and the encapsulation of carbon black for the environmental removal of chromium.9 Polymer and inorganic microparticles coated with conducting polymers have been studied in connection with electrorheological fluids,10,11 cosmic-dust impact simulations,12 and organic catalysis.13 Membranes coated with conducting polymers14,15 have been used in separation science. All these materials have a common denominator: a conducting polymer film grown on a suitable support. Polyaniline (PANI) is usually prepared by the oxidation of aniline hydrochloride with ammonium peroxydisulfate * To whom correspondence should be addressed. E-mail: [email protected] Fax: 420-296809410. Phone: 420-296809351. (1) Martin, C. R. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Dekker: New York, 1998; pp 409-421. (2) Malinauskas, A. Polymer 2001, 42, 3957. (3) Park, J. H.; Ko, J. M.; Park, O. O.; Kim, D.-W. J. Power Sources 2002, 105, 20. (4) Falca˜o, F. H. L.; de Azveˆdo, W. M. Synth. Met. 2002, 128, 149. (5) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480. (6) Goren, M.; Lennox, R. B. Nano Lett. 2001, 1, 735. (7) Jin, Z.; Su, Y.; Duan, Y. Sens. Actuators, B 2000, 71, 118. (8) Jin, Z.; Su, Y.; Duan, Y. Sens. Actuators, B 2001, 72, 75. (9) Wampler, W. A.; Rajeshwar, K.; Pethe, R. G.; Hyer, R. C.; Sharma, S. C. J. Mater. Res. 1995, 10, 1811. (10) Trlica, J.; Sa´ha, P.; Quadrat, O.; Stejskal, J. J. Phys. D: Appl. Phys. 2000, 33, 1773. (11) Cho, Y. H.; Cho, M. S.; Choi, H. J.; Jhon, M. S. Colloid Polym. Sci. 2002, 280, 1062. (12) Burchell, M. J.; Willis, M. J.; Armes, S. P.; Khan, M. A.; Percy, M. J.; Perruchot, C. Planet. Space Sci. 2002, 50, 1025. (13) Stejskal, J.; Quadrat, O.; Sapurina, I.; Zemek, J.; Drelinkiewicz, A.; Hasik, M.; Krˇivka, I.; Prokesˇ, J. Eur. Polym. J. 2002, 38, 631. (14) Sata, T.; Sata, T.; Yang, W. K. J. Membr. Sci. 2002, 206, 31. (15) Elyashevich, G. K.; Kuryndin, I. S.; Rosova, E. Yu. Polym. Adv. Technol. 2002, 13, 725.

in an aqueous medium.16,17 The control of film thickness18 and morphology19,20 is needed for the successful fabrication of various advanced materials. Termination of the film growth at a suitable moment is the most simple way.19,21,22 The selection of concentrations of reactants and of the polymerization temperature is another way to produce a PANI film of a desired thickness.18,23,24 As the reaction temperature was reduced from +50 to -30 °C, the thickness of the PANI film increased from 50 to 360 nm.18 Besides the factors inherent to PANI, the properties of the surface to be coated are important. The nucleation density in the growth of both PANI and polypyrrole films is enhanced by increasing the hydrophobicity of the surface,5,6,25,26 and it depends also on the surface morphology.6,13 The formation of PANI has two distinct phases: an induction period, in which mainly oligomeric species are produced, followed by the polymerization. It has been demonstrated that the surface polymerization, giving rise to a polymer film on the surfaces immersed in the reaction mixture, precedes the polymerization of aniline in the bulk of the reaction mixture, which produces a PANI precipitate.27-29 The growth of the film during the induction period is virtually terminated by the onset of precipitation poly(16) Trivedi, D. C. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: Chichester, U.K., 1997; Vol. 2, pp 505-572. (17) Stejskal, J., Gilbert, R. G. Pure Appl. Chem. 2002, 74, 857. (18) Stejskal, J.; Sapurina, I.; Prokesˇ, J.; Zemek, J. Synth. Met. 1999, 105, 195. (19) Sapurina, I.; Riede, A.; Stejskal, J. Synth. Met. 2001, 123, 503. (20) Zhang, Z.; Wei, Z.; Wan, M. Macromolecules 2002, 35, 5937. (21) Avlyanov, J. K.; Josefowicz, J. Y.; MacDiarmid, A. G. Synth. Met. 1995, 73, 205. (22) Riede, A.; Helmstedt, M.; Sapurina, I.; Stejskal, J. J. Colloid Interface Sci. 2002, 248, 413. (23) Riede, A.; Helmstedt, M.; Riede, V.; Zemek, J.; Stejskal, J. Langmuir 2000, 16, 6240. (24) Ayad, M. M.; Salahuddin, N.; Sheneshin, M. A. Synth. Met. 2003, 132, 185. (25) Mazur, M.; Krysinˇski, P. Electrochim. Acta 2001, 46, 3963. (26) Liao, C.; Gu, M. Thin Solid Films 2002, 408, 37. (27) Orlov, A. V.; Kiseleva, S. G.; Yurchenko, O. Y.; Karpacheva, G. P. Polym. Sci., Ser. A 2000, 42, 1292. (28) Fedorova, S.; Stejskal, J. Langmuir 2002, 18, 5630.

