Towards a Rate-Dependent Material Model for a Polyetherimide Copolymer

Polyimides are a robust class of materials that are an industry staple in aerospace and electronics manufacturing for applications involving extreme conditions such as rapid temperature changes, irradiation, and dynamic mechanical loads including shocks. We perform all-atom molecular dynamics simulations of a common polyimide, i.e., poly (4-4'-oxydiphenylene-pyromellitimide), to predict and understand its response to high-rate uniaxial loads, isotropic compression, and thermal cycling. We find that the pressure-temperature-volume response is highly rate- and path-dependent when the material is subjected to rapid heating and compression rates typical of shocks. At low degrees of polymerization, the flow response to ultrafast uniaxial tension and compression is predicted to be initially strain hardening. Rate dependent structural changes are identified for large deformations, including apparent strain softening and failure due to the formation of voids under tension. These simulations are used to identify factors governing material equation of state and strength needed for accurate constitutive models at larger length scales.