A molecular configuration-driven interfacial engineering strategy for constructing high-performance carbon fiber/epoxy composites

Interfacial chemical bonding between carbon fibers (CFs) and the resin matrix plays a critical role in enhancing the mechanical properties of carbon fiber reinforced polymer composites (CFRPs). While grafting functionalized molecules onto CFs can improve their interfacial properties, previous studies have focused on grafting efficiency and content rather than the influence of molecular configurations. Herein, three electrochemical surface treatment methods for the grafting of piperazine (PIP) molecules onto CFs were evaluated. Among them, the cycling electrode-switching electrochemical treatment (C-ESET) was identified as optimal and used to investigate the effect of interfacial molecules on the mechanical properties. The P-s-CF@PIP (Periodic-switching-CF@PIP) was obtained after 210 s of treatment and had a high surface N content of 19.46 at% and a surface energy of 51.23 mN m− 1. When combined with the epoxy resin (EP) matrix, the average interfacial thickness of the CFRP reached 506.7 nm, resulting in an interlaminar shear strength (ILSS) of 127.9 MPa and a tensile modulus of 244.2 GPa. Furthermore, the distinct configuration-dependent reinforcement mechanisms of linear, heterocyclic and aromatic diamine molecules at the CF/EP interface were elucidated. Thus, this work presents not only a universal surface treatment method for amine-reactive systems but also a molecular-level interfacial design principle for constructing CFRPs with rigid-flexible interphases.