Molecular modeling of EPON-862/graphite composites: Interfacial characteristics for multiple crosslink densities

Many thermo-mechanical properties of fiber-reinforced epoxy composites strongly depend on the conditions at the fiber/matrix interface. Because it is difficult to experimentally characterize the interface region, computational molecular modeling is a necessary tool for understanding the influence of interfacial molecular structure on bulk-level properties. The objective of this study is to determine the effect of crosslink density on the conditions of the interface region in graphite/epoxy composites. Molecular dynamics models are developed for the fiber/matrix interfacial region of graphite/EPON 862 composites for a wide range of crosslink densities. The mass density, residual stresses, and molecular potential energy are determined in the epoxy polymer in the immediate vicinity of a graphite fiber. It is determined that a surface region exists in the epoxy in which the mass density is different than that of the bulk mass density. The effective surface thickness of the epoxy is about 10 Å, irrespective of the crosslink density. A high-resolution TEM image is obtained for the interfacial region of carbon nanofiber/EPON 862 composites which clearly shows that the interface region thickness is about 10 Å, thus validating the molecular modeling technique. The simulations also predict residual stress levels in the surface region of the epoxy that are slightly higher than in the bulk, yet far below the ultimate load for the epoxy system considered herein. Furthermore, the simulations predict elevated levels of molecular potential energy in the interface region relative to the bulk epoxy, with the magnitude of energy decreasing for increasing crosslink densities.