Positional Isomerism Controls Epoxy Network Mechanics: Meta-Substitution Enhances Toughness via Adaptive Loop...

Positional isomerism is a key determinant of mechanical performance in polymer networks, yet its underlying molecular mechanisms remain insufficiently understood. In this work, we employ a multiscale simulation approach─integrating density functional theory with a stochastic coarse-grained reaction model─to investigate how meta- and para-substituted diamine curing agents influence the curing kinetics, network architecture, and mechanical properties of epoxy resins. Simulations reveal that the meta-substituted system exhibits higher ultimate strength and fracture toughness than its para-substituted counterpart, consistent with experimental observations. Mechanistic analysis shows that the enhanced performance of the meta-system stems from strain-induced conformational adaptation within loop structures, facilitated by progressive bond-angle relaxation. This dynamic response promotes energy dissipation and effectively suppresses void growth during deformation. In contrast, the para-system undergoes accelerated failure due to its restricted structural flexibility. Our findings highlight positional isomerism as a powerful molecular design strategy for achieving simultaneous improvements in strength and toughness in epoxy thermosets, providing a foundation for rational material design beyond empirical approaches.