A Structural Investigation of Polypyrrole as a Function of Oxidation State
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A Structural Investigation of Polypyrrole as a Function of Oxidation State Mya R. Warren and John D. Madden University of British Columbia ABSTRACT Polypyrrole exhibits actuation under electrochemical doping and undoping of the polymer matrix. Active strain and x-ray diffraction measurements are performed in order to correlate the microscopic and macroscopic structural changes occurring during actuation. Under galvanostatic reduction, the film shows first a linear contraction with oxidation state due to expulsion of small anions. At 50% of the as-grown doping level, the film reverses direction and begins to expand, possibly due to incorporation of the larger cations. X-ray diffraction, however, does not show a linear contraction or expansion in the crystalline portions of the polymer. The polymer crystallites go through a sudden contraction in one dimension of approximately 14%, and otherwise remain constant. The contraction in the polypyrrole crystals occurs at the 50% doping level, and may be correlated with the reversal of macroscopic actuation. INTRODUCTION Conducting polymers are known to experience volume changes as they are electrochemically doped and undoped under low voltages. This effect has made these materials good candidates for application as light-weight linear actuators, or artificial muscles. Polypyrrole has been the most studied conducting polymer for this application due to its chemical stability, high conductivity and ease of manufacture. Polypyrrole actuators exhibit peak stresses one hundred times that of mammalian muscle, but fall short of real muscle when it comes to strain and strain rate. Strains of 2-5% are typical, and actuation usually occurs at rates of 1 Hz or less [1,2]. Despite much progress in the design of conducting polymer actuators, the mechanisms of actuation are still not well understood. While the volume changes can almost certainly be attributed to the flux of ions during redox cycling [3], the full electromechanical response depends on a complex interconnection of the chemical, electrical and mechanical properties. The one common factor affecting all of these properties is the structure of the polymer matrix [4-9]. The evolution of the microscopic structure during actuation is unknown. In this study, x-ray diffraction is used to investigate the structural changes in the polypyrrole matrix as it is electrochemically doped and undoped. The active strain occurring under the same electrochemical conditions is also measured in order to understand the relationship between the microscopic structural changes and the macroscopic actuation. EXPERIMENT Polypyrrole is polymerized through electrochemical oxidation of the pyrrole monomer using the method of Yamaura [8]. The electrolytic solution is composed of 0.06M distilled pyrrole, 0.05M tetraethylammonium hexafluorophosphate (TEA·PF6) and 1%vol water in propylene carbonate (reagents from Sigma-Aldrich). The solution is deoxygenated by bubbling with nitrogen before growth. A polished, glassy carbon crucible is used as the work
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