Shear Piezoelectricity in Poly(vinylidenefluoride-co-trifluoroethylene): Full Piezotensor Coefficients by Molecular Modeling, Biaxial Transverse Response, and Use in Suspended Energy-Harvesting Nanostructures

The intrinsic flexible character of polymeric materials causes remarkable strain deformations along directions perpendicular to the applied stress. The biaxial response in the shear piezoelectricity of polyvinylidenefluoride copolymers is analyzed and their full piezoelectric tensors are provided. The microscopic shear is exploited in single suspended nanowires bent by localized loading to couple flexural deformation and transverse piezoelectric response. Piezoelectric materials and associated nanostructures accumulate electrical charges on their surfaces in response to an applied mechanical stress, through the change in their spontaneous electric polarization.[1, 2] Exploiting deformations induced by motion, mechanical vibrations, and environmental noise,[3, 4] these systems are extremely attractive for energy harvesting in information and communications technologies and personalized electronics.[5-7] Solid-state materials such as crystals[8] and ceramics[9] have been integrated in complex networks for the internet of things[10] as actuators, sensors, and transducers,[11] and as switches in memory devices. Recently, the emerging of magnetoelectric data storage,[12] self-power sources for smart wearables,[13] or implantable biomedical devices[14, 15] fostered the conjugation of mechanical energy harvesting with simply shaped, biocompatible flexible materials.[16, 17] In particular, the need for bendable and stretchable systems can be fulfilled with elongated nanostructures (e.g., nanowires and nanotubes). In this framework, organics show an unequalled processing flexibility, light weight, large area, low-cost manufacturing methods, biocompatibility, and low acoustic and mechanical impedance, which make them ideal for underwater and medical applications.[14, 15, 18] For instance, copolymers of vinylidenefluoride (VDF, [CH2[BOND]CF2]n) with trifluoroethylene (TrFE), are stable and can achieve a high degree of crystallinity (>90%).[19] In addition, they do not need to be poled because they directly crystallize from melt or solution into the ferroelectric (?-) phase. Piezoelectricity in these materials is related to the electronegativity difference in hydrogen and fluorine atoms, which determines an effective dipole moment in the direction normal to the carbon backbone. Consequently, these films or nanostructures are often utilized with top/bottom contacts.[20-23] Instead, the special piezoelectric properties of polymers such as the poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] might lead to the development of much more versatile nanogenerator architectures. Unlike crystalline inorganic solids, for which normal piezoelectricity is generally exploited and conveniently achieved by strain along the spontaneous polarization (P, Figure 1a), in flexible polymeric systems the stress applied along one axis also causes remarkable deformations along perpendicular directions.[24] This effect, along with the reduced alignment of the polymeric chains and the presence of glassy grains, strips out the concept of uniaxial piezoelectricity and more complex transverse contributions are to be taken into account, in polarization and in the undergone distortions. Most studies assume only uniaxial models for polymers and organic-inorganic nanocomposites,[12, 25] which may result in a limiting description of the response because of the assignment of a reduced number of piezoelectric parameters, as pertaining to systems much more symmetric than the actual ones. In this work, we demonstrate the biaxial shear activity in P(VDF-TrFE), which show the presence of two net components of the electronic polarization (Px, Py according to the geometry displayed in Figure 1b) in the plane perpendicular to the chains of macromolecules. Findings reported he