Molecular Simulation Study of Orientational Dynamics in a Cross-linked Epoxy Network

Subtle changes in the chemistry of cross-linked epoxy networks have a drastic impact on the thermomechanical properties of cross-linked epoxy, which indicates the strong connection between chemical topology and polymer physics in these materials. Here, we study the orientational dynamics of cross-linked epoxy using atomistic molecular dynamics simulations. Specifically, we characterize the reorientation of vectors defined at atomic and molecular length-scales using first and second order Legendre polynomials (C₁ and C₂, respectively) at temperatures across the glass transition. For atomic length scales, we use the bond vectors of selected groups of atoms. For molecular length scales, we use end-to-end vectors of the epoxy monomer and the cross-linker that comprise the network. We then use the time—temperature superposition (TTS) principle to create master curves of C₁ and C₂. We find that while TTS can be successfully applied at molecular length scales, the topological location of the atoms can cause a distinct signature in the orientational dynamics of the bond vectors, which can be teased out using TTS. We are interested in using atomistic simulations to connect molecular insight with macroscopic properties.