Interface Enhancement of Unidirectional Carbon Fiber/Epoxy Resin Composites Using Bio‐Based Hyperbranched Waterborne Polyurethane With Biomimetic Rhizome Structure

ABSTRACT The inert nature of carbon fiber (CF) surfaces leads to inadequate bonding strength with resin matrices, thereby constraining the overall performance of its composite materials. Sizing agents are pivotal for enhancing the wettability and interfacial adhesion of CF surfaces. However, most linear‐structured sizing agents fail to fulfill the interfacial performance requirements of composite materials. Inspired by the rhizome structure, a bio‐based hyperbranched waterborne polyurethane sizing agent (WPU‐M) with a hyperbranched structure was synthesized successfully to address this limitation. A key intermediate, polyether–polyester polyol (MEP), was first synthesized via the esterification of L‐malic acid and polyethylene glycol (PEG). The hyperbranched architecture of WPU‐M arises from the multifunctionality of L‐malic acid, which facilitates the formation of a highly branched structure during the polyurethane synthesis. When WPU‐M is applied to carbon fiber/epoxy resin (CF/EP) composites, its distinctive biomimetic rhizome structure significantly enhances its interfacial properties. Compared with the untreated composites, the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) increased by 71.20% and 39.16%, respectively. The biomimetic rhizome architecture of WPU‐M not only effectively mitigated microscopic defects on the CF surface but also amplified the density of polar functional groups, thereby strengthening the intermolecular interactions between CF and EP. This study presents a novel approach for optimizing the interfacial design of high‐performance carbon fiber composites.