Mesophase disclination structures

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 4.26 Sketches of various wedge dislocations. Source. Reprinted with permission from Zimmer JE, White JL, Disclination structures in the carbonaceous mesophase, Advin Liq Crysts, 5, Academic Press, New York, 157, 1982. Copyright 1982, Elsevier.

The most common morphology observed in current mesophase carbon fibers of moderate modulus (55 to 75 Mpsi, 379 to 517 GPa) is a cylindrical filament with a random-structured core and a radial rim (12) Given the fracture section of Figure 3, with its scroll-like features, the core appears to be an array of +2ir and -ir disclinations. The radial rim of heavily wrinkled layers usually constitutes half or more of the cross section. 

The spatial geometry of disclination reactions in bulk mesophase has recently been presented by Zimmer and Weitz (29) Working with coarse-structured mesophase prepared by lengthy pyrolysis of A240 petroleum pitch at 400°C, they defined the disclination arrays on a succession of polished sections spaced at about 7 ym. In this way a +tt disclination was traced through a branching point (i.e., a reaction point) to form a -tt and a +2tt disclination. Thus a reaction... 

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... 

On the basis of this discussion, the mechanisms of mesophase carbon fiber formation are closely related to those of needle coke, the principal differences being the extent to which the deformation and relaxation mechanisms are able to act. Because delayed coking involves relatively gentle but random deformation processes by bubble percolation and the long dwell times in the coke drum afford opportunity for extensive disclination annihilation and micro-structural relaxation, the structure of needle coke can be well defined by polarized-light microscopy (2,36). 

The study of defects in liquid crystal systems is rooted in the understanding of defects in the solid state. For instance, crystals are rarely perfect and usually contain a variety of defects, e.g., point defects, line defects, or dislocations, and planar defects such as grain boundaries. In addition to these typical imperfections of the solid state, liquid crystals can also exhibit defects known as disclinations. These defects are not usually found in solids and result from the fact that mesophases have liquid-like structures that can give rise to continuous but sharp changes in the orientations of the molecules, i.e., sharp changes in orientation occur in the director field. 

Fibers with a radial transverse microstructure can exhibit a crack, which Volk [54] reports will tend to spiral around the fiber axis. The crack is able to transform into a true radial structure along a single filament and vice versa (Figure 7.13). White et al [55] believe that this is due to the annihilation of disclinations, as depicted in Figure 7.14. Yoon et al [56,57] have investigated crack formation in mesophase pitch based carbon fibers. 

FIGURE 1.33 (a) Disclinations in mesophase (b) aromatic molecular distribution (basic structural units [BSUs]). (Adapted from a sketch in Ref. 99.)... 

The links between the dimeric units are assumed to broken at the transformation fi om the solid to the mesophase. It is worthwhile noting that the mesophase formed from the isotropic melt appears first as bright bands which resemble those observed in PDFS. Earbom and Hartsbom regarded these bands as rodlike structures and referred to these bands as battonets . On the other hand, these structures can be interpreted as disclination lines From our point of view this question needs further consideration with allowance made for the fact, observed by Earborn and Hartshorn, that battonets can develop from striated bands arising after melting of acircular crystals and for the fact that the mesojdiase is not liquid, but a semi-solid phase. 

The distribution of defects in mesophases is often regular, owing to their fluidity, and this introduces pattern repeats. For instance, square polygonal fields are frequent in smectics and cholesteric liquids. Such repeats occur on different scales - at the level of structural units or even at the molecular level. Several types of amphiphilic mesophase can be considered as made of defects . In many examples the defect enters the architecture of a unit cell in a three-dimensional array and the mesophase forms a crystal of defects [119]. Such a situation is found in certain cubic phases in water-lipid systems [120] and in blue phases [121] (see Chap. XII of Vol. 2 of this Handbook). Several blue phases have been modeled as being cubic centred lattices of disclinations in a cholesteric matrix . Mobius disclinations are assumed to join in groups of 4x4 or 8x8, but in nematics or in large-pitch cholesterics such junctions between thin threads are unstable and correspond to brief steps in recombinations. An isotropic droplet or a Ginsburg decrease to zero of the order parameter probably stabilizes these junctions in blue phases. 

In liquid crystalline mesophases, there also exist domain boundaries in polydomain samples, in addition to domain disclinations. However, judging from the fact that neither the mobility nor the p,x-product depend on the size of the domains in a polydomain sample, these structural defects hardly affect the carrier transport properties of smectic mesophases [48-50]. Until now, the exact reason why structural defects in smectic mesophases are less harmful to carrier transport has not been explained. It is possible that the flexibility of the molecular orientation in mesophases, or the soft structure of mesophases, makes local carrier transport possible at defect sites. This is another outstanding feature of carrier transport in the mesophases, which distinguishes mesophases from crystalline materials. It provides... 

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