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

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 T3GT3G′ 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.