Stretching-Induced Relaxor Ferroelectric Behavior in a Poly(vinylidene fluoride-co-trifluoroethylene-co-hexafluoropropylene) Random Terpolymer

Relaxor ferroelectric (RFE) polymers exhibiting narrow hysteresis loops are attractive for a broad range of potential applications such as electric energy storage, artificial muscles, electrocaloric cooling, and printable electronics. However, current state-of-the-art RFE polymers are primarily poly(vinylidene fluoride-co-trifluoroethylene-co-X) [P(VDF-TrFE-X)] random terpolymers with X being 1,1-chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE). Potential dehydrochlorination at elevated temperatures can prevent the melt-processing of these Cl-containing terpolymers. It is desirable to achieve the RFE behavior for Cl-free terpolymers such as P(VDF-TrFE-HFP), where HFP stands for hexafluoropropylene. Nonetheless, HFP units were mostly excluded from the crystalline structure because of their large size, and thus no RFE behavior was observed when crystallized from the quiescent melt. Intriguingly, mechanical stretching could effectively pull the HFP units into the P(VDF-TrFE) crystals, forming nanosized ferroelectric (FE) domains with a strong physical pinning effect. Consequently, the RFE behavior was observed for the uniaxially stretched P(VDF-TrFE-HFP) film. Thermal annealing above the Curie temperature (ca. 50 °C) without tension led to the return of the normal FE behavior with broad hysteresis loops. However, thermal annealing above Curie temperature under tension prevented the exclusion of HFP units from the crystalline structure, and thus relatively stable RFE behavior was achieved. Various characterization techniques were utilized to unravel the structure–property relationships for these P(VDF-TrFE-HFP) films. In addition, the RFE behavior of P(VDF-TrFE-HFP) was compared to those of other terpolymers. This study provides a unique and simple strategy solely based on film processing to achieve the RFE behavior for P(VDF-TrFE)-based terpolymers.