(359f) Simulating the Dynamic Response of Nanocomposites Using Dissipative Particle Dynamics

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 the energy transfer processes are atomistically governed. While modeling and simulation of such phenomena can offer fundamental insight, it is computationally challenging. Computing power and resources continue to grow, but there still exists much behavior that cannot be studied using atomistic simulation techniques. Moreover, field-based mesoscale modeling techniques have fidelity limitations with respect to gaining a fundamental understanding. In an attempt to overcome these challenges, we recently embarked on a study of the thermo-mechanical response of nanocomposites using a particle-based modeling technique, namely, the constant-energy Dissipative Particle Dynamics method (DPD-E) [1, 2]. A constant energy approach is required since materials subjected to mechanical stimuli will inevitably respond via energy exchange and transfer processes. In contrast to the standard Dissipative Particle Dynamics method [3, 4] that conserves only momentum, DPD-E conserves both momentum and energy by assigning an internal energy to each particle, allowing particles to exchange both momentum and thermal energy. This particle internal energy, which contains the resolved atomic degrees-of-freedom, is included as a separate equation of motion along with the equations of motion for the particle's position and momentum. The predictability of DPD simulations can be improved by introducing realistic conservative interactions, which accurately represent the free-energy surface of the underlying atomistic system. In the present work we explore this possibility and apply multiscale coarse-graining method (MS-CG), [6,7] in which a pairwise decomposition of atomistic free energy is derived through force-matching of atomistic level interactions, to develop a new generation of coarse-grain conservative forcefields suitable for a use with the DPD formalism. A number of modeling challenges exist for nanocomposites, including the development of accurate models that can capture the following known thermo-mechanical responses: (i) phase transitions; (ii) structural rearrangements (e.g., plastic deformation); and (iii) chemical reactions. Moreover, it is desirable that the coarse-grain model parameters have a physical basis so that direct links to real material properties or higher resolution modeling are possible. Technical challenges also exist for such studies. While current numerical integrator algorithms work quite satisfactorily for DPD [5] and DPD-E [1,2] simulations under normal conditions, at extreme conditions such as high densities, much shorter integration timesteps are required to ensure numerical stability. In order to maintain a reasonable timestep that allows for the simulation of micro- and mesoscale events, we have implemented higher quality integrators. In this talk, we will present the status of this ongoing project, including progress on coarse-grain model development using the MS-CG method [6,7] implementation of improved integrator algorithms for various DPD methods [8], and the development of a constant-pressure, constant-enthalpy DPD method [8]. To date, progress has been encouraging, where we find the DPD method coupled with the accurate coarse-grain models to be a viable tool for simulating the thermo-mechanical response of nanocomposites to mechanical stimuli. 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. ML acknowledges funding through a Cooperative Agreement with the U.S. Army Research Laboratory.

References: [1] Bonet Avalos, J., and A.D. Mackie, Europhysics Letters, 40, 141 (1997). [2] Español, P., Europhysics Letters, 40, 631 (1997). [3] Hoogerbrugge, P.J., and J.M.V.A. Koelman, Europhys. Lett., 19, 155 (1992). [4] Koelman, J.M.V.A., and P.J. Hoogerbrugge, Europhys. Lett., 21, 363 (1993). [5] Groot, R.D., and P.B. Warren, J. Chem. Phys., 107, 4423 (1997). [6] S. Izvekov, P.W. Chung, and B.M. Rice J. Chem. Phys., to be published, (2010). [7] S. Izvekov and G.A. Voth,, J. Chem. Phys., 123, 134105 (2005). [8] M. Lisal, J.K. Brennan, and J. Bonet-Avalos, J. Chem. Phys., to be submitted (2010).