Electrostrictive polymers having a large strain are desirable for actuation, sensing, and energy harvesting in wearable electronics and soft robotics. However, a high electric field (>100 MV/m) is usually required for current electrostrictive polymers. To realize large electrostriction at reduced electric fields, the fundamental electrostriction mechanism needs to be better understood. In response to this need, the structure and electrostrictive properties of relaxor ferroelectric (RFE) poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] random terpolymers films with different thermal annealing histories were studied in this work. First, the semicrystalline structure of the P(VDF-TrFE-CTFE) terpolymer films was studied by combined small-angle X-ray scattering and wide-angle X-ray diffraction analyses. A three-phase model was employed, namely, crystals and oriented and isotropic amorphous fractions (OAF and IAF). The bulky CTFE units generated taut-tie molecules (TTM) in the crystalline lamella, dividing it into many nanosized crystals (∼1.3 nm thick). It is this unique crystalline structure with nanocrystals and mobile TTM/OAF that enabled the RFE behavior for the P(VDF-TrFE)-based terpolymers. Through electrostriction measurements and nonlinear dielectric analysis, an inverse correlation was observed between the ferroelectric nonlinearity and the electrostrictive coefficient under a high poling electric field (>100 MV/m). This suggested that higher electrostriction performance could be achieved by decreasing the ferroelectric nonlinearity of the RFE terpolymer. Indeed, above the Curie temperature, the paraelectric terpolymer films achieved a high electrostrictive performance with the transverse strain being ∼5% at 200 MV/m. This was attributed to the strong electrostatic repulsion among electric field-induced ferroelectric nanodomains. The finding from this work provides a viable way to design new electrostrictive polymers with higher performance at low driving fields.
Keywords: electrostriction; ferroelectric nonlinearity; nanodomains; poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifloroethylene) random terpolymer; relaxor ferroelectric behavior.