Molecular dynamics investigation of solvent diffusion through nitrocellulose matrices in single-base propellants under water leaching environment

A detailed molecular dynamics study was conducted to investigate the diffusion mechanisms of ethanol-ether and ethanol-acetone mixed solvents in nitrocellulose (NC) during water leaching for solvent removal. This work presents the first exploration of the impact of NC nitrogen content and heterogeneous nitrogen distribution on solvent diffusion. Although results show hydrogen bonding predominates in ethanol-NC interactions, ethanol exhibits the highest diffusion coefficient due to its small molecular volume, which enables high jump frequency and low steric hindrance. In contrast, ether and acetone primarily interact with NC via van der Waals forces. Solvent diffusion coefficients significantly increase with elevated temperature, higher water content, increased NC nitrogen content, or heterogeneous nitrogen distribution, resulting from weakened solvent-NC hydrogen bonding. The increase of temperature intensifies solvent molecular thermal motion and increases the free volume of system, thus enhancing solvent diffusion. An increase in water content drives the formation of new interaction networks among water, solvent and NC molecules, weakening solvent-NC interactions whereas strengthening solvent-H₂O interactions. These changes enable water molecules with greater mobility to carry solvent molecules, facilitating solvent molecules diffusion. The increase of nitrogen content and heterogeneous nitrogen distribution enhances hydrogen bonding of NC intrachain. These lead to chain contraction and reduces the number of hydroxyl groups available for hydrogen bonding between NC with solvent, enhancing solvent diffusion. At 298 K and 11.08 % nitrogen content, heterogeneous nitrogen distribution can increase solvent diffusion coefficients up to 46.19 %. This work provides a theoretical basis for understanding the solvent removal process in single-base propellants.