Hierarchically Structured ANF @ BN Hybrid Fillers With Dual Phosphorus Functionalization for Low‐Coefficient of Thermal Expansion and Mechanically Robust Epoxy Composites

ABSTRACT Epoxy resins are widely used in structural and electronic applications; however, their high coefficient of thermal expansion (CTE) and intrinsic brittleness limit their reliability under thermal and mechanical stresses. In this study, a multifunctional epoxy composite with a reduced CTE and enhanced mechanical performance was developed using a hierarchically structured hybrid filler composed of aramid nanofibers (ANFs) and boron nitride (BN), followed by dual‐phosphorus‐based surface functionalization with phenylphosphonic acid (PPA) and phosphoric acid (PA). BN incorporation effectively suppressed self‐assembly of the ANFs into dense films, enabling the formation of a well‐dispersed powder‐type hybrid structure. Subsequent PPA and PA functionalization significantly enhanced the interfacial adhesion through hydrogen bonding and π–π interactions, leading to improved stress transfer and restricted polymer chain mobility. Consequently, the epoxy composites exhibited a substantial reduction in the CTE (up to 28%), with a simultaneous improvement in the tensile strength and adhesion performance. The hybrid filler also contributed to enhanced thermal stability and flame retardancy, as evidenced by the reduced peak heat release rate (pHRR) and increased char formation. These findings demonstrate that synergistic hybridization and interfacial engineering are effective strategies for designing mechanically robust low‐CTE epoxy composites for advanced structural and electronic applications.