The thermotropic liquid crystalline behavior of mesophase pitches with different chemical structures

Chemical structure-dependent liquid crystalline behaviors of two commercial mesophase pitches, M-AR and T-MP, which are synthesized with different raw materials of naphthalene and coal tar, respectively, were comparatively examined. The NMR analysis together with preparation procedure enabled us to suggest that M-AR consisted of linear semi-rigid molecules and T-MP, irregularly shaped rigid molecules. Between 320 and 370 °C M-AR showed marbled texture whereas T-MP, a change from schlieren/marbled to schlieren texture. WXRD results indicated that M-AR exhibited a gradual change of d002 and Lc with temperature but T-MP, a drastic change. A point shear stress made M-AR produce oriented flow domains but T-MP retained schlieren texture even in the oriented structure. In the temperature range producing biphase, M-AR showed a shoulder on the plot of viscosity versus temperature but T-MP, a peak. As temperature was increased M-AR gave an increase of loss tangent but a decrease of yield stress while T-MP gave a saturation point at 330 °C, at which temperature loss tangent was maximum and yield stress was minimum. Both mesophase pitches gave viscosity curves matched with the Asada-Onogi model. In the temperature range producing dispersed polydomain system, however, T-MP showed almost shear rate-independent plateau while shear-dependence was observed in M-AR.