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Surface and Precipitation Polymerization of Aniline 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 February 26, 2002. In Final Form: April 30, 2002
Introduction Polyaniline (PANI), one of the most common conducting polymers, is prepared by the oxidation of aniline in an acidic aqueous medium and obtained as a precipitate.1,2 It has been often observed that glass surfaces immersed in the reaction mixture become coated with a PANI film.1,3-5 Typical film thickness is about 40-500 nm depending on polymerization conditions.6 Films are produced not only on glass but on virtually any material that is stable in the reaction medium.7 Conducting and colored PANI films find uses as sensors for ammonia8-10 and in the surface modification of various substrates.11 It has been proposed that aniline molecules or lowmolecular-weight intermediates adsorb at surfaces and initiate the growth of PANI chains.12 This concept assumes that chains grow preferably perpendicularly to the surface and produce polymer brushes. Experimental evidence based on FTIR spectra,13,14 optical anisotropy, and molarmass measurements14 supports the feasibility of such structure formation. The polymerization of aniline proceeds at the surface (giving rise to a PANI film), or in an aqueous medium (yielding a PANI precipitate) or both. It was experimentally observed that various materials introduced into the mixturessuch as fibers, textiles, and inorganic and polymer particlessaccelerate the formation of PANI.11,15 At the same time they become coated with a PANI overlayer.7 This suggests that the presence of interfaces * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +420-2-3535-7981. (1) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss. Chem. Soc. 1989, 88, 317. (2) Stejskal, J.; Kratochvı´l, P.; Jenkins, A. D. Polymer 1996, 37, 367. (3) Avlyanov, J. K.; Josefowicz, J. Y., MacDiarmid A. G. Synth. Met. 1995, 73, 205. (4) Rajapakse, R. M. G.; Chandani, A. D. L.; Lankeshwara, L. P. P.; Kumarasiri, N. L. W. L. Synth. Met. 1996, 83, 73. (5) Riede, A.; Helmstedt, M.; Riede, V.; Zemek, J.; Stejskal, J. Langmuir 2000, 16, 6240. (6) Stejskal, J.; Sapurina, I.; Prokesˇ, J.; Zemek, J. Synth. Met. 1999, 105, 195. (7) Malinauskas, A. Polymer 2001, 42, 3957. (8) Jin, Z.; Su, Y.; Duan Y. Sens. Actuators, B 2001, 72, 75. (9) Nicho, M. E.; Trejo, M.; Garcı´a-Valenzuela, A.; Saniger, J. M.; Palacios, J.; Hu, H. Sens. Actuators, B 2001, 76, 18. (10) Hu, A.; Trejo, M.; Nicho, M. E.; Saniger, J. M.; Garcı´a-Valenzuela, A. Sens. Actuators, B 2002, 82, 14. (11) Kuhn, H. H.; Child, A. D. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A, Elsenbaumer, R. L., Reynolds, J. R., Eds.; Dekker: New York, 1998; Chapter 35, pp 993-1013. (12) Sapurina, I.; Riede, A.; Stejskal, J. Synth. Met. 2001, 123, 503. (13) Wu, C.-G.; Yeh, Y.-R.; Chen, J.-Y.; Chiou, Y.-H. Polymer 2001, 42, 2877. (14) Sapurina, I.; Osadchev, A. Yu.; Volchek, B. Z.; Trchova´, M.; Riede, A.; Stejskal, J. Synth. Met. 2002, 129, 29. (15) Tzou, K.; Gregory, R. V. Synth. Met. 1992, 47, 267.
plays an important role in the polymerization process. It was further reported that, surprisingly, the polymerization at the surfaces precedes the precipitation polymerization in the bulk.12,16,17 The films are therefore produced on the surfaces even before the polymerization in the whole reaction volume has started. The explanation, based on the concept of heterogeneous catalysis, has only recently been offered.14,17 Two centuries ago, Faraday observed some organic reactions proceed more readily, or even exclusively, at surfaces.18 Langmuir later suggested that the adsorption on the solid surface involves forces similar to those concerned in chemical valence. Because of the alteration of the electron-density distribution in adsorbed molecules, the reactivity of such molecules thus may be higher than of that in the surrounding medium. We expect that aniline or low-molecular-weight oxidation intermediates, e.g., an aniline cation-radical, would adsorb at any available surfaces (interfaces). Adsorbed molecules thus may have an enhanced ability to initiate the growth of polymer chains. If this were the case, one should observe an increase in the oxidation rate after the introduction of material with a high surface area. Such an experiment is demonstrated in the present study by using silica gel as a chemically inert template with a high specific surface area. Experimental Section Aniline hydrochloride (Fluka, purum, 0.02 mol, 2.59 g) was dissolved in water to 50 mL of solution, and a similar solution of ammonium peroxydisulfate (Fluka, purum 0.025 mol, 5.71 g) was prepared. Both solutions were kept at 20 °C and then mixed in a free-standing beaker to start the oxidation of aniline to PANI. Silica gel (Fluka, unmodified type 100 for chromatography; particle size 63-200 µm, specific surface 400 m2 g-1, apparent density 0.4 g cm-3) was used as the material providing the surface for the adsorption of aniline oxidation intermediates. A 100 mL portion of the reaction mixture was poured over 10 g of silica gel. In one experiment the reaction mixture was left at rest; in the other, the silica gel suspension was stirred during the polymerization with a magnetic stirrer at about 150 rpm. The course of aniline polymerization was followed by monitoring the temperature of reaction mixture with one or two digital thermometers. The content of silica in the coated particles was determined as an ash.
