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

Abstract

Poly­(vinylidene fluoride) (PVDF) is a well-known polymer with a (−CH2–CF2−) n chemical formula that is used, in particular, in electronic devices. The spatial arrangements of −CH2– and −CF2– units and the spatial alignment of PVDF chains determine the ferroelectricity, pyroelectricity, and piezoelectricity of condensed-phase PVDF. PVDF can be fabricated with Al metal as an energetic composite for rocket propellants. To better understand how PVDF molecular structures affect the properties in the condensed phase and the chemical reaction mechanisms with Al, we have developed a C/H/O/Al/F ReaxFF force field through parameterization against data from quantum mechanical (QM) calculations and experiments. This ReaxFF force field demonstrates good transferability in both low-temperature regimes, dominated by nonreactive conformational changes, and high-temperature regimes, dominated by PVDF chemical conversion. In the low-temperature regime, we investigated the α → β crystalline phase transition kinetics induced by a poling electric field or mechanical deformation. The molecular dynamics (MD) simulations show that electric field magnitude thresholds for the α → β crystalline phase transitions are 5.0 and 7.5 GV/m in the y and x directions, respectively. In addition, we found that the trans-gauche+–trans-gauche– conformation in the α crystalline structure transforms to the all-trans conformation through mechanical deformation. However, the all-trans chains are arranged in an antiparallel pattern in the stretched structure, resulting in zero polarity. We also observed that the poling electric field threshold can be reduced by combining it with mechanical deformation. In the high-temperature regime, we analyzed the reactions between PVDF and surface-oxidized Al nanoparticles. Results indicate that the reactions are triggered by the absorption of an H or F atom from PVDF to the alumina surface, followed by HF formation from PVDF pyrolysis. The produced HF molecules rapidly react with the surface alumina to form OH and AlF x . The activation energy of AlF x formation is estimated using Arrhenius analysis. In addition, OH groups combine to produce H2O vapor, whereas AlF x aggregates. Moreover, AlC x is also produced. The developed C/H/O/F/Al ReaxFF force field can serve for future studies of composite materials involving Al, alumina, PVDF, and its copolymers.

Combustion of Al Nanoparticles Coated with Nitrocellulose/Ethanol/Ether Molecules by Equilibrium Molecular Dynamics Simulations

“…This is because the thermal decomposition of NC is limited by the time scale of ReaxFF MD simulations, which has been confirmed in a large number of literature studies. 29 − 31 Figures 4 and 5 show the pyrolysis process and the variation curve of the number of key product molecules with time in the thermal decomposition process of the NC single molecular chain model. The first gas phase product is NO 2 , which is consistent with the fact that the –NO 2 base strength measured in the heating process of the NC thermal decomposition experiment decreases first.…”

The combustion-supporting mechanism of fluoropolymers on aluminum particles studied using reactive dynamics simulations

“…To perform the RMD simulations for the Al/fluoropolymer composites, a ReaxFF force field for C/H/O/F/Al was chosen. 32 Then, an aluminum sphere with a diameter of 5 nm was placed in the center of a box with a cell length of 10 Â 10 Â 10 nm. The simulation box was filled with an O 2 atmosphere.…”

Effect of Hyperthermal Hybrid Gas Composition on the Interfacial Oxidation and Nitridation Mechanisms of Graphene Sheet

“…The RMD simulations proposed in this work are all based on the open-source LAMMPS MD simulator by using the reactive force field ReaxFF C/H/O/N . , The validity of ReaxFF C/H/O/N is demonstrated by simulating combustion between hydrocarbon/O 2 mixtures and then further applied to investigate the ablation of graphene under femtosecond pulse illumination successfully . The overall system energy by ReaxFF can be divided into various partial energy contributions as follows: − E system = E bond + E over + E val + E pen + E tors + E conj + E vdWaals + E coulombic where E system is the energy of the overall system, E bond is the bond energy, E over is the overcoordination energy, E under is the undercoordinated energy, E val is the valence angle energy, E pen is the penalty energy, E tor is the torsion energy, E conj is the conjugation energy, E vdWaals is the van der Waals interactions energy, and E Coulombic is the Coulombic interactions energy.…”