Microstructural effects on the dynamic rheology of a discotic mesophase pitch

The dynamic rheology and accompanying microstructure of a synthetic mesophase pitch (AR-HP) is reported. The loss modulus (G″) was found to be higher than the storage modulus (G′) at all frequencies (∼0.1 to ∼100 rad/s) and temperatures (280–305 °C) tested. The slope of the low-frequency terminal zone for G′ was found to be approximately 0.8, much lower than a value of 2 that is observed for flexible chain polymers. Cross-polarized optical microscopy with full-wave retardation plate confirmed the presence of different textures of mesophase pitch processed under different conditions. While loss moduli remained fairly unchanged, finer textures led to significantly lower storage moduli. Consistent with this trend, coarsening of the microstructure during textural relaxation led to an increase in storage moduli. Therefore, for the discotic mesophase pitch, the viscous component was found to remain unaffected by the microstructure, but the elastic modulus was dependent on the orientation of layer-planes and size of the texture.