Results and Discussion The properties of silica gel decorated with PANI have been reported in several papers,19-22 but the process of coating has not yet been followed. In the present experiment, the aqueous reaction mixture prepared by mixing the solution of aniline hydrochloride and ammonium peroxydisulfate was poured over the silica gel in a beaker. The oxidation of aniline is exothermic and can be monitored by temperature changes.23,24 One thermometer (16) Orlov, A. V.; Kiseleva, V. G.; Yurchenko, O. Y.; Karpacheva, G. P. Polym. Sci. A 2000, 42, 1292. (17) Riede, A.; Helmstedt, M.; Sapurina, I.; Stejskal, J. J. Colloid Interface Sci. 2002, 248, 413. (18) Atkins, P. W. In Physical Chemistry, 3rd ed.; Oxford University Press: Oxford, 1986; pp 762-789. (19) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325. (20) Kuramoto, N.; Yamazaki, M.; Nagai, K.; Koyama, K.; Tanaka, K.; Yatsuzuka, K.; Higashiyama, Y. Thin Solid Films 1994, 219, 169. (21) Orlov, A. V.; Yurchenko, O. Y.; Kiseleva, S. G.; Razuvaeva, V. S.; Karpacheva, G. P. Polym. Sci. A 2001, 43, 572. (22) Stejskal, J.; Quadrat, O.; Sapurina, I.; Zemek, J.; Drelinkiewicz, A.; Hasik, M.; Krˇivka, I.; Prokesˇ, J. Eur. Polym. J. 2002, 38, 631. (23) Fu, Y.; Elsenbaumer, R. L. Chem. Mater. 1994, 6, 671.
10.1021/la025665o CCC: $22.00 © 2002 American Chemical Society Published on Web 06/07/2002
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Figure 1. The course of surface polymerization is monitored by the increase of temperature, T1, in the silica gel phase. The precipitation polymerization takes place in the supernatant liquid, the temperature of which, T2, also increases. Figure 3. Temperature profile of aniline polymerization carried out in a stirred medium in the presence (full circles) and in the absence (full triangles) of silica gel. Reaction conditions are the same as in Figure 2. The dependences for the coexisting phases in the unstirred system (Figure 2) are shown for comparison (open symbols).
Figure 2. Temperature profile observed during the unstirred polymerization of aniline hydrochloride in the presence of silica gel (a) in a silica gel sediment (open circles) and (b) in a supernatant liquid (open triangles): 0.2 M aniline hydrochloride, 0.25 M ammonium peroxydisulfate, 10 g of silica gel per 100 mL of liquid mixture.
was placed in the lower part of the beaker in the silica gel phase, the second in the upper part in the supernatant reaction mixture (Figure 1), and the temperatures were recorded as functions of time. During this experiment the mixture was not stirred. The temperature in the silica gel phase starts to increase earlier than that in the supernatant liquid (Figure 2), indicating that the polymerization takes place preferentially in the silica gel phase. The temperature reaches a maximum and the silica gel phase starts to cool. This means that the polymerization at the silica gel surface came to an end before a similar process in the supernatant liquid had started. The depletion of reactants in the silica gel phase seems to be responsible for the termination of polymerization. Aniline and peroxydisulfate, however, are still present and ready for the polymerization in the supernatant liquid. This is obvious from the marked increase of the temperature recorded on the second thermometer placed in the supernatant liquid after 5 min of reaction time. The temperature of the silica gel also slightly increases due to heat transfer between the phases. The polymerization in the upper phase is finished in 10 min, and the mixture starts to cool. The decrease in the temperature is more marked in the upper phase; because of the larger heat capacity afforded by the silica gel, the temperature decrease is less pronounced there. If we make an analogous experiment in which the mixture is stirred, the onset of the surface polymerization is the same as that in the preceding case (Figure 3, circles). The temperature of the agitated suspension is about the average of the temperatures recorded in the individual (24) Fong, Y.; Schlenoff, J. B. Polymer 1995, 36, 639.
