Piezoelectric activity and field-induced crystal structure transitions in poled poly(vinylidene fluoride) films

Publisher Website
Google Scholar
Abstract
Unoriented and biaxially oriented phase‐II poly(vinylidene fluoride) films were poled at very high fields up to 4.0×106 V/cm at room temperature. Changes in molecular orientation and crystal structure induced by the high‐field poling were studied using x‐ray and DSC methods. The piezoelectric coefficients d31 and e31 together with the dielectric response (ε′ and ε″) were determined using a Piezotron Model U (Toyo Seiki, Tokyo) for the poled films. The piezoelectric activity did not increase steadily with poling field, but showed four distinct regions depending on poling field. At low fields (Ep6 V/cm), when x‐ray and DSC data indicated that the films were predominantly in the phase‐II structure, the piezoelectric activity was small (e31−3 C/m2). For poling fields between 1×106 and 2×106 V/cm the piezoelectric activity was observed to rise rapidly with poling field until a value of e31∼14.5 C/m2 was observed. In this region the x‐ray scans indicated that phase‐II crystallites were being transformed to a polar phase. At poling fields from 2 ×106 to 3.2×106 V/cm a plateau region was observed with very little change in the value of the piezoelectric coefficient. X‐ray and DSC data showed that very little change in the structure and molecular orientation occurred in this region. At the very highest poling field applied (4×106 V/cm) the piezoelectric data indicated the commencement of a very rapid increase in piezoelectric activity (e31∼19.5×10−3 C/m2). At this poling field the x‐ray and DSC data indicated that a crystal transformation to phase I had occurred. The piezoelectric coefficients of the unoriented films was not measured. X‐ray diffraction data indicated that, in addition to the structural and orientation changes occurring in the biaxially oriented film, an x‐ray reflection, which could not be indexed using phase I, II, or III unit cells, increased in intensity with poling field. The DSC data showed the appearance of a new endothermic peak which also increased with poling field. These data indicated that molecular chains oriented perpendicular to the plane of the films transformed to some new structure under the action of the high applied fields.