Molecular and Nanoscale Anisotropic Shock Response Mechanisms of Aramid Fibers

Aramid fibers are used in a wide range of applications due to their low density, high strength, and shock resilience, which originates from the properties of poly(p-phenylene) terephthalamide (PPTA). Notwithstanding their wide applications, e.g., in Twaron, Kevlar and other high-performance fabrics, there are still gaps in the understanding of the intrinsic deformation mechanisms of this material under shock loading. Here, we perform molecular dynamics simulations with a reactive force field, ReaxFF, to characterize the PPTA shock response for loading along the [100] and [010] directions, perpendicular to the polymer backbone/fiber axis. The plastic deformation for shocks along [100] preserves hydrogen bonding, while the shock is released with the generation of shear bands, where the PPTA structure becomes planarized. Amorphization is induced for shocks along [010] promoting massive hydrogen bond scission.  These shock regimes occur until cross-links between polymer chains are triggered, starting at U_p = 2.18 km/s for [010] direction and U_p = 2.46 km/s for [100] direction. These results demonstrate the underlying molecular and nanoscale deformation mechanisms of PPTA furthering our understanding of shock-loading of strong, high-performance polymers.