In the Laboratory
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Electropolymerized Conducting Polymer as Actuator and Sensor Device An Undergraduate Electrochemical Laboratory Experiment María T. Cortés* and Juan C. Moreno Department of Chemistry, Universidad de los Andes, Carrera 1 #18A-10, Bogotá, Colombia; *
[email protected] Since the discovery of the intrinsically conducting polymers in the 1970s, these materials have been extensively studied owing to their potential applications. In fact, the Nobel Prize in Chemistry for the year 2000 was awarded to Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa for the discovery and development of conductive polymers (1). The conducting properties, volume changes, ability to store charge, color changes, and porosity changes among others make of these polymers multifunctional materials (2, 3). There is currently a tremendous increase in both academic and corporate research on technological applications of these polymers. A number of recent articles have appeared in multidisciplinary journals summarizing the technological applications of these polymers (4); a few lab experiments based on these polymers have been outlined in this Journal (5–7). At the beginning of the last decade, the first actuator devices based on intrinsically conducting polymer (CP) were proposed (8, 9). The performance of these actuators focused on the volume changes undergone by the polymer when submitted to an electrical current (3, 10). The volume change of the CPs can be exploited in actuators in numerous ways: in layered devices that bend when one or more CP layers is the active layer, in fibers that elongate and contract, in bundles, valves, and so forth. The use of these polymers as actuator materials has been widely studied in the recent years owing to the resemblance to muscles. The CPs have an array of potential applications as artificial muscles since they are capable of producing moderate displacement when they are submitted to an electrochemical reaction. For a detailed review of artificial muscles based on these polymers see ref 11. Experimental Overview We describe the synthesis of ClO4−-doped polypyrrole (PPy) films and the fabrication of a trilayer device: PPy film兾adhesive polymer兾PPy film that shows actuating and sensing properties. This device bends under the influence of about 10 mA of current or 1 V potential. The bending angle can be up to ±180 in a LiClO4 aqueous solution. The performance of this device is comparable to the bimetal effect in thermostats and is discussed in detail in the Supplemental Material.W No external electrode is required in the trilayer device. One of the PPy films acts as the working electrode and the other PPy film as counter and reference electrode. This is a compact system that does not require auxiliary electrodes. The movement of the trilayer actuator is controlled by an electrochemical reaction. The direction and movement rate of 1372
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this actuator are controlled by the applied current and the position by oxidation depth of PPy films (12). This can be demonstrated by calculating the consumed charge during different extent of bending under several electrical conditions. The consumed energy during a given bending can be related to the cell temperature or to the electrolyte concentration. In this way, the trilayer is a compact device working as actuator and sensor simultaneously. To fabricate a trilayer actuator (PPy film兾adhesive polymer兾PPy film), it is necessary to obtain ClO4−-doped PPy films. The electrochemical method is advantageous in terms of the ease of controlling the growth rate and the film thickness, the enhanced electrical properties, and the relatively inexpensive polymerization procedure. The experiment can be adjusted to fit either one or two lab periods: one lab of six hours or two labs of three hours. One lab can be used to synthesize the doped PPy films (three hours) and the other lab to fabricate the trilayer device and to evaluate the sensing and actuating properties (three hours). If conducted over two lab periods, the polymer should be evaluated within 72 hours of its synthesis. The coated working electrode should be kept in a desiccator after the synthesis. These experiments are appropriate for visualizing and understanding smart materials, electrochemistry and polymer science concepts. They are suitable for courses dealing with electroactive material and electrochemical synthesis and characterization. The synthesis of conducting polymers as actuating and sensing devices is suitable for undergraduate materials laboratory and electrochemical laboratory experiments. The objectives of this experiment are: (i) electrochemical synthesis of ClO4− doped PPy, (ii) fabrication of a sensing and actuator device based on PPy, (iii) observation of the bending ability of this device in aqueous solution, and (iv) performance of temperature monitoring by this device. Summary of the Procedure
Synthesis of ClO4−-Doped PPy Films In the first part of this procedure ClO4−-doped PPy films are electrochemically synthesized. The setup requires a fourcompartment cell and a power source that generates potential square waves (potentiostat). The potentiostat should generate square waves of potential from 300 mV for 2 s to 872 mV for 8 s for a period of 90 minutes, versus Ag兾AgCl reference electrode (13). The power source is capable of generating either a constant voltage or a constant current to the working electrode for the evaluation of the trilayer (potentiostat–galvanostat).
