Atsushi Izumi

Atomistic molecular dynamics study of cross-linked phenolic resins

In this study, we analyzed cross-linked phenolic resins by using atomistic molecular dynamics simulations. Cross-linked structures consisting of a network of three functional phenols and two functional methylenes with degrees of cross-linking of 0.70, 0.82, and 0.92 were prepared by cross-linking reactions of linear novolac-type phenolic resins in a unit cell under three-dimensional periodic boundary conditions. Uniaxial elongations of the cross-linked structures to a strain of 0.03 were performed at 300 K. At this temperature, all structures were apparently in a glassy state, which was confirmed by the analysis of specific volume as a function of the temperature. The uniaxial elongation did not cause a significant change in the distribution of bonding potential energies (i.e., bond stretching, angle bending, and torsion angle potentials). On the other hand, the change in the potential energies owing to the uniaxial elongation indicated that cross-links suppressed local segmental motions in the cross-linked structure, probably at the region around the linear and terminal phenols, which resulted in an increase in the degree of cross-linking accompanied by a decrease in Poissons ratio and an increase in Youngs modulus.

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 13C-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 cross-linked network appear, followed by an increase in the size of the domains.

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.

Network structure evolution of a hexamethylenetetramine-cured phenolic resin

AbstractThe 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.The network structure evolution of a hexamethylenetetramine-cured novolac-type phenolic resin was investigated using 1H-pulse NMR spectroscopy, SAXS and WAXS to elucidate the mechanism responsible for the apparent absence of inhomogeneity after curing, in which the inhomogeneity was first observed at the gel point.