Mechanism of the impact of the sizing agent chain length on the interfacial mechanical properties of carbon/epoxy composites

Abstract

The mechanical properties of carbon fiber reinforced composites can generally be improved by interfacial design, and one of the possible strategies involves the manipulation of the sizing agent polymer. Although this approach can be industrialized with relative ease compared to grafting functional groups and chemical deposition nano-fillers on the surface of fibers, elucidating its regulatory mechanisms remains a challenge. Herein, molecular simulations are utilized to study the interfacial modification of typical carbon fiber/epoxy composites with different sizing chain length structures, and the physical and chemical characteristics of the interface are firstly quantified. An inhomogeneous distribution of the interfacial phase is further divided into three layers, and each layer is correlated with the evolution of the mechanical properties of the composite. The first layer in the interfacial phase is closest to the fiber surface, which is highly restrained by physical diffusion and exhibits poor mechanical properties. However, the first layer can be considered as an equivalent to a “softener”, which is beneficial for conferring better pull-out and bending strength. The second layer has the strongest interaction between the sizing agent and matrix, is controlled by a combination of physical diffusion and chemical interaction, which are particularly significant for interfacial transfer. The interfacial properties of the third layer are controlled by chemical reactions, which are closer to that of the resin matrix. The present study emphasizes the importance of gradient design of mechanical properties between the first and second layers in the hierarchical interface, paving the foundation and direction for a systematic framework of gradient design, optimization, and matching of interfaces in carbon fiber composites.