Effects of stretching on phase transformation of PVDF and its copolymers: A review

Poly(vinylidene fluoride) (PVDF) and its copolymers are promising candidates for energy-harvesting devices because of their flexibility, environmental friendliness, lightweight, and high halogen and acid resistance. However, the relatively low piezoelectricity limits their applications. The piezoelectricity of PVDF and its copolymers highly depends on the polar ss-phase, while the non-polar a-phase is the most common one. As a result, the ss-phase formation and a- to ss phase transformation have attracted much attention in recent years. Stretching is a widely used method to induce the a- to ss-phase transformation for the improvement of piezoelectricity. In this work, the influences of the parameters during stretching on phase evolution and piezoelectricity are discussed and summarized. Besides, nontraditional stretching methods are also introduced and discussed. This work will provide important information for preparing highperformance piezoelectric polymer films.