Enhanced Piezoelectricity in Poly(vinylidene fluoride- co-trifluoroethylene- co-chlorotrifluoroethylene) Random Terpolymers with Mixed Ferroelectric Phases

Piezoelectric polymers are attractive as next-generation flexible, wearable, and implantable electronic materials for energy, sensor, and medical applications. Here, we report the enhancement of the longitudinal piezoelectric coefficients (d33) of poly(vinylidene fluoride) (PVDF)-based ferroelectric polymers via the synthesis and characterization of poly(vinylidene difluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] terpolymers with coexisting mixed normal ferroelectric and relaxor phases. Specifically, the terpolymer with a composition of VDF/TrFE/CTFE = 64.5/33.1/2.4 mol % exhibits a d33 of -55.4 pC/N, corresponding to an 85% increase compared with the well-known P(VDF-TrFE) 65/35 mol % copolymer. The microstructures, chain conformations, Curie transitions, and crystal structures of the terpolymers have been investigated as a function of the CTFE content. The structural and conformational analyses show that the incorporation of CTFE into the ferroelectric P(VDF-TrFE) facilitates the formation of the relaxor phase. The maximum d33 is present in the terpolymers with the CTFE content ranging from 1.7 to 5.0 mol %. A further increase in the CTFE concentration renders the terpolymer a ferroelectric relaxor without the polar ferroelectric phase. The phase transition induced by varying the CTFE content of the terpolymers is evidenced by the dielectric and electromechanical characterizations. This work demonstrates a new structural modification approach to improve the piezoelectricity of the ferroelectric polymers.