| Thermosetting binder systems formed by crosslinked polymer chains offer mechanical framework of the composite propellant,which is the power source of the aerial vehicle like rockets and missiles.Oxidizers offered the oxygen during the combustion of solid propellants,which made the aerial vehicle still work without oxygen in the airs.The interface strength between the binder and oxidizers was an important factor affects the mechanical properties of solid propellants.Actually,the“dewetting phenomenon” could happen,which results from the interface fracture between the binder and oxidizers under the stimulation of external forces or heat.Therefore,bonding agents were added into the solid propellant to reinforce the interface strength between the binder and oxidizers,and further improve the mechanical properties of solid propellants.There are two main groups on the neutral polymeric bonding agent,one bonding with the oxidizer through physical absorption,and the other one crosslinked with the binder through chemical reaction.ε-CL-20(hexanitrohexaazaisowurtzitane)/azido polyether solid propellant systems are one of the most promising propellants at present,which are high energy density and environment-friendly.ε-CL-20/azido polyether solid propellant systems are made of ε-CL-20,which is high enthalpy of formation and density,and the crosslinked azido polyether binder.Also,the azide-rich binder can not only bring high energy level to solid propellants,but also introduce the humidity insensitivity and highly efficient huisge reaction based on click chemical reaction into the curing system.Therefore,it is very important to design a specific matching neutral polymeric bonding agents for ε-CL-20/azido polyether solid propellant to improve the mechanical properties of solid propellant.Molecular dynamics simulation,as a new method of studying materials,is widely used in the study of energetic materials,which could not only investigate the interaction between molecules,but also provide guidance for the synthesis of experiments and save costs.Therefore,based on the Flory-Huggins polymer solution theory and the Hildebrand solubility parameter concept,neutral bonding agents suitable for the system of ε-CL-20/azido polyether systems were designed.The effect of neutral bonding agent on the overall mechanical properties of solid propellant based on the system of ε-CL-20/azido polyether systems was investigated by molecular dynamics simulation.The specific details are as follows:(1)2 NPBAs with alkynyl group were designed to investigate the interaction between ε-CL-20 and NPBA by all-atom molecular dynamics(MD)simulations.The results showed that introducing 2 alkynyl groups(-C≡C-)into every 1000 g/mol NPBA polymer chain could increase the binding energy from 346.85 to 391.64kcal/mol.The increase was mainly from the change in van der Waals(vd W)force,which was about 10% greater than that of electrostatic interaction.The sensitivity analysis showed that the mechanical sensitivity of ε-CL-20 coated by NPBA reduced by 11.06% compared to neat ε-CL-20.These results indicate that NPBAs with alkynyl group is expected to be applied in ε-CL-20/azido polyether systems.(2)The NPBA with alkynyl group should further interact with the azide group on the binder.Therefore,based on the previous work,a trifunctional azide molecule was introduced into the system to react with the alkynyl terminated NPBA.The effect of the number of triazole rings on the binding ability of NPBA with ε-CL-20 was investigated by MD.The results show that when five alkynyl groups are reacted,that is,50% of the alkynyl groups are reacted,the interfacial binding force between the polymer and ε-CL-20 is the strongest,which is 389.59 k J/mol,when 10 alkynyl groups are reacted,that is,100% of the alkynyl groups are reacted,the interfacial binding force between the polymer and ε-CL-20 is the lowest,which is-307.76 k J/mol.(3)The trifunctional azido curing agents react with the alkynyl terminated NPBA to construct the polymer crosslinking network,and studied the effect of different alkynyl and azido functional ratios on the binding ability between the polymer network and the ε-CL-20.Define the functional ratio of alkynyl and azido group as α.When α = 1:1,the binding force between the polymer network and the interface is the weakest,which is-269.34 k J/mol.In addition,when α > 1:1 or α < 1:1,the binding ability between the polymer network and the interface of ε-CL-20 is relatively strong.Especially,when α = 5:3,the maximum binding force between the polymer network and the interface of ε-CL-20 is-379.17 k J/mol.when α = 4:3,the binding ability between the polymer network and the interface of ε-CL-20 is almost the same as that between α = 1:3 and α = 2:3,which are-334.16 k J/mol,-351.61 k J/mol,-338.02 k J/mol.The work enriches the design theory of energetic materials,especially polymeric bonding agents in solid propellants,revealing the specific interaction mechanism of polymeric bonding agents and their cross-linking systems with oxidizers,and promotes the theoretical design of energetic materials and the development of solid propellants.At the same time,the research and simulation method proposed in this work can also be applied to other dangerous systems. |
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