Atomistic Simulations on Structural Characteristics of ZnO Nanowire-Enhanced Graphene/Epoxy Polymer Composites: Implications for Lightweight Structures

Improving the interfacial properties between fiber and polymer is critical for developing high-performance, lightweight composite materials. One approach is growing nanomaterials on the fiber surface as the secondary reinforcement to enhance the fiber/matrix bonding. Atomistic investigations of the interfacial properties of the ZnO nanowire (NW)-enhanced carbon fiber polymer composite are presented in this study. Molecular dynamics (MD) simulation is conducted to evaluate the effect of the ZnO NW diameter, ZnO NW length, ZnO NW/graphene crystal twisting angle, loading temperature, and separation rate on the traction–separation behavior of the hybrid structure. A representative volume element is developed at the atomic scale, containing a ZnO NW aligned on the fiber surface and embedded in the cross-linked epoxy polymer. The cohesive parameters, such as penalty stiffness, interfacial strength, and cohesive energy, are extracted from the separation of the graphene sheet from the ZnO NW/epoxy in the opening mode (normal separation). Improved adhesion properties in ZnO NW-enhanced composites are observed by comparing the results of the hybrid structures with the bare one (no ZnO NW). Higher interfacial properties are achieved by reducing the diameter of ZnO NW in the enhanced structures. A negligible effect of changing the ZnO NW length on the interfacial stiffness is observed, while incorporating shorter NWs result in higher interfacial strength and cohesive energy values. Results show that the interface in the hybrid composite is sensitive to the twisting angle of the ZnO’s hexagonal-shaped plane with respect to the graphene’s crystal. For instance, the interfacial strength of 0° twisting angle is 25% higher than the 45° angle. The MD results reveal that increasing the loading temperature leads to a weaker interface. Interfacial properties are initially improved by increasing the loading rate and then become rate-independent at separation rates higher than 25 Å/ps.