Large effective piezoelectric response from the spontaneously polarized surface layer in P(VDF-TrFE) arch films

In this work, we show that a d(33) similar to 150 pC/N can be obtained in nonpoled poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) copolymer films with an arch structure. The copolymer films, which are often thought to be homogeneous, are in fact inhomogeneous in microstructure and physical properties after film fabrication. Although a large proportion of the copolymer film is nonpolar, as expected in a nonpoled ferroelectric film, the surface regions of the film are spontaneously polarized. We propose that inhomogeneous stress in the surface regions, which is either from the constraint of the substrate or skin layer effect formed during the film fabrication, generates a flexoelectric response and orients the spontaneous polarization of the ferroelectric film. As a result of the polar surface regions, the nonpoled films exhibit a piezoelectric response. The piezoelectric response is further amplified by the special arch structure of the films, leading to the observed large effective piezoelectric response. This study not only discovers the polar surface effect in ferroelectric polymer films, but also proposes an approach to design polymer materials with a strong piezoelectric response.