Toshio Nakao

Volume transition of nematic gels in nematogenic solvents

Equilibrium swelling and phase behavior of liquid crystalline (LC) networks swollen in miscible nematogenic solvents has been investigated by polarizing microscopy as a function of temperature. Four systems, i.e., each of two different LC networks comprising dissimilar mesogens in two different nematic solvents, exhibit essentially the same swelling and phase characteristics. The swelling characteristics strongly correlate with the phases of the LC molecules inside and outside the gel. The two independent nematic-isotropic transition temperatures for the gel (TNIG) and the surrounding pure solvent (TNIS; TNIG>TNIS for all the systems examined) yield three characteristic temperature regions. In the totally isotropic and nematic phases (T>TNIG and T<TNIS, respectively), the degree of equilibrium swelling (Q) is almost independent of T, and the magnitudes of Q in these phases are comparable. Meanwhile, Q strongly depends on T in the region TNIS<T<TNIG where the LC phases inside and outside the gel are differ...

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.

Rubber elasticity for incomplete polymer networks

We investigated the relationship between the elastic modulus, G and the reaction probability, p for polymer networks. First, we pointed out that the elastic modulus is expressed by G = {(fp∕2 - 1) + O((p - 1)(2))} Nk(B)T∕V (percolated network law), which does not depend on the local topology of the network structure or the existence of the loops. Here, N is the number of lattice point, V is the system volume, f is the functionality of the cross-link, k(B) is the Boltzmann constant, and T is the absolute temperature. We also conducted simulations for polymer networks with triangular and diamond lattices, and mechanical testing experiments on tetra-poly(ethylene glycol) (PEG) gel with systematically tuning the reaction probability. Here, the tetra-PEG gel was confirmed to be a potential candidate for ideal polymer networks consisting of unimodal strands free from defects and entanglements. From the results of simulations and experiments, it was revealed, for the first time, that the elastic modulus obeys this law in the wide range of p (p(c) ≪ p ≤ 1), where p(c) is the reaction probability at gelation threshold.

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.