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
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.
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