Probing dynamics and crosslink morphology of thermosets during cure via XPCS

Epoxy-based systems have been widely used for decades, from aircraft parts to consumer goods. However, there are still gaps in the fundamental understanding of their crosslinking (curing) behavior, specifically heterogeneities in crosslink topology depending on cure cycle rates. Optimization of cure cycles is key to agile polymer matrix processing in industry and government. Here, we use X-ray photon correlation spectroscopy (XPCS) to study the crosslinking of epoxies in real time, with dilute nanoparticles as tracers to study the effect of the progressing crosslink topology as it confines the dynamics of the tracer. A series of six epoxy systems are presented, wherein the ratio of two curing agents is systematically varied. The fully cured systems range from linear polymers to densely crosslinked resins, enabling a deeper understanding of their cure mechanisms and crosslink topologies. XPCS shows diffusive motion at low degrees of cure, and a sharp transition to ballistic motion at higher cure, which depends on composition. DSC, in situ near-FTIR, and MC simulations complement the XPCS data. XPCS can be used as a tool to interpret crosslinking mechanisms without dynamically perturbing the system as is done in rheology.