Mesophase microstructure

During this period, useful reviews were published. White (15) and Zimmer and White (16) describe the disclinations present in mesophase microstructures and their relation to coke properties. 

The carbonization experiments emphasize the need to fix or stabilize an oriented mesophase microstructure that has been imposed by mechanical deformation. The array of wedge disclinations imposed by drawing during cooldown is not thermally stable, and disclination reactions will proceed to coarsen the microstructure and reduce the preferred orientation if the mesophase softens sufficiently during carbonization. 

The effect of elemental sulfur and organic sulfur compounds and organometallics on the mesophase microstructure was investigated. Elemental and organic sulfur coiqtounds and organometallic compounds were added to Catalytic Incorporated coal liquid asphaltene and A240 petroleum pitch. 

The mesophase microstructure was examined using polarized light microscopy. It was found that organic sulfur compounds have less effect on mesophase microstructure than elemental sulfur but some organometallic compounds greatly inhibit mesophase formation. 

The major conclusion of this work so far is that the trace inorganics Al Fe and Cu organometallic additives will increase the pyrolysis yield without affecting the mesophase microstructure. Addition of organometallics can either have inhibitory or promotional effects. The prosiotional effect (e.g. Cu) is perhaps due to the termination of radical formation whereas the inhibitory effect (e.g. V and Ni) is due to the initiation of radical formation. This will be studied further using the temperature dependence of electron spin resonance (ESR) studies. 

J.L. White, G.L. Guthrie, and J.O. Gardner. Mesophase microstructures in carbonized coal-tar pitch. Carbon 5, 517-518 (1967). 

The double architectures shown in Table 1.1 correspond to LCPs more commonly referred to as combined , indicating that their structures combine the features of main chain and side chain LCPs. Thus mesogenic entities are located both in the side chains and in the polymer backbone, and mesophase microstructures tend to contain mesogens from each of these environments. Ordering that involves main chain components implies a tendency away from the random-coil conformation associated with flexible, non-mesogenic backbones this is possibly why double LCP materials are often crystalline and tend to exhibit at least one smectic mesophase (sometimes several) in their thermal profiles. 

It has been established that, when mesophase pitch is carbonized, the morphology of the pitch is the primary factor [20] in determining the microstructure of the resulting graphitic material. This may be attributed to the stacking behavior of mesophase molecules (quite similar to the planar stacking in turbostratic graphite), which may be visualized as shown in Fig. 5. 

Fathollahi, B. and White, J. L., Polarized-light observations of flow-induced microstructures in mesophase pitch, JRheol, 1994,38(5), 1591-1607. 

Understanding the factors that govern the formation of mesogens will assist in determining the processing conditions for the production of materials with specified amounts, sizes, and distribution of such crystalline microstructures. Mesophases can be local or permeate the entire structure. They can be large or small, and present in a random or more ordered arrangement. 

Simmons BA, Irvin GC, Agarwal V, Bose A, John VT, McPherson GL, Balsara NP (2002) Small Angle Neutron Scattering Study of Microstructural Transitions in a Surfactant-Based Gel Mesophase. Langmuir 18 624-632... 

Chemical and plastic behavior of the carbonaceous mesophase, which is the liquid crystalline phase where the microstructure of coke and many other carbon products is established... 

Coke microstructure is determined by the conditions in the coalesced mesophase prior to solidification. It is determined by the combined effects of convection currents, bubble percolation, and imposed shear stresses, all of which tend to deform the mesophase. 

The relative magnitudes of the Tg values in the two-phase region should be related to the development of microstructure in mesophase pitch, as discussed earlier. Tg will also be a useful parameter in defining the maximum temperature at which fibres can be oxidized without molecular motion causing some decrease in extent of preferred orientation. 

Microstructure Formation in Mesophase Carbon Fibers and Other Graphitic Materials... 

This paper commences with evidence for lamelliform morphologies in mesophase carbon fiber, summarizes relevant information on disclination structures in the carbonaceous mesophase, and then reviews what we learn of disclination behavior from hot-stage observations and from deformation and carbonization experiments. The results indicate that disclination interactions that occur before the mesophase is fully hardened play an important role in determining the microstructures of mesophase carbon fibers, as well as those of cokes and graphites that form through the carbonaceous mesophase. 

Figure 11. The increase in viscosity and refinement in microstructure as a mesophase pitch (from A240 petroleum pitch) is pyrolyzed within a rheometer (24). Crossed polarizers.

Fine deformed microstructures with strong preferred orientations could be produced by a single stroke of the wire probe (25), as illustrated by Figure 17. A vertical section made on a specimen quenched immediately after deformation confirmed that the underlying structure was fibrous with tt and 2tt wedge disclinations (30). The relaxation or coarsening after deformation indicated that the mesophase was sufficiently fluid for disclination motion... 

Figure 20. The microstructures of mesophase rods, as extruded (above) and as drawn after extrusion (below). Crossed polarizers.

Figure 21. Transverse microstructures of drawn mesophase rods (a) as extruded and drawn to a draw ratio of 3.6, (b) after oxidation at 300°C, and (c) after carbonization under inert atmosphere. Crossed polarizers.

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