Beyond the ring flip: A molecular signature of the glass–rubber transition in tetrafunctional epoxy resins

The glass–rubber structural transition of epoxy resins is a critical quantity in the design of resins for thermally demanding applications. Currently, many high T_g epoxy resins exhibit reduced mechanical performance and are not suitable for structural applications. A present lack of understanding regarding the molecular origins of the glass–rubber transition, together with the cost of resin synthesis, has limited progress in the targeted development of novel, mechanically strong, high-T_g resins. Here, molecular dynamics simulations are used to predict the molecular-level structure and thermo-mechanical properties of three tetrafunctional epoxy resins. Via introduction of a conceptual framework of monitoring local molecular motions during molecular dynamics simulations, a characteristic molecular signature of segmental dynamics, correlating with the glass transition temperature, is identified. This computational framework provides a cost-effective strategy for rapidly assessing the comparative thermal performance of novel epoxy resins. Additionally, the impact of ring substitution on the thermo-mechanical properties of the naphthalene-based resins reveals promising directions for future design to yield desirable mechanical and thermal properties.