Surface fluctuations on epoxy resin thin films during the curing process studied by GI-XPCS

Epoxy resins are the essential materials for modern industry because of their excellent mechanical properties, chemical resistance, and moldability. In thermosetting epoxy resins, heating temperature and curing time significantly affect the physical properties. Using x-ray photon correlation spectroscopy (XPCS), we have recently studied the dynamics of the curing process in bulk systems and discussed the formation of crosslinked structures and their temperature dependence [1]. On the other hand, the dynamics during the curing process of thin films, which are widely used as semiconductor protective films or adhesives, has not been well studied. In this study, thermal fluctuations on the surface of epoxy resin thin films were investigated by grazing incidence XPCS (GI-XPCS) for understanding the dynamical properties during the curing process. From the measurements, we obtained the capillary wave spectra on the thin films during the thermal curing process and found they slowed down with the reaction process. We analyzed those spectra and showed that they can be explained by the theory of capillary waves for viscous liquids. In addition to these capillary wave spectra for the viscous liquids, we also found a sudden rise and increase in the baseline of the time autocorrelation function at the later stage. Those behavior is assumed to represent the gelation process. Finally, almost no relaxation behavior was observed, indicating that the gelation was almost complete, and the thin films solidified. We succeeded in observing the freezing process of capillary waves during the thermal curing process. This research is widely applicable to the study of the dynamics of the curing process of thin films in non-equilibrium conditions and is expected to lead to various developments in the understanding of the curing mechanisms of various thin films.