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C/H/O/F/Al ReaxFF Force Field Development and Application to Study the Condensed-Phase Poly(vinylidene fluoride) and Reaction Mechanisms with Aluminum

Summary

PVDF exhibits ferroelectric/piezoelectric response tied to chain conformation and packing; it is also used with aluminum in energetic composites where fluoropolymer–metal reactions matter for ignition and energy release. The paper develops a C/H/O/F/Al ReaxFF model from QM and experimental references, then applies it to condensed-phase PVDF under electric/mechanical loading and to high-temperature reaction of PVDF with oxidized Al nanoparticles. The condensed-phase section discusses competition between polar β-phase formation and antiparallel packing that can yield negligible net dipole even when local conformations change, while the reactive section follows hydrogen and fluorine abstraction pathways that produce HF and surface fluoride/oxide species on alumina-coated aluminum. The galley PDF in the corpus should be checked against the J. Phys. Chem. C version-of-record for final thresholds, barriers, and SI cross-references.

Methods

2 — Force-field training (C/H/O/F/Al). A new C/H/O/F/Al ReaxFF is built from merged prior ReaxFF modules and DFT/QM and (where cited) experimental data for condensed PVDF (crystal and response), Al/oxide surfaces and NPs, and high-temperature PVDF+Al reaction channels (HF, fluoride, oxide). Details: J. Phys. Chem. C and SI; weighted least-squares and genetic-algorithm fitting; LAMMPS-compatible parameter file in the article.

1 — MD application (ReaxFF, LAMMPS). (A) Condensed-phase PVDF: NVT (or as stated) ReaxFF MD probes α → β-type polar order under static E\(_x\) and E\(_y\) (magnitudes in Findings), uniaxial stretch/deformation, and combined (E + mechanical) loading. (B) High T, PVDF + oxidized Al: ReaxFF MD of PVDF in contact with alumina-coated Al nanoparticles follows H or F abstraction, HF, AlF\(_x\), OH recombination, H\(_2\)O, AlC\(_x\), and an Arrhenius-style E\(_A\) for AlF\(_x\) (abstract). Engine / code: LAMMPS with ReaxFF. PBC: 3D supercells for polymer and NP assemblies. Timestep, thermostat, temperature, pressure handling, and long-range / QEq details: not duplicated here—N/A in this short note; the repo holds a galley; use the JPCC version-of-record PDF and SI. Duration / stages: N/A in this short note (equilibration/production segmentation and per-case run lengths are not reproduced from the galley text here; confirm in the version-of-record article and SI). Barostat: N/A in typical NVT poling—see PDF for any NPT. Umbrella / metadynamics / replica: N/A in summary scope. Shear / strain (beyond the stated uniaxial /poling protocols): N/A unless the article reports them. E-field (static, for poling in A): specified in the main text with the GV m⁻¹ values in Findings.

3 — Static DFT and 4 — new experiments: N/A as the primary outcome—the paper’s new results are the ReaxFF and ReaxFF MD; context to experiments is in the main text.

Findings

Low-temperature regime (PVDF, poling, mechanics). Electric-field thresholds near 5.0 GV m⁻¹ (y) and 7.5 GV m⁻¹ (x) drive α → β transitions in the model; mechanical stretch can convert TGTG⁻ motifs toward all-trans chains, but antiparallel packing can yield zero net polarity. Combined electric and mechanical loading lowers the poling field needed compared with either loading mode alone. High-temperature regime (PVDF + Al/oxide NPs). Reactions start with H or F abstraction from PVDF to the alumina surface, followed by HF and reaction with alumina to give OH and AlF\(_x\); Arrhenius analysis reports an activation energy for AlF\(_x\) formation (abstract). H\(_2\)O from OH recombination; AlF\(_x\) aggregates and AlC\(_x\) appear among products in the abstract narrative. N/A here to restate all barrier heights—see the JPCC PDF/SI.

Limitations

Galley PDF in corpus; verify final numerical thresholds against the published issue PDF.

Relevance to group

C/H/O/F/Al ReaxFF development and application co-authored by van Duin, linking polymer phase behavior with aluminum fluoride/oxide surface chemistry relevant to energetic and interface modeling.

Citations and evidence anchors

Reader notes (navigation)