Title: Multiscale Computational Polymer Research at the Army Research Laboratory

Presenter: Timothy Sirk

Authors: Timothy Sirk, Y. Sliozberg, T. Chantawansri, J. Andzelm

Affiliation: Army Research Laboratory

Abstract: This poster highlights particle-based computer simulations of polymeric materials in the Macromolecular Science and Technology Branch at the U.S. Army Research Laboratory (ARL). We will review our recent work on glassy polymers under high pressure 1 and mechanical properties of polymer gels 2-4.

The behavior of polymers under high pressure and temperature is of interest for a variety of applications, such as polymer-bonded explosives, coatings, adhesives, and light-weight armor. Simulation methods such as molecular dynamics (MD) and quantum mechanics (QM) were used to provide insight into atomic-level phenomena. Using classical MD and QM, we have calculated the principle shock Hugoniot curves for four polymers: poly(methyl methacrylate), poly(ethylene), poly(styrene), and poly(carbonate). In the MD calculations, we considered both a non-reactive (i.e. PCFF) and reactive (i.e. ReaxFF) forcefield, respectively, where calculations were performed in LAMMPS. The QM calculations were performed with density functional theory (DFT) using CP2K code. Overall, results obtained by QM show better agreement with available experimental data than classical force fields, particularly at low pressures, up to 20GPa.

Particle-based models of gel materials must consider high molecular weight chains, and therefore must capture the large length- and time- scales associated with chain entanglement. An aggressive coarse-grain approach is needed in the calculation of transport and mechanical properties of gels. Our work shows that Dissipative Particle Dynamics is useful 2,3,4 and computationally practical 3 in understanding the reptation dynamics and virtual network of entangled polymer melts if an addition potential is considered to prevent chain crossings, i.e., a segmental repulsion potential (SRP). We modify the parameters and functional form of a previous SRP to essentially eliminate chain crossing while maintaining the thermodynamic and structural properties of the DPD bead-spring model 2.

1 T. Chantawansri, T. Sirk, E. Byrd, J. Andzelm, B. Rice. “Shock Hugoniot Calculations of Polymers using Quantum Mechanics and Molecular Dynamics”. Journal of Chemical Physics, 137, 204901 (2012).

2 T. Sirk, Y. Sliozberg, J. Brennan, M. Lisal, J. Andzelm. “An enhanced entangled polymer model for dissipative particle dynamics”. Journal of Chemical Physics, 136 134903 (2012).

3 Y. Sliozberg, T. Sirk, J. Brennan, J. Andzelm. “Bead Spring Models of Entangled Polymer Melts: A Comparison of Hard-core and Soft-core Potentials”. Journal of Polymer Science Part B: Polymer Physics, 50 (24) 1694-1698, 2012.

4 T. Chantawansri, T. Sirk, Y. Sliozberg. “Entangled triblock copolymer gel: Morphology and Mechanical Properties”. Journal of Chemical Physics, 138, 024908 (2013).