Comparative study of isotropic and mesophase pitches derived from C9 alkylbenzenes

A series of pitches prepared from C9 aromatic hydrocarbons through CH₂ coupling and successive heat treatments of variable severity were analyzed by NMR and FD-MS. The molecular species of mesophase pitch (toluene soluble fraction) are cata-condensed aromatic hydrocarbons, principally with 7 to 11 rings. Detailed ¹³C-NMR analyses indicate some bends in cata-type ring arrangement. The heat treatment decreased the number of methyl groups and di-methylene bridges in the ring skeleton of unchanged size, thereby reducing the solubility and enhancing the ordered stacking of aromatic planes. The major differences in the isotropic and anisotropic pitches were found principally in their hexane soluble (HS) fractions. The isotropic pitch fraction contained many more methyl groups and methylene bridges, which hinder the stacking of hexane insoluble (HI) aromatic molecules by occupying interlayer positions. In contrast, the same fraction in the anisotropic pitch is included between the layers of HI molecules to exhibit anisotropy. Natures of lyotropic liquid crystal are emphasized, being influenced slightly by the structure of its components.