New Insight into Dynamic Mechanical Relaxation in N-butyl-N-(2-nitroxy-ethyl) Nitramine Plasticized Nitrocellulose through Molecular Dynamic Simulations

Abstract We performed dynamic mechanical analysis (DMA) on nitrocellulose (NC) plasticized by an insensitive plasticizer N-butyl-N-(2-nitroxy-ethyl)nitramine (Bu-NENA). NC/Bu-NENA blend shows two mechanical relaxation processes in the temperature ranges of -50~-40oC and 30~40oC, and their variations with deformation frequencies were studied. To explore further the effect of temperature on relaxation, the binary mixture model of NC/Bu-NENA was constructed, and molecular dynamic simulations were conducted. The simulated mean square displacements (MSD) show abrupt increase in the temperature range of -50~-40oC and 30~40oC, which are consistent with those of the two relaxation processes observed in the DMA curves. Moreover, the free volume (Vfree) and torsion energy obtained from molecular dynamic simulations exhibit distinct increase at the temperature above 30oC and -50oC respectively, reflecting the sudden enhancements on the mobility of polymer chain elements and the rotation of molecular bonds. Furthermore, the radial distribution function (RDF) associated with the intermolecular interactions reveals that the intensities of both hydrogen bond and van-der-vaals forces decrease with the increase of temperature, which is responsible for the decrease of storage modulus at high temperature. These computational and experimental studies reveal guidance to strengthening the NC base propellants in broad temperature range.