Multiscale Flat Glass Fiber/Aramid Nanofiber Epoxy Composites With Enhanced Mechanical Performance and Low Permittivity

ABSTRACT Developing epoxy composites that simultaneously exhibit low dielectric response and high mechanical reliability remains challenging because conventional fiber reinforcement often increases interfacial polarization and dielectric permittivity. In this study, a multiscale hybrid architecture is designed by combining flat glass fibers (GF) with aramid nanofibers (ANF) as nanoscale interfacial modifiers. ANFs are prepared from aramid fibers through controlled deprotonation‐assisted fibrillation. Increasing the fibrillation time progressively refined the hierarchical fibrillar morphology, with the 72 h‐treated sample exhibiting the finest submicrometer structures while largely preserving the characteristic aramid chemical structure. The selected ANF and flat GF are incorporated into a diglycidyl ether of bisphenol A epoxy/amine system, followed by hot‐press curing. The flat GF provides primary load‐bearing reinforcement, while ANF incorporation is associated with changes in matrix/interfacial morphology, reduced segmental relaxation, and enhanced fracture‐energy dissipation. The best‐balanced composite containing 5 wt% GF and 0.5 wt% ANF exhibits a flexural strength of 170.72 MPa, flexural toughness of 1.45 MJ/m 3 , and impact strength of 4.58 kJ/m 2 , corresponding to increases of 68.4%, 229.5%, and 103.6%, respectively, compared with neat epoxy. It also achieves a low relative permittivity of 2.34 and a dielectric loss of 0.0102 at 1 kHz. The enhanced mechanical–dielectric balance is consistent with the combined contributions of flat GF reinforcement, ANF‐induced matrix/interfacial modification, and changes in polymer segmental relaxation and dielectric polarization. These results demonstrate a multiscale design strategy for mechanically robust, low‐ k epoxy composites with potential for advanced electronic packaging and electrical insulation applications.