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

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, Ts), 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 Ts 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.