Infrared Spectroscopic Studies of Thin Polyaniline Films
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INFRARED SPECTROSCOPIC STUDIES OF THIN POLYANILINE FILMS CAROL KORZENIEWSKI AND DAVID M. SEEGER Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109.
ABSTRACT The electrochemistry of polyaniline depends upon the pH of the deposition solution and the nature of the anion present during deposition. When the pH of the deposition solution is below 1.0, the response is typical of the para-substituted phenyleneamine structure. When the pH of the deposition solution is greater than 1.0, an additional voltammetric wave appears, and the response is a function of the dopant anion. This report presents results of an in situ infrared spectroscopic study aimed at elucidating structural features which determine the anion dependent voltammetric response. INTRODUCTION Polyaniline is an organic material which exhibits near metallic conduction when partially oxidized [1]. In contrast to other conducting organic polymers, nitrogen atoms along the polyaniline backbone act as sites for oxidation and form an integral part of the conducting structure. As these nitrogen sites can be protonated, pH dependent electrical properties are observed [2]. Shown in Scheme I are proposed structures for the neutral and oxidized states of polyaniline [2-5]. The most highly conducting form has been assigned to structure Ila [2-4]. However, a continuum of oxidation states is possible [6] which gives rise to a repeat unit of varying length, and the extent of backbone protonation will change depending upon the pH of the surrounding solution. Indeed, conductivity is observed only over a narrow range of pH between pH 1 and pH 4 [2]. Further, internal redox processes can occur which result in the formation of semiquinone radical cations which can delocalize along the chain [3,6]. In recent years, identification of the structures which are important for the electrical properties of polyaniline have been sought using electronic absorption spectroscopy [2,7], electron spin resonance [8,9], infrared [10-16], Raman [17-19] and solid state nmr [3]. The electrochemical response of polyaniline is complicated by the many structures possible for the oxidized form. The cyclic voltammetry of polyaniline is characterized by two oxidation waves, the positions of which depend upon solution pH. The half wave potential of the first oxidation wave appears at about +0.2 V vs SCE and is independent of pH above pH 0 [6]. (All potentials reported herein are with respect to the saturated calomel electrode (SCE)). In strongly acidic media (at pH 0 and below), this first oxidation wave shifts to lower potential by 59 mV/pH unit [6]. The second wave appears in the range between +0.2 and +0.6 V and is a strong function of pH, shifting to more positive potentials by about 120 mV/pH unit as the pH is decreased. Decomposition limits the oxidative limit to about +0.5 V so that the second oxidation wave cannot be observed below about pH 0 [6]. The precise voltammetric response of polyaniline is a function of the preparation conditions. Typically, films are prepared from
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