Reactive Molecular Dynamics Simulations of the Atomic Oxygen Impact on Epoxies with Different Chemistries
Summary¶
Low-Earth-orbit (LEO) environments expose polymer components to hypervelocity atomic oxygen (AO), producing erosion and property loss that depend on cure chemistry, cross-link density, and aromatic content. This J. Phys. Chem. C article uses ReaxFF MD to simulate AO impacts on cross-linked thermosetting epoxies with different curatives and tunable cross-linking, ranking relative damage rates and highlighting aromatic curatives as more AO-resistant owing to persistent benzene moieties. The modeling angle is explicitly high-energy impact: the introduction notes that many prior ReaxFF parametrizations lack adequate short-range repulsion for AO encounters, motivating a parametrization path suited to hypervelocity O collisions. The corpus PDF is a proof; confirm final figures in the publisher VOR PDF. For space materials contexts, the paper’s emphasis on short-range repulsion for AO impacts is central: standard organic ReaxFF fits optimized for thermal chemistry may misrepresent hypervelocity encounters where close O–C approaches dominate damage. The manuscript therefore couples virtual curing (to make realistic thermoset topology) with impact studies that rank curative chemistry and cross-link density effects on disintegration rates. The overarching demonstration is that cure-structure generation and AO damage can be studied within one ReaxFF workflow, avoiding a stitched pipeline where curing is nonreactive and impact is reactive. Confirm impact energies and damage metrics in the published JPCC PDF.
Methods¶
The workflow combines an accelerated ReaxFF curing strategy to build virtually cured thermosetting epoxies with controlled cross-link density, then subjects slabs to AO projectiles at kinetic energies in the ~4.5–5 eV range cited in the introduction as representative of LEO-relevant impacts. Simulations compare aromatic versus aliphatic curative chemistries and sweep cross-link density and temperature to map disintegration kinetics as described in the abstract. Analysis focuses on bond-breaking statistics, fragmentation, and qualitative damage rates rather than continuum ablation modeling.
MD application (J. Phys. Chem. C proof §2). Engine / code: all molecular dynamics in LAMMPS with the authors’ ReaxFF for curing and hypervelocity atomic oxygen (AO) bombardment (cited in Methods). System / PBC: periodic slabs of cured epoxy in 3D PBC cells; non-periodic z uses LAMMPS fix wall/reflect at the bottom interface as described. Curing stages use inert Ar-based protocols noted in the article before exposure to AO flux. Ensemble / temperature: after preparation, NVT MD (including NVE microcanonical impact segments for bombardment at the documented T of ~300 K, plus additional temperatures like ~103 K and ~396 K in §4). Duration: NVE bombardment trajectories for tens of ps (e.g., 40 ps with 0.00005 ps = 0.05 fs time step for the AO impacts per the Methods text); post-impact analysis tracks ~10 ps mass-loss / temperature transients in Results figures. Barostat / pressure: N/A for the NVE / NVT bombardment protocol described. Timestep (impact): 0.05 fs for the high-energy AO segment (proof Methods). Thermostat: not applied during the NVE impact stage (microcanonical). AO injection: projectiles are introduced with ~0.2 fs spacing between successive O atoms along −z in the method described, targeting 4.5–5 eV-class kinetic energies in the Introduction. Electric field / enhanced sampling: N/A — LEO mimic is kinetic AO impact, not bias fields; no metadynamics in the core workflow.
Findings¶
Aromatic-cured networks show higher AO resistance in the comparisons reported, attributed to stable benzene units that survive aggressive oxidation longer than purely aliphatic networks under the simulated impacts. Lower cross-link density and higher temperature both accelerate polymer breakup under AO in the trends summarized. The authors argue ReaxFF can serve as a practical screening tool for spacecraft polymer candidates once parametrized for these repulsive high-energy encounters, while acknowledging that LEO environments include flux distributions, charging, and multi-impact histories beyond single-impact atomistic studies. For materials selection, the ranking story—aromatic vs aliphatic curatives and cross-link density—is meant to be interpreted at the level of relative damage, not as an absolute erosion yield prediction without calibration.
Limitations¶
Proof PDF; LEO mimics are partial. Excited-state chemistry is not treated explicitly.
Relevance to group¶
Demonstrates ReaxFF for environmental degradation of engineered polymers and accelerated curing workflows in the Penn State reactive MD ecosystem.