Effect of Diamine Isomerism on Mechanical and Thermal Properties of TGAP Epoxy Systems: Molecular Dynamics Simulation Approach

Molecular dynamics simulations were conducted to investigate the thermomechanical behavior of epoxy networks based on triglycidyl p–aminophenol (TGAP) cured with two positional isomers of diaminodiphenyl sulfone (DDS), 3,3′–DDS and 4,4′–DDS. Thermal analysis revealed that the 4,4′–DDS system exhibited a higher glass transition temperature (533.35 K) and lower coefficient of linear thermal expansion (49.4 × 10⁻⁶ K⁻1), while the 3,3′–DDS system showed a lower Tg (506.55 K) and higher CLTE (52.7 × 10⁻⁶ K⁻1). Conversely, the 3,3′–DDS system exhibited a higher Young’s modulus of 4.05 GPa, compared to 3.87 GPa for the 4,4′–DDS system. To better understand these differences, analyses of fractional free volume, cohesive energy density (CED), and two types of molecular motions were performed, with molecular mobility measured via mean square displacement (MSD) reflecting overall translational dynamics, and segmental dynamics such as ring rotations capturing localized flexibility. The 3,3′–DDS displayed a lower fractional free volume and higher CED, indicating a more tightly packed network contributing to its greater mechanical stiffness. In contrast, the para–substituted geometry of the 4,4′–DDS system enabled localized molecular motions, which may enhance thermal adaptability and contribute to its higher thermal performance. These findings suggest that even subtle geometric differences in curing agents can influence molecular dynamics and the macroscopic performance of epoxy networks, providing useful insight for the design of materials tailored to specific engineering requirements.