Liquid crystallinity, molecular requirements

NMR Self-Diffusion of Desmopressin. The NMR-diffusion technique (3,10) offers a convenient way to measure the translational self-diffusion coefficient of molecules in solution and in isotropic liquid crystalline phases. The technique is nonperturbing, in that it does not require the addition of foreign probe molecules or the creation of a concentration-gradient in the sample it is direct in that it does not involve any model dependent assumptions. Obstruction by objects much smaller than the molecular root-mean-square displacement during A (approx 1 pm), lead to a reduced apparent diffusion coefficient in equation (1) (10). Thus, the NMR-diffusion technique offers a fruitful way to study molecular interactions in liquids (11) and the phase structure of liquid crystalline phases (11,12). 

This means that the partial molar area may directly be determined from the change in molecular area, when an amphiphilic molecule is introduced into a host liquid crystalline pattern. Of course, this area is the change of area per molecule at the introduction of one molecule of the substance in question and may be influenced by the interaction between the host molecules and the guest molecules. Since this interaction is an essential part of the present problem, it appears obvious that the method exactly meets the requirements. 

Since organogelators are in crystalline or lyotropic aggregate states in a gel. the nature of their intermolecular interactions becomes a factor of paramount importance in determining the nature and the intensity of emitted radiation from a gelled sample. Molecular proximity opens possible reactive channels for the excited states that are not available in dispersed solutions. For example, it has been shown that CAB dimerizes in its neat solid, liquid-crystalline, and gelled (fiber) states when exposed to UV radiation [47,48]. (See Structure I.) In dilute isotropic solutions, no photoreaction is observed because the time required for an... 

Aromatic polyamides are generally made by low-temperature reactions of aromatic diamines and aromatic diacid chlorides in special solvents such as a 1 3 molar mixture of hexamethylphosphoramide A-methylpyrrolidone, as in reaction (4-50). Intensive stirring is required to attain high molecular weights because the polymer precipitates. These macromolecules are very rigid and rodlike. They form oriented liquid crystalline arrays in solution and require little postspinning orientation to produce extremely strong and stiff fibers. The polymer would not be made in the melt because it is infusible. It must be synthesized and handled in solution, and this requires the use of reactive precursors. 

Thermotropic polyesters derived from unsubstituted aromatic diols and diacids usually have melting points which approach or exceed the thermal decomposition point. Thus it is reasonable to expect that some modification in molecular structure would be required to render them melt-processable, even though some adverse effects on liquid crystallinity and mechanical properties of the polymers would result. 

There are occasions when specimens exhibit a paramorphotic texture, i.e. one that reflects order inherited from the parent phase. At a molecular level, the transition between liquid crystalline phases typically occurs via a route that requires the minimum instantaneous rearrangement of molecules. Because textures are dictated by molecular order, the immediate post-transition texture may not easily be distinguishable from its pre-transition counterpart. Stable textures that are characteristic of the new phase may require a long time, sometimes months, to form. Transitions between highly otdered smectics are especially likely to favour paramorphoses. 

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