MOLECULAR DYNAMICS MODELING OF PPTA CRYSTALLITE MECHANICAL PROPERTIES IN THE PRESENCE OF DEFECTS
B. Mercer, E. Zywicz, and P. Papadopoulos Polymer, 114, pp. 329–347, (2017

Abstract
The mechanical properties of PPTA crystallites, the fundamental building blocks of aramid polymer fibers such as Kevlar and Twaron, are studied here using molecular dynamics simulations. The ReaxFF interatomic potential is employed to study crystallite failure via covalent and hydrogen bond rupture in constant strain-rate tensile loading simulations. Emphasis is placed on analyzing how chain-end defects in the crystallite influence its mechanical response and fracture strength. Chain-end defects are found to affect the behavior of nearby chains in a region of the PPTA crystallite that is small relative to the typical crystallite size in manufactured aramid fibers. The central C--N bond along the backbone chain is identified as the weakest in the PPTA polymer chain backbone in dynamic strain-to-failure simulations of the crystallite. It is found that clustering of chain-ends leads to reduced crystallite strength and crystallite failure via hydrogen bond rupture and chain sliding, whereas randomly scattered defects impact the strength less and failure is by covalent bond rupture and chain scission. The crystallite modulus increases with increasing chain length and is independent of chain-end defect locations. On the basis of these findings, a theoretical model is proposed to predict modulus as a function of chain length.


(If your institution subscribes to the electronic version of the journal, click here for a copy of this article.)