Thermal-Field-Tuned Heterogeneous Amorphous States of Poly(vinylidene fluoride) Films with Precise Transition from Nonpolar to Polar Phase

Abstract

In this work, we explore how melt memory, induced by a previous crystalline state, can be used to tune the polymorphs of poly(vinylidene fluoride) during the recrystallization process. In situ experiments were conducted to monitor the melting and chain relaxation of highly oriented α-PVDF crystals during the heating process. Accordingly, a series of heterogeneous amorphous states (induced by self-nucleating the sample) can be made by controlling the thermal treatment temperature (i.e., the self-nucleation temperature, T s ), and the effect of the relaxation state of oriented crystals on the subsequent polymorphic crystallization behavior is studied in depth. A complete α → γ phase transition occurs during the cooling process if the sample is thermally treated in the self-nucleation temperature range of 181−186 °C within the Domain IIa region (i.e., within the melt memory Domain). Only if the oriented molecular chains relax to a certain degree and the 3D structures of the α lamellae are molten, can the retained ordered segments develop γ nuclei in the subsequent crystallization process. If the samples are heated to T s within Domain I (i.e., isotropic melt) or Domain IIb (self-seeding Domain), an α phase is preferred upon subsequent cooling. Hence, a temperature region of 181−186 °C leads to a special heterogeneous self-nucleated melt that can cultivate the rich γ nuclei developed from the relaxation along with segmental flip-flop and inversion motions of oriented molecular chains with a TGTG′ conformation. As a result, the rich γ nuclei enable the formation of pure γ crystals during the cooling process.

Different Crystallization Behaviors of Poly(vinylidene fluoride) Blended with or Coated with Cetyltrimethylammonium Bromide

“…Polyvinylidene fluoride (PVDF) has been attracting intensive interest of academic and industrial research, due to its excellent piezoelectric, pyroelectric, and ferroelectric properties. − It is well-known that PVDF possesses five crystalline structures, i.e., α, β, γ, δ, and ε, in which the β - phase with TTTT conformation and the γ - phase with T 3 GT 3 G′ conformation are the polar phases exhibiting strongest electroactive properties. − They are, however, not easy to fabricate through traditional melt-crystallization processes, because the more kinetically favorable α - phase with TGTG′ conformation is usually formed from the melt state . Consequently, the effective fabrication of PVDF polar phases, especially the β - phase, through a simple approach is desired but remains a challenge.…”

Mechanistic Study of Rhodamine B Piezocatalytic Decomposition Using Poly(vinylidene difluoride) and Related Polymers

“…Fabrication of PVDF with a high content of the polar phases is a challenge since the α-phase is the most stable. The most common method to obtain the β-phase is electric field poling while stretching and cooling the melt. , The γ-phase can be obtained by slow recrystallization from the melt or by deposition from polar solvents. ,− Synthetic modifications can also be used to create polymers that have favorable properties. Substitution of some of the H atoms in PVDF to form the co-polymer [(CHFCF 2 ) x (CH 2 CF 2 ) (1– x ) ] n leads to materials that only crystallize in the β-phase. − The additional dipoles from the increased number of C–F bonds are sufficient to stabilize the β-phase as the ground state in the co-polymers.…”

The γ → β phase-transition behavior of polyvinylidene fluoride under uniaxial drawing

“…We melted PVDF films initially with mixed polar and nonpolar crystals at a specific temperature and then cooled down to room temperature. [31][32][33] Films could be stretched over a wide temperature range and, thus, be used to study the mechanism of the γ-to-β phase transformation in the present work. For comparison, pure α-PVDF films were prepared through a regular melt-crystallization process.…”