Abstract: Glass fiber reinforced epoxies (GREs) are widely used in aerospace, wind power, and other fields, where the interface is a critical factor determining overall mechanical performance. Modifying glass fibers by sizing agents with phase separation structures to construct phase separation structures at interface can enhance both interfacial strength and toughness, resolving the inherent strength-toughness trade-off. However, the high-temperature curing required for GREs preparation disrupts intermolecular hydrogen bonds in sizing agents and destroys their phase separation structures. To address this challenge, we developed a novel polyurea (PU40) as sizing agent with phase separation structures for glass fiber modification and proposed a “room-temperature/high-temperature synergistic curing” strategy for GREs fabrication. During room-temperature curing, PU40 reacts with epoxy to form interpenetrating polymer networks (IPNs), locking the phase separation structures. In the subsequent high-temperature curing, hydrogen bonds weaken, but the IPNs maintain structural integrity. This successfully constructs the phase separation structures at interface. GREs prepared with PU40-modified glass fibers using this synergistic strategy achieved an interfacial shear strength (IFSS) of 30.2 MPa and interfacial fracture toughness of 59.7 J·m^-2. Compared with both unmodified systems and those cured solely at high temperature, interfacial performance is significantly enhanced. This work provides a new pathway for developing high-performance GREs.

Key words: sizing agents with phase separation structures, interfacial fracture toughness, interfacial strength, glass fiber, epoxy resin