Meta -kinked Polyphenylenes with Ethylene Oxide Side Chains as Flexible Solid Polymer Electrolytes: Thermomechanical Properties and Ion Dynamics

Abstract Kinked polyphenylenes, poly(meta, meta, para-phenylene) (PmmpP), with ethylene oxide (EO) side chains are used as solid polymer electrolytes (SPEs) for Li+ conduction. The meta-kinks in P(EO-mmpP) effectively suppress side-chain crystallization and enhance backbone flexibility, resulting in amorphous materials with outstanding ductility and tunable toughness. Varying the EO chain length from three to five units enables tuning of the glass transition temperature (Tg), mechanical properties, and ionic conductivity. Dynamic mechanical thermal analysis reveals two transitions for the side and main chains. While the temperature of side-chain relaxation is independent of side-chain length, the temperature of main-chain relaxation, which is closely related to Tg, decreases with increasing side-chain length. The SPEs exhibit excellent miscibility with LiTFSI upon simple kneading, forming transparent, flexible, and free-standing films. Ionic conductivities increase with increasing side-chain length following Arrhenius behavior. The activation energies (EA) decrease with increasing side-chain length. However, the EA of local Li+ dynamics from temperature-dependent 7Li NMR measurements increases with increasing EO side-chain length due to stronger complexation. As a result, the Tg is dominating local effects of local Li+ dynamics and thus governs bulk ionic conductivity. This effect is further enhanced by external plasticization, furnishing a conductivity of 0.483 mS/cm at 80 °C.