Fiber–Matrix Interface Engineering in Polymer Composites: Linking Surface Chemistry to Multiscale Mechanical Performance

Fiber–matrix interface engineering plays a decisive role in determining the mechanical performance of fiber-reinforced polymer composites (FRPCs). Despite extensive research on surface modification strategies, translating improvements in interfacial properties into reliable structural performance remains a major challenge. This review critically examines how surface chemistry, interphase architecture, nanomodification, and processing conditions collectively govern load transfer, interfacial shear strength, damage evolution, and overall mechanical behavior. Unlike conventional reviews that primarily summarize modification techniques, this work emphasizes the coupled relationship between interface design and manufacturing, demonstrating that interfacial performance is strongly process-dependent rather than an intrinsic material property. Reported improvements in interfacial metrics are critically evaluated against macroscopic structural performance, revealing persistent limitations arising from dispersion quality, resin rheology, processing defects, and scalability. A multiscale framework is proposed to connect physicochemical modifications with laminate-level failure mechanisms, including delamination, fiber pull-out, and fatigue degradation. The review identifies major research gaps in interphase characterization, standardized evaluation methods, and scalable interface engineering, and outlines future directions based on process-integrated design, predictive modeling, and multifunctional interphases for next-generation composite structures.