Controlled stress, oscillatory rheometry of mesophase-pitches

The rheology of mesophase-pitches, having different percentage mesophase content, has been investigated by means of controlled stress, oscillatory rheometry. Three thermally transformed mesophase pitches (prepared from Ashland A240), with mesophase contents in the range 60-95%, together with a 100% optically anisotropic synthetic mesophase-pitch from Mitsubishi Gas-Chemical Company were studied. With the application of the time-temperature superposition principle, the viscoelastic behaviour of these mesophase-pitches was revealed in the form of the G', G '', eta' and delta master curves. At high reduced frequencies (or low temperatures), all the samples behave elastically, as is typical of a glassy solid. With decreasing reduced frequency (or increasing temperature), they become viscoelastic, their behaviour tending towards that of a Newtonian fluid, i.e. simple viscous behaviour. However, elasticity returned at very low reduced frequency (or high temperature); its extent increased with increasing mesophase content. This result indicates the presence of a long time constant relaxation process, which is tentatively ascribed to the reorienting of the discotic liquid crystalline mesophase molecules. This work has demonstrated unusual elastic behaviour in mesophase-pitches. Information gained by this approach can greatly assist the determination of processing conditions for these materials.