Gelation and cross-link inhomogeneity of phenolic resins studied by small- and wide-angle X-ray scattering and 1H-pulse NMR spectroscopy

Abstract

The gelation mechanism and cross-link inhomogeneity of phenolic resins prepared via polycondensation of phenol and formaldehyde under acidic conditions were studied using small- and wide-angle X-ray scattering and 1H-pulse nuclear magnetic resonance spectroscopy. The solvent-swelling technique was applied for both measurements at the initial stage of gelation to enhance the local fluctuations of the cross-link density. The change in the static and dynamic structures obtained through observations of the X-ray scattering functions and the spin–spin relaxation functions, respectively, during the polycondensation reaction indicates the presence of two different mechanisms for the formation and growth of the inhomogeneity that depend on the amount of cross-linker. (i) When there is a stoichiometrically insufficient amount of the cross-linker, inhomogeneous domains with a loosely cross-linked network appear at the initial stage of gelation. The intradomain reactions become dominant in the growth of the inhomogeneous domain and the degree of cross-linking in the domain increases by bridging two unreacted sites in the network structure via the cross-linker. (ii) When there is a stoichiometric amount of the cross-linker, inhomogeneous domains with a tightly cross-linked network appear at the initial stage of gelation. The interdomain reactions become dominant in the growth of the domain and the size of the domain increases by incorporating new polymer chains into the domain via the cross-linker.