STRONG PIEZOELECTRIC ACTIVITY IN SIMULTANEOUSLY STRETCHED AND POLED POLY (VINYLIDENE FLUORIDE)

Piezoelectric properties of Poly (vinylidene fluoride) films in stretched, annealed and poled states were investigated in terms of function of annealing conditions. With increasing the annealing temperature, a lattice spacing, d (110, 200), in form I crystal decreases and the crystallinity increases; however, the dipole orientation induced by the field becomes imperfect. Consequently, for the roughly packed crystal, it could be expected that poling would facilitate the dipole orientation.From the above result, the new method of electret preparation in PVDF and its related copolymer is proposed. This method, which consists of simultaneous drawing and poling, can apply the field to necking zone where the transition of form II to form I occurs. The sample with unstable crystal structure can easily be modified to have strong piezoelectric constant (d31_??_40pC/N) by this method.