A Hotspot’s Better Half: Non-Equilibrium Intra-Molecular Strain in Shock Physics

Abstract

Shockwave interactions with material microstructure localizes energy into hotspots, which act as nucleation sites for complex processes such as phase transformations and chemical reactions. To date, hotspots have been described via their temperature fields. Nonreactive, all-atom molecular dynamics simulations of shock-induced pore collapse in a molecular crystal show that more energy is localized as potential energy (PE) than can be inferred from the temperature field and that PE localization persists through thermal diffusion. The origin of the PE hotspot is traced to large intra-molecular strains, storing energy in modes readily available for chemical decomposition.

Many-Body Mechanochemistry: Intra-molecular Strain in Condensed Matter Chemistry

“…To address this gap, we develop Many-Bodied Steered Molecular Dynamics (MB-SMD) in which we use four-body external biasing potential to deform molecular groups of interest along torsional and out-of-plane degrees of freedom. Four-body terms are motivated by our observation of the large molecular distortions observed under shock loading 35 as well as their general applicability to explore the conformational space of a molecule. This external biasing potential is implemented in the form of four-body harmonic inter-atomic potential, 𝐸 = 𝐾(𝜙 𝑖𝑗𝑘𝑙 − 𝜙 𝑜 ) 2…”

“…We have recently shown that a possible origin of this increased reactivity is the presence of highly strained and deformed molecules in the hotspot. 35,39 This energy localization, in the form of intra-molecular potential energy, is persistent well into reaction timescales. The increase in strain energy in TATB is caused primarily by torsional and out-of-plane deformations of the nitro and amino groups, shown in Section S5 of the SI, and incurs a local amorphization of the material that affects kinetics 59 and heat transport 60 .…”

“…We show that straining the central torsional angle by ~50 reduces the energy barrier associated with the mechanical activation of the mechanophore by a factor of two. The second example couples external many-body potentials that mimic the molecular strains observed under shock loading 35 using reactive molecular dynamics in the molecular crystal TATB. A Kissinger analysis of the MD simulations shows marked changes in chemical kinetics and decomposition paths that can explain previous inconsistencies between mechanical and thermal insults in this class of materials 8,9,38 .…”

“…To date, computational and experimental studies designed to single out the effects of mechanochemistry have focused on elongation (two-body forces), while in most cases of extemporaneous mechanochemistry in condensed matter, such as shock-induced chemistry or the deformation of polymers, the mechanical loads are complex and many-bodied. 3,10,34,35 The importance of many-bodied mechanochemistry (MB-MC) effects has been revealed by several studies that tracked the reaction kinetics and pathways for organic systems under uniform shear loads 5,36,37 . These studies have demonstrated that dynamic mechanical loading can reduce activation energies and speed up chemistry.…”

Strongly Anisotropic Thermomechanical Response to Shock Wave Loading in Oriented Samples of the Triclinic Molecular Crystal 1,3,5-Triamino-2,4,6-Trinitrobenzene (TATB)

“…Shock collapse of pores leads to even larger molecular-level strains than seen in shear bands. 57 These strained states were shown to persist across typical hot-spot reaction time scales and are postulated to give rise to a mechanochemical acceleration of decomposition kinetics.…”