Structure and Spontaneous Polarization in Fast-Quenched Copolymers of Vinylidene Fluoride and Trifluoroethylene

Abstract

ABSTRACT:Fast quenching from the melt was found to effect greatly the structures and ferroelectric properties of vinylidene fluoride and trifluoroethylene copolymers (P(VDF /TrFE)) in contrast to slow cooling. The results are discussed for high-and low-VDF content divided at about 50 mol% VDF. For the high-VDF copolymers, the quenched samples showed high T, like poled samples, implying an ordered ferroelectric phase. Poling the quenched samples at elevated temperatures produced a ferroelectric polarization the direction of which was restored after cooling from above T,. For the low-VDF copolymers, the quenched samples showed a crystalline phase different from that of the slow-cooled samples, which was found to exist in a metastable state. The quenched samples exhibited no ferroelectricity unlike the slow-cooled samples but poling at elevated temperatures induced pyroelectricity with anomalous temperature dependence. From the present results, the fast quenching seems to produce a phase stable at low temperatures and the poling at elevated temperatures causes crystal growth under an electric field resulting in thermallystable spontaneous polarizations. Crystal structures of the low-VDF copolymers are also speculated based on the present results and published structure models.KEY WORDS Vinylidene Fluoride-Trifluoroethylene Copolymers I Fast Quenching I Melt Crystallization I Ferroelectric Phase Transition I Spontaneous Polarization I Pyroelectricity I Copolymers of vinylidene fluoride and trifluoroethylene (P(VDF/TrFE)) have been extensively studied since the discovery of ferroelectricity in samples with high-VDF content as the first clear example of a ferroelectric polymer.1.2 Ferroelectricity originates in the structural characteristics of the PVDF form I crystal, namely, the all-trans chain conformation for the highest polarization and the pseudohexagonal chain packing to facilitate chain rotation by 60° steps. 3.4 The high-VD F copolymers are of great advantage to the formation of ferroelectricity because the form I crystal is adopted by as-crystallized samples and a high degree of crystallinity is attained by heat treatment. On the other hand, copolymers with VDF content lower than 50 mol% have been found by our study to exhibit intricate ferroelectric behavior, for example, a D-E double hysteresis loop and an abnormally low amount of remanent polarization. 5 Ferroelectricity is a crystal property which is expected to be strongly effected by a method of melt crystallization in semicrystalline polymers, i.e., slow cooling or fast quenching. We have developed a method of fast quenching * Present address:

Crystal‐structure dependence of electroactive properties in differently prepared poly(vinylidene fluoride/hexafluoropropylene) copolymer films

“…Oka et al17 explained that in fast quenching, a sample passes through a high‐temperature region too fast to induce nucleation in an appreciable amount; hence, nucleation mostly proceeds at low temperatures. In slow‐cooling, on the other hand, a sample stays in a high‐temperature region during cooling long enough to complete nucleation at high temperatures.…”

Ferroelectric and piezoelectric properties of a quenched poly(vinylidene fluoride-trifluoroethylene) copolymer

“…The remanent polarizaagreement with the work of Tanaka et al 22 and others. 12,13 These authors concluded that an ortion measured from the first cycle of poling was considerably larger (Ç 100 mC/m 2 ) than that dered ferroelectric phase is formed when the copolymer is crystallized with large undercooling, seen in copolymer films prepared by slow cooling from the melt, after six cycles ( Ç 75 mC/m 2 ; see below T c . The response of ordered ferroelectric copolymer films subjected to electric field poling cy- Fig.…”

“…6 These copolymers also show increases in ied by many researchers. Oka et al 12 and Yang and the Curie transition temperature as the vinyliChen 13 have shown that although PVDF samples dene fluoride content increases. [7][8][9] In addition, prepared by crystallization from the melt generally they generally crystallize from the melt into a result in formation of the nonpolar phase II, which crystal form analogous to the polar phase I or bmust then be mechanically oriented in order to produce the polar phase I, thin films (õ30 mm) prepared by rapid quenching from the melt to infrared (FTIR) studies show that subsequent anstructure of the ferroelectric phase in P ( VDFTrFE) strongly depends on the crystallization nealing of these samples for 50 h at 120ЊC results in a complete transformation to phase I.…”

“…These defects make transformation to the comto that seen in low VDF (Ӷ50 mol %) copolymers. 12 plete ferroelectric phase more difficult on cooling through T c . For the copolymers of VDF and TrFE, the structure and electrical response strongly depend on…”

“…perfection, and high ferroelectric and piezoelectric response. [16][17][18][19][20] In their study, Oka et al 12 concluded from the results of their DTA and x-ray EXPERIMENTAL diffraction experiments that an ordered ferroelectric phase is formed when high VDF content copol-…”