Electron dynamics of shocked polyethylene crystal

Electron force field (eFF) wave-packet molecular-dynamics simulations of the single shock Hugoniot are reported for a crystalline polyethylene (PE) model. The eFF results are in good agreement with previous density-functional theories and experimental data, which are available up to 80 GPa. We predict shock Hugoniots for PE up to 350 GPa. In addition, we analyze the structural transformations that occur due to heating. Our analysis includes ionization fraction, molecular decomposition, and electrical conductivity during isotropic compression. We find that above a compression of 2.4 $\mathrm{g}/{\mathrm{cm}}^3$, the PE structure transforms into an atomic fluid, leading to a sharp increase in electron ionization and a significant increase in system conductivity. eFF accurately reproduces shock pressures and temperatures for PE along the single shock Hugoniot.

Figure 1

(a) The principal Rankine-Hugoniot for PE. Experimental data from the LASL shock compression handbook[24] and Nellis[25] are provided along with data for the classical MD potentials (OPLS and AIREBO).[4] A reactive force field (ReaxFF[4]) and quantum-mechanical approaches (DFT/AM05 and tight binding[26]) are included for comparison. (b) An expansion of the low compression region of the Hugoniot .

Figure 2

(a) The pressure-temperature locus of the Hugoniot curve for the eFF, DFT/AM05, OPLS, AIREBO, and ReaxFF methods. (b) An expansion of the pressure-temperature seam for lower pressures.

Figure 3

Radial distribution functions for (a) C-C atom pairs, (b) C-H pairs, and (c) H-H pairs. Each curve corresponds to a different density point (g/cm${}^3$) defined by the colors in the legend.

Figure 4

Structural decomposition along the PE Hugoniot. The circles correspond to the % intact C-C backbone for the eFF simulations. The open diamonds show the results from DFT/AM05. The secondary axis shows the % ionization along the Hugoniot calculated from the eFF simulations in open circles.

Figure 5

The direct current electrical conductivity of points along the eFF Hugoniot curve (circles) and finite-temperature DFT (diamonds) from Horner.[1] (a) Conductivity plotted against temperature with densities (g/cm${}^3$) provided. (b) Conductivity plotted against density with temperatures (K) provided.