(13c) Study of Orientational Dynamics in a Cross-Linked Epoxy Network Using Atomistic Simulations

Subtle differences in the chemistry of cross-linked epoxy networks can lead to substantial changes in their thermomechanical properties, indicating the strong connection between chemical architecture and polymer physics in these materials. Atomistic molecular dynamics (MD) simulations leveraged with time-temperature superposition (TTS) are well-suited for studying these systems, as the combination enables integration of molecular length and time scale insight with experimental measurement at times scales relevant to commercial applications and measurement science. In this work, we study the orientational dynamics of cross-linked epoxy using the combination of atomistic MD and TTS. Specifically, we characterize the rotational diffusion autocorrelation function for network features at both atomic and molecular length scales at temperatures spanning from the rubbery state across the glass transition. At atomic length scales, we study the bond vectors of the central carbon atom of the epoxy monomer and the nitrogen atom of the cross-linker. At molecular length scales, we study end-to-end vectors of the epoxy monomer and the cross-linker. Using TTS, we show that all the molecular level data can be collapsed onto master curves spanning over nine orders of magnitude in reduced time. The temporal features of master curves can be quantitatively compared with the trend in experimental creep compliance for the same system. At the atomic level, TTS shows that the topological location of the atoms results in distinctly different signatures in the orientational dynamics for the two bond vectors. The results are relevant to the behavior of the molecular probes (mechanophores) being used for damage metrology by our experimental collaborators.