Mesophase coalescence

The preferred orientation of the matrix parallel to the fiber bundles appears to be affected by processing pressure during pyrolysis (400-500°C), becoming more random as impregnation pressures are increased. Even transverse lamellar orientations can result (Figure lb). Because pitch mesophase coalescence has been inhibited by the pressure, grains of matrix may be transversely oriented to the filaments. 

When the elementary spherules of mesophase coalesce to form anisotropic domains, the microtexture of the carbonaceous mesophase becomes more complex. Disdinations (rotational defects) in the arrangement of the discotic molecules are often present. Disdinations in 2-D media play a role similar to that of dislocations in crystals and the evolution of gas bubbles results from condensation reactions in a medium which is still fluid. Disdinations, which occur during the mesophase formation, remain after carbonization and are key to understanding the relationships between the microtexture and the properties of carbon fibers formed from mesophase pitches. 

CARBONACEOUS MESOPHASE is a liquid crystaUine state of PITCH which shows the optical birefringence of disc-like (discotic) nematic crystals. It can be formed as an intermediate phase during thermolysis (p5Tolysis) of an isotropic molten PITCH, or by precipitation from PITCH fractions prepared by selective extraction. With continuous heat treatment, the CARBONACEOUS MESOPHASE coalesces to a state of BULK MESOPHASE before solidification to GREEN COKE, with further loss of hydrogen or other low molecular weight compounds. 

Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molecules in a mesophase sphere. B, a) isolated mesophasc spheres in an isotropic fluid pitch matrix b) coalescence of mesophase c) structure of semi-coke after phase inversion and solidification.

Figure 5 presents the results of tensile tests for the HPC/OSL blends prepared by solvent-casting and extrusion. All of the fabrication methods result in a tremendous increase in modulus up to a lignin content of ca. 15 wt.%. This can be attributed to the Tg elevation of the amorphous HPC/OSL phase leading to increasingly glassy response. Of particular interest is the tensile strength of these materials. As is shown, there is essentially no improvement in this parameter for the solvent cast blends, but a tremendous increase is observed for the injection molded blend. Qualitatively, this behavior is best modeled by the presence of oriented chains, or mesophase superstructure, dispersed in an amorphous matrix comprised of the compatible HPC/OSL component. The presence of this fibrous structure in the injection molded samples is confirmed by SEM analysis of the freeze-fracture surface (Figure 6). This structure is not present in the solvent cast blends, although evidence of globular domains remain in both of these blends appearing somewhat more coalesced in the pyridine cast material.

In the course of liquid-phase carbonization of pitches, optically anisotropic spheres are formed first, which are called mesophase spheres. By further heating, the spheres grow and coalesce with... 

FIGURE 2.23 (a) Formation, (b) growth and (c) coalescence of mesophase spheres in a pitch. 

Heat Treatment Temperature and Soak Time. A study of Mochida and Marsh (53) indicates, unlike classical kinetics, that time and temperature for mesophase formation are not interdependent. The reason for this is the controlling influence of viscosity (not found for reactions in the gas or solution phase). Maximum size of optical texture and coalescence results if the mesophase is formed under conditions which provide a minimum viscosity as quickly as possible. Probably, rate controlling processes for mesophase growth are not the dehydrogenated polymerization reactions. Therefore, the attainment, relatively quickly, of temperature of 400°C has provided the necessary size of molecule and consequently the resultant mesophase sbbws minimum viscosity because it is at a high temperature ( 400°C). Mesophase formed at lower (relative) temperatures can have a higher viscosity and coalescence behaviour can be restricted. 

Pressure of Carbonization. The effect of a pressurized carbonization is to create a closed system preventing loss of volatile materials. Hence, carbon yields increase. Further, the material normally lost as volatiles in open systems is now retained and the effect of this, by reducing turbulence and bubble formation, is to enhance the size of resultant optical textures. Hiittinger and Rosenblatt (54) report such effects when gas pressures up to 15 MPa pressure (150 bar) were applied to the carbonization of a coal-tar pitch. If higher pressures are used, the pressure being applied hydraulically to the carbonization system, then the effect of pressures at, say, 300 MPa, is to enhance the viscosity of the total system and this prevents coalescence of the mesophase. The resultant appearance of the carbon has been described as botryoidal (55, 56) and an example is Figure 8. 

This is a serious misnomer as these inert constituents of pitch are certainly not inert during the carbonization processes. It is well-established that the size of the optical texture of a coke can be reduced by the presence within the pitch of primary QI material (102-105). The QI material within the pitch becomes adsorbed on the surfaces of the growth units of mesophase. This thereby prohibits coalescence of these growth units into the larger sized optical textures. When this process is viewed by hot-stage optical microscopy (106) this lack of coalescence is seen to reduce markedly the flow characteristics of the mesophase - it becomes almost static. 

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). ... 

Figure 1. Three steps in the development of optical anisotropy via the mesophase. (a) Nucleation of mesophase spherules. (b) Growth and coalescence of mesophase. (c) Flow anisotropy.

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. 

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. 

Figure 13 depicts a coalescence event that took place in 7 sec. Like other coalescence events in which new disclinations are produced, a 2tt disclination appeared at the position of the former boundary between the mesophase spherules. Frenkel (27) has pointed out that the time constant t for such coalescence phenomena is given approximately by... 

Figure 13. Coalescence of mesophase spherules to produce a new 2tt disclination. Hot-stage microscopy, crossed polarizers.

---