Thermal Decomposition Mechanisms Of Three Kinds Of Plastic Bonded Explosive: ReaxFF-lg Molecular Dynamics Simulations

Energetic materials are widely used in aerospace,military and civil fields,so it is of great significance to understand their pyrolysis reaction mechanism.The thermal decomposition of energetic materials is not only related to the ignition and detonation performance of explosives,but also to the sensitivity of explosives to the surrounding environment.In this paper,based on the reaction field（ReaxFF-lg）,the molecular dynamics method was used to simulate the pyrolysis of PBXs explosives composed of three different types of energetic compounds and HTPB with them.These three energetic compounds are divided into:1,3,5,7-tetrachitro-1,3,5,7-tetrazolium（HMX）chemical formula C₄H₈O₈N₈;Stilbene（HNS）chemical formula C₁₄H₆O₁₂N₆;And 1-ox-2,6-diamino-3,5-dinitropyrazine（LLM-105）chemical formula C₄H₄O₅N₆.To assess the various categories of each type of PBXs under different temperature pyrolysis performance,after we statistics a molecular dynamics simulation model of lattice parameter and density,finishing their potential energy curve,main material within 10 ps before change curve and of the main reaction pathway and frequency,and calculate their initial reaction rate and activation energy.By comparing the activation energy and lattice parameters with the reference values,the feasibility of the force field is proved.Their potential mechanisms of thermal decomposition were systematically compared by means of material transformation diagram,reaction pathway table and molecular snapshot analysis.The activation energies of HMX and HMX/HTPB are calculated to be 30.62kcal/mol and 21.38 kcal/mol respectively.The activation energy of HMX decreased with the addition of binder.We believe that the number of C-NO₂ bond decomposition increases significantly in the HMX/HTPB decomposition process under the influence of the polymer binder HTPB,so the activation energy of thermal decomposition decreases.However,HTPB still plays a positive role in reducing the reaction sensitivity due to the change of the main chain cracking mode,which promotes the carbon aggregation of the pyrolytic sheet.For LLM-105 and LLM-105/HTPB,the activation energies are 35.58 kcal/mol and 38.87 kcal/mol,respectively.As an insensitive elemental explosive,ll M-105 has a lower activation energy than HMX.It was found that the initial pyrolysis process of pure LLM-105 and ITS PBX was mainly controlled by c-NO2 and C-NH2 bond breaking,dimerization,nitroso rearrangement,amino elimination and H atom transfer.Some pyrolysis methods were not found in the related reactions of HMX.Through further comparison,we believe that H mainly causes the difference in the pyrolysis process of the two energetic compounds and inhibits the decomposition rate of LLM-105.This inhibition is caused by two factors.On the one hand,the hydrogen transfer provided by HTPB competes with the intramolecular hydrogen transfer of LLM-105,forcing the LLM-105 molecules in PBX to choose a reaction path with a higher barrier at the initial stage rather than dimerization or polymerization.This effect is especially pronounced at low temperatures,when the hydrogen atoms in LLM-105 do not have enough energy to transfer from the ground.On the other hand,the hydrogen atmosphere in HTPB will enhance the stability of the C-NO2 bond,which is the main way of decomposition without H transfer.In addition,the reversible movement of amino group,distribution and aggregation of nitro group caused by H transfer may affect its sensitivity and explosive performance.Because reversible reactions can store energy from external stimuli and internal stresses in the gap between forward and reverse barriers.In this respect,the decomposition mode of LLM-105 is different from that of HMX.The activation energies of HNS and HNS/HTPB are 38.06 kcal/mol and 47.24kcal/mol respectively.HNS activation energy is higher than LLM-105,but its reaction rate is lower than LLM-105.This is because HNS are diphenyl organic compounds with a dimerization tendency.However,HTPB can inhibit the group exchange and dimerization of HNS,which is favorable for dissociation,and turn to the complex monomolecular decomposition reaction with higher dissociation energy barrier,which does not occur in pure phase.This further explains the phenomenon and more strongly confirms our conjecture about the mechanism of the pyrolysis process of energetic compounds.This paper is expected to provide insights into the synthesis and modification of PBXs,so as to find highly explosive materials with high thermal stability and explosive properties.