Structural analysis of cured phenolic resins using complementary small-angle neutron and X-ray scattering and scanning electron microscopy

The structure of cured phenolic resins prepared by compression molding of a deuterated phenolic resin oligomer and nondeuterated hexamethylenetetramine as a curing agent was investigated using complementary small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and scanning electron microscopy (SEM). Cured thermosetting resins have been considered to have an inherent inhomogeneity of the cross-links with sizes ranging from tens to hundreds of nanometers based on SEM observations of fracture surfaces. However, such spatial inhomogeneity has not been observed for the phenolic resins by either SANS or SAXS. The present observation with SANS and SAXS indicates that the phenolic resins have an inhomogeneity associated with internal fractal interfaces between voids and phenolic resins, with a fractal dimension equal to 2.5-2.6 in the range of 3-1600 nm. The presence of voids in phenolic resins with sizes ranging from tens to hundreds of nanometers is clearly confirmed by an evaluation of the difference in scattering length densities between the SANS and SAXS functions and by SEM observations of etched surfaces prepared by focused-ion beam milling. Therefore, it can be concluded that (i) cross-links are randomly distributed over the range and (ii) the spatial inhomogeneity of the cross-links in that range is very small and negligible in comparison with the inhomogeneity associated with the internal fractal interfaces in terms of the fluctuations of the neutron and X-ray scattering length densities. Understanding cross-linked structure and inhomogeneity has been one of the major objectives of the structural analyses of insoluble and infusible thermosetting resins. We report the observations from complementary SANS, SAXS, and SEM for cured phenolic resins to elucidate their inhomogeneity.