Revealing the toughening mechanism of graphene-polymer nanocomposite through molecular dynamics simulation

By employing united atom molecular dynamics simulation, we have investigated the effects of polymer-graphene interaction epsilon(np), volume fraction of graphene phi, thermodynamics of polymer matrix (rubbery versus glassy), interfacial interaction in the case of the same dispersion state, shape of nanoparticles (NPs) such as C-60, CNT and graphene at the same loading on the toughening efficiency of polymer nanocomposites. By beginning with the pure polymer, we observe that a plateau stress occurs at long chain length because entangled polymer chains in fibrils cannot become broken. We find that the work needed to dissipate during the failure increases with the increase of epsilon(np) and phi, and the yield point in the stress-strain behavior occurs at a smaller strain for an attractive NPs filled system compared to the pure case, attributed to the more mechanically heterogeneous environment. The thermodynamics of the polymer matrix (below and above T-g) seems to have a significant effect on the toughening efficiency of graphene sheets. In the case of the same dispersion state, stronger interfacial interaction always induces long and highly orientated polymer fibrils along the deformation direction, with graphene sheets being encapsulated in these fiber-like bundles. By characterizing the interaction energy between polymer-polymer and polymer-graphene as a function of the strain, we find that the separation of polymer chains from the graphene sheets cease immediately after the yield point, followed by the continuous propagation of the cavities by excluding surrounded polymer chains and graphene sheets together. We also find that at the same attractive interfacial interaction and same loading, the toughening efficiency exhibits the following order: graphene > CNT > C-60. Generally, the toughening mechanism of graphene sheets results from the formation of long and highly orientated polymer fibrils to prevent the occurrence of the rupture, which can be greatly improved by the strong interfacial interaction and the large surface area compared to CNT and C-60. This also indicates that polymer matrices with high flexibility and mobility of polymer chains tend to be better toughened. It is hoped that this simulation work will provide rational guidance for fabricating high performance of polymer nanocomposites with excellent toughness.