(104a) Particle Based Multi-Scale Modeling of the Dynamic Response of Hexahydro-1,3,5-Trinitro-s-Triazine (RDX): Constant Energy Dissipative Particle Dynamics with Coarse-Grained Density Dependent Potentials

Moore, J. D., U.S. Army Research Laboratory
Lisal, M., Academy of Sciences of the Czech Republic
Brennan, J. K., U.S. Army Research Laboratory

Nanocomposites are inherently heterogeneous, containing several species types of disparate shapes and sizes.  For example, energetic materials are often comprised of micro-sized crystallites (or grains) held together by a small amount of polymer binder, and in some instances contain metal nanoparticles.  Accordingly, mechanical stimulation (e.g., shock or shear) of these materials can incite responses over a wide range of spatial and temporal scales.  Localized regions of elevated thermal energy, or hot spots, often occur near microscale defects in the material such as voids and grain boundaries.  However, some energy transfer processes are atomistically governed, for which field-based microscale, mesoscale, and continuum modeling techniques are currently unable to provide the required level of fidelity.  Modeling the thermal and mechanical response of these materials atomistically, despite growing computational power and resources, remains a challenge due to the length and time scales required, as often billions of molecules are necessary to be modeled on micron-size scales.

To overcome some of these challenges, we have implemented multiscale techniques to bridge the atomistic and mesoscale levels of description.  The multiscale coarse-graining method (MS-CG) [1,2], a bottom-up approach based on force-matching, has been utilized to derive particle-based coarse-grained potentials for hexahydro-1,3,5-trinitro-s-triazine (RDX), where one RDX molecule is mapped onto a single interaction site.  To improve transferability of the MS-CG model (required to accurately capture thermodynamic responses during mechanical and thermal stimulation), explicit MS-CG mapping from the atomistic to the microscale is made from ambient to high pressures [2].  The resulting density dependent MS-CG model reproduces several atomistic properties within reasonable agreement from ambient to high pressures for the molecular crystal including the lattice structure, melting point, elastic and vibrational properties, and the phase diagram [3].  For the molten state, the 2-body and often multi-body structure are also well described with the model [3]. 

Despite the successes of the MS-CG model of RDX to describe static properties, it inevitably cannot account for accurate energy and momentum exchange in response to mechanical stimulation using traditional molecular dynamics.  This is due to intramolecular degrees of freedom (i.e., intramolecular bonding) being coarse-grained out, and thus, the heat capacity is underestimated by nearly an order of magnitude [3].  To correct this deficiency, we account for momentum and energy transfer under mechanical shock by utilizing the constant-energy Dissipative Particle Dynamics method (DPD-E) [4-6].  Unlike traditional Dissipative Particle Dynamics [7-9], which conserves only momentum, the DPD-E method conserves both energy and momentum by assigning an internal energy to each particle, which for the MS-CG RDX case, helps to minimize the effect of coarse-graining the intramolecular degrees of freedom.

In this talk, we will present results for mechanical and thermal shock loading of our MS-CG model of RDX using DPD-E.  Various modeling parameters have been investigated for sensitivity including spatial interaction cutoffs used to determine local densities for the density dependent potentials, the mesoparticle equation of state that accounts for the particle internal energy and the overall dynamics through energy dissipation.  Results will be compared directly to atomistic simulation for both mechanical shock and the response due to thermal heating of the material. 

This effort is supported by the recently established Institute for Multi-Scale Reactive Modeling of Insensitive Munitions (MSRM), which is a multi-team effort led by the U.S. Army Research Laboratory and the U.S. Army Armament Research, Development, and Engineering Center, involving various other national laboratories and academic groups totaling over 20 scientists.  J.D.M. gratefully acknowledges support, in part, by an appointment to the Internship/Research Participation Program for the U.S. Army Research Laboratory administered by the Oak Ridge Institute of Science and Education through an agreement between the U.S. Department of Energy and U.S. ARL (Project ID: 201003211).

[1] S. Izvekov and G.A. Voth, J. Chem. Phys., 123, 134105 (2005).

[2] S. Izvekov, P.W. Chung, and B.M. Rice, J. Chem. Phys., 133, 064109 (2010).

[3] S. Izvekov, P.W. Chung, and B.M. Rice, Submitted (2011).

[4] J. Bonet Avalos and A.D. Mackie, Europhys. Lett., 40, 141 (1997).

[5] P. Español, Europhys. Lett., 40, 631 (1997).

[6] M. Lísal, J.K. Brennan,  and J. Bonet Avalos, Submitted (2011).

[7] P.J. Hoogerbrugge and J.M.V.A. Koelman, Europhys. Lett., 19, 155 (1992).

[8] J.M.V.A. Koelman and P.J. Hoogerbrugge, Europhys. Lett., 21, 363 (1993).

[9] R.D. Groot and P.B. Warren, J. Chem. Phys., 107, 4423 (1997).