Influence of Fiber Surface Treatment on the Elevated Temperature Interfacial Properties of Fiber‐Reinforced Phthalonitrile Composites

Phthalonitrile (PN) resin is an ideal matrix for high‐temperature resistant composites due to its exceptional thermal stability (> 300°C). However, a critical incompatibility arises from the use of commercial sizing agents, which are thermally unstable at high temperatures and become a weak link. To fundamentally investigate this interfacial incompatibility and reveal the intrinsic bonding behavior between fibers and resin, this study employed a desizing approach to systematically isolate the influence of conventional sizing agents. The interfacial shear strength (IFSS) between PN resin and various fibers (T700, T800, T1100 carbon fibers, and a quartz fiber) was evaluated from 25°C to 250°C. The results revealed that the removal of sizing agents significantly enhanced the IFSS of carbon fiber/PN systems, with a maximum increase of 42.4% at 250°C. This enhancement stemmed from the elimination of the thermally degraded weak boundary layer and the subsequent dominance of mechanical interlocking afforded by the intrinsic roughness of the exposed fiber surface. Through a comparative analysis of sized and desized fiber interfaces, this work elucidates the degradation mechanisms of conventional sizing at high temperatures and clarifies the intrinsic interfacial bonding behavior. The findings provide crucial theoretical insights and practical guidance for constructing the next generation of high‐performance composites.