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

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 AlFx. The activation energy of AlFx formation is estimated using Arrhenius analysis. In addition, OH groups combine to produce H2O vapor, whereas AlFx aggregates. Moreover, AlCx 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.