Molecular Dynamics Insights into Diamine Isomer Effects on TGDDM Epoxy Thermomechanical Properties

Epoxy resins are widely used in aerospace and defense composites owing to their excellent mechanical strength and thermal resistance, and the structural characteristics of curing agents are critical in determining final network properties. In this study, the high-functionality epoxy resin tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) was cured with two positional isomers of diaminodiphenyl sulfone (3,3′-DDS and 4,4′-DDS), and the resulting networks were investigated by molecular dynamics (MD) simulations, while complementary density functional theory (DFT) calculations were conducted on the curing agents to elucidate their molecular structures. Under identical curing conditions, the 3,3′-DDS network exhibited a higher tensile modulus with denser packing, whereas the 4,4′-DDS network showed broader free volume distribution and stronger π-electron resonance interactions. Thermal characterization further revealed that the 4,4′-DDS system possessed a higher glass transition temperature ( T g ) and lower coefficient of linear thermal expansion (CLTE), consistent with molecular dynamics results such as mean square displacement and ring-flip behavior. Overall, this study identifies positional isomerism in DDS as a key factor governing the balance between mechanical stiffness and thermal stability in TGDDM-based networks, thereby providing molecular-level insights for the rational design of high-performance epoxy materials.