GRAPHENE CONFINED POLYMER THIN FILMS SUBJECTED TO SUPERSONIC IMPACT

Due to their high specific penetration energy and high-strain-rate dependence, polymers are good candidates for lightweight protection against high-velocity impacts. Recent micro-impact experiments have shown enhanced abilities of polymer thin films to absorb remarkable amounts of energy during ballistic impacts. However, in situ observation of the molecular energy dissipation mechanisms is difficult or impossible, resulting in a gap in understanding. During confinement, the critically connected relationship between chain mobility, temperature, and pressure becomes more enigmatic as shock waves travel through the material. In this work, we use large-scale Molecular Dynamics (MD) simulations with reactive bond-breaking capabilities to gather insights into the nanoscale energy dissipation mechanisms of unconfined and confined polymers. Multi-layered graphene (MLG) sheets are used to create various MLG-polyethylene (PE) configurations, such as a PE with an MLG backing layer, and confined PE sandwiched between graphene. By varying the number and configurations of PE and MLG layers, the level of confinement on the PE can be increased. The resulting PE thin films, graphene films, and confined PE composites are subjected to a range of high-velocity impacts (500 - 7,500 m/s) to determine the amount of energy dissipation for the given material system. Unconfined PE experiences large void formation and chain pullout due to high entanglement density and low internal chain friction (small monomeric friction coefficient). With increasing levels of confinement, the energy dissipation mechanism transitions from chain disentanglement to chain scission, subsequently dissipating higher amounts of energy.