Simulations of Bond Rupture in Normal Alkanes Under Tensile Molecular Deformation Using Reactive Potentials

Nouranian, S., Gwaltney, S. R., Tschopp, M. A., Baskes, M. I., & Horstemeyer, M. (2013). Simulations of Bond Rupture in Normal Alkanes Under Tensile Molecular Deformation Using Reactive Potentials. Conference Abstract, The 2013 Annual Meeting of the American Institute of Chemical Engineers (AIChE), November 2-8. San Francisco, CA.

Abstract

The molecular modeling of failure and fracture phenomena, such as crazing and crack formation, in polymers is of key interest in the multiscale modeling of these materials incorporating damage evolution and progression at different scales. In this study, preliminary simulations of bond rupture in single normal alkane molecules under tensile molecular deformation were performed using three reactive potentials, i.e., modified embedded-atom method (MEAM), second-generation reactive empirical bond order (REBO), and reactive force field (ReaxFF). The higher alkanes in the series provide reasonable representation of a polyethylene chain. The MEAM potential, a semi-empirical many-body potential traditionally used for metals, was recently parameterized for saturated hydrocarbons and is now suitable for the atomistic simulations of complex reactive and non-reactive metallic/organic material systems. The MEAM-, REBO-, and ReaxFF-reproduced energetics of molecular deformation and bond rupture as well as the structural breakdown of a homologous series of alkane molecules from normal propane (n-C3H8) to normal undecane (n-C11H24) were compared to those reproduced by first-principles (FP) methods. The molecules were initially energy-minimized using the Polak-Ribiere conjugate gradient method. For convenience, the coordinates of the first carbon atom was fixed at the origin, while all the atomic coordinates were scaled in the direction of the molecular backbone (the vector between the first and terminal carbon atoms) with a distance increment (ΔL) of 0.1 Å. The molecules were energy-minimized after each affine scaling of the atomic coordinates, while the coordinates of the first and terminal carbon atoms were fixed. The deformation continued for 100 distance increments (ΔL =10 Å). The bond rupture energies and the resulting molecular fragments (free radicals) differed significantly among the three potentials. While both MEAM and ReaxFF predicted a smooth rupturing instance and fragment formation for all the molecules, REBO predicted relatively high strain energy before rupture that would increase with increasing size of the molecules as well as the formation of many smaller molecular fragments, especially for higher alkanes. Furthermore, REBO allowed significantly bent C-C-C bond angles (almost 180°) to manifest before rupture. ReaxFF almost exclusively led to a methyl radical formation as the smaller molecular fragment resulting from bond rupture, while MEAM predicted the formation of an ethyl radical, especially for the rupture of higher alkanes. To further study the strain distribution in the molecules during molecular deformation, another set of simulations were run, where only the coordinates of the terminal carbon atoms in the molecules were changed as the molecules were strained. At each intermediate step, the first and terminal carbon atom coordinates were again fixed, while the energies of the molecules were minimized. The average C-C bond distances and C-C-C bond angles were calculated for the molecules up to the moment of rupture and compared with FP calculations. This study provides insight into the behavior of different alkane molecules under tensile deformation as simulated using the currently available reactive potentials with the potential applicability to the fracture in high polymers, such as polyethylene. Advantages and disadvantages of these potentials in capturing the essential energetics and structural details of the bond rupture phenomena are further compared and contrasted. The results of this study further highlight the essential characteristics of bond breaking for the three reactive potentials.