Atomistic Molecular Dynamics Simulations of Cross-linked Epoxy using LAMMPS

Due to the vast mismatch between the timescales accessible with atomistic simulations and experiment, quantitative comparison of their results can be challenging. For cross-linked epoxy, the preparation of realistic model structures is non-trivial due to the high density of covalent cross-links between relatively short and rigid constituent molecules that show extremely slow diffusion. Further, the thermomechanical properties of such networks show strong rate dependence. Here, we will demonstrate the utility of the Simulated Annealing and the Directed Diffusion techniques for the creation of the model structures for an epoxy network. We will then present the volumetric and dynamic properties of the created network obtained using atomistic simulations. For volumetric properties, we have developed the Specific Volume-Cooling Rate analysis that enabled quantitative comparison with experimental data sets. Finally, for the translational dynamics of the network, we have used the time-temperature superposition principle to obtain master curves. The timescale accessed by such master curves extends to macroscopic values. Thus, the simulation dynamics trends can be directly compared and quantitatively integrated with experimentally obtained creep compliance measurements. We will also discuss some of the features in LAMMPS that have been valuable for our work.