*3.7. Electrostrictive Properties*

The longitudinal strain (*S*3) behavior of the P(VDF-HFP) film, fibers, and fiber mats compressed at 30◦, 60◦, and 80◦C and induced by an external electric field (*E*3) at a frequency of 1 Hz, is presented in Figure 9a. At a low electric field, the electrostriction behavior demonstrated that the total thickness strain had an approximately quadratic relation to the applied electric field. This can be illustrated by the electrostriction which is shown in Equation (7) and Maxwell stress. The Maxwell stress effect involves electrostatic attractions and interactions with the charges on the electrodes showing SM = <sup>ε</sup>0εr<sup>E</sup>23/Y. This equation can be used to estimate the value of the Maxwell stress which can be neglected due to its small and negligible value in the case of a low dielectric constant at an applied low electric field. The induced strain of all samples based on the electrostrictive behavior can be observed. The linear relationships between the induced strain and the electric field, shown in Figure 9b, display that their slope can be assigned as the electrostrictive coefficient (*M*33). As a result, the electrostrictive behavior of the samples can be expressed according to Equation (7). The M33 increased with the fraction of temperatures compressed and the increased number of polymer–polymer interfaces may play a key role, affecting both the electrical and mechanical properties as shown in Figure 9c. This was reflected by the abruptly increased space charge distribution, and the increasing dielectric constant increased with the compressed fibers. Clearly, this study indicates that fibers compressed by temperature are promising electrostrictive materials for actuation and can be fabricated with a simple preparation.

**Figure 9.** *Cont*.

**Figure 9.** Strain behaviors of film, fibers, and compressed fibers as a function of the (**a**) electric field and (**b**) square of the electric field at 1 Hz. (**c**) The effect of the compression fibers on the electrostrictive coefficients and absolute β fraction.