10.1021/la034667l CCC: $25.00 © 2003 American Chemical Society Published on Web 07/25/2003

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merization, which consumes the available monomer. The precipitate may become attached to the film surface, thus increasing its roughness. As a working hypothesis, we propose that, by controlling the induction period, that is, the time allowed for the film growth, we also predetermine the film thickness. Besides the temperature and concentration of the reactants, the course of aniline oxidation can be accelerated by the addition of so-called mediators which, in minute concentrations, accelerate the formation of conducting polymer.30,31 Introductory experiments have, indeed, shown that the thickness of the simultaneously produced films is reduced many times. Efficient mediators comprise organic compounds that are capable of cation radical formation when oxidized, for example, 1,4-phenylenediamine and its N,N′-disubstituted derivatives.31 Relatively stable cation radicals participate in the electron transfers in redox reactions leading to PANI. PANI itself is a mediator in this sense;32 that is why the polymerization of aniline is autoaccelerated.33 Among the inorganic species, protons are the best-known example because the increasing acidity of the medium promotes the oxidation of aniline.16,32 Iron(III),34 cerium(IV),35 and rare-earth cations36 or hexachloroiridate anions37 have also been reported to promote the oxidation of aniline. As far as we are aware, there is only a single reference to the reverse effect, the prolongation of the induction period in aniline oxidation caused by low concentrations of 1,3-phenylenediamine.31 During tests on the ability of various inorganic compounds to affect the oxidation of aniline, we have observed that the presence of sodium tungstate similarly extends the induction period and, thus, remarkably delays the precipitation polymerization of aniline. The use of this effect in the control of the thickness of PANI films produced in situ during the induction period is demonstrated in the present paper. Experimental Section Aniline hydrochloride (0.2 M; Fluka, Switzerland) was oxidized17 with ammonium peroxydisulfate (0.25 M; Lachema, Czech Republic) in the presence of various concentrations of sodium tungstate. Aniline hydrochloride and sodium tungstate dihydrate (Na2WO4‚2H2O; Fluka, Switzerland) were dissolved in water. The polymerization of aniline was started at 20 °C by introducing an aqueous solution of oxidant, ammonium peroxydisulfate, in a single charge. The mixture was gently stirred to avoid temperature gradients. The course of exothermic polymerization was monitored by recording the temperature of the reaction mixture17,22,38 and, in some cases, also its acidity.31,39 The thickness of PANI films, d, deposited on circular glass supports of 13-mm diameter fixed on adhesive tape and placed in the reaction medium, was assessed from the optical absorption at 400 nm, A400, by using the relation d (nm) ) 185A400 determined earlier.18 The PANI precipitate was collected on a filter, rinsed with 0.2 M HCl and then acetone, and dried in a vacuum at ambient temperature. Its conductivity was determined on a compressed pellet with the four-point van der Pauw method. (29) Stejskal, J.; Trchova´, M.; Fedorova, S.; Sapurina, I.; Zemek, J. Langmuir 2003, 19, 3013. (30) Wei, Y.; Hsueh, K. F.; Jang, G.-W. Polymer 1994, 35, 3572. (31) Stejskal, J.; Kratochvı´l, P.; Sˇ pı´rkova´, M. Polymer 1995, 36, 4135. (32) Stejskal, J.; Sˇ pı´rkova´, M.; Kratochvı´l, P. Acta Polym. 1994, 45, 385. (33) Gregory, R. V.; Kimbrell, W. C.; Kuhn, H. H. Synth. Met. 1989, 28, C823. (34) Segal, E.; Aviel, O.; Narkis, M. Polym. Eng. Sci. 2000, 40, 1915. (35) Fong, Y.; Schlenhoff, J. B. Polymer 1995, 36, 639. (36) Cai, L.-T.; Yao, S.-B.; Zhou, S.-M. Synth. Met. 1997, 88, 205. (37) Kogan, I. L.; Knerelman, E. I.; Shunina, I. G.; Fokeeva, L. S.; Estrin, Y. I.; Sokolov, D. N. Synth. Met. 1995, 69, 133. (38) Fu, Y; Elsenbaumer, R.L. Chem. Mater. 1994, 6, 671. (39) Nakao, H.; Nagaoka, T.; Ogura, K. Anal. Sci. 1997, 13, 327.

