Simultaneous iterative boltzmann inversion for coarse-graining of polyurea

Polyurea is an alternating copolymer with excellent viscoelastic properties for dissipating shock and impact loads; however, a molecular-level understanding of how its chemistry relates to its performance remains elusive. While molecular dynamics simulations can in theory draw connections between molecular structure and viscoelastic properties, in practice the long relaxation times associated with polymer dynamics make such calculations prohibitively expensive. To address this issue, we have developed a coarse-grained (CG) model of polyurea in which each of the phenylmethane-aminobenzoate and tetramethylene-oxide units making up the polyurea chains are treated using individual CG beads. The parameters for the intra- and intermolecular force field of the CG model have been obtained in a rigorous manner by using the iterative Boltzmann inversion approach. We have validated the CG model against densities, heat capacities, and chain conformations obtained from fully atomistic MD simulations for oligomeric polyurea chains. A time-dependent dynamic rescaling method is proposed that allows for quantitative predictions of stress relaxation beyond microsecond time scales. The CG model introduced here opens up avenues to study the molecular structure-function relationship of polyurea and polyurea-based materials.