Mechanical and Viscoelastic Properties of Wrinkled Graphene Reinforced Polymer Nanocomposites - Effect of Interlayer Sliding within Graphene Sheets.

Multilayer graphene sheets (MLGSs) are promising nano-reinforcements that can effectively enhance the properties of polymer matrices. Despite many studies on MLGSs-reinforced polymer nanocomposites, the effect of wrinkles formed in MLGSs on the reinforcement effect and the viscoelastic properties of polymer nanocomposites has remained unknown. In this study, building upon previously developed coarse-grained models of MLGSs and poly(methyl methacrylate) coupled with molecular dynamics simulations, we have systematically investigated nanocomposites with different numbers of graphene layers and various wrinkle configurations. We find that with decreasing degree of waviness and increasing numbers of layers, the elastic modulus of the nanocomposites increases. Interestingly, we observe a sudden stress drop during shear deformation of certain wrinkled MLGSs-reinforced nanocomposites. We further conduct small amplitude oscillatory shear simulations on these nanocomposites and find that the nanocomposites with these specific wrinkle configurations also show peculiarly large loss tangents, indicating an increasing capability of energy dissipation. These behaviors are attributed to the activation of the interlayer sliding among these wrinkled MLGSs, as their interlayer shear strengths are indeed lower than flat MLGSs measured by steered molecular dynamics technique. Our study demonstrates that the viscoelastic properties and deformation mechanisms of polymer nanocomposites can be tuned through MLGS wrinkle engineering.