Gelation and cross-link inhomogeneity of phenolic resins studied by 13C-NMR spectroscopy and small-angle X-ray scattering

The gelation mechanism and cross-link inhomogeneity of phenolic resins prepared via polycondensation of phenol and formaldehyde under acidic conditions were studied by using C-13-NMR spectroscopy and small-angle X-ray scattering. The structural analysis of the gelation process indicated the presence of two different mechanisms of the formation and growth of the inhomogeneity that depend on the initial formaldehyde-to-phenol molar ratio: (i) when there is an insufficient amount of a cross-linker at the initial stage of gelation, inhomogeneous domains with a loosely cross-linked network appear and the degree of cross-linking in the domain increases with the reaction time. (ii) When there is a sufficient amount of a cross-linker at the initial stage of gelation, inhomogeneous domains with a tightly crosslinked network appear, followed by an increase in the size of the domains.