An Insight into the Rate Dependent Viscoelastic Behavior of PVDF

Polyvinylidene fluoride (PVDF) is widely used in soft piezoelectric devices, where mechanical stability plays a critical role in long-term performance. In this work, the rate-dependent elastic and viscoelastic behavior of pristine PVDF films is investigated using nanoindentation. Experiments are carried out at room temperature with a maximum load of 30 mN using four different loading times (5, 10, 15, and 20 s), corresponding to loading rates of 6, 3, 2, and 1.5 mN/s, while maintaining a constant holding time of 10 s. The results show a clear loading-rate dependence, with the elastic modulus and hardness increasing by approximately 14.7 and 11.7%, respectively, as the loading rate increases. Creep compliance data obtained from the indentation hold segment are analyzed using the three-element Standard Linear Solid (Kelvin–Voigt form) and the four-element Burger model to extract the viscoelastic parameters. Comparative analysis reveals that the instantaneous elastic modulus (E₁) remains independent of loading rate in both models, whereas the delayed elastic modulus (E₂) decreases with increasing loading rate by ~31% in the SLS–K.V model and ~40% in the Burger model. Furthermore, the viscous parameters show significant rate sensitivity, with η₁ decreasing by ~57% and η₂ reducing to nearly half of its initial value at higher loading rates in both models. These results highlight the pronounced rate-dependent viscoelastic response of PVDF and provide insights relevant to the mechanical reliability of PVDF-based piezoelectric devices.