Resolving Complex Multiscale Structure of Magneto- and Electroactive Polymer Composites With an Ionic Liquid

A multiscale understanding of the structure of ionogels - nanoparticle-free polymer composites incorporating ionic liquids - is essential for enhancing their macroscopic functional properties and unlocking their potential in critical applications such as energy storage, sensing, and actuation. We establish a complete picture of the nano- and microstructuration of an ionic liquid within the matrix of a practically relevant electroactive copolymer poly(vinylidenefluoride-co-trifluoroethylene), by combining neutron scattering with cryogenic scanning electron tomography assisted by focused ion beam milling and cryogenic transmission electron microscopy with elemental analysis. We show that the ionic liquid is primarily located in the polymer amorphous phase and forms nanostructures with 10-12 nm size. It does not penetrate into the crystalline lamellae or the polymer amorphous phase confined between them, and it does not affect the polymer degree of crystallinity nor its complete crystallization in the highly electroactive β-phase. Saturation of the unconfined amorphous phase with ionic liquid is identified as the key factor enabling high ionic conductivity while preserving mechanical integrity. At high ionic liquid concentrations, its excess microphase-separates during the composite processing and accumulates in predominantly interconnected micrometer-sized pores, providing the magnetoelectric response and further increasing the ionic conductivity.