Strong impact of LiNbO3 fillers on local electromechanical and electrochemical properties of P(VDF-TrFe) polymer disclosed via scanning probe microscopy

In this work, we demonstrate an alteration of mechanical, electrophysical, piezo- and ferroelectric properties of polyvinylidene fluoride/trifluoroethylene P(VDF-TrFE) copolymer at the composition of 70/30 mol% in the presence of lithium niobate (LiNbO3) fillers. The micro- and nanoscale measurements of the elastic modulus suggest a two-fold increase in the mechanical rigidity of the P(VDF-TrFE) film after embedding the LiNbO3. The enhancement of local piezo- and ferroelectric properties of the modified polymer is evidenced by the increase of the direct piezoelectric coefficient from 27.1 pm/V to 36.1 pm/V. This increase is supposed to be associated with a significant contribution from Li-ion diffusion (the surface nanoelectrochemistry effect). This suggestion is supported by the observation of irreversible dc bias voltage-induced Li-ion extraction contributing to the locally-measured piezoresponse. The dynamics of the Li-ion diffusion, studied by electrochemical strain time spectroscopy, shows a decrease of the diffusion coefficient for an area poled by dc bias voltage as compared with a pristine one. This decrease confirms the existence of irreversible electrochemical processes during the local piezoelectric measurements. At the same time, X-ray diffraction and micro-Raman data suggest the integrity of the intramolecular structure of the P(VDF-TrFE) copolymer after embedding the LiNbO3 fillers. This is a key achievement allowing to keep the structure-related parameters specific to the pure P(VDF-TrFE) copolymer. The obtained results reveal a crucial role of the fillers in attaining the desired functional behavior, thus paving the way towards the development of advanced sensors, transducers, actuators, and piezoelectric devices.