Understanding Nonlinear Dielectric Properties in a Biaxially Oriented Poly(vinylidene fluoride) Film at Both Low and High Electric Fields

Understanding nonlinear dielectric
behavior in polar polymers is crucial to their potential application
as next generation high energy density and low loss dielectrics. In
this work, we studied nonlinear dielectric properties of a biaxially
oriented poly­(vinylidene fluoride) (BOPVDF) film under both low and
high electric fields. For fundamental nonlinear dielectric constants
at low fields (<30 MV/m), Novocontrol high-voltage broadband dielectric
spectroscopy (HVBDS) was accurate enough to measure up to the third
harmonics. It was observed that the low-field dielectric nonlinearity
for the BOPVDF disappeared above 10 Hz at room temperature, suggesting
that the low-field dielectric nonlinearity originated from ionic migration
of impurity ions rather than dipolar relaxation of the amorphous segments.
Above the coercive field (EC ≈
70 MV/m), bipolar electric displacement-electric field (D–E) loop tests were used to extract the nonlinear
behavior for pure PVDF crystals, which had a clear origin of ferroelectric
switching of polar crystalline dipoles and domains and nonpolar-to-polar
(α → δ → β) phase transformations.
By using HVBDS, it was observed that the ferroelectric switching of
polar crystalline dipoles and domains in BOPVDF above the EC always took place between 20 and 500 Hz regardless
of a broad range of temperature from −30 to 100 °C. This
behavior was drastically different from that of the amorphous PVDF
dipoles, which had a strong dependence on frequency over orders of
magnitude.