Potentialities of flexoelectric effect in soft polymer films for electromechanical applications

Abstract

Among the transduction mechanisms of interest for sensing and/or actuation applications at nano/micro scale, the piezoelectric effect has been widely exploited owing to the solid state nature of piezoelectrics, the large ability of specific classes of materials for the mechanical-to-electrical energy conversion and easy integration. However, every piezoelectric (also generally ferroelectric) presents well-known intrinsic drawbacks such as required poling step and related aging. In contrast, uniquely flexoelectric materials do not suffer from these disadvantages because flexoelectricity, a universal effect in all dielectric solids defined as the electrical polarization induced by a strain gradient, does not imply preliminary electric field-induced macroscopic polarization. Besides, strain gradient may be easily obtained by bending plate or cantilever-shaped structure and in this case it is nothing but the local curvature of the flexible system. Thus, as strain gradient (curvature) inversely scales with both elastic stiffness and thickness, this study will focus on the evaluation of the potentialities of flexoelectric effect in soft polymer films for electromechanical applications, with an emphasis on the thickness influence. In this way, analytical results combined to experimentally obtained effective flexoelectric coefficients for some typical polymer classes may provide guidelines for the development of soft and low frequency flexoelectric mechanical transducers.

Tuning the Mechanoelectrical Transduction Performance of Multifunctional Polymer Electrolyte Membranes via Variation of Precursor Molecular Weight

“…As illustrated in Figure b, the present strategy of increasing the precursor molecular weight has been shown to offer enhanced conversion efficiency. More importantly, as can be witnessed in Table , the observed flexoelectric coefficient and mechanoelectrical energy conversion efficiency of the present PEI-co-PEGDGE-based PEM composites are significantly higher than those of ceramics, ,, liquid crystal elastomers, , electroactive polymers, − solid polymer electrolyte membranes, ,, and biomaterials, including auditory hairs and human bones, hitherto reported. Hence, it opens future investigations on other effects such as salt type (e.g., ionic liquid), network architecture, and temperature on the flexoelectric energy harvesting performance.…”

“…The proposed strategy of increasing the molecular weight of the ion-solvating polymer as a means of enhancing energy conversion efficiency was found to be successful, i.e., reaching 13.7% for the PEI-co-PEGDGE1000/ LiTFSI (95/5) laminate, which is 6-and 34-fold enhancement relative to those of the PEMs based on PEGDGE500 and PEGDGE200, respectively. 32 1.4 μC/m auditory hair 29 ∼0.1 nC/m human bone 9 ∼2.3 nC/m PVDF 26,27 5−13 nC/m polythiophene-based compound 28 ∼7 μC/m bent-core nematic liquid crystal 18 60 nC/m ionic liquid crystal elastomer (i-LCE) 31 25−220 μC/m PEM(PEGDA/SCN/LiTFSI) 30 29−323 μC/m 1−2 PEM(TS-PEGDA/IL) 48 84−154 μC/m ∼1 PEGDA/(K-, Na-, Mg-, Al-(TFSI) 3 ) 50 18−45 μC/m 0.4−2.6 PEGDA-co-PEGMEA/LiTFSI 95/5 33 64−230 μC/m 0.9−6.…”

“…17,18 Recently, it has been noticed that flexoelectricity generation amplifies by shrinking the scale of the strain gradient, intensifying the demand and focusing on the utilization of flexoelectric materials in various applications including energy harvesters, sensors, soft robotics, and actuators. 19−25 The flexoelectric effect was shown to exist in many polymers and biomaterials including poly(vinylidene fluoride) (PVDF), 26 epoxy-based polymers, 27 oriented poly-(ethylene terephthalate) (PET) and polyolefins, 28 auditory hairs, 29 and bones. 9 However, the flexoelectric coefficients originating from the induced dipole moments (as in crystalline ceramics) or from the reorientation of directors of liquid crystals or electroactive crystalline polymers are significantly low for targeted applications.…”

“…In ferroelectric ceramics, the flexoelectric effect that originated from the induced dipole moments of electrons is a fraction of the electrical signal produced by the piezoelectric effect. Later, the flexoelectric effect was shown to exist in liquid crystals and electroactive polymers, where the generated electricity was driven by the molecular reorientation or more specifically director orientation and polarization, leading to a change in dipole moment. , Recently, it has been noticed that flexoelectricity generation amplifies by shrinking the scale of the strain gradient, intensifying the demand and focusing on the utilization of flexoelectric materials in various applications including energy harvesters, sensors, soft robotics, and actuators. − The flexoelectric effect was shown to exist in many polymers and biomaterials including poly­(vinylidene fluoride) (PVDF), epoxy-based polymers, oriented poly­(ethylene terephthalate) (PET) and polyolefins, auditory hairs, and bones . However, the flexoelectric coefficients originating from the induced dipole moments (as in crystalline ceramics) or from the reorientation of directors of liquid crystals or electroactive crystalline polymers are significantly low for targeted applications.…”

Large Curvature Sensors Based on Flexoelectric Effect in PEDOT:PSS Polymer Films

“…This is particularly the case with polymers, which have the advantage of being naturally flexible and, therefore, tolerate large deformation. In other words, flexoelectricity may compete with piezoelectricity in flexible materials like soft polymer films with Young’s modulus lower than 1 GPa. , Marvan et al first studied flexoelectricity in dielectric polymers like rubber. Many other insulating polymers exhibit a significant flexoelectric effect when subjected to a deformation gradient such as polyurethane (PU), poly­(vinylidene fluoride) (PVDF), , etc.…”