Direct evidence for electrostriction‐enhanced piezoelectricity in poly(vinylidene fluoride‐ <i>co</i> ‐trifluoroethylene) random copolymers

Abstract A ferroelectric poly(vinylidene fluoride‐ co ‐trifluoroethylene) [P(VDF‐TrFE)] 52/48 mol.% random copolymer was recently shown to exhibit large piezoelectric responses when properly processed to achieve an extended‐chain crystal structure. Although relaxor‐like secondary crystals (SCs) within the oriented amorphous fraction (OAF), that is, SC OAF , was proposed to explain the high direct and converse piezoelectric coefficients in the quenched‐stretched‐annealed‐poled sample (denoted as coP‐52/48QSAP), direct structural verification remained unavailable. In this work, we employed in‐situ time‐resolved small‐angle scattering and wide‐angle X‐ray diffraction to study the nanoscale structural evolution of SC OAF under mechanical loading. We find that even a small strain up to 5.84% triggered significant lamellar thickening, together with stretching‐induced crystallization (i.e., 10% increase in crystallinity) by merging SC OAF into the poled primary crystal (PC) lamellae. Due to the templating effect from PCs with poled/aligned dipoles, the overall polarization increased. Upon removing the mechanical loading, the lamellar spacing, SC OAF , and polarization largely recovered, demonstrating the direct piezoelectric effect. These results established direct structural evidence linking reversible SC OAF crystallization/melting upon mechanical loading/unloading to the giant piezoelectricity of coP‐52/48QSAP. The knowledge obtained from this study will provide design principles for engineering next‐generation high‐performance piezoelectric polymers.