Effect of Annealing temperatures and time on Structural Evolution and dielectric Properties of PVdF Films

There are multiple polymorphic crystalline forms (such as α, β, γ, δ) of Poly(vinylidene fluoride) (PVDF). These crystal phases are different from each other due to two main factors: the relative orientations of the polymorphic molecules in crystal structures and the sequential orders of the torsion angles. Most of the previous research has been focused on the αand β-phases, since α-crystal is thermodynamically stable at room temperature and β-phase exhibits large piezoelectric and dielectric properties. Thus β-phase is also known as the electro-active phase. The β-phase with all-trans bond (TTTT) conformation is comprised of fluoride atoms on one side and hydrogen atoms on the opposite side of the polymer chain backbone, leading to the highest net dipole moment among these four different polymorphs1-2. There are two main factors which are critical for a crystal material to demonstrate high ferroelectric response: the net dipole moment of crystal structure and the identical oriented dipoles in the bulk material3. Hence β-phase PVDF displays large piezoelectric and dielectric properties. Owing to wonderful mechanical flexibility, high dielectric strength, low cost and also processing advantages, the β-phase PVDF has wide application in piezoelectric sensors and actuators and energy conversion/storage4-6.