Coupling between polarization and the strain gradient, namely, flexoelectricity, is a universal phenomenon that widely exists in all solid dielectrics and polymers. At a low level of the applied strain gradient, this electromechanical coupling is strictly a linear effect. In this Letter, a strong nonlinearity between the polarization and the strain gradient was experimentally found in polyvinylidene fluoride when the strain gradient was higher than a material-dependent threshold value. Such nonlinear behavior was in good accordance with an ion chain theory and could be fitted by a nonlinear equation. The observed flexoelectric nonlinearity in this work will help in the understanding of the discrepancy between the previous findings about the bulk materials and their nanoscale counterparts.

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