Polypyrrole Actuators: Properties and Initial Applications

Polypyrrole actuators are low-voltage (1–3 V), moderate to large strain (2–35%), and relatively high stress (up to 34 MPa) actuator materials. Strain rates are moderate to low, reaching 11%/s, and frequency response can reach several hertz. Faster response (> 1 kHz) is anticipated in nanostructured materials. Forces can be maintained with minimal power expenditure. This chapter reports on the current status and some of the anticipated properties of conducting polymer actuators. Applications investigated to date include braille cells, shape changing stents, and variable camber foils. Situations where low voltage operation is valuable and volume or mass are constrained favor the use of conducting polymers.

Polypyrrole and other conducting polymers are typically electrochemically driven and can be constructed in linear or bending (bilayer) geometries. Synthesis can be by chemical or electrochemical means, and raw materials are generally very low in cost. These polymers are electronically conducting organic materials. They also allow ions to diffuse or migrate within them. An Increase in the voltage applied to a polymer electrode leads to removal of electrons and an increasingly positive charge within the volume of the polymer. This charge is balanced by negative ions that enter the polymer from a neighboring electrolyte phase (or by positive ions that leave). Ion insertion is generally accompanied by expansion of the polymer. The ions, solvent, and synthesis conditions determine the extent of this expansion, which can be anisotropic. A change in modulus has also been observed as a function of the oxidation state.

Models relating charge, strain, voltage, stress, and current have been developed that allow designers to evaluate the feasibility of designs. One of these modeling approaches is presented with the aim of enabling selection of appropriate device geometry.

The field of conducting polymer actuators is developing rapidly with larger strains, stresses, cycle lifetimes, and rates reported every year. The background needed to understand these developments and to decide if polypyrrole and in general conducting polymer actuators are appropriate for use in a given application is provided.