Induced polarization imparts piezoelectricity in noncrystalline polymer films

Piezoelectric materials produce a linear deformation in response to an applied electric field and are essential for precision-control devices. The conventional view is that polymers must be crystalline and possess remanent polarization after artificial poling to gain piezoelectricity. For the ferroelectric polymer poly(vinylidene difluoride), the required poling field is exceptionally high, up to 150-200 kV mm-1. Here, we circumvent this limitation by utilizing the elastic displacement of electric dipoles and creating a net polarization. We find that a subcoercive field of 49 kV mm-1 can induce a high piezoelectric coefficient, d33, of -33 pm V-1 in an unpoled poly(vinylidene fluoride-trifluoroethylene) film. In this case, the dielectric acts as a piezoelectric, as long as a bias electric field is applied, with its piezoelectric coefficient increasing proportionally to the strength of the electric field until polarization saturation sets in. The proposed methodology is further extended to amorphous polymers, providing an opportunity to discover alternative piezoelectrics within the dielectric family.