Influence of unipolar electric fields on the behavior of dielectric elastomer actuators based on plasticized acrylonitrile‐butadiene rubber (NBR)

Commercial acrylonitrile‐butadiene rubber (NBR) containing large amount of plasticizer was used as base material for dielectric layers and compliant electrodes of dielectric elastomer actuators (DEA). The deformational behavior and dielectric properties of the DEA were characterized by combined measurements of polarization‐electric field (P‐E) as well as deformation‐electric field (S‐E) hysteresis loops. Unipolar electric fields were used to stimulate the actuators since theoretical considerations reveal that with unipolar fields larger deformations will be achieved compared to bipolar fields. Surprisingly, it turned out that the P‐E‐ and S‐E hysteresis loops exhibit a strong time dependency if a unipolar electric field is applied for actuators with a dielectric containing plasticizer. In contrast, no time dependency was observed for samples without plasticizer and in the case of bipolar fields. By means of additionally performed DC loss current measurements indication were obtained, that due to the unipolarity of the electric fields polar or ionized plasticizer molecules can be slightly accumulated at the anode. Finally, it is shown that application of unipolar electric fields results in larger actuator deformations compared to the case of simple bipolar sinusoidal stimulation. The experimental results are in good agreement with a theoretical approach which reveals that the resulting deformation is composed of the first harmonic and the second harmonic oscillation, superimposed by a constant offset value.