Stochastic System Identification of the Compliance of Conducting Polymers

Conducting polymers such as polypyrrole, polythiophene and polyaniline are currently studied as novel biologically inspired actuators. The actuation mechanism of these materials depends upon the motion of ions in and out of the polymer film during electrochemical cycling. The diffusion of ions into the bulk of the film causes the dynamic mechanical compliance (or modulus) of the material to change during the actuation process. The mechanism of this change in compliance is not fully understood as it can depend on many different factors such as oxidation state, solvation of the film and the level of counter ion swelling. In-situ measurement of the dynamic compliance of polypyrrole as a function of charge is difficult since the compliance depends upon the excitation frequency as well as the electrochemical stimulus. Pytel et al [1] studied the effect of the changing elastic modulus in-situ at a fixed frequency. In this study we describe a technique to measure the compliance response of polypyrrole as a function of frequency and electrochemistry. A voltage input and a simultaneous stress input was applied to polypyrrole actuated in neat 1butyl-3-methylimidazolium hexaflourophosphate. The stress input was a stochastic force with a bandwidth of 30 Hz and it allows us to compute the mechanical compliance transfer function of polypyrrole as function of the electrochemistry. Our studies show that the low frequency compliance changes by 50% as charge was injected into the polymer. The compliance changes reversibly as ions diffuse in and out of the film which indicates that the compliance depends upon the level of counter ion swelling. INTRODUCTION Electroactive conducting polymers are currently studied as materials that can have a wide range of applications including novel biologically inspired actuators, sensors, valves and pumps. They have a number of attractive properties such as being lightweight, inexpensive and are easy to mold and shape into a wide range of forms. Various electroactive polymers that are activated by ion diffusion are studied for use in a wide variety of applications. The actuation mechanisms in these materials are based on the diffusion of ions in and out of the polymer film. The films are capable of generating large strains of between 5-10% at low voltages (1 to 2 V) which make them ideal elements to build artificial devices that can mimic biological elements [2]. During electrochemical stimulus as charge is injected into the polymer there is an expansion of the polymer film that accounts for a large portion of the overall strain. However, there are a number of underlying mechanisms that can also govern polymer actuator behavior. Changing material properties during the actuation cycle is one such mechanism that has not been studied well. For instance, as ions diffuse into the polymer the compliance of the polymer also changes [1, 3-6]. If the polymer is held in tension then this changing compliance also contributes to the overall strain. This can be best understood from Equation 1 where εT is the total strain on the polymer, α is the strain to charge ratio, q is the charge injected into the polymer, Y is the compliance which can depend on the charge and excitation frequency and σ is the preload stress. Mater. Res. Soc. Symp. Proc. Vol. 1134 © 2009 Materials Research Society 1134-BB07-03