Cross-link inhomogeneity in phenolic resins at the initial stage of curing studied by 1H-pulse NMR spectroscopy and complementary SAXS/WAXS and SANS/WANS with a solvent-swelling technique

Abstract

Abstract The cross-link inhomogeneity of phenolic resins at the initial stage of curing in a temperature range of 110–130 °C was investigated through structural analysis of the network structure evolution mechanism using 1 H-pulse nuclear magnetic resonance spectroscopy and complementary small- and wide-angle X-ray and neutron scattering methods. Two types of phenolic resins, PR06 and PR12, were prepared with stoichiometrically insufficient and sufficient amounts of cross-linker, respectively, via curing of a novolac-type phenolic resin oligomer with hexamethylenetetramine as the curing agent. Their network structures comprised three different structural domains because of the cross-link inhomogeneity: the high-cross-link-density domain (HXD), the low-cross-link-density domain (LXD), and the interface region between these domains. Percolation of the HXD occurred at the beginning of the curing. Intradomain reactions inside both HXD and LXD proceeded as the dominant reactions accompanying minor interdomain reactions between the HXDs, resulting in no significant change in the spatial location and size of the HXDs and LXDs. Intradomain reactions inside the LXD involved reactions with dangling chains, which would not affect the average mesh size of the domain significantly. These behaviors of the network structure evolution mechanism at the initial stage of the curing are a general feature of phenolic resins that does not depend on the amount of cross-linker. The difference between the amount of the cross-linker present in PR06 and PR12 was manifested as a difference in the degree of cross-linking in the percolated HXDs, i.e., the HXD of PR12 exhibited a tightly cross-linked, well-developed network structure since the beginning of the curing process; however, that of PR06 exhibited a loosely cross-linked network structure, with the degree of cross-linking increasing as the curing proceeded.