ABSTRACT:Yinylidene Fluoride/Trifluoroethylene copolymers with compositions 47/53 and 37/63mol% exhibit a ferroelectric square D-E hysteresis loop at low temperatures (below -40'C) and undergo transition to a paraelectric motional phase near T, = 70"C. At intermediate temperature they exhibit a double-hysteresis loop suggesting that polarization reversal occurs through a non-polar phase. Time domain measurement of polarization reversal shows two-step switching transient consisting of fast depolarization and slow re-polarization processes. The remanent polarization imparted in the ferroelectric phase shows gradual decay on heating and disappears completely well below T,. We conclude that these copolymers take an antifcrroelectric-likc non-polar phase between low temperature ferroelectric and high temperature paraelectric phases.KEY WORDS Yinylidene Fluoride/Trifluoroethylene Copolymer ! Ferroelectricity ! Antiferroelectricity I Double Hysteresis I Switching I Depolarization I Copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE) have attracted much attention for the past fifteen years because of their ferroelectric properties and associated piezoelectric and pyroelectric activities. t -3 Of special importance are copolymers containing 50-80mol% VDF which exhibit fast polarization reversal as well as a clear Curie point Tc. When VDF content exceeds 80mol%, Tc is not observable at atmospheric pressure because it is overtaken by melting. When TrFE becomes a major component, ferroelectricity is obscured and finally disappears in polytrifluoroethylene.The ferroelectric phase of a VDF-rich copolymer consists of all-trans molecules packed in parallel to each other in a pseudo-hexagonal lattice. In the paraelectric phase, molecules undergo active rotational motion by adopting a random sequence ofTT, TG, and TG' bonds. Thus, spontaneous polarization is lost intramolecularly. The transition is of first order judging from thermal hysteresis. The 70/30mol% VDF/TrFE copolymer has Tc at llOoC on heating and 70oC on cooling. As VDF content decreases to 50mol%, thermal hysteresis is lost and the transition becomes second-order.A copolymer with equi-molar composition is complex in that it takes a diffuse phase when cooled from the paraelectric phase. The X-ray diffraction profile associated with (110)/(200) interchain reflections consists of broad double peaks. Tashiro et at. 3 designated such a phase as the "cooled phase" and assigned it to an aggregate of tilted monoclinic crystalline domains connected by gauche or skew bonds. Lovinger et at. 4 and Horiuchi et at. 5 proposed a two-phase structure consisting of well-ordered and less-ordered pseudohexagonal phases. In any case, such a diffuse phase is transformed into a regular ferroelectric phase showing a single (110)/(200) reflection peak by applying a sufficiently high electric field if VDF content is slightly larger than 50 mol%. When VDF content becomes smaller than 50 mol%, the diffuse phase is maintained and ferroelectricity is obscured.Koizumi et at. 6 re...