Plasma modification of the internal polymer surfaces and polarization fatigue in cellular-polypropylene ferroelectrets

Dielectric barrier discharges (DBDs) in the voids of ferroelectrets generate a physico-chemically active plasma. Due to memory effects, repeated DBDs can facilitate the charging process in cellular PP ferroelectrets. It was found that the threshold voltage may be reduced and that the charging efficiency is obviously improved after only 10³ DBD cycles. On the other hand, the micro-plasma phenomena of the DBDs modify the inner polymer surfaces through chain scission and surface oxidation, and thus deteriorate the chargeability of the voids, leading to polarization fatigue. Also, the stability of the piezoelectricity in fatigued cellular PP ferroelectrets can be much worse than that of a freshly charged film. The piezoelectric d₃₃ coefficient that fatigued films can hold quasi-permanently decays according to a stretched exponential function that directly depends on the number of alternating cycles of the driving voltage.