Probing the Structure-Dynamics-Process-Property Relationship of a Curing Epoxy Resin via X-ray Photon Correlation Spectroscopy

Industrial adhesives are dispensed as viscous fluids, which then undergo a liquid-solid transition due to the formation of a cross-linked polymer network, known as "curing". However, the development and testing of adhesives is largely empirical owing to the lack of experimental tools to directly probe the internal molecular structure and dynamics of the material during the curing process. Here, we used X-ray photon correlation spectroscopy (XPCS) to investigate the structure/dynamics changes of a commercially available thermally-cured two-component adhesive (epoxy and acrylic resin) under operando conditions closely simulating industrial processing (i.e., nozzle extrusion and thermal-assisted crosslinking). The adhesive contained a small amount of fumed silica fillers for viscosity modification, though as a control we also measured the same epoxy-resin without silica fillers. Numerical analysis of the XPCS data demonstrated direct evidence of non-equilibrium structural and dynamical properties of the cross-linking network during the curing process. We will further discuss the relationship between structure-dynamics of the adhesive during curing, the manufacturing process, and the mechanical properties of the final adhesive.