Deformed mesophases

Deformed Mesophases, Low-order Mesostructures, and Other Possible Mesophases... 

Thermotropic liquid crystalline (LC) phases or mesophases are usually formed by rod-like (calamitic) or disk-like (discotic) molecules. Spheroidal dendrimers are therefore incapable of forming mesophases unless they are flexible, because this would allow them to deform and subsequently line up in a common orientation. However, poly(ethyleneimine) dendrimers were reported to exhibit lyotropic liquid crystalline properties as early as 1988 [123],... 

Next is a group of materials with a mesophase which shows orientational disorder , but positional order. These materials are widely known as plastic crystals 16) because of the ease of deformation of such crystals. Again, this name is well accepted and much less cumbersome than orientationally disordered crystals7). 

Condis crystals and glasses of macromolecules are a newly recognized type of mesophase. The mobility in this mesophase may lead to chain extension, and as a corollary, it may be possible that mechanical deformation can cause the stabilization of the condis state. Several examples of stable condis crystals are documented, but there seem to be also examples of metastable condis crystals which are produced as intermediates to crystallization. The size of the condis crystal transitions vary depending on the number of conformational isomers involved in the cooperative transitions. 

A major effort should be undertaken to clarify the open questions. It is hoped that the classification as offered in this review can serve to focus future work in mesophases of macromolecules. Of particular interest is the question of the involvement of condis crystals as intermediates in polymer crystal deformation. 

The possibility for the existence of mesophase in a rubbery state 36,46), typical only for macromolecular compounds with their natural ability to display big reversible deformations, reveals interesting prospects from the viewpoint of creation of new types of liquid-crystalline materials in the form of elastic films, as well as for development of the theory of viscoelastic behaviour of such unusual elastomers. 

Discussion. We can now propose a coarse description of the paraffinic medium in a lamellar lyotropic mesophase (potassium laurate-water). Fast translational diffusion, with D 10"6 at 90 °C, occurs while the chain conformation changes. The characteristic times of the chain deformations are distributed up to 3.10"6 sec at 90 °C. Presence of the soap-water interface and of neighboring molecules limits the number of conformations accessible to the chains. These findings confirm the concept of the paraffinic medium as an anisotropic liquid. One must also compare the frequencies of the slowest deformation mode (106 Hz) and of the local diffusive jump (109 Hz). When one molecule wants to slip by the side of another, the way has to be free. If the swinging motions of the molecules, or their slowest deformation modes, were uncorrelated, the molecules would have to wait about 10"6 sec between two diffusive jumps. The rapid diffusion could then be understood if the slow motions were collective motions in the lamellae. In this respect, the slow motions could depend on the macroscopic structure (lamellar or cylindrical, for example)... 

Mesophase is susceptible to chemical reactions other than those induced by pyrolysis. Modifications to enhance fusibility or solubility for easier spinning (see Preparation of Mesophase Pitch) and to induce thermosetting for carbonization without deformation are both practical steps in carbon fiber manufacture. 

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

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. 

In contrast to conventional nematic liquid crystals, the molecular units are disk shaped and range widely in size, even when the mesophase is produced by the pyrolysis of pure organic compounds (17). Many of the molecules are volatile or reactive in the temperature range over which the mesophase is fluid, and the evolution of gaseous species usually causes the mesophase to be extensively deformed by bubble percolation before it congeals to a solid semicoke. 

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... 

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. 

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). 

Figure 22 Uniaxial deformation of a disclination loop bounding a mesophase fold. Reproduced with permission from reference 35.

As the mesophase has a much lower volatile content than the original binder pitch, and because the mesophase is still deformable by molding, a carbonization process was developed by Bruckmann (19) in which mechanical pressure is applied after the green composite has been pyrolyzed at 450°C for 5 h. A tensile strength of about 800 MPa and a Young s modulus of about 150 GPa were achieved without subsequent impregnation processes. These values were measured after final baking to 950°C. 

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