phases in the previous experiment. One would expect to observe two temperature steps: the first, corresponding to surface polymerization, and the second, related to precipitation polymerization. This is not the case, and the increase in temperature is smooth. It has been well established that the polymerization of aniline is autoacceleratedsthe presence of PANI accelerates the oxidation of aniline.15,25 Obviously, PANI produced on the silica gel surface stimulates the polymerization in the surrounding suspension medium by this mechanism. That is why the polymerization in the bulk proceeds under these conditions faster than that in the supernatant liquid over the silica gel in the preceding experiment. By comparing the course of precipitation polymerization in the absence of silica gel and in the supernatant liquid over the silica gel in the unstirred experiment, we see a close similarity (Figure 3, triangles). This means that the concentration of reactants in the supernatant liquid has not been affected by the surface polymerization of aniline in the silica gel phase and by depletion of reactants there. In an unstirred medium, the diffusion of reactants between phases is too slow to cope with the concentration gradients in the time allowed for the reaction. The slightly higher temperature of the supernatant liquid, observed between 5 and 7 min of reaction time, is caused by the diffusion of warmer liquid from the lower silica gel phase to the cooler medium above. Finally, the maximum temperature of the supernatant liquid at the end of polymerization is higher compared with that of the stirred reaction medium in the absence of silica gel. In a stirred experiment, the heat loss at the beaker walls is higher and thus the total temperature increase is lower. Moreover, in an unstirred supernatant, the temperature gradient is anticipated to form in the supernatant liquid, the temperature being higher close to the center of the reaction mixture, where the thermometer has been located, and lower at walls. Properties of Coated Silica Gel. PANI content in the coated silica was 12.9 and 10.8 wt % PANI in unstirred and stirred experiments, respectively. A 0.84 g quantity of PANI hydrochloride is produced theoretically from 1 g of aniline hydrochloride, and the experimentally achieved conversion is close to 100%.26 If the same amount of PANI is formed also in the presence of silica, then 1.48 and 1.21 g of PANI, respectively, would be deposited on 10 g of (25) Stejskal, J.; Sˇ pı´rkova´, M.; Kratochvı´l, P. Acta Polym. 1994, 45, 385. (26) Stejskal, J.; Gilbert, R. G. Pure Appl. Chem., in press.
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silica gel. This means that 68 and 56 wt % of the total amount of produced PANI is on silica gel, and the rest is obtained as an accompanying PANI precipitate. Both components are easily separated by the difference in the sedimentation rate. Obviously, by increasing the content of silica gel, the fraction of the free PANI precipitate would be reduced. A PANI film produced on glass under the same reaction conditions has a thickness6 of 133 nm. The density of PANI hydrochloride26 is 1.33 g cm-3; the specific mass of the film is thus 1.77 × 10-5 g cm-2. If silica gel particles having an average size of 130 µm and apparent density of 0.4 g cm-3 were assumed to be spherical, they would have an external surface of 1150 cm2 g-1 (the total surface afforded by pores is much higher, 400 m2 g-1). The fraction of PANI in the silica gel particles coated with a 133 nm PANI overlayer would thus be 2.3 wt %, i.e., much less than we find. This means that PANI coats also the internal pores of the silica gel. If we assume the above apparent density and the true SiO2 density to be27 2.1 g cm-3, the estimated fraction of pores is 81 vol %. If the pores were completely filled with PANI, the particles would contain 76 wt % PANI. The fact that the observed content of PANI is much (27) Stejskal, J.; Kratochvı´l, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstedt, M.; Prokesˇ, J.; Krˇivka, I. Macromolecules 1996, 29, 6814.
Notes
lower suggests that the most of porosity has been preserved. The BET surface analysis indeed indicated that the silica gel surface was reduced by 13% after coating with PANI. Conclusions The oxidative polymerization of aniline at surfaces precedes the precipitation polymerization in the bulk volume of the aqueous phase. This is illustrated by the faster formation of PANI in the presence of a substrate with a high specific surface area, such as silica gel. The effect is explained by the heterogeneous catalysis of the PANI-chain initiation afforded by a surface. The coating of silica gel with a PANI overlayer does not take place only on the external surface of particles but also in the pores. The better understanding of the coating mechanism is likely to improve the quality of conducting polyaniline films deposited on the surface of various substrates. Acknowledgment. This work was supported by Academy of Sciences of the Czech Republic (K 4050111). S.F. participated in the UNESCO and IUPAC-sponsored Postgraduate Course in Polymer Science organized by the Institute of Macromolecular Chemistry in Prague. LA025665O
