Network structure evolution of a hexamethylenetetramine-cured phenolic resin

The network structure evolution of a hexamethylenetetramine (HMTA)-cured novolac-type phenolic resin over a curing temperature range of 135–155 °C was investigated using 1H-pulse nuclear magnetic resonance spectroscopy and small-angle and wide-angle X-ray scattering techniques. The aim was to elucidate the mechanism responsible for the apparent absence of inhomogeneity after curing at 175 °C, in which the inhomogeneity was first observed at the gel point below 130 °C. The HMTA-cured phenolic resin exhibited high-cross-link and low-cross-link density domains (denoted as HXD and LXD, respectively). The LXD was a minor structure having a cross-link fraction of 0.2, which was 5–6 nm in size and comprised a few meshes. As curing proceeded, intradomain reactions in the LXD occurred, and the electron density in the domain increased, decreasing the electron density difference between the HXD and LXD. This reduction in the electron density difference decreased the cross-link inhomogeneity in the phenolic resins in terms of electron density fluctuations. This structural evolution caused the apparent absence of inhomogeneity in the fully HMTA-cured phenolic resins.