Synthetic Metals 151 (2005) 25–42 Polyaniline actuators Part 1. PANI(AMPS) in HCl Elisabeth Smela a,∗ , Wen Lu b , Benjamin R. Mattes b a Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA b Santa Fe Science and Technology, Inc., Santa Fe, NM 87505, USA Received 27 December 2004; accepted 14 February 2005 Available online 17 May 2005 Abstract Drawn polyaniline films and fibers doped with 2-acrylamido-2-methyl-propane-1-sulfonic acid, PANI(AMPS), were electrochemically cycled in HCl and their material properties and actuation performance comprehensively characterized. The Young’s modulus was obtained as a function of applied voltage. Actuator figures of merit were derived from isotonic and isometric measurements, including strain, stress, work, power, creep, and efficiency. The effects of sample length, solution pH, electrochemical driving method, frequency, and load were studied, as well as the response of current to applied load for sensing applications. This work presents a complete picture of a polyaniline actuator for the first time. The behavior of the actuator is discussed in terms of the changes in the oxidation and protonation states of polyaniline. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Fiber; Actuator; Strain; Modulus; Metrics 1. Introduction Polyaniline (PANI) has attracted considerable interest as an electroactive polymer actuator (EAP) [1–35]. Actuators have been fabricated from both chemically and electro- chemically synthesized polyaniline, as well as substituted polyanilines. Various material and actuator properties have been studied, primarily in HCl, although other acids and ionic liquids have also been used. Because of the disparate materials, preparation methods, and cycling conditions, a full picture of the performance metrics and behavior of one material–electrolyte system has been unavailable. In this paper, we present a thorough characterization of the mechan- ical and actuation properties of highly conducting stretched polyaniline films and fibers doped with 2-acrylamido-2- methyl-propane-1-sulfonic acid, PANI(AMPS). To allow comparison with most previously published studies, the samples were cycled in 1 M HCl. This work should allow potential actuator users to evaluate the merits of this material, ∗ Corresponding author. Tel.: +1 301 405 5265; fax: +1 301 314 9477. E-mail address: smela@eng.umd.edu (E. Smela). as well as to understand some of the limitations of polyaniline actuators. The currently accepted model for the molecular structure and oxidation levels of polyaniline is represented in Fig. 1, showing explicitly for the first time all the pathways that give rise to actuation. Electronic and ionic charges and protons are transferred when the pH and oxidation levels are changed [36–39]. Solvent transfer also occurs, but this is not yet suf- ficiently understood to be included in the model. Polyaniline is electroactive in acids below pH 3–4, at which it can undergo electrochemical reduction and oxidation (redox) between three states: leucoemeraldine (an insulator), emeraldine salt (a conductor), and pernigraniline (an insu- lator). The cyclic voltammogram therefore has two pairs of peaks corresponding to the two transitions (see Fig. 4a). In aqueous electrolytes, the extent of protonation depends on the solution pH (increasing left to right in Fig. 1). For leucoemeraldine, the pK a is between 0 and 1 [36]: pro- tonation begins at approximately pH 2 and is completed at pH -1 [40,41]. The pH at which the polymer proto- nates/deprotonates depends on the acid, however [42]. In naphthalene sulfonic acid, for example, leucoemeraldine is 0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.03.009