Development of an Atomistic Model to Predict the High-Rate Strength of Polyimide Copolymers

Aromatic polyimides are known for their chemical resistance and thermal stability, and are used in a variety of contexts, including situations where they're subjected to mechanical shocks. While the high strain-rate behavior of polymers is expected to be sensitive to segmental and chain-level dynamics, and conformational changes, the molecular-level mechanisms operative in polyimides are not well characterized. In this work we validate an all-atom molecular dynamics model for a pyromellitic dianhydride (PMDA)-based polyimide with 4,4′-oxydianiline (ODA) separators to enable direct numerical simulation of these materials under high-rate loading. Protocols are developed to accelerate the creation of initial amorphous configurations with chain lengths representative of commercial PMDA-ODA copolymers. Even at equilibrium, we find relatively slow dynamics outside of high temperature regimes, which is consistent with the high glass transition temperatures characteristic of PMDA-based polyimides. Simulations measuring the stress-strain response of polyimides under ultrafast uniaxial loading are used to assess factors governing material strength.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Approved for unlimited release, LLNL-ABS- 845021.