Microphase Separation Promoting High‐Temperature Energy Storage Properties of Polymer Alloys

High-temperature polymer dielectrics must meet the requirements of modern power electronic systems, particularly high energy storage density under elevated-temperature conditions. However, polyetherimide (PEI), one of the most promising candidate materials, suffers from significant deterioration in energy storage density due to high conductive loss under high temperatures. In this study, we designed a polymer alloy comprising PEI and semi-aromatic benzimidazole polyimide (SPBII), where intermolecular hydrogen bonding acts as a compatibilizer to stabilize a nanoscale microphase-separated structure. The SPBII polymer chains confined within nanoregions exhibit a nanoconfinement effect, substantially enhancing the thermal and mechanical properties of the polymer alloy, with T_g and Young's modulus of 50% SPBII reaching 287.5°C and 5.32 GPa. Furthermore, the high positive electrostatic potential of SPBII introduces charge traps at the PEI/SPBII interface, effectively suppressing high-temperature conduction loss. As a result, the 50% SPBII polymer alloy demonstrates remarkable improvements in breakdown strength and energy storage density. For instance, at 200°C, it achieves an energy density of 4.53 J cm^-3 with an efficiency of 90%, outperforming that of pure PEI (1.71 J cm^-3). This breakthrough provides a promising solution for achieving superior capacitive performance of dielectrics under high-temperature conditions, significantly advancing the applications of polyimide-based dielectric materials.