Modeling of the bending of an electroactive pseudo trilayer based on PEDOT, a semiconductor polymer

Abstract

Electroactive polymers (EAP) are smart materials that can be used as actuators, sensors or energy harvesters in many fields. We had previously studied an ionic metal-polymer composites (IPMC), which consists in an ionic polymer film such as Nafion saturated with water and coated on both sides with a thin layer of metal acting as electrodes. This system bends when it is subject to an electric field orthogonal to the film and can thus be used as an actuator. Conversely, the deflection of the film generates a potential difference between the electrodes ; the same system can therefore be used as a sensor.We have developed a ''continuous medium'' model for this system. The thermodynamics of linear irreversible processes had enabled us to establish its constitutive equations.We are currently interested in a system of close properties based on PEDOT, a semiconductor EAP. The central part of the device consists in two interpenetrating polymers playing the role of an ions reservoir. The PEDOT is polymerized on each side and forms an interpenetrating network with the two other polymers. A pseudo trilayer is obtained, the two outer layers containing the PEDOT acting as electrodes. It is then saturated with an ionic liquid. When the blade thus obtained is placed in an electric field orthogonal to its faces, the PEDOT undergoes a reduction reaction (or dedoping) on the side of the negative electrode, which attracts cations from the central part and therefore swells ; the blade ultimately bends towards the positive electrode.We have first adapted our model to this two-components system : the cations on the one hand, and the three polymers and the anions on the other hand. We have written its balance equations and thermodynamic relations first at the microscopic scale for each phase, then at the macroscopic scale for the whole material using an averaging technique. The thermodynamics of linear irreversible processes then provides its constitutive relations : a Kelvin - Voigt type stress-strain relation and generalized Fourier's and Darcy's laws. The equations obtained were applied to the case of a cantilevered blade subject to a continuous potential difference at constant temperature. The numerical resolution of the equations system enabled us to draw the profiles of the different quantities, which are very steep functions near the electrodes. We also evaluated the tip displacement and the force that must be exerted on the free end of the beam to prevent its displacement (blocking force). The results obtained are in good agreement with the experimental data published in the literature.