Antiferroelectric-Like Behavior of Vinylidene Fluoride/Trifluoroethylene Copolymers with Low Vinylidene Fluoride Content

Abstract

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...

Polarization of block textured films of ferroelectric copolymers of vinylidene fluoride with tetrafluoroethylene in an external field

“…6a) is usually observed in dipole glass [17] and in classical antiferroelectrics; however, it occurs often in polymer ferroelectrics as well [17,32]. Moreover, this shape of the D(E) curve allowed the authors of [33] to claim that antiferroelectricity exists in this class of polymers. This claim should be verified, and, more precisely, we should talk about the apparent similarity because the structural differences between the antifer roelectric crystals and the studied crystallizing poly mers are obvious.…”

Ferroelectric Polymers Exhibiting Negative Longitudinal Piezoelectric Coefficient: Progress and Prospects

“…Considering these curve evolution with the decrease of VDF content, polarization hysteresis loops evolve from ferroelectric to antiferroelectric‐like type (Figure 4a), which is accompanied by a remarkable smearing of the butterfly peaks (i.e., VDF = 45 mol%, Figure 4b). The copolymers with antiferroelectric‐like loops (VDF < 49 mol%) are not in a true antiferroelectric phase due to the absence of typical strain response characteristic of antiferroelectric materials (Figure 4b). In addition, Figure 4b shows that the emergence of relaxor behavior for VDF ≤ 55 mol% largely improves the strain response, as relaxor ferroelectrics are known to exhibit remarkably larger electrostriction strain than normal ferroelectrics …”

Nonlinear Dielectric Investigation of Trifluoroethylene-Rich Copolymers of Vinylidene Fluoride

“…In the range of 30-50 mol% VDF content, copolymers exhibit a double hysteresis loop at room temperature, implying that polarization reversal occurs through a nonpolar state. [4][5][6] It has been shown that the double loop is converted into a ferroelectric square loop as the temperature is decreased. At high temperatures, they undergo transition to a paraelectric motional phase.…”