Molecular Dynamics Modeling of PPTA Crystals in Aramid Fibers

Abstract

Author(s): Mercer, Brian Scott | Advisor(s): Papadopoulos, Panayiotis | Abstract: 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.