Molecular dynamics simulation: The roles of silane coupling agent structural configurations on quartz fiber-epoxy interface

Due to the complexities of γ-aminopropyl triethoxysilane (KH550) existed-forms (including graft bonding, physical attachment, and chemical crosslinking) on reinforcing fiber surface, the enhancement mechanism of KH550 obtained from limited experimental methods are still understood in a simple and surficial level. Herein, for maximizing the effects of KH550 in reinforcing the mechanical properties of fiber-reinforced polymeric composites, the roles of KH550 forms in interface enhancement are clarified at the micro-nano level based on molecular dynamics methods. Firstly, three situations including only grafted KH550, grafted KH550 with physically attached KH550, and grafted KH550 with chemical cross-linked KH550 are constructed respectively at the interface between quartz fiber and epoxy resin. Subsequently, the structural characteristics of the interface are elucidated by examining the distribution of atoms and molecules present at the interface. Calculations of interfacial binding energy and hydrogen bond analysis are performed to evaluate the robustness of interfacial interactions. The resilience of various interfacial structures against separation is probed using tensile and shear simulations. Among these simulations, the interface performance improved with the increase in the number of grafts, but remained lower than that achieved with the KH550 interface layer. Particularly, higher hydrogen bonding number and interfacial energy between quartz fibers and epoxy resin were attained with a 10% crosslinking degree of KH550, leading to stronger tensile and shear strength. Nevertheless, as the crosslinking degree increased, silicon-oxygen skeletons formed, and hydrogen bonding decreased, resulting in a decrease in the tensile and shear properties of the interface.