The tuning of crystallization behavior of ferroelectric poly(vinylidene fluoride‐co‐trifluoroethylene)

Abstract

Poly (vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)] has three crystal forms, including paraelectric α, ferroelectric β, and γ phases. In previous studies, the properties and performances of P(VDF‐TrFE) have been the focus of research. However, the formation mechanism and regulation mode of various crystal forms remain unclear. Therefore, it is an important topic for further research to elucidate, summarize, and prospect the polymorphism of P(VDF‐TrFE) and regulate the crystal forms. This review systematically summarizes the crystalline structure and phase transition between ferroelectric and paraelectric phase of P(VDF‐TrFE) crystals; discusses the influence of annealing, blending and electric field on the crystallinity, selection of polymorphic crystals, and phase transition behavior between them; reviews the effects of annealing, melt‐recrystallization, substrate and nanoconfinement on the crystal orientation. Finally, the effects of the crystal structure of P(VDF‐TrFE) on its properties are briefly summarized.

Disclosing Solid-Phase-Transition Mechanism from Nonpolar to Polar Poly(vinylidene fluoride) via In Situ Real-Space Visual Methods

“…The manipulation of polymorphic phases of polymers has consequently received widespread concerns. − Poly­(vinylidene fluoride) (PVDF) is a typical polymorphic semicrystalline polymer with at least three common crystal forms: α, β, and γ. − The α form is thermodynamically most stable and accessible. , It is, however, not wanted owing to its nonpolar nature. Therefore, either avoidance of its formation or conversion of it into the polar phase for piezo-, pyro-, or ferroelectric application has to be made. − Among the various polar phases, the β phase exhibits the highest polarization, resulting in excellent electrical properties. − Despite the superior polarization performance of the β phase, its practical application and development are often limited because of the harsh conditions required for its formation, including the use of nucleating agents, , high electric fields, , rapid quenching, or strong external force fields. , By contrast, the γ phase also displays higher polarity and can be obtained by a simple thermal treatment, such as direct crystallization from the melt at a high temperature, , or annealing the α phase at proper temperature through the solid phase transition. − Even though the γ crystals obtained via solid phase transition exhibit exactly the same crystal structure, they possess yet a much higher melting temperature (around 185–190 °C depending on the chemical structure) than those directly obtained from melt crystallization and thus been referred to as the γ′ phase. ,,− Notably, the Curie temperature of PVDF is expected to be higher than the melting temperature; thus, the polarity of the γ′ phase is expected to be kept above its melting temperature, which is significantly higher than that of the widely used piezoelectric poly­(vinylidene fluoride- co -trifluoroethylene) (P­(VDF-TrFE)) copolymers in the range of 55–128 °C. − Hence, it enables the development of heat-proof piezoelectric …”