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

Abstract

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][6][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 to conjugate mechanical energy harvesting with easy shaped, biocompatible flexible materials. [16,17] In particular, the request of bendable and stretchable systems can be fulfilled with elongated nanostructures (e.g. nanowires and nanotubes).In this framework, organics show an unequalled processing flexibility, lightweight, largearea and 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, [CH 2 -CF 2 ] n ) with trifluoroethylene (TrFE), are stable and 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][21][22][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. Differently from crystalline inorganic

Shear deformation dominates in the soft adhesive layers of the laminated structure of flexible electronics

“…It seems that the two categories would not yield big difference, but one will see in the following that this is not true. Actually, the importance of shear contribution in electronic devices based on polymeric flexible materials has been confirmed, which was used for understanding the shear piezoelectricity ( Persano et al, 2016 ). For the laminated structure of flexible electronics, however, a problem of extreme importance arises: should the shear deformation be neglected in the soft adhesive layers?…”

Mechanical Energy Harvesting Performance of Ferroelectric Polymer Nanowires Grown via Template‐Wetting

“…While there have been recent reports of ferroelectric polymers, such as polyamides (odd‐numbered nylons), having found applications in mechanical energy harvesting, polyvinylidine fluoride (PVDF) and its co‐polymers have received by far the most interest for ferroelectric polymer nanogenerator applications due to their superior electromechanical properties. PVDF is a fluoro‐polymer known to exhibit piezoelectricity since 1969 and ferroelectricity since 1981 .…”

“…While there have been recentr eportso ff erroelectric polymers,s ucha sp olyamides (odd-numbered nylons), [8,9] having found applications in mechanical energy harvesting, polyvinylidine fluoride (PVDF) and its co-polymers have received by far the most interest for ferroelectric polymer nanogenerator applications [4,[10][11][12][13][14][15][16][17][18][19] due to their superiore lectromechanical properties.P VDF is af luoro-polymer knownt oe xhibit piezoelectricity since 1969 [20] and ferroelectricitys ince 1981. [21] PVDF consists of ac arbon backbonew ith each carbon in the chain alternatively binding two fluorineo rt wo hydrogen atoms oriented on opposite sides of the carbon chain.…”

Polymer nanogenerators: Opportunities and challenges for large‐scale applications

“…Individual suspended fibers of poly(vinylidenefluoride‐ co ‐trifluoroethylene) (PVDF‐TrFE) with diameter 400–600 nm and length in the millimeter scale, generate open‐circuit voltage of 60 μV under well‐defined levels of displacement in the submicroscale. Density functional theory has been used to calculate the piezoelectric coefficients and the full piezoelectric tensor for different copolymer configurations, also highlighting the role of shear forces contributions in flexible polymeric nanobeams . By using properly designed collectors, fibers can be arranged in various forms such as random mats, aligned strands, highly aligned arrays, ribbonds and yarns realized by twisting ribbons.…”

“…The electrode design depends mainly on the piezoelectric material properties and on the mechanism used to transduce electricity. Unlike crystalline inorganic solids, for which piezoelectricity is generally achieved by strain along the direction of the spontaneous polarization vector and is theoretically described by an uniaxial model, in polymeric systems, which are intrinsically flexible, the stress applied along one axis also causes remarkable deformations along perpendicular directions . More complex transverse contributions can be then taken into account while designing the device, and the electrodes can be positioned to collect piezo‐voltage along a direction either perpendicular [Figure (a)] or parallel to the applied stress .…”