Sapurina et al.

Figure 1. Temperature profile of the oxidation of 0.2 M aniline hydrochloride with 0.25 M ammonium peroxydisulfate in the presence of sodium tungstate. The molar ratio of sodium tungstate to aniline hydrochloride, [WO42-]/[Ani], is specified at the individual curves.

Figure 2. Acidity profiles corresponding to the polymerization of aniline shown in Figure 1.

Figure 3. Temperature profiles of aniline hydrochloride polymerizations carried out in the presence of sodium tungstate in 1 M HCl. The polymerization in water is shown for comparison (circles).

Results and Discussion Effect of Sodium Tungstate on the Course of PANI Formation. The process of PANI formation consists of two parts: (1) an induction period followed by (2) the exothermic polymerization of aniline. The evolution of reaction heat allows for monitoring of the polymerization by simply recording the reaction temperature.17,28,35,38 The temperature slightly increases throughout the induction period, rapidly grows during the subsequent polymerization of aniline, and slowly decreases after the poly-

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Figure 4. PANI film prepared in situ on glass in the absence (top) and in the presence (bottom) of 0.002 M sodium tungstate at two magnifications (left and right). 0.2 M aniline hydrochloride was oxidized with 0.25 M ammonium peroxydisulfate in water at 20 °C; [WO42-]/[Ani] ) 0 (top) and 0.01 (bottom).

merization has been completed (Figure 1). When sodium tungstate (2 mol % per aniline monomer) is added to reaction mixture, that is, the tungstate concentration is 0.004 M at a 0.2 M aniline content, the induction period is prolonged about sixfold. The onset of polymerization is, thus, considerably delayed. The slopes of the time dependences of the temperature during polymerization itself do not change, indicating that the polymerization per se has not been affected by the presence of tungstate. This is further supported by the fact that the temperature maxima are about the same. If the polymerization were slower, the maximum temperature would be reduced as a result of enhanced heat dissipation. Similarly, the maximum temperature would be lower if the polymerization yield were reduced. Obviously, this is also not the case. The preliminary experiments have shown that a similar, although less pronounced, retarding effect is also afforded by sodium molybdate. The addition of sodium chromate had no effect on the rate of aniline oxidation. Molybdenum and tungsten salts have a common feature that distinguishes them from other elements: they have the ability to form polyacids in acidic media, whereas a chromate yields only a dichromate. It has been reported that PANI prefers to interact with larger anions.39 The interaction of the voluminous polytungstate anions with reaction intermediates may be the reason for the delayed onset of the polymerization. Acidity Changes During the Oxidation of Aniline. Protons are released during the polymerization of aniline. The course of polymerization can also be followed by an increase in the acidity32,40-43 (Figure 2). The time

dependences of the pH are almost mirror images of the temperature profiles, thus confirming the observations discussed previously. It should be noted that the addition of sodium tungstate does not affect the pH of the starting reaction mixture. The observed prolongation of the induction period, thus, cannot be caused by the potentially altered acidity of the reaction medium after the introduction of tungstate. The formation of aniline is accelerated by the presence of protons. Besides the mediating activity of PANI itself, the increasing proton concentration observed during the polymerization (Figure 2) is another reason for the autoacceleration of aniline polymerization. When the oxidation of aniline has been carried out in a more acidic medium, 1 M HCl instead of water, the formation of PANI is substantially faster (Figure 3). Even under these conditions, however, the influence of tungstate on the prolongation of the induction period is marked. A more detailed inspection of the temperature profiles (Figure 1) reveals a small but steady increase in the temperature during the induction period. This may be attributed simply to the leveling of the temperature to ambient conditions or may reflect a slow and slightly exothermic reaction preceding the polymerization; as a similar response, a decrease in the pH has also been observed in the pH dependences (Figure 2), making the latter possibility more likely. The following explanation is offered: during the polymerization of aniline, a PANI film grows on the substrates immersed in the reaction mixture.18,21 It has been well established that the polymerization at the surfaces precedes the precipitation polymerization.19,27-29 Both a thermocouple and pH elec-

(40) Gospodinova, N.; Mokreva, P.; Terlemezyan, L. Polymer 1993, 34, 2438. (41) Li, D.; Ding, W.; Wang, X.; Lu, L.; Yang, X. Appl. Surf. Sci. 2001, 183, 259.

(42) Stejskal, J.; Sˇ pı´rkova´, M.; Riede, A.; Helmstedt, M.; Mokreva, P.; Prokesˇ, J. Polymer 1999, 40, 2487. (43) Minami, H.;, Okubo, M.; Murakami, K.; Hirano, S. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4238.

