Dual Interfacial Engineering of Glass Fiber‐Reinforced Composites: Synergistic Enhancement via Hyperbranched Polyester‐Modified Resin and Rigid/Flexible Fiber Coatings

ABSTRACT Glass fiber‐reinforced polymer (GFRP) composites were prepared by hand lay‐up using unsaturated polyester resin (UPR) matrices in which a hyperbranched polyester (HBP) synthesized via a polyethylene glycol (PEG) core route was incorporated. Glass fibers (GFs) were functionalized to create “rigid” (HEA/TDI‐modified, m‐GF) and “flexible” (KH570‐modified, KH570‐GF) interfaces. HBP‐modified UPR composites exhibited enhanced tensile, flexural, and impact strengths by 62.2%, 17.5%, and 95.0%, respectively, versus unmodified counterparts. Introducing the rigid m‐GF interface further amplified these improvements, achieving strength gains of 96.1% (tensile), 90.9% (flexural), and 80.4% (impact). The flexible KH570‐GF interface yielded the most pronounced synergistic effect: the optimum system (KH570‐GF + 5 wt% HBP) achieved a tensile strength of 336.8 MPa (a 113% increase), a flexural strength of 622.1 MPa (93.6% increase), and an impact strength of 142.2 kJ m −2 (187% increase) relative to the baseline GF/UPR composite. Dynamic mechanical analysis confirmed increased glass transition temperatures, indicating enhanced thermal stability from improved crosslinking. Scanning electron microscopy demonstrated reduced fiber‐matrix debonding and suppressed crack propagation in the modified systems. The rigid interface primarily enhanced load transfer efficiency, whereas the flexible interface promoted energy absorption via interfacial slippage. These findings illustrate that independent tailoring of rigid or flexible interfaces provides distinct pathways for performance optimization, effectively overcoming the traditional strength–toughness trade‐off in GFRPs and offering customizable solutions for aerospace and structural applications.