Synergistic strengthening-toughening of composite interphase via in-situ tensile load-triggered 2D flake-like polydopamine on carbon fiber and 0D polydopamine nanospheres in epoxy matrix

Polydopamine (PDA) is renowned for its outstanding adhesiveness, however, the high stiffness imparted by its rich aromatic structure tends to sacrifice interfacial toughness, limiting the long-term application of composites. In this study, without resorting to any additional flexible materials, a discontinuous 2D flake-like PDA structure is formed on the surface of single carbon fiber by simply applying an in-situ tensile load. This leads to a 32.6% increase in interfacial shear strength and a 190.6% growth in interfacial toughness of epoxy composites, relative to untreated fiber composites. The enhanced strength is primarily associated with good wettability that boosts interfacial interactions, while the enhanced toughness arises from mechanisms including plastic yielding and deformation of resin in the gap regions, along with the obstruction of crack propagation. Moreover, upon addition of 0D PDA nanospheres to epoxy matrix, interfacial strength and toughness are further improved, with increments of 55.8% and 338.9% as opposed to untreated fiber composites. This results from the additional performance-enhancing mechanisms, such as improved resin cohesive strength, particle debonding, and frictional sliding. The dual-phase synergistic effect of PDA sheets and nanospheres endows the composite with excellent interfacial properties, offering a new strategy for the design of advanced composites.