Simultaneous Improvement in the Elastic and Fracture Properties of Graphene-Epoxy Nanocomposites ̶ a Computational Perspective

Abstract

The incorporation of stiff nano-additives (such as graphene) into a relatively soft polymer material (such as epoxy) usually leads to an improvement in elastic properties (i.e., Young’s modulus) at the expense of fracture properties (i.e., tensile strength and toughness). Despite over a decade of research in polymer nanocomposites, we still lack a clear understanding of their structure-property relationships, which limits us from enhancing both elastic and fracture properties concurrently. Here, we performed large-scale reactive molecular dynamics simulations to study the deformation and fracture of model graphene/epoxy systems under uniaxial tension. A computationally efficient reactive force field was developed for graphene-epoxy system, allowing covalent bond formation, and breaking, which is crucial to model cross-linking in model epoxy as well as fracture. It was found the mechanical properties of the nanocomposite are very sensitive to the strength of the graphene-epoxy interface. As expected, elastic modulus increases with the interfacial strength. However, there appears to be an optimal interfacial strength to enhance the tensile strength and toughness. This is due to stress concentration occurring near the graphene edges at high interfacial strength, which leads to premature fracture. We show that by appropriately selecting an intermediate interface strength, one can simultaneously improve the ultimate tensile strength, toughness and the Young’s modulus of the nanocomposite epoxy at ultra-low (∼0.2% by weight) loading fraction of graphene additives. Our findings highlight the critical importance of properly engineered additive-matrix interfacial strength to develop high-performing polymer nanocomposites that are concurrently stiff, strong as well as tough.