Mesophase formation enhancement

The importance of unsaturation is illustrated by the fact that 2,4-nonadienoic acid [21643-39-0] forms a Hquid crystal phase, whereas the aHphatic carboxyHc acids do not. The two double bonds enhance the polarizabiHty of the molecule and bring iatermolecular attractions to a level that is suitable for mesophase formation. The overall linearity of the molecule must not be sacrificed ia poteatial Hquid crystal candidates. For example, whereas /n j - -aIkoxyciaaamic acids (5) are mesomorphic, the cis isomers (6) are not, a reflection of the greater anisotropy of the trans isomer. 

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

Solubility is governed not only by the solubility limits of the individual pitch species, but will also be affected by associations known to occur in pitch solutions. These may enhance or reduce the solubility of a particular pitch component. Precipitation of associated clusters or micelles broaden the MWD of the extract pitch and may provide the mechanism for direct mesophase conversion upon melting. The lower molecular weight species reduce viscosity to permit low temperature realignment of the larger, more aromatic species(10. From the MWD s, a larger fraction of pitch may be capable of mesophase formation if an optimized separation technique could be developed. 

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. 

That is, the ordered structure of the cholesteric mesophase affects the formation of the traTO-adduct advantageously. Furthermore, the trans/cis product ratio depends significantly on the initial acenaphthylene concentration. In isotropic solutions, the dimerization of singlet-excited acenaphthylene molecules is known to yield exclusively the czv-adduct, whereas a mixture of cis- and traTO-adducts results from triplet-excited solute molecules. The lowering of cu-adduct production in the mesophase has been attributed to the enhanced efficiency of the triplet reaction in comparison with the singlet reaction, as shown by quantum yield measurements [732]. The increase in triplet reaction efficiencies has been ascribed to the increase in the fraction of acenaphthylene-acenaphthylene collisions which have coplanar or parallel-plane orientations with respect to the surrounding solvent molecules, and not to the increase in the total number of collisions per unit time [732]. See references [713, 732, 733] for a more detailed discussion of this photodimerization reaction. 

Another example of a rigid macrocycle used as the central core of a den-drimer is the porphyrin. First and second generation phenylene-based dendritic porphyrins with 8 and 16 long alkyl chains on their periphery have been studied (Fig. 80). Only the second generation exhibits mesomorphic behavior with the formation of a rectangular columnar mesophase upon heating (Cr 39 Coir 110 I) [334]. A stable supramolecular 1 1 complex was formed between the second generation and Ceo, which led to the enhancement of the mesophase stabiUty of the formed (imidentified) columnar phase (Cr 99 Col 250 I). 

Rodriguez-Spong, B. Enhancing the Pharmaceutical Behavior of Poorly Soluble Drugs Through the Formation of Cocrystals and Mesophases. Ph.D. thesis. University of Michigan, 2005. 

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