Molecular Dynamics (MD) and Coarse Grain Simulation of High Strain-Rate Elastomeric Polymers (HSREP): Molecular and Coarse-Grained (CG) Modeling of Shock Wave Mechanics in HSREP

In this chapter, an overview is provided of the all-atom and coarse-grained computational methods and tools used to investigate the superior shock wave dispersion/attenuation capacity of nanosegregated polyurea. This class of HSREP consists of (high glass-transition temperature, T_g) nanometer-sized rod-shaped, discrete, so-called hard domains which are embedded into a (low T_g) highly compliant, so-called soft matrix. Direct simulations of the interactions of shock waves (of different strengths) with the host material using all-atom and coarse-grained molecular-dynamics methods enables identification and quantification of the key viscous or inelastic deformation and microstructure-altering processes occurring at the shock front. Among these processes, the ones which are most likely responsible for the dispersion of fully supported shock waves in polyurea have been identified as shock-induced coordinated shuffle-like motion of the soft-matrix chain-segments behind the shock front, densification and crystallization of hard domains, and breakage and regeneration of the hydrogen bonds within the hard domains. In addition, in the case of blast-induced shock waves, it is shown that the "shock wave capture and neutralize" process by the release-waves can play an important role and that the efficacy of this process is greatly affected by the polyurea molecular weight and its synthesis route. This chapter presents developments in the molecular modeling and simulation of elastomeric polymers to link the microstructural and chemical features of polyurea to its macroscopic mechanical properties. A bead-spring model is first presented to qualitatively illustrate the importance of the hard domains of polyurea to its mechanical performance and demonstrate how the interaction energy between hard segments influences the microphase separation and morphology of hard domains. Next, a more sophisticated coarse-grained model of polyurea, calibrated in a bottom-up fashion from fully atomistic molecular dynamics simulations, is presented. This systematically coarse-grained model is parameterized to match structural distributions calculated from atomistic simulations by using the iterative Boltzmann inversion method. A method for dynamically rescaling the coarse-grained simulations to accurately predict the viscoelastic stress relaxation function is developed by matching the atomistic and coarse-grained diffusion rates. Analysis shows that quantitative predictions of the stress relaxation response of a complicated polymer can be computed from such multiscale approaches with reasonable accuracy. In conclusion, several outstanding challenges in coarse-grained modeling of polymers and bridging such models with continuum models are presented.