Molecular Simulation Study on the Impact of a Cross-Linked Network Structure on the Tensile Mechanical Properties of PBT Substrates

This study investigates the correlation between the cross-linked network structure and the macroscopic mechanical properties of 3,3-bis(azidomethyl)oxetane-tetrahydrofuran copolymer (PBT)-based solid propellants through molecular dynamics (MD) simulations. A multi-component system comprising PBT molecular chains, toluene diisocyanate (TDI), trimethylolpropane (TMP), tetraethylene glycol (TEG), and sodium perchlorate (AP) was constructed. Perl script programming was utilized to precisely control the dynamic cross-linking reaction. Molecular models with cross-linking densities of 0%, 50%, 60%, 70%, 80%, and 90% were established, and their mechanical properties were analyzed under varying cross-link densities and strain rates through uniaxial tensile simulations. The results indicate that the formation of the cross-linked network significantly alters the energy distribution and microstructural characteristics of the system. As the cross-linking density increases from 50% to 90%, the total energy of the system decreases by approximately 40%, primarily due to reductions in non-bonded energy. The radial distribution function (RDF) and root mean square displacement (MSD) curves reveal that the cross-linking reaction enhances covalent bond formation between molecular chains, reduces their degrees of freedom, and increases the glass transition temperature (<i>Tg</i>). Under identical strain conditions, the models with higher cross-link densities exhibit greater stress resistance. Specifically, the stress growth rate of the 90% cross-link density system increases by 42.1% as the stretching rate rises from 1.0 × 10¹¹ s^-1 to 2.0 × 10¹¹ s^-1, compared to an 18.7% increase for the 50% cross-link density system. These findings have significant implications for optimizing processing parameters and predicting the mechanical properties of propellants.