The tuning of crystallization behavior of ferroelectric poly(vinylidene fluoride‐<i>co</i>‐trifluoroethylene)

Bai, Xiao; Li, Haikuo; Chu, Xiao; Liu, Mengling; Li, Huihui; Wang, Haijun; Sun, Xiaoli; Yan, Shouke

doi:10.1002/pol.20230296

Cited by 6 publications

(1 citation statement)

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“…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 …”

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confidence: 99%

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

Li,

Liu

et al. 2024

Macromolecules

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Poly(vinylidene fluoride) (PVDF) is a polymorphic semicrystalline polymer. To utilize its piezo-, pyro-, and ferroelectric applications, it is ideal to fabricate the polar phases directly. Another way to obtain a polar PVDF is to transform the original nonpolar α crystals into their polar γ counterparts through the solid α−γ′ phase-transition. Therefore, a better understanding of the solid α−γ′ phase-transition is of great importance and significance. Here, the PVDF α−γ′ phase-transition process has been tracked in situ in real space using a polarized optical microscope based on the synchronous phase-transition accompanied by the change in birefringence. Thus, the propagation of the phase-transition along and perpendicular to the radial directions of the spherulites was quantitatively determined. It is found that the phase transition along the radial direction is almost 3 times faster than that in the tangential direction (1.07 vs 0.39 μm/min). Moreover, the effect of the crystallization temperature and subsequent annealing temperature on the phase-transition behavior has been explored. It is confirmed that annealing at a high temperature below the onset melting temperature provides thermal energy for the phase-transition and thus endows a high phase-transition efficiency and speed. The transition speed decreases, however, when the annealing temperature is close to the peak melting temperature. This is associated with the melting of the lamellae, which is adverse to the transformation of TGTG′ to T 3 GT 3 G′ conformation and consequently hinders the phase-transition. It is further demonstrated that the crystallization temperature of the sample affects the solid α−γ′ phase-transition in the subsequent annealing process as well. The sample crystallized at a higher temperature with an increased crystallinity and thickened lamellae facilitate the phase-transition. These results provide more insight into the solid α−γ′ phase-transition of PVDF.

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