Flow properties of mesophase pitches were examined using a capillary rheometry at the temperature range of their spinning to clarify influences of temperature, dimension of capillary, and structure and molecular assembly of the mesophase pitch. The viscosity-shear rate correlation curves of mesophase pitches exhibited two or three regions according to the temperature. At lower temperature around 270°C, the viscosity-shear rate correlation curves of mesophase pitches showed non-Newtonian type flow at low shear rates, the viscosity sharply decreasing with increasing shear to a certain limit. Above the limit of the shear rate, the viscosity stayed unchanged to another limit of the shear rate, exhibiting a plateau region. Above the second limit of the shear rate, the viscosity again decreased sharply with increasing shear, exhibiting again non-Newtonian type flows. The smaller the diameter of the capillary, the narrower the plateau region. At higher temperature, the pitch showed non-Newtonian type of flow regardless of shear rates, however, the dependence of the viscosity on the shear rate was certainly larger in the lower shear rate region than that in the higher shear rate region. Methylnaphthalene-derived pitch showed stronger dependence of its visocsity on the shear rate than naphthalene-derived mesophase pitches. Three kinds of the latter pitches showed similar correlations in spite of their very different softening points, solvent solubilities, and the ratios of H C. The viscosity-shear rate correlation appears to reflect the changes of domain texture and molecular assembly in the mesophase pitch, which have been suggested by X-ray diffraction and optical microscopy, respectively. At the low temperature, the deformation of the texture and slippage of the layer stacking take place at lower and higher shear rate ranges, respectively, since the viscosity changes with the shear rate in both regions. At the intermediate stage, stability of layer stacking allows Newtonian flow. At higher temperature, there is no stable region of layer stacking, therefore no plateau correlation. The difference between methylnaphthalene- and naphthalene-derived mesophase pitches is ascribed to their different orders of stacking. Much higher stacking of the former pitch may be more sensitive to the change of its height by the shear rate. Naphthalene-derived pitches possessed the same order of stacking height regardless of their softening points, and exhibited different correlations from that of methylnaphthalene-derived pitch.
All Science Journal Classification (ASJC) codes
- Materials Science(all)