Mechanism of glass transition temperature enhancement in multicomponent epoxy resins incorporating triazine rings

Epoxy resins are widely used in electronic and structural applications. However, their use at high temperatures is limited by their glass transition temperature (T_g). Although the addition of multifunctional resins increases T_gvia crosslink density, the molecular-scale contribution of specific intermolecular interactions, such as π-π stacking, remains poorly understood in multicomponent systems. Here, we investigated the molecular-level mechanism of T_g enhancement in a multicomponent epoxy resin system incorporating a triazine-based epoxy resin, tris(2-epoxypropyl)isocyanurate (TEPIC), into a conventional epoxy resin. The experimental results revealed a non-monotonic dependence of T_g on TEPIC content, with an initial decrease at low concentration followed by a pronounced increase at higher concentration, which cannot be explained solely by changes in crosslink density. Structural analysis using wide-angle X-ray scattering and molecular dynamics (MD) simulations showed that the amorphous halo originating from intermolecular packing splits into two peaks, suggesting that one of these peaks includes a contribution from π-π stacking interactions. MD simulations showed that ring pairs with centre-of-mass distances of 3.4-5.8 Å form stable π-π stacking structures, and that the population of these interactions correlates strongly with the observed variation in T_g. Quantum chemical calculations further demonstrated that benzene-triazine stacking interactions introduced by TEPIC are significantly stronger than benzene-benzene stacking. These results indicate that the enhancement of T_g arises not only from an increase in crosslink density but also from localized intermolecular constraints induced by strong stacking interactions, providing a molecular-level design guideline for high-performance epoxy resins.