Mesoscopic Heterogeneity in the Curing Process of an Epoxy–Amine System

Epoxy resins are composed of a three-dimensional network formed by chemical reactions between epoxy and amino compounds, which plays an important role in the mechanical properties. Thus, to use epoxy resins in various applications, it is necessary to gain a better understanding of their network structure. Here, we study the structural heterogeneity evolved in an epoxy–amine mixture during the curing process on the basis of a particle tracking technique, in which the thermal motion of probe particles in the mixture was tracked, small-angle X-ray scattering measurements in conjunction with coarse-grained molecular dynamics simulation. The heterogeneous environment was generated even at the initial stage of the curing process. Notably, the characteristic length scale was on the order of several hundreds of nanometers down to several tens of nanometers, depending on the extent of curing. Once a reaction occurs between a pair of epoxy and amino groups, the temperature at the site is locally elevated due to the heat of formation, accelerating a subsequent reaction nearby. Repeating such a situation, actively and scarcely reacted domains are formed. This is the main origin of the structural heterogeneity in epoxy resins.