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In the Laboratory
The electrodes are made of stainless steel, an inexpensive material. A Ag兾AgCl reference electrode is used, which can be made easily (14). The polymerization solution is 0.25 M pyrrole and 0.1 M LiClO4 in acetonitrile with 2% water (all reagent-grade) according to the method reported by Otero (8). The solution should be deaerated for 10 minutes by bubbling of nitrogen. A sketch of the setup is shown in Figure 1. Once the polymerization is finished, the PPy films are submitted to voltage of 800 mV for 100 s in polymerization solution (LiClO4 in acetonitrile, 2% water, without monomer) to obtain the oxidized polymer.
Fabrication and Evaluation of the Trilayer The fabrication method of the trilayer requires a doublesided adhesive tape sandwiched between two PPy films. The movement evaluation of the trilayer allows observing its actuating and sensing properties. Copper wire can be used to make the electrical contacts between the trilayer and a power source. To evaluate the movement, a current or voltage is applied to the trilayer in LiClO4 aqueous solution (any concentration from 0.5 to 1 M). To observe the sensing properties, the influence of the temperature or the electrolyte concentration on the movement can be studied. In the first case, a thermostated cell is needed to control the temperature.
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This experiment is written to serve as a laboratory project and to provide experience in smart materials and electrochemistry. It is used to demonstrate the potential application of conducting polymers to convert electrical energy into mechanical energy at low voltage or current. The performance of the device is explained using electrochemistry and solidstate chemistry. In this article, a PPy-based actuator is shown; however, other conjugated conducting polymers can be studied and evaluated as actuating and sensing materials. The ClO4−-doped PPy is easily synthesized and the experiment can be adapted to sense temperature or electrolyte concentration. Acknowledgment MTC wishes to thank Science Faculty of Universidad de los Andes for financial support. She also wishes to thank Toribio Fernández Otero for leading her doctoral thesis in the University of the Basque Country and for his valuable guidance and continuous support. www.JCE.DivCHED.org
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N2
CE: counter electrode RE: reference electrode
deposited polypyrrole
Figure 1. A schematic drawing of the electrochemical cell and potentiostat used to synthesize polypyrrole.
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Summary
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WE: working electrode
Hazards Appropriate precautions should be taken in working with reagents for the synthesis of the polypyrrole and the characterization of the trilayer. Pyrrole is harmful by inhalation, ingestion, or skin absorption. Acetonitrile is highly flammable, an irritant, and toxic by inhalation, ingestion, or skin absorption. It may cause serious damage to the eyes and is a possible teratogen. Lithium perchlorate is a strong oxidizer and incompatible with combustible materials. It is a skin, eye, and respiratory irritant. Sulphochromic mixture is a corrosive combination of strong, extremely corrosive and highly toxic reagents.
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Supplemental Material
Background material on conducting polymers, a detailed experimental procedure, and a student handout are available in this issue of JCE Online. Literature Cited 1. The Nobel Prize in Chemistry Home Page. http:// www.nobelprize.org/chemistry (accessed May 2005) 2. Mac Diarmid, A. G. Angew. Chem. 2001, 113, 2649. 3. Otero, T. F. In Handbook of Organic Conductive Molecules and Polymers, Vol. 4; Nalwa, H. S., Ed.; John Wiley & Sons: New York, 1997; p 519. 4. Saxena, V.; Malhotra, B. D. Current Appl. Phy. 2003, 3, 293. 5. Sadik, Omowunmi A.; Brenda, Sharin; Joasil, Patrick; Lord, John. J. Chem. Educ. 1999, 76, 967. 6. Lewis, T. W.; Wallace, G. G. J. Chem. Educ. 1997, 74, 703. 7. Bunting, Roger K.; Swarat, Karsten; Yan, DaJing; Finello, Duane. J. Chem. Educ. 1997, 74, 421. 8. Otero, T. F.; Angulo, E.; Rodríguez, J.; Santamaría, C. J. Electroanal. Chem. 1992, 341, 369. 9. Baughman, R. H. Makromol. Chem., Macromol. Symp. 1991, 51, 193. 10. Jager, E. W. H.; Inganäs, O.; Lundström, I. Science 2000, 288, 2335. 11. Cortés, M. T.; Moreno, J. C. e-polymers 2003, No. 41. http:// www.e-polymers.org (accessed May 2005). 12. Otero, T. F.; Cortés, M. T. Chem. Commun. 2004, 3, 284– 285 13. Cortés, M. T.; Vera, E.; Duran, O.; Moreno, J. C. Instrum. Sci. Technol. 2004, 32, 479. 14. East, Gaston A.; del Valle, M. A. J. Chem. Educ. 2000, 77, 97.
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