Reversal of Glass Transition Temperature at Buried Interfaces in Thermosetting Epoxy Reveals Competition Between Network Maturity and Solid-Induced Confinement

The glass transition at buried interfaces critically influences the reliability of thermosetting adhesives and composites, yet direct determination of interfacial glass transition behavior remains challenging. Here, we report a label-free fluorescence lifetime approach that directly excites aromatic backbones of an epoxy resin using third-harmonic generation pulses, eliminating probe-induced artifacts and enabling thermally robust and noninvasive measurements. By combining this approach with evanescent wave illumination, we selectively probed the buried adhesion interface and found that the interfacial glass transition temperature was lower than the bulk value under conventional curing conditions. Upon additional thermal annealing, however, the interfacial glass transition temperature exceeded the bulk value. These results resolve the interplay between chemical network maturity and substrate-induced confinement, establishing a mechanistic framework for interfacial glass transition in thermosetting polymers.