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Sapurina et al. Table 1. Conductivity, σ, and Density, G, of PANI Hydrochloride Prepared in the Presence of Sodium Tungstate at 20 °Ca “standard” polymerizationa,b

polymerization in 1 M HCl

[WO42-]/[Ani]

σ (S cm-1)

F (g cm-3)

σ (S cm-1)

F (g cm-3)

0 0.005 0.01 0.02 0.05 0.1

4.37b

1.333b

11.9b

1.326b

3.12 2.51 1.84 1.68 0.23

1.369 1.376 1.408 1.425

10.4 2.98

1.369 1.764

a Molar concentration of aniline hydrochloride was [Ani] ) 0.2 M, and that of ammonium peroxydisulfate was 0.25 M in all polymerizations. b Average value, taken from ref 17.

Figure 5. (a) Temperature profile of the oxidation of 0.2 M aniline hydrochloride with 0.25 M ammonium peroxydisulfate in the presence of 0.01 M sodium tungstate, that is, at [WO42-]/ [ANI] ) 0.05, and (b) the thickness of a PANI film produced at the same time by surface polymerization on glass supports immersed in the reaction mixture.

trode represent “surfaces” where a PANI film grows. This is obvious at the end of an experiment31,44 when these components have to be cleaned. The observed temperature increase and simultaneous pH decrease during the induction period may, thus, reflect the surface polymerization of aniline on the probes rather than the processes occurring in the bulk of the reaction medium. Properties of PANI Films. The globular surface morphology, typical of PANI films prepared on various substrates in situ during the oxidation of aniline (Figure 4, top), changes to a coral-like porous appearance after the addition of a small amount of sodium tungstate (1 mol % relative to aniline; Figure 4, bottom). The formation of nonspherical nanostructures in PANI films has also been observed by Zhang et al.20 and Oh and Im,45 who carried out the polymerization in the presence of surfactants. A similar morphology appears to be quite commonly produced under a variety of reaction conditions.41,46-48 The thickness of the PANI film increases from 130 nm18 to 300-400 nm in the presence of tungstate salt (Figure 5). Improved coating of various substrates with a conducting polymer overlayer, after the addition of tungstate into (44) Manohar, S. K., MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1991, 41, 711. (45) Oh, S.-G.; Im, S.-S. Curr. Appl. Phys. 2002, 2, 273. (46) Stejskal, J. J. Polym. Mater. 2001, 18, 225. (47) Huang, K.; Wan, M. Chem. Mater. 2002, 14, 3486. (48) Pich, A.; Lu, Y.; Adler, H.-J. P.; Schmidt, T.; Arndt, K.-F. Polymer 2002, 43, 5723.

the reaction mixture, is, thus, the obvious practical consequence of the present experiments. Conductivity and Density of PANI. Because PANI is a conducting polymer, the conductivity is a parameter of prime interest. Although the kinetics of PANI formation are greatly affected by the presence of sodium tungstate, the conductivity of PANI products decreases only moderately with an increasing content of tungstate in the reaction mixture (Table 1). As is expected, the conductivity of PANI prepared in more acid conditions, 1 M HCl, is higher. Here again, the trend towards the decrease in the PANI conductivity with increasing tungstate concentration is visible (Table 1). The density of the products increases at the same time. Preliminary studies indicate that this effect is not connected with the incorporation of a tungstate counterion into the PANI but rather with changes in the organization of the PANI chains and a consequent increase in the degree of crystallinity. A detailed study of the polymerization products will be reported elsewhere. Conclusions The oxidation of aniline in an acidic aqueous medium is substantially retarded by the presence of sodium tungstate. The induction period of the reaction is prolonged several times, but the course of subsequent precipitation polymerization is not affected. The conductivity of the resulting PANI is only slightly dependent on the presence of tungstate. The delay in the precipitation polymerization creates better conditions for the preceding polymerization of aniline at surfaces immersed in the reaction mixture. As a consequence, PANI films produced on glass supports in the presence of tungstate were considerably thicker than films produced in its absence, and their globular morphology changed to porous and coral-like. The use of tungstate, thus, may be of benefit when a more efficient coating of substrates with conducting polymer is required. Acknowledgment. The authors wish to thank the Chemistry Department of Materials Sciences, Russian Academy of Sciences (1-03/391), Grant Agency of the Academy of Sciences of the Czech Republic (A 4050313), and the Ministry of Education, Youth and Sports of the Czech Republic (ME 539) for financial support. The conductivity of the samples was kindly determined by Dr. J. Prokesˇ from Charles University in Prague. LA034667L