KH550 ‐Modified CaCu ₃ Ti ₄ O ₁₂ @ Al ₂ O ₃ Core–Shell Fillers: Interfacial Modification for High‐Dielectric and Mechanically Enhanced Polyimide Composites

ABSTRACT Core–shell CaCu ₃ Ti ₄ O ₁₂ @Al ₂ O ₃ (CCTO@Al ₂ O ₃ ) fillers were synthesized via a hydrothermal method and subsequently functionalized with KH550 to obtain K‐CCTO@Al ₂ O ₃ . The formation of the core–shell structure was confirmed by transmission electron microscopy (TEM), while the successful grafting of KH550 and the presence of a stable interfacial layer were further verified by FT‐IR spectroscopy, X‐ray diffraction (XRD), surface energy measurements, and particle size analysis. Polyimide (PI) composites were fabricated through in situ polymerization of 4,4′‐(4,4′‐isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA) and 4,4′‐oxydianiline (ODA), with various loadings of K‐CCTO@Al ₂ O ₃ incorporated into the matrix. The dielectric, mechanical, water resistance, breakdown strength, and thermal properties of the resulting PI composite films were systematically investigated. Among all samples, the composite containing 10 wt% K‐CCTO@Al ₂ O ₃ showed the best overall performance, showing a dielectric constant of 8.93 at 1 kHz before water absorption and 9.10 after water absorption, with dielectric loss remaining below 0.01 under both conditions. In addition, this composite exhibited a tensile strength of 117.41 MPa, a breakdown strength of 246.1 kV/mm, improved thermal stability, and excellent water resistance. The KH550 functionalization renders the filler surface energy closer to that of the PI matrix, thereby enhancing interfacial compatibility and promoting more uniform filler dispersion. Consequently, interfacial defects are effectively suppressed, facilitating more efficient charge transport and stress transfer within the composite. This study demonstrates that combining the core–shell structure with surface functionalization is an effective approach for developing high‐performance dielectric polyimide composites.