Reevaluating the Condensed-Phase Enthalpy-of-Formation of Nitrocellulose from Oligomers to Bulk: Addressing Variability in Prior Studies through Computational Modeling

Author(s):

Jeffrey D. Veals, Chiung-Chu Chen, Joshua L. Lansford, and Christopher P. Stone

Abstract:

This study presents state-of-the-art modeling predictions for the condensed-phase enthalpy-of-formation of neat mono-, di-, and trinitrated nitrocellulose (NC) oligomers (n) and various oligomer “blends.” Deriving estimates for the blends using a previously developed Monte-Carlo weighting approach, we also obtained estimates for various weight-percent nitrogen contents that correspond to several important forms of NC commonly used in propellant formulations. Gas-phase enthalpies of formation were estimated with quantum chemical calculations, employing an isodesmic reaction scheme. Equilibrium molecular dynamics (EMD) simulations were performed with the AMBER force field, incorporating in-house optimized parameters tailored to the nitrate ester functionality. EMD-based density, cohesive energy, solubility parameter, and enthalpy-of-sublimation estimates were obtained. Dependences of these property predictions on the number of monomers per oligomer (n), degree of nitration, and temperature were evaluated and found to have merit in comparison with available measured values. Additionally, enthalpies of vaporization were estimated using a previously developed machine learning (ML) model. Condensed phase enthalpy-of-formation estimates were obtained by combining the gas-phase enthalpy-of-formation estimates with enthalpy-of-sublimation and enthalpy-of-vaporization estimates from the EMD simulations and ML model, respectively. Lastly, independent estimates of the condensed-phase enthalpy-of-formation were obtained by using a previously developed quantum mechanics-based program (EDAT). Predictions based on these three approaches were evaluated in context with measured values, and it was found that coupling EMD estimates of the enthalpy-of-sublimation with gas phase enthalpy-of-formation estimates provides a systematic means of obtaining accurate computational estimates of the condensed phase enthalpy-of-formation of NC with respect to nitrogen content.