Crosslink inhomogeneity in epoxy resins cured with a phenolic hardener under organophosphine catalysts revealed by small-angle X-ray scattering

In this study, the crosslink inhomogeneities in two types of epoxy resins cured with a phenolic hardener under organophosphine catalysts were successfully clarified by small-angle X-ray scattering (SAXS) using a solvent-swelling technique. Herein, triphenylphosphine (TPP) and tetraphenylphosphonium tetra-p-tolylborate (TPPTTB) were investigated as the curing catalysts. The SAXS profiles in a fully tetrahydrofuran (THF)-swollen state clearly revealed a difference between the two types of resins. The profiles at the early, middle, and late stages of gelation were effectively explained by a sum of structural functions representing the crosslinking inhomogeneity and concentration fluctuations of the polymer chains. Furthermore, the change in the profiles indicated that the characteristic size of inhomogeneity was associated with the size of the minor, low-crosslink density region. The correlation lengths of the mesh size decreased to 1 nm for both resins. These results confirmed the formation of a well-developed crosslinked network structure. Moreover, the correlation lengths of the crosslink inhomogeneity at the late stage of gelation were 1 and 5 nm for the TPP- and TPPTTB-cured resins, respectively. Thus, the phenolic-cured epoxy resins using an organophosphine catalyst system exhibited a THF-swellable, low crosslink density region due to inhomogeneity at the late stage of gelation.