Mesophase rheology

To date, there has been relatively little work reported on the mesophase pitch rheology which takes into account its liquid crystalline nature. However, several researchers have performed classical viscometric studies on pitch samples during and after their transformation to mesophase. While these results provide no information pertaining to the development of texture in mesophase pitch-based carbon fibers, this information is of empirical value in comparing pitches and predicting their spinnability, as well as predicting the approximate temperature at which an untested pitch may be melt-spun. 

Suto et al. (102) studied the effects of salts on the turbidity and viscometric behavior of HPC mesophases in water and the rheology of liquid crystalline solutions of HPC in m-cresol (103). Suto (104) found that crosslinking HPC mesophases in water destroyed their order. 

For a nematic polymer in a transition region from LC to isotropic state, maximal viscosity is observed at low shear rates j. For a smectic polymer in the same temperature range only a break in the curve is observed on a lgq — 1/T plot. This difference is apparently determined by the same reasons that control the difference in rheological behaviour of low-molecular nematics and smectics 126). A polymeric character of liquid crystals is revealed in higher values of the activation energy (Ef) of viscous flow in a mesophase, e.g., Ef for a smectic polymer is 103 kJ/mole, for a nematic polymer3 80-140kJ/mole. 

It should be emphasised that the micellar structures themselves are still too small to be seen under an optical microscope but they form domains of uniform orientation that can be observed. Other methods for the characterisation of mesophases are scattering methods, e.g. neutron scattering, x-ray diffraction or rheology as pointed out in Section 3.6. For a more detailed description see the literature relevant to this subject [19]. 

The Pitch-Mesophase-Coke Transformation As Studied by Thermal Analytical and Rheological Techniques... 

As outlined earlier, the rheological properties of pitch and mesophase pitch are important in the processing of these materials to carbon products. The rheology of isotropic pitch will be considered briefly and then the effects of pyrolysis to mesophase will be described. 

Mesophase Pitch. Mesophase pitch is a generic term for those products of pyrolysis that contain mesophase, or are entirely mesophase, but which on reheating still pass through a fluid phase prior to the formation of semi-coke. The following are areas of scientific or industrial interest where the rheological properties of mesophase pitch are of interest. ... 

Coalescence of mesophase is often said to be determined by the mesophase viscosity. This aspect requires much further investigation. However, it is clear that, amongst other factors, the rheological behaviour (including viscoelastic effects) of each phase is important in mesophase growth and coalescence. Diffusion of molecular species through the isotropic pitch to the mesophase spheres is likely to be related to the viscosity of the isotropic med i urn. 

The extent to which the mesophase pitch is deformed, as well as the relaxation after deformation, depends on the basic rheological behaviour. 

Fibre manufacture - The conditions for spinning of fibres from mesophase pitch are determined by the pitch rheology which may also influence the degree of preferred orientation. The rheological properties of the spun fibres are important in defining the conditions for thermosetting. 

Other manufacturing routes for carbon products based on mesophase are almost certain to be devised in the future and will depend upon an understanding of the rheological behaviour of the mesophase pitch. 

Rheological behaviour of chemically stable mesophase-pitch. 

The rheology of low molecular weight thermotropic compounds has been a subject of considerable theoretical and experimental analysis In general, liquid crystals are easily oriented by surfaces, electromagnetic fields and mechanical stress or shear, and the degree of orientation, in turn, affects their melt viscosity. The rheological behavior of a liquid crystal is known to be greatly dependent on the nature and also on the texture of its mesophase. 

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