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 phenylmethaneaminobenzoate 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 polyureabased materials.

A new technique is presented to study fracture in nanomaterials by coupling quantum mechanics (QM) and continuum mechanics (CM). A key new feature of this method is that broken bonds are identified by a sharp decrease in electron density at the bond midpoint in the QM model. As fracture occurs, the crack tip position and crack path are updated from the broken bonds in the QM model. At each step in the simulation, the QM model is centered on the crack tip to adaptively follow the path. This adaptivity makes it possible to trace paths with complicated geometries. The method is applied to study the propagation of cracks in graphene which are initially perpendicular to zigzag and armchair edges. The simulations demonstrate that the growth of zigzag cracks is self-similar whereas armchair cracks advance in an irregular manner. The critical stress intensity factors for graphene were found to be 4.21 MPa √ m for zigzag cracks and 3.71 MPa √ m for armchair cracks, which is about 10% of that for steel.

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In this work we apply a material-frame, kernel-based estimator of continuum fields to atomic data in order to estimate the J-integral for the analysis of an atomically sharp crack at finite temperatures. Instead of the potential energy appropriate for zero temperature calculations, we employ the quasi-harmonic free energy as an estimator of the Helmholtz free energy required by the Eshelby stress in isothermal conditions. We employ the simplest of the quasi-harmonic models, the local harmonic model of LeSar and co-workers, and verify that it is adequate for correction of the zero temperature J-integral expression for various deformation states for our Lennard-Jones test material. We show that this method has the properties of: consistency among the energy, stress and deformation fields; path independence of the contour integrals of the Eshelby stress; and excellent correlation with linear elastic fracture mechanics theory.

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a b s t r a c tAn anisotropic strain energy function is proposed for tensile loading in graphene that provides a nonlinear, hyperelastic constitutive equation. In the proposed function, the energy depends on the principal invariants of the right Cauchy-Green tensor and the strains in the zigzag and armchair directions. The use of the zigzag and armchair strains gives the model the ability to account for anisotropic behavior at moderate deformations. The constitutive law parameters are determined by a least squares fit to the energies predicted by density functional theory (DFT) calculations, and a good match is obtained to the DFT results for zigzag and armchair graphene sheets with various loading combinations. The law is applied in a continuum calculation of nanoindentation of a graphene membrane. The force-deflection predicted with this model show excellent agreement with analogous experimental results, thus providing a strong link between DFT calculations and nanoexperiments.

To explore the relationship between microscopic structure and viscoelastic properties of polyurea, a coarsegrained (CG) model is developed by a structure matching method and validated against experiments conducted on a controlled, benchmark material. Using the Green-Kubo method, the relaxation function is computed from the autocorrelation of the stress tensor, sampled over equilibrium MD simulations, and mapped to a real time scale established by matching self-diffusion rates of atomistic and CG models. Master curves computed from the predicted stress relaxation function are then compared with dynamic mechanical analysis experiments mapped to a wide frequency range by time-temperature superposition, as well as measurements of ultrasonic shear wave propagation. Computational simulations from monodisperse and polydisperse configurations, representative of the benchmark polyurea, show excellent agreement with the experimental measurements over a multidecade range of loading frequency. V C 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 797-810 KEYWORDS: coarse-grained molecular dynamics; mechanical properties; polyurea INTRODUCTION Knowledge of the connections between chemistry, structure, and properties is needed to develop improved polymers with a materials-by-design approach. Computational models offer promise in identifying these relationships, but unfortunately they typically lack predictive capability beyond a small range of properties. Molecular dynamics (MD) simulations can provide tremendous insight into how the fine details of chemistry, chain architecture, and microstructure affect many physical properties; however, they face well-known limitations in both time and length scales. The goal of this work is to develop coarse-grained (CG) models that enable molecular simulations to reach more representative time and length scales to investigate the viscoelastic properties of polyurea, a thermorheologically complex block copolymer, for which theoretical rheological models are difficult to apply.

We investigate the thermomechanical response of semi-crystalline polyethylene under shock compression by performing molecular dynamics (MD) simulations using a new coarse-graining scheme inspired by the embedded atom method. The coarse-graining scheme combines the iterative Boltzmann inversion method and least squares optimization to parameterize interactions between coarse-grained sites, including a many-body potential energy designed to improve the representability of the model across a wide range of thermodynamic states. We demonstrate that a coarse-grained model of polyethylene, calibrated to match target structural and thermodynamic data generated from isothermal MD simulations at different pressures, can also accurately predict the shock Hugoniot response. Analysis of the rise in temperature along the shock Hugoniot and comparison with analytical predictions from the Mie-Grüneisen equation of state are performed to thoroughly explore the thermodynamic consistency of the model. As the coarse-graining model affords nearly two orders of magnitude reduction in simulation time compared to all-atom MD simulations, the proposed model can help identify how nanoscale structure in semi-crystalline polymers, such as polyethylene, influences mechanical behavior under extreme loading.