Anisotropic atomistic shock response mechanisms of aramid crystals

EJ Gurniak and SC Tiwari and SW Hong and A Nakano and RK Kalia and P Vashishta and PS Branicio, JOURNAL OF CHEMICAL PHYSICS, 157, 044105 (2022).

DOI: 10.1063/5.0102293

Aramid fibers composed of poly(p-phenylene terephthalamide) (PPTA) polymers are attractive materials due to their high strength, low weight, and high shock resilience. Even though they have widely been utilized as a basic ingredient in Kevlar, Twaron, and other fabrics and applications, their intrinsic behavior under intense shock loading is still to be understood. In this work, we characterize the anisotropic shock response of PPTA crystals by performing reactive molecular dynamics simulations. Results from shock loading along the two perpendicular directions to the polymer backbones, 100 and 010, indicate distinct shock release mechanisms that preserve and destroy the hydrogen bond network. Shocks along the 100 direction for particle velocity U-p < 2.46 km/s indicate the formation of a plastic regime composed of shear bands, where the PPTA structure is planarized. Shocks along the 010 direction for particle velocity U-p < 2.18 km/s indicate a complex response regime, where elastic compression shifts to amorphization as the shock is intensified. While hydrogen bonds are mostly preserved for shocks along the 100 direction, hydrogen bonds are continuously destroyed with the amorphization of the crystal for shocks along the 010 direction. Decomposition of the polymer chains by cross-linking is triggered at the threshold particle velocity U-p = 2.18 km/s for the 010 direction and U-p = 2.46 km/s for the 100 direction. These atomistic insights based on large-scale simulations highlight the intricate and anisotropic mechanisms underpinning the shock response of PPTA polymers and are expected to support the enhancement of their applications.(C) 2022 Author(s).

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