Interferometric measurement of the transverse strain response of electroactive polymers

Abstract

Some electroactive polymers produce large electric-field-induced strains that can be used for electromechanical actuation. The measurement of the strain response, especially the dynamic response under high driving fields, is difficult. We have developed a transverse strain measurement system based on the Zygo laser Doppler interferometer. The system can measure transverse strain responses of polymer samples of different sizes over a wide displacement range and a frequency range from DC up to 100 Hz. We have used this interferometric system to investigate the strain response of Maxwell stress actuators fabricated from silicone (Dow Corning HS III RTV) and thermoplastic polyurethane (Dow Pellethane 2103) films. The static and dynamic strain responses of the materials to a variety of driving electric fields such as step fields, AC fields and DC bias fields have been measured as functions of amplitude and frequency. The strain response has a quadratic relationship with the driving field and shows a strong dependence on the frequency of the applied field. Of the two kinds of polymers investigated, HS III silicone polymer shows higher strain and breakdown fields. High transverse strains of 3.25 % (static) and 2.08 % (dynamic at 1 Hz) for HS III silicone polymers have been obtained. In addition, the effect of mechanical tensile load on the transverse strain has also been studied. The experimental data are interpreted in terms of measured material properties and small strain models for dielectric film actuators.

The strain response of silicone dielectric elastomer actuators

“…This happens because the electrostatic attraction that causes the Maxwell stress only occurs over the electroded area whereas the elongation is measured over the entire film including the non-electroded inactive areas and so, when the inactive areas are relatively smaller the elongation is larger. Figures 10 and 11 also show that the elongation first increases with pre-strain, reaches a maximum at a pre-strain of around 15% and then decreases as the pre-strain is further increased, which is similar to earlier observations 5,6,7,8 . In Figure 12 the FEA model predictions for the electric-field-induced elongation as a function of pre-strain for the HSIII silicone specimen with the narrow inactive edge are compared with our observed values for various applied electric fields.…”

“…This shows that the stress -strain relation of a hyperelastic material can be represented by Hooke's law when the strain is very small, and it explains why the linear elastic model describes the behavior of the load-free actuator fairly well 2,5 given that the deformation due to applied electric field is normally not very large (the highest deformation reported was 3.25%). Figure 2 shows the agreement between our experimental data and Equation (7), which is represented by the continuous line in the figure.…”

“…This assumption is fairly good if the film actuator is narrow enough (the ratio of the length to the width is 10:1). Our detailed experimental investigations of the transverse strain response of silicone and polyurethane elastomers 5,6,7,8 suggested that the geometry as well as the material properties determined the measured strains. High transverse strain required not only good material properties, but also optimized actuator geometry.…”

The transverse strain response of electroactive polymer actuators

“…According to Equations (1) 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50th Anniversary Conferencenot only on the applied electric field, but also on the dielectric constant. The frequency dispersion of the dielectric constant of the silicone film over the frequency range from 0.01 Hz to 100 kHz, measured with the impedance analyzer, shows that the dielectric constant decreases with frequency [7], which leads to a decrease of the strain response. In addition to changing material properties, the decrease in dynamic strain with increasing frequency is also due to inertial effects, i.e.…”

“…The maximum strain value measured is 2.08% at a field of 26.3 MV/m, which is smaller than the static strain value (3.25%) at a similar amplitude of the electric field as shown in Figure 2. The transverse strain generally decreases with frequency [7]. According to Equations (1) 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50th Anniversary Conferencenot only on the applied electric field, but also on the dielectric constant.…”