Impact of cross-linking on the time-temperature superposition of creep rupture in epoxy resins.

Epoxy resins are an important class of thermosetting resins, and their network structure, formed by the curing reaction of epoxy and amine compounds, plays a crucial role in determining material properties, including creep behavior. We here applied the time-temperature superposition (TTS) principle to analyze the creep behavior of epoxy resins with well-defined network structures that were systematically varied based on the length of the n-alkyl diamine used. The superposition of isothermal creep curves under small stress was achieved through horizontal and vertical shifting, regardless of the length of the n-alkyl diamine. The temperature dependence of the horizontal shift factor was well described by the Williams-Landel-Ferry equation. Creep rupture measurements under large stress conditions revealed specimen rupture, and the time to rupture was plotted against the imposed stress. These plots, acquired at various temperatures, could be superimposed through horizontal shifting. As the diamine length decreased-namely, the distance between cross-linking points-the temperature dependence of the horizontal shift factors deviated from the WLF equation and exhibited Arrhenius-type behavior. The deviation was associated with differences in the fracture process involving chain scission, which became more pronounced as the diamine length decreased. The insights gained in this study should be valuable for controlling creep response and predicting the long-term durability of epoxy resins.