Enhanced Dielectric and Ferroelectric Properties of Poly(vinylidene fluoride) through Annealing Oriented Crystallites under High Pressure

The structural manipulation of the electroactive β phase of poly(vinylidene fluoride) (PVDF) is particularly important in sensor and actuator applications. Herein, an efficient way to enhance dielectric and ferroelectric properties of PVDF films by annealing preoriented PVDF films through thermal treatment combined with the pressure field is proposed. During annealing processing, an appropriate pressure is attributed to the efficient dipole rotation and results in complete phase transformation of the nonpolar α phase (TGTG′) into the polar β phase (TTTT). Moreover, the appropriate pressure and temperature fields synergistically promote a more perfect alignment of the main chain in β-crystallites along the stretching direction. Thus, the pure β phase with ultrahigh orientation (Herman's orientation factor >0.97) is successfully obtained. Moreover, the enhanced mobility of molecular chains with the increase of temperature contributes to the perfection of β-crystallites with high crystallinity. The relaxation of oriented chains in the amorphous region at high temperature during the annealing process is obviously inhibited by high pressure, leading to increased density of dipoles capable of efficient rotation under an electric field. The unique structure obtained imparts a distinctly enhanced dielectric and ferroelectric properties to the PVDF films. The highest dielectric constant at room temperature is observed in preoriented films annealed at 160 °C due to the optimal chain orientation. Moreover, the film with more perfect and tightly packed β-crystallites annealed at 180 °C shows clearly ferroelectric switching and the maximum remnant polarization (5.8 μC/cm2). The outcomes of this work indicate that a rational combination of pressure and temperature fields could effectively achieve optimal dielectric and ferroelectric properties of PVDF oriented films.