Are cellular polypropylene ferroelectrets ferroic?

Non-polar polymer foams can be rendered piezoelectric by internal charging of their cavities through a series of dielectric barrier discharges (DBDs). The cavities with their positive and negative charges are man-made macroscopic dipoles that function in analogy to the dipoles in ferroelectrics and that also constitute a macroscopic electric polarization. In ferroelectric materials, polarization-versus-electric-field (P(E)) hysteresis curves are observed. Here, P(E) hysteresis curves are obtained on cellular polypropylene ferroelectrets by means of voltage-voltage-mode measurements. The charge transferred during the internal DBD charging is determined from the voltage on a standard capacitor that is connected in series with the sample and that has a much larger capacitance than the sample. Another standard capacitor, with a capacitance much smaller than that of the former, but still substantially larger than that of the sample, is also connected in series and serves as a high-voltage divider that protects the electrometer in case of destructive breakdown in the sample. It will be shown how the DBDs inside the cellular foam lead to a phenomenological hysteresis curve that is the same as the hysteresis loops found in other ferroic materials.