Formation of mesophase

There is a large number of stages of orientation in the production of carbon products with a graphitic crystalline structure from high-boiling, aromatic residues with the concurrent elimination of hydrogen. 

The mesophase stage is of particular importance. It occurs in the temperature range between 300 and 500 C and is characterized by the close association of 

After the semi-coke phase, which extends up to around 700 °C, further increase in temperature brings about the formation of anisotropic coke, which can be used as the feedstock for the production of graphite, especially graphite electrodes. 

Insufficient pre-orientation in the mesophase stage, as a result of low aromaticity of the respective residue, or through too rapid a heating to temperatures in excess of 500 °C, leads to isotropic cokes, which are principally used for the manufacture of anodes for aluminum production by electrolysis. 

Unlike liquid-crystal formation in synthetically produced mesogens, such as e.g. 4 -octylbiphenyl-4-carbonitrile, reversible liquid-crystal formation is generally not 

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The specification requirements for electrode binder pitch, eg, high C/H ratio, high coking value, and high P-resin content, effectively ruled out pitches from gasworks or low temperature tars. The cmde tar is distilled to a medium-soft pitch residue and then hardened by heating for several hours at 385—400°C. This treatment increases the toluene-insoluble content and produces only a slight increase in the quinoline-insoluble (Ql) material, the latter by the formation of mesophase. 

Besides this, the remarkable properties of gold(I) compounds, which often give rise to aurophilic interactions and/or to luminescence, are of interest when these properties are transported into the liquid crystal field. Although there is much still to be studied, it is already clear that luminescence can survive in the condensed but mobile state of a mesophase, and even in the isotropic liquid state of a molten gold compound. It also seems that aurophilicity can contribute in some cases to the formation of mesophases. 

Another interesting feature pertains to the melting points cited for the polymers having m — 6-9. The side chains are now long enough to crystallize themselves, which is apparently the reason that formation of mesophases is suppressed. Such side-chain crystallization has also been involved in polysilane homopolymers... 

A lattice model that takes such attractions between parallel bonds into account provides a reasonable prediction of polymer melting points [13] and of their interplay with liquid-liquid demixing in polymer solutions [14]. The same factors that favor freezing do affect to a greater or lesser extent the formation of mesophases hence, there is a close relation between polymer crystallization and the formation of mesophases, which are frequently observed before polymer crystallization (see other papers in this issue). 

Ribo, J. M., Crusats, J., Sagues, E, Claret, J., and Rubires, R. (2001). Chiral sign induction during the formation of mesophases in stirred solutions. Science, 292, 2063-6. 

The world availability of pitch materials is such that there is an abundance of pitch which produces cokes of little industrial value. The economic pressures within the industry are to upgrade the commercial value of these pitches. Three ways of doing this appear as possibilities - (i) from fractionation separation of pitch systems to select molecular species (a process of "molecular cropping") most amenable to formation of mesophase. (ii) to add, by blending, materials which have been proven to up-grade the quality of resultant cokes. 

Miyazawa et al. (92) related rates of decrease of aliphatic hydrogen protons during pyrolysis of ethylene tar pitch to formation of mesophase. Yokono et al, (93) used the model compound anthracene to monitor the availability of transferable hydrogen. Co-carboniza-tions of pitches with anthracene suggested that extents of formation of 9,10-dihydroanthracene could be correlated with size of optical texture. The method was then applied to the carbonization behaviour of hydrogenated ethylene tar pitch (94). This pitch, hydrogenated at 573 K, had a pronounced proton donor ability and produced, on carbonization, a coke of flow-type anisotropy compared with the coarse-grained mosaics (<10 ym dia) of coke from untreated pitch. 

This influences the structural features of the mesophase which remains more disordered, a point made by Cranmer et al. (43). Stadelhofer (107) found that the presence of QI did not change rates of formation of mesophase. Romovacek et al. (108) consider that pyrolytic particles in pitch (primary QI) retard the development of mesophase and suppress coalescence. Decrease in size of optical texture, as brought about by mechanical modification as distinct from chemical modification of pitch properties can increase both the strength and reactivity to oxidising gases of the resultant coke, as recently put forward by Markovic et al. (109). ... 

The complexity of formation of mesophase must not be underestimated. With the exception of a few model compounds, it is the industrial pitch which is the source of mesophase. Such materials contain thousands of reactive molecules and there is an interdependence in the carbonization system which currently is known to us but not analyzed in depth. This is an area for further research. Formation of mesophase is further complicated because it involves chemistry within a fluid/plastic system of increasing viscosity. And in the delayed coker, volatile release and liquid turbulence are yet additional factors in influencing final structure in mesophase. 

It is now known that the rate of shear prevailing in such "in situ" studies can influence the rate of formation of mesophase (23) and this has recently been explained by Sorensen and Diefendorf (24) as being due to the release of volatiles being facilitated during flow of the hot pitch. 

The direct interaction between surfactants and inorganic precursors was later found to be not the only pathway for the formation of mesophases. A major discovery following Mobil s work is the synthesis of mesophases through the assembly of cationic inorganic species with cationic surfactants in acidic solutions. Here, the interaction between cationic silica species and cationic surfactant headgroups is suggested to be mediated by halide anions. 

In this chapter lyotropic liquid crystalline polymers are considered, where micelle-like organizations of the macromolecules cause the formation of mesophases in defined concentration and temperature regions. For these polymers it has to be assumed that the polymer backbone or the monomer units must contain an amphiphilic character. 

Hence, the tremendously high equilibrium rigidity and the ordered structure of para aromatic polyamides favouring the formation of mesophases in the concentrated polymer solutions and permitting their use for the manufacture of ultrahigh-modulus fibers is ensured by both the frans-structure of the amide groups and the para position of the phenyl rings. 

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