Superior Protection Conferred by Multi-Layered Graphene-Polyethylene Nanocomposites under Shock Loading

With the emerging need for lightweight, protective materials such as armor, polymer nanocomposites are being continuously developed with tailored properties to enhance energy-absorbing and dissipating capacity. Developing strong and tough materials is of paramount importance within space and weight constraints. Understanding the high-strain rate deformation mechanisms, shock propagation, energy dissipation, and failure is a critical design consideration for armor materials. High-performance natural materials, such as nacre, show remarkable strength and toughness due to their hierarchical layered architecture across multiple length scales. However, such natural materials often experience high impact loads and thus offer good design criteria for artificial armor materials. Here, we report on the shock response of multilayered graphene polyethylene nanocomposites through large-scale coarse-grained molecular dynamics simulations. Simulations for multiple piston speeds (0.3-3.5 km/s) were conducted to study shock propagation, dissipation, and eventual failure. The multilayered organization of graphene significantly increases the impact strength of the composites, as reflected in the spallation strength of the composites. This study also shows the effect of physical grafting of polyethylene chains on shock dissipation and mitigation. The spall strength of the grafted MLG-PE is approximately 10-40% greater than that of its corresponding counterparts. We also elucidated the underlying molecular mechanisms involved in shock deformation. The foremost mechanisms driving dissipation in the grafted composite are the intricate scattering of shock waves at the interface, a substantial difference in the acoustic impedance between graphene and polyethylene, and the occurrence of visco-plastic deformation involving numerous stress-transfer sites. Our research revealed that introducing grafted polymer chains to the fillers results in significant morphological alterations in the polymers, primarily attributed to bond compression, stretching, and bending. The study offers promising design strategies for designing high-performance lightweight materials.