In this work, molecular dynamics modeling is used to study the mechanical
properties of PPTA crystallites, which are the fundamental microstructural
building blocks of polymer aramid fibers such as Kevlar. Particular focus is
given to constant strain rate axial loading simulations of PPTA crystallites,
which is motivated by the rate-dependent mechanical properties observed in
some experiments with aramid fibers. In order to accommodate the covalent
bond rupture that occurs in loading a crystallite to failure, the reactive
bond order force field ReaxFF is employed to conduct the simulations.
Two major topics are addressed: The first is the general behavior of PPTA
crystallites under strain rate loading. Constant strain rate loading
simulations of crystalline PPTA reveal that the crystal failure strain
increases with increasing strain rate, while the modulus is not affected by
the strain rate. Increasing temperature lowers both the modulus and the
failure strain. The simulations also identify the C--N bond connecting the
aromatic rings as weakest primary bond along
the backbone of the PPTA chain. The effect of chain-end defects on PPTA
micromechanics is explored, and it is found that the presence of a chain-end
defect transfers load to the adjacent chains in the hydrogen-bonded sheet in
which the defect resides, but does not influence the behavior of any other
chains in the crystal. Chain-end defects are found to lower the strength of
the crystal when clustered together, inducing bond failure via stress
concentrations arising from the load transfer to bonds in adjacent chains near
the defect site. The second topic addressed is the nature of primary and
secondary bond failure in crystalline PPTA. Failure of both types of bonds is
found to be stochastic in nature and driven by thermal fluctuations of the
bonds within the crystal. A model is proposed which uses reliability theory
to model bonds under constant strain rate loading as components with
time-dependent failure rate functions. The model is shown to work well for
predicting the onset of primary backbone bond failure, as well as the onset of
secondary bond failure via chain slippage for the case of isolated
non-interacting chain-end defects.