Structure Formation and Depolarization Relaxation in Porous Polyvinylidene Fluoride Piezofilms

Piezoactive porous polyvinylidene fluoride films obtained by melt extrusion followed by isometric annealing, uniaxial stretching, and thermal fixation have been studied. It is shown that two competing orientational processes occur under the uniaxial extension of the annealed films: the polymorphic transition of the nonpolar crystalline α-phase to the polar piezoactive β-phase and the formation of a porous structure. It has been established that the degree of orientation of the extruded films is a key factor determining the efficiency of both processes. The thermally stimulated depolarization method has been used to study the dipole relaxation in the oriented structure of the films and to determine the activation energy of the investigated processes. The films have been polarized by a corona discharge and a high-voltage contact method; the dependences of the piezoelectric modulus on the polarization conditions have been obtained. The maximum piezoelectric modulus d 31 = 30.1 pC/N has been reached in the films polarized in a corona discharge field.