Molecular origin of acid-triggered degradation in recyclable hexahydro-s-triazine epoxy thermosets

Understanding the molecular mechanism of chemically triggered degradation in thermosetting polymers is essential for developing recyclable high-performance resins. In this study, we investigated the acid-triggered degradation mechanism of epoxy resins cured with a hexahydro-s-triazine-based hardener using first-principles molecular dynamics simulations combined with density-functional theory calculations. Calculations of acid dissociation constants revealed that the hardener undergoes double protonation under strongly acidic conditions around pH 0–1, which initiates network-degrading bond cleavage reactions. Free-energy analyses using the blue moon method demonstrated that degradation proceeds through sequential cleavage of two C–N bonds accompanied by structural rearrangements. The first C–N bond cleavage exhibited the highest free-energy barrier of 1.50 ± 0.04 eV in water at 298 K, decreasing to 1.32 ± 0.05 eV at 363 K, and was identified as the rate-determining step, whereas subsequent degradation processes proceeded with substantially lower barriers. Water solvation strongly influenced the free-energy landscape and stability of intermediates. Additional protonation of decomposed intermediates produced stable fragmented species, indicating a computed pathway by which the doubly protonated hardener can decompose into three protonated molecular fragments. The calculated free-energy barrier at 363 K (1.32 ± 0.05 eV) was in semi-quantitative agreement with the effective experimental barrier (1.18–1.23 eV) inferred from decomposition in 20 wt% phosphoric acid solution at the same temperature. These findings provide atomistic insight into acid-triggered degradation in recyclable epoxy thermosets and offer molecular-level design guidelines for chemically recyclable thermosetting materials.