New insights on the crystal structure of P(VDF-co-TrFE) copolymer (55/45 mol%) and influence on the high piezoelectric response

Abstract

Copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE), P(VDF-co-TrFE), exhibit remarkable electroactive properties. More precisely, the copolymers with a molar fraction of VDF between 49 % and 55 % present high piezoelectric properties. However, the literature on P(VDF-co-TrFE) with this composition range is limited, and there are divergences regarding the attribution of crystal phases, unlike the copolymers containing around 70 %–80 % of VDF, which have been extensively studied. In this study, wide-angle X-ray scattering (WAXS) experiments were conducted on a 55/45 P(VDF-co-TrFE) film to gain a better understanding of its crystal structure in correlation to its high piezoelectric properties. For comparison, a 80/20 P(VDF-co-TrFE) film is used as a reference. The results reveal that 55/45 P(VDF-co-TrFE) is mainly composed of two defective ferroelectric (DFE) phases, unlike 80/20 P(VDF-co-TrFE), which contains a mixture of a small fraction of DFE phase and a high fraction of ferroelectric (FE) phases. The stretching of 55/45 copolymer film induces the transformation of DFE phases to FE phase to obtain a structure very similar to that of the unstretched 80/20 copolymer. After poling, the unstretched 55/45 P(VDF-co-TrFE) film exhibits a high piezoelectric coefficient d₃₃ of −41 pC/N, compared to more classical values of −22 pC/N for both the stretched 55/45 P(VDF-co-TrFE) and the unstretched 80/20 P(VDF-co-TrFE). This result is mainly explained by the exclusive presence of DFE phases in the unstretched 55/45 P(VDF-co-TrFE), which have greater mobility and then a better ability to polarize compared to the FE phase. This study thus highlights the importance of the DFE phase presence impacted by the film elaboration process in achieving a high piezoelectric coefficient in copolymers. This investigation contributes to a better understanding of the relationships between elaboration, structure and properties of P(VDF-co-TrFE) films, which is a key issue for the design of advanced electroactive organic devices.