Liquid crystallinity nature

To date, there has been relatively little work reported on the mesophase pitch rheology which takes into account its liquid crystalline nature. However, several researchers have performed classical viscometric studies on pitch samples during and after their transformation to mesophase. While these results provide no information pertaining to the development of texture in mesophase pitch-based carbon fibers, this information is of empirical value in comparing pitches and predicting their spinnability, as well as predicting the approximate temperature at which an untested pitch may be melt-spun. 

In the foregoing sections, the rate-enhancing effect of alkylammonium micelles has been extensively described. Similar effects can be expected for bilayer membranes of dialkylammonium salts. In addition, specific catalytic processes may be realized in this new system by taking advantage of the peculiar membrane structure. For example, catalyst molecules which are anisotropically bound to the membrane may act in very specific manners, and the liquid-crystalline nature of the bilayer membrane should provide unique microenvironments for catalysis. These are particularly interesting in relation to the mode of action of membrane-bound enzymes. 

The families of materials discussed in this section have also been deposited from solution to form films having a thickness of the order 1 fim, orientational order being introduced by the application of a strong magnetic field. The diamagnetic nature of these materials is made use of, its effect being very much magnified by their liquid crystalline nature. This topic will be returned to in Chapter 7. 

For polymer concentrations < 3 % the mixtures remain isotropic during polymerisation and become a rubbery mass that is too viscous to stir in the later stages of heating. In contrast polymer concentrations of 5-10% form yellow green stir-opalescent solutions within 30 minutes of the dissolution of the terephthalic acid. Such solutions remain stirrable throughout the polymerisation despite the higher concentrations — an indication of the liquid-crystalline nature of the medium. 

It is noteworthy that even in the isotropic phase (in the coil as well as in the globule) there exists a weak orientational ordering (not of the liquid-crystalline nature), due to the inhomogeneity of the spatial density distribution in the macromolecule. This ordering will be considered in the Appendix. 

Recent physico-chemical investigations of such materials are somewhat contradictory. Japanese workers, with a strong tradition in such work, suggest that certain silkworm fibroins are branched, whereas different workers, stress the liquid-crystalline nature of other materials, such as spider silks, and the parallels vwth linear synthetic liquid-crystalline polymers. Perhaps it is fair to say, as in other areas discussed in this article, that there is still a severe shortage of basic information. 

The results and discussion presented here focus on two major points which are (i) the number of phases coexisting in the CB such as it is found in tempered chocolate and (ii) the liquid crystalline nature of the variety obtained from rapid quenching of CB. A more detailed reexamination of CB polymorphism is given in Reference 4. 

Because of the liquid crystalline nature of the melts in which the polymer chains are arranged in domains of parallel arrays, fibers are readily obtained with an appreciable degree of molecular orientation in the direction of the long fiber axis. This orientation of polymer chains leads to high tenacity and high modulus without a drawing step. 

Owing to high viscosity, broad molecular weight distribution, existence of poly-crystalline and amorphous material, the liquid crystalline nature of thermotropic polym.ers is usually established through a combination of these methods. 

Weber, R, D. Guillon, A. Skoulios, and R.D. Miller. 1989. Liquid-crystalline nature of poly(dinor-mal-hexylsilane). J Physq 50 793. 

Non-linear behavior of aqueous formulations containing 1.5-2.0% NaCl at low shear rates, is attributed to liquid crystalline nature of these phases. Screen factor measurements further corroborate these findings. 

In addition to these main-chain liquid crystalline polymers, there are also side-chain liquid crystalline polymers, where the liquid crystalline nature arises from the presence of rigid straight side-chain units (called the mesogens) chemically linked to an existing polymer backbone either directly or via flexible spacer units. 

The molecular motion of dialkylammonium bilayers is highly restricted due to their liquid crystalline nature, and the presence of the crystal-liquid crystal phase transition was confirmed by diverse techniques such as differential scanning calorimetry(14), NMR spec-troscopy(13), fluorescence polarization(13,15) and positron annihi-lation(15). 

Dichromatic dyes when introduced in LC elastomers can be used to investigate the LC orientation behavior. The stress-induced orientational elastomers could be developed to form a color polarizer. It can be designed by chemically attaching dye molecules in side chains of LC elastomer [54]. Also, dichromatic dye-containing liquid crystalline elastomers can be synthesized by reacting a terpolymer, as a precursor, with hexamethylene diisocyanate. The elastomers, even with 10 mol% of cross-linkage, remain nematic in their liquid crystalline nature [55]. 

Type - 3 polyesters were synthesized by copolymerization of 4-acetoxybenzoic acid (HBA) and PET. The properties of PET/HBA copolymers with varied composition are tabulated in Table 8.6. It was found that the reaction mixture become turbid for HBA composition higher than 35%, this was due to liquid crystalline nature of copolymer. It can be observed from the table that non-liquid crystalline PET / 30-HBA is stronger than that of compositions with liquid crystalline nature. Low production cost of PET/HBA with good mechanical properties and processability made it very attractive. However, because of the tendency toward homopolymerization of the acyl ester of 4-hydroxybenzoic acid to form poly(4-oxybenzoyl)... 

The properties of CNs can be broadly described under three categories namely mechanical properties, liquid crystalline nature, and rheological properties. They are briefly discussed as follows ... 

Whilst tilted smectic phases of chiral materials are in general electro-optically switchable, many chiral tilted columnar systems show no optical effect whatsoever when an applied voltage is reversed (Figure 11.12). This resilience to the reorientation of dipoles by an electrical field may in some cases be due to the more crystalline than liquid crystalline nature of the systems, but even in the quite conventional high-temperature tilted phase of the first chiral material (triphenylene-2,3,6,7,10,ll-hexayl ( S )-3-... 

The liquid-crystalline nature of nucleic acids in vivo. In lower organisms, the nucleic acids are naked and exist for much of the time as concentrated droplets of cholesteric phase. In electron micrographs of the nuclei of some dinoflagel-lates, for example, the characteristic bands of nested arcs, Bouligand Pat-... 

The bimolecular leaflet is not a rigid structural entity. In keeping with its liquid-crystalline nature, the hydrocarbon interior of the bi-... 

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