Development of Chemical Kinetics Models from Atomistic Reactive Molecular Dynamics Simulations: Application to Iso-octane Combustion and Rubber Ablative Degradation

Modeling the complex chemical phenomena
resulting from multiple
active species and long-chain polymers is limited by uncertainties
in the reaction rate parameters, which increase rapidly with the number
of active species and/or reaction pathways. Reactive molecular dynamics
simulations have the potential to help obtain in-depth information
on the chemical reactions that occur between the polymer (e.g., ablative
material) and the multiple active species in an aggressive environment.
In this work, we demonstrate that molecular dynamics (MD) simulations
using the ReaxFF interatomic potential can be used to obtain the reaction
kinetics of complex reaction pathways at high temperatures. We report
two recently developed tools, namely, MolfrACT and KinACT, designed to extract chemical kinetic pathways by
postprocessing reactive MD simulation data. The pathway extraction
is based on a new algorithm, Consistent Reaction Stoichiometry via
an Iterative Scheme (CReSIS), for the automated extraction of reactions
and kinetics from MD trajectories. As a validation of the methodology,
we first report the kinetic analysis and mechanisms for the high-temperature
combustion of iso-octane. The observed reaction pathways are consistent
with experimental models. In addition, we compare the activation energies
of select iso-octane combustion pathways with experimental data and
show that nanosecond timescale molecular dynamics simulations are
sufficient to obtain realistic estimates of activation energies for
different fuel consumption reaction pathways at high temperatures.
The framework developed here can potentially be combined with time-series
forecasting and machine learning methods to further reduce the computational
complexity of transient molecular dynamics simulations, yet yielding
realistic chemical kinetics information. Subsequently, the CReSIS
scheme applied to ethylene–propylene–diene–monomer
(EPDM) rubber ablative reveals that H2O, C2H4, and HCHO are the major products during the initial stages
of the polymer degradation in high-temperature oxidative environments.
While prior work involving ReaxFF simulations is restricted to overall
rates of formation of any species, we extract kinetic information
for individual reaction pathways. In this paper, we present several
reaction pathways observed during the EPDM rubber degradation into
the dominant products and report the pathway-specific reaction rates.
Arrhenius analysis reveals that the dominant reaction pathway activation
energies for the formation of water, ethylene, and formaldehyde are
34.42, 27.26, and 6.37 kcal/mol, respectively. In contrast, the activation
energies for the overall formation (across all reaction pathways)
of these products are in the 40–50 kcal/mol range.