Effects of Chain Length on the Dynamics and Structure of a Model Polyimide Under Uniaxial Loading*

Aromatic polyimides are known for their chemical resistance and thermal stability, and have long been used in high-temperature applications such as the manufacturing and soldering of electronic components. The mechanical properties of these materials can depend sensitively on the molecular composition and length of the polyimide chains, but specific underlying mechanisms are poorly understood. In this work we validate an all-atom molecular dynamics model for a pyromellitic dianhydride (PMDA)-based polyimide with 4,4′-oxydianiline (ODA) separators that include flexible ether linkages. This model is used to predict the chain-level and segmental dynamics, and the molecular structure of amorphous polyimides across a range of temperatures and chain lengths. We find relatively slow dynamics outside of high temperature regimes, even for low molar masses, 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.