Structural Aspects at Phase Transitions in Ferroelectric Copolymers of Vinylidenefluoride and Tetrafluoroethylene

Abstract

Permittivity and conductivity low-voltage temperature dependences of a ferroelectric copolymer of vinylidenefluoride VDF with tetrafluoroethylene TFE in the ferroelectric to paraelectric phase transition range were investigated. The as received films obtained by crystallization from a solution had the metastable structure shown in particular in coexistence of ferroelectric and paraelectric phases below a Curie point. Temperature dependences of electrical characteristics in the first and second cycles of heating differ. Low glass transition (−40 ºC) facilitate intensive micro-Brownian chains mobility in the amorphous phase at ambient temperatures and above. As per the x-ray and spectroscopic data, the first heating cycle will result in a recrystallization process for molecular structure improvement. Asymmetry of temperature dependence of a dielectric susceptibility in a vicinity of "order-disorder" phase change is found out. For the first time paid attention to the fact that for crystalline polymer ferroelectrics interpretation of the parameters of the temperature dependences of the dielectric susceptibility in the cycle of heating near the phase transition point should take into account the possibility of the premelt processes. First the method of IR spectroscopy shows that the melting of ordered structures, emerging secondary crystallization is accompanied by a change of the basic parameters (frequency position, the integral intensity and half-width) component of the doublet of valence vibrations of С-Н groups. Such structural changes lead to the emergence of "anomalies" on the temperature dependences of conductivity.The phase transition ferroelectric to paraelectric mechanism research is usually done on copolymers P(VDF-TrFE), in which Curie point T c and melting point T m considerably differ (Wang, 1988; Nalva, 1995;Kochervinskii, 1999) more often. In this paper, the order-disorder transition is investigated on copolymer VDF-TFE of composition 71/29 for which both a chain chemical microstructure , and crystallographic parameters (Kochervinskii, 2010) had being studied earlier. It was shown, that in the copolymer isotropic films been crystallized from a low-boiling solvent at room temperature, T c and T m temperate values differ slightly. It was founded out, that the order-disorder transition having the first kind phase transition type. The electric parameter anomalies in the close proximity to a ferroelectric-paraelectric transition are depends on www.ccsenet.org/jmsr

Local piezoelectric response, structural and dynamic properties of ferroelectric copolymers of vinylidene fluoride–tetrafluoroethylene

“…As can be seen, samples #1 and #2 are significantly different. Previously, we have shown [25,26] that both the copolymers at the selected film forming method crystallized in a metastable state. The copolymer composition VDF-TFE 94:6 (sample #1) shows metastability since the crystallization proceeds as a β-and γ-phase [25].…”