Can Relaxor Ferroelectric Behavior Be Realized for Poly(vinylidene fluoride-co-chlorotrifluoroethylene) [P(VDF-CTFE)] Random Copolymers by Inclusion of CTFE Units in PVDF Crystals?

Relaxor ferroelectric (RFE) polymers are attractive for various electrical applications such as electrostrictive actuation, electromechanical sensors, electric energy storage, and electrocaloric cooling because of their high dielectric constants and low hysteresis loss. Current state-of-the-art RFE polymers include poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]-based random copolymers and terpolymers. However, the high costs due to a safety concern of TrFE make their near term commercialization difficult. It is highly desirable to explore the opportunity of TrFE-free PVDF copolymers [e.g., P(VDF-CTFE); CTFE is chlorotrifluoroethylene] to achieve the RFE behavior by inclusion of CTFE in PVDF crystals (i.e., isomorphism). In this work, two strategies were employed to include CTFE in PVDF crystals. First, high-pressure crystallization was used to obtain extended-chain crystals via the pseudohexagonal paraelectric phase. Structural analyses indicated that CTFE units were largely excluded from the gamma unit cells and ferroelectric domains of PVDF but located as kinks inside the extended chain lamellae. As a result, no RFE behavior was observed because of large ferroelectric gamma domains. The second strategy utilized mechanical stretching at low temperatures (-20 to 0 degrees C) to obtain oriented small beta crystallites (ca. 5-7 nm). Structural analyses indicated that CTFE units were excluded from the beta unit cells, locating at the crystal-amorphous interfaces. Although the hysteresis loops became somewhat slimmer as a result of small crystallite sizes, the RFE behavior with slim hysteresis loops was still not achieved. This study demonstrated that CTFE units were too large to be included in the tightly packed PVDF unit cells, whether the alpha, gamma, or beta phase. In the future, it is desirable to explore other PVDF copolymers with a smaller comonomer such as 1-chloro-1-fluoroethylene.