A New Technique to Study Kinetics of Chain Conformation in Polyaniline Films

Abstract

Conducting polymers are well known to expand/contract upon electrochemical oxidation/reduction. Such dimensional changes induced by an electrochemical stimulus originate from incorporation of solvated dopant ions into the film and/ or conformational change of -conjugated polymer chains. An increasing number of studies have been devoted to the potential-induced deformations of conducting polymers in an effort to fabricate novel actuators and artificial muscles. Polyaniline (PANI) and its derivatives are a family of conducting polymers that exhibit a significant extent of deformations at low voltages. They have been intensively studied by Kaneto et al. as a key element of soft actuators. Recently, we have found a new type of a potential-induced change in film property of PANI and its derivatives, i.e., a change in the intensity of light reflected from the polymer-coated electrode with an applied potential. The change in reflection was interpreted as below. In the reduced state of the PANI film where polymer chains have a coiled structure, a light scattering or a diffuse reflection takes place in the polymer layer and thus a negligible intensity of light reaches the photodetector placed at a right position of specular reflection. By increasing the potential, polymer chains stretch and the coiled structure relaxes as has been demonstrated by MacDiarmid and Epstein. Consequently, Rayleigh or Mie scattering is attenuated in the polymer film and the incident light can go deep into the polymer layer to reach a highly reflective Pt surface. Then, specular reflection at the Pt surface occurs effectively and the reflected light gains its intensity. When the potential is raised further, the PANI film turns a blue-black color. At the higher potentials, therefore, the PANI film starts to absorb the incident 633 nm light and the intensity of light coming to the Pt surface reduces. In the present study, a new method for investigating kinetics of chain conformation for the PANI film is proposed on the basis of the in situ reflection technique. Aniline of reagent grade from Aldrich was distilled under Ar atmosphere and stored in a Schlenck tube filled with Ar gas. PANI was deposited on Pt-coated glass plates by a potential-sweep method with 1.3M aniline in 2.3M perchloric acid (HClO4). Electrode potentials were referred to Ag/AgCl/ NaCl(satd.). After polymerization by cycling the potential between 0:1 and 0.9V at 50mV s , the PANI film was rinsed thoroughly with distilled water, dried in air lightly, and transferred to a thin layer cell of Figure 1 for in situ reflection measurements. Thicknesses of PANI films were evaluated by measuring the amount of electricity during a potential-step doping in HClO4. The oxidation level was defined as the number of unit charges per aniline unit and calculated from the polymer weight, a molecular weight of an aniline unit, and the doping charge. In situ reflection technique with a He-Ne laser (633 nm) is described earlier. The optical arrangement is schematically depicted in Figure 1, where the PANI electrode is slightly tilted from the two parallel glass plates to avoid a light reflected directly at the front glass surface. A current signal of a Si photodiode was converted to a voltage using a current/voltage preamplifier and the voltage signal was fed to a lock-in amplifier. Before starting measurements, the potential of the PANI electrode was kept at a sufficiently negative potential so as to obtain a reduced form of PANI, i.e., leucoemeraldine. Just prior to the measurement, the system was switched to a galvanostatic mode and a constant anodic current was applied to the electrode. A time course of the intensity of a reflected light was measured till the electrode potential reaches 0.5V. The electrode potential was rested at 0:2V till the cathodic current density fell down to 0:2 mAcm 2 or below. Then the next current pulse was applied to the PANI electrode. Figure 2 depicts a set of light intensity vs. charge curves for a PANI film in HClO4 (pH 1) obtained in the galvanostatic Si photodiode Slit PANI/Pt/Cr/glass Electrolyte solution Silicone spacer