The origin of piezoelectricity in ferroelectric polymers*

One important property of ferroelectric polymers is their piezoelectricity. Despite decades of research on piezoelectric polymers, the origin of polymer piezoelectricity is still unclear. Combined with the fact that their piezoelectric performance is much worse than the ceramic counterparts, such as barium titanate and lead zirconate titanate, this has hampered the development of high-performance piezoelectric polymers and their practical applications. Recently, we have discovered that the complex semicrystalline structure of ferroelectric polymers plays an important role. Namely, the electrostriction of the oriented amorphous fraction (OAF), which links between the primary crystalline lamellae and the isotropic amorphous fraction (IAF), is the origin of piezoelectricity of ferroelectric polymers. In addition, if relaxor-like secondary crystals can further grow in the OAF, the piezoelectricity will be further enhanced. For example, we have achieved high piezoelectric coefficients (both d₃₁ and d₃₃) in the range of 60-80 pC/N for poly(vinylidene fluoride) and its random copolymers with trifluoroethylene. This understanding will help us design new ferroelectric polymers with even higher performance and better applications.