Role of charge carriers in long-term kinetics of polyurethane electroactuation

Abstract

Previous work has shown that dielectrophoretic body forces resulting from permittivity and conductivity heterogeneities only partially contribute to the overall electromechanical deformation of segmented polyurethanes (PUs). In this work, we studied the experimental kinetics and electric current of PU thin films over a long time period (1–105 s) for different applied electric fields. Then, we thoroughly analyzed the drift behavior of electric carriers and its macroscopic effect using simple modeling and numerical simulation. The main assumption is that the macroscopic deformation results from the accumulation of electric charges near the electrodes, leading to local stretching. Assuming that the mobilities of negative and positive carriers are different, their migration towards the electrodes will have different kinetics. A good agreement between simulation and experimental data was obtained for bending amplitude and current evolution. Furthermore, the resulting compression is consistent with the observed electrostriction.

Electric space charge threshold observation in polyurethane under high electric fields

“…This result agrees well with our previous study on the long-term kinetics of PU, where a linear relationship was observed between current density and electric field below 8 MV/m. 19 It is clear that I middle and I last were obtained arbitrarily by turning off the voltage, and that these values could be lower if a longer step voltage was applied. Nevertheless, this figure quantifies the observation that the current response was rectangular (similar to the applied step voltage) at low voltage, but was more trapezoidal at high voltage.…”

“…2, and the polymer conductivity σ can be estimated from the fitted line to reveal an electrical conductivity of σ = J/E 0 ≃ 2.7 × 10 −9 S/m, a value similar to the results of a prior investigation. 19 To analyze the effect of time on a PU sample under a high field, the step voltage was applied on a longer time scale. The plot in Fig.…”

“…The J-E plot in Ref. 19 was restricted to 10 MV/m, and linearity was assumed, although the points above 8 MV/m were a little higher than the fitted curve and might indicate non-linearity. More generally, Fig.…”

“…This effect takes 1-10 2 s to fully develop. 2,4,[14][15][16][17][18][19] The theoretical expressions of M m have an implicit time dependency through the permittivity ε and Young modulus Y. Dynamic characterization of PU reveals relaxations of the dielectric constant and the mechanical shear modulus.…”

“…A polymer chemically doped with NaCl has been shown to exhibit a slow drift of Na + and Cl − ions along its thickness due to the electric field. 19 Two charge layers were created close to the electrodes. A numerical calculation estimated that each charge layer would expand laterally, thus reducing the polymer thickness.…”