Polarization Reversal and Double Hysteresis Loop in Copolymers of Vinylidene Fluoride and Trifluoroethylene

The polarization reversal and the ferroelectric hysteresis loop have been studiedfor copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) with 47.2, 63.0, 70.7, and 86.8 mo1% TrFE at high electric fields of up to 100 MVm-1 and at frequencies of 5 to 100 mHz over a wide range of temperatures. The copolymers exhibited two current peaks associated with the polarization processes at different field intensities: one at a higher field Eh was due to the polarization reversal; the other at a lower field E¿ occurred in the opposite polarity at high temperature, indicating the depolarization process. These two current peaks were related with a double hysteresis loop in the copolymers. As the temperature was lowered, the field E¿ decreased crossing zero and changed in the polarization field. The temperature T¿ at which E¿ reduced to zero was 321, 277, 253, and 239 K for 47.2, 63.0, 70.7, and 86.8 mo1% TrFE content, respectively. It is suggested that the crystalline form associated with the ferroelectric polarization in these copolymers is slightly different above and below the temperature T¿.