Decomposition and Reaction of Polyvinyl Nitrate Under Shock and Thermal Loading: a Reaxff Reactive Molecular Dynamics Study Md Mahbubul Islam and Alejandro Strachan*

Article pubs.acs.org/JPCC Decomposition and Reaction of Polyvinyl Nitrate under Shock and Thermal Loading: A ReaxFF Reactive Molecular Dynamics Study Md Mahbubul Islam and Alejandro Strachan* School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States *S Supporting Information ABSTRACT: We use molecular dynamics (MD) simulations with the reactive force field ReaxFF to investigate the response of polyvinyl nitrate (PVN), a high-energy polymer, to shock loading using the Hugoniostat technique. We compare predictions from three widely used ReaxFF versions, and in all cases, we observe shock-induced, volume- increasing exothermic reactions following a short induction time for strong enough insults. The three models predict NO2 dissociation to be the first chemical, and relatively similar final product populations; however, we find significant differences in intermediate populations indicating different reaction mechanisms due to discrepancies in the relative stability of various intermediate fragments. A time-resolved spectral analysis of the reactive MD trajectories enables the first direct comparison of shock-induced chemistry between atomistic simulations and experiments; specifically, ultrafast spectroscopy on laser shocked samples. The results from one of the ReaxFF versions are in excellent agreement with the experiments both in terms of threshold shock strength required for the disappearance of NO2 peaks and in the time scale associated with the process. Such direct comparison between physical observables is an important step toward a definite determination of detailed chemistry for high-energy density materials. I. INTRODUCTION interactions with molecular monolayers.11 The experiment on 12 Shock-induced chemistry of high-energy (HE) materials and PVN revealed that 18 GPa shocks resulted in chemical the shock to detonation transition involve complex and inter- reactions within approximately 150 ps indicated by the disappearance of a peak associated with NO2. Despite such related physical and chemical processes through which the fi energy in the shockwave is transferred to chemical bonds with signi cant accomplishments, these experimental techniques are characteristic lengths of Ångstroms and vibrational periods of not without limitations. For example, experiments do not enable a direct characterization of chemical mechanisms nor the few femtoseconds. In many cases the microstructure of the fi material plays a key role in localizing the energy in the shock in identi cation of all intermediates and products. Current space, and these hotspots1,2 initiate chemical reactions, spatiotemporal and analytical resolution preclude in situ fl evaluation of the detailed behavior of molecules behind a de agration and eventually detonation. At the same time, 8 inter- and intramolecular relaxation processes transfer energy detonation front. On the simulation side, ab initio and from long wavelength, low-frequency modes of the shock to quantum-based molecular dynamics (MD) simulations have high-frequency bond vibrations, a process called up-pump- been used to identify initiation mechanisms, reaction barriers, 3−5 and rates associated with the decomposition of various energy ing. To complicate matters, these processes occur under 13−20 extreme conditions of temperature, pressure and strain rate and materials. However, the computationally intensity of these involve short time scales. It is, thus, not surprising that despite simulations limits the spatial and temporal scales achievable. fi 21−23 significant efforts in experiments and simulations a definite a Reactive force elds, like ReaxFF, provide a less molecular-level description of shock-induced chemistry is still computationally intensive alternative and enable multimillion atom simulations and capturing the role of microstructure and lacking even in relatively simple materials like nitromethane, a 24,25 liquid at room temperature, or polyvinyl nitrate (PVN), an defects with scales approaching those in experiments. ff amorphous polymer. Recent e orts provide an atomic picture of the shock to fl 24 Recent experimental efforts are yielding insight and de agration transition. While these simulations provide quantitative information about the molecular level processes unparalleled resolution in space and time, they involve 6−10 ff responsible for shock-induced chemistry. For example, approximations whose e ect on predictions remain poorly ultrafast spectroscopy study of laser shocked PVN enabled the detection of chemical reactive events under shock loading,7 Received: June 23, 2017 broadband multiplex vibrational sum-frequency generation Revised: September 5, 2017 (SFG) technique allows real-time observation of shock-front Published: September 6, 2017 © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.7b06154 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article understood. To different degrees, ab initio, tight-binding, and uses the concept of partial BOs between pairs of atoms to reactive potentials approximate atomic interactions and forces; describe covalent interactions including 2-, 3-, and 4-body in addition, the use of classical ionic dynamics is also an bonded interactions. BOs are a continuous, many body, and approximation. functions of the atomic coordinates. The nonbonded van der We note that shock-induced chemistry is an extremely Waals and Coulomb interactions are calculated between every challenging problem for an atomistic model as it needs to pair of atoms, within the respective cutoffs, regardless of capture two interdependent and complex processes: (i) The covalent interactions.33,34 In the nonbonded energy expres- first is the thermo-mechanics of shock loading, the amount of sions, shielding parameters are introduced to circumvent energy the shock deposits into the system and how this energy excessive repulsion at short distances, and a seventh order is distributed among different degrees of freedom via variety of taper function is used to eliminate any energy discontinuity.35,36 relaxation processes: volumetric compression and heating as The electronegativity equalization method (EEM)37 is utilized well as plasticity, fracture, phase transformations, void collapse to obtained environment dependent partial atomic charges that and interfacial sliding that can lead to energy localization and are updated at every step during the MD simulations. For a the formation of hot spots. (ii) The model should also capture more detailed description of the ReaxFF method, see refs the thermodynamics and kinetics of chemical decomposition at 21−23, 38, and 39. various conditions of pressure and temperature; including uni- Over the past decade, a number of ReaxFF parameter sets and multimolecular processes. Given the complexity of the have been developed to describe high-energy density materials. problem, a definite understanding of shock-induced chemistry The accurate prediction of the complex chemistry of these will likely require a synergistic combination of experimental and materials at extreme conditions of pressure and temperature as theoretical investigations26 where the experiments validate the well as the description of physical, thermodynamic, and simulations and simulations help interpret the experimental spectroscopic properties are challenging. Therefore, several results. versions are currently available, each emphasizing different In this paper, we use MD simulations with three different properties. Importantly, the uncertainties associated with versions of the ReaxFF15,27,28 force field to establish the initial different versions have not been characterized, and comparisons decomposition of PVN following shock loading. We build on across different force fields are not common. Therefore, it is an the successful use of ReaxFF in a family of high-energy evident necessity to systematically evaluate the performance of molecular crystals RDX, HMX, PETN, and NM14,17,19,27,29,30 the available force fields in terms of their predictions of and apply it to an energetic polymer. This is motivated by the mechanical, chemical and spectroscopic properties. In this existence of spectroscopic data on shocked PVN which enables study, we used three widely used ReaxFF C/H/N/O force the first direct comparison of ReaxFF predictions of shock- fields to formulate a detailed chemical and mechanical induced chemistry against experiments. Importantly, the description of the PVN polymer. The original C/H/N/O amorphous nature of PVN simplifies the comparison between ReaxFF force field was developed and applied for RDX20,40 and experiments and simulations due to the lack of grain boundaries since then several updates have been proposed. The standard and other crystal defects that can significantly affect shock ReaxFF C/H/N/O force field resulted in a less than ideal response. In addition, PVN is of interest as a binder in explosive description of lattice parameters and equations of states due to and propellant formulations and understanding its physics and the inadequate description of the London dispersion. Rom et chemistry can contribute to their safe operation. A high energy al.27 incorporated additional inner-core repulsions for the van density binder provides not just adhesion but also contributes der Waals interactions to improve the description of lattice to the total energy release.6 Despite the interest of the PVN parameters and updated the previous C/H/N/O force field.40 polymer in explosive applications, only a limited number

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