Shock-induced paracrystallinity in PPTA

The outstanding strength-to-weight ratio of para-aramid fibers, such as Kevlar and Twaron, is largely attributed to their high content of crystalline p-phenylene terephthalamide (PPTA). Atomistic simulations of shock loading on PPTA are performed along the [100] and [010] crystallographic directions, using reactive molecular-dynamics simulations. The reactive forcefield utilized is fitted to PPTA properties using first principles data and the results are validated by ab-initio molecular dynamics (QMD) simulations. Simulation results reveal an anisotropic shock response displaying elastic, crosslinking, and phase transformation from crystalline to para-crystalline phases. While QMD simulations show elastic to amorphous planar transformation for shocks along [010] direction, long time simulations accessible by reactive molecular dynamics indicate the formation of a para-crystalline phase initiated by an amorphous planar transformation, which displays H-bond scission and rotation of chains. The rotation process reorients the polymer chains such that vdW interactions dominate chain-chain interactions leading to the formation of local domains where new H-bond interaction forms leading to the para-crystalline phase.