UC Berkeley

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.