Flexoelectricity of Multifunctional Polymer Electrolyte Membranes: Effect of Polymer Network Functionality

Abstract

The phenomenon of flexoelectricity generation in flexible, thermally cross-linked, ionic multifunctional polymer electrolyte membranes (PEMs) was investigated subjected to mechanical deformation by bending. The effect of copolymer ratios of functional branched poly(ethylene imine) (PEI) and poly(ethylene glycol) diglycidyl ether (PEGDGE) on the mechanoelectrical transduction in their PEMs was demonstrated at a fixed lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) content. By changing the PEI and PEGDGE copolymer ratios, the constructed co-networks comprised of various amines (primary, secondary, and tertiary) to ethylene oxide ratios, revealed distinctively different flexoelectric responses. With increasing the amount of PEGDGE, the multifunctional PEM exhibited not only lower glass transition temperature but also higher ionic conductivities. This finding may be attributed to the higher content of the linear ethylene oxide chain, which promotes both higher segmental motion and lithium ion complexation with ether oxygen. The examination of the flexoelectric response of the various multifunctional PEMs as a function of composition was performed under intermittent square-waves and sinusoidal bending modes along with flexoelectric responses as a function of relaxation time or frequency. A remarkable flexoelectric coefficient (μ = 210.1 μC/m) was obtained without requiring plasticization or network modification. The energy conversion efficiency of various PEMs was determined in order to evaluate potential applications in energy harvesting devices and sensors. The developed process of PEM fabrication involving thermal curing is compatible and scalable to current commercial processes such as tire vulcanization and shape conformable 3D printing for soft sensors and energy harvesters.