The inherent inertness of carbon fiber surfaces severely limits the interfacial bonding with epoxy resin matrices, while existing modification methods often damage the fiber's intrinsic strength or involve complex processes. In this work, domestic T700 carbon fibers were modified via a two-step mild strategy: carboxylation activation with 65% concentrated nitric acid followed by polyethylenimine (PEI) covalent grafting, to construct a functional interface synergized by rigid carboxyl groups and flexible PEI segments. Surface chemistry, wettability, and mechanical properties were systematically characterized by X-ray photoelectron spectroscopy (XPS), contact-angle measurement, and single-filament tensile testing. Results show that concentrated nitric acid etching efficiently introduces oxygen-containing polar groups (e.g., carboxyl groups) for surface activation. PEI is stably grafted via amidation, converting oxygen-containing groups to amino/amide bonds, which significantly enhances surface polarity and wettability, turning fibers from strongly hydrophobic to highly hydrophilic. Moreover, PEI grafting covers and fills the nanoscale etching pits and rough areas on the fiber surface, reducing stress concentration points on the fiber surface and improving the overall surface integrity. This simple and reliable strategy provides a feasible approach and theoretical support for the interfacial reinforcement of high-performance carbon fiber/epoxy composites.
Li et al. (2026) studied this question.