Experimental studies on the direct flexoelectric effect in α-phase polyvinylidene fluoride films

Abstract

We report observation of the giant direct flexoelectric effect in α-phase polyvinylidene fluoride (PVDF) films. This unexpected phenomenon was detected in the α-phase PVDF film undergoing elastic tensile stretching. Our results have shown that the physical mechanism behind the flexoelectricity in polymers might be more complicated than the one proposed for solid crystalline dielectrics.

Measurement of the flexoelectric response in ferroelectric and relaxor polymer thin films

“…We did not find evidence for giant flexoelectric coefficients of up to 82 lC/m as reported by Baskaran et al with nominally unpoled samples of PVDF. [15][16][17] Although they corrected their data for piezoelectric contributions, a small error in the correction may have had a large effect on the extracted value of the flexoelectric coefficient, as we found in our studies.…”

“…14 Nevertheless, Baskaran et al have reported high values for the flexoelectric coefficient l up to 82 lC/m in nominally nonferroelectric PVDF samples. [15][16][17] It is difficult, however, to rule out piezoelectric contributions from residual ferroelectric beta and delta phases, because PVDF is, in general, a polymorphous material, 14,18,19 containing a substantial amorphous component 20 and various crystalline phases that depend strongly on synthesis and sample preparation procedures. [21][22][23] Therefore, we have made a study of thin films of VDF copolymer and a VDF terpolymer, which allow us to compare the flexoelectric response in three distinct states-ferroelectric, paraelectric, and relaxor.…”

Relationship between direct and converse flexoelectric coefficients

“…The investigation regarding the flexoelectric effect was mainly focused on crystalline solid materials, 7 polymers, 8 liquid crystals 9 as well as biological materials such as biomembranes. 10 The concept of flexoelectricity in liquid crystals stems directly from the reorientation of irregularly shaped polarized molecules under strain gradients caused by splay-or bent-deformations, which is different from its cousin effect produced by uniform strain or stress, namely piezoelectric effect.…”

Flexoelectric Effect in Solids

“…Again,μ 11 is an effective coefficient, which in the case of an isotropic material is related to the flexoelectric tensor components throughμ 11 = µ 11 − 2νµ 12 . In practice, the pyramid-compression approach is complicated by the fact that the strain gradient is strongly inhomogeneous [55] (concentrated mainly at the sample edges) making it difficult to extract reliable values ofμ.…”