Molecular orientation, liquid crystalline

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. 

This value coincides in the order with that proposed for low molecular-weight liquid crystalline substances in the swarm theory (6), 10 . In PBLG-dioxane preparation, orientation of the solution is not observed at 1125 V/cm, and is still weak at 2500 V/cm (29) a similar result is obtained from the X-ray diffraction photc raph of the sunilar liquid crystalline solution (52). There is every possibility that the orientation is caused by induced dipoles of the molecular cluster and/or by an electric current through the solution. This may be in accordance with the su estion that molecular association is of the antiparallel type in a low dielectric solvent... 

PTFE is a special case. It has been shown that rubbed PTFE layers induce the oriented crystallization of a number of materials crystalline and liquid-crystalline polymers as well as low molecular weight liquid-crystalline materials [55]. The underlying orientation process is not elucidated in every case. In many instances, it seems that some form of graphoepitaxy may be involved since no clear-cut structural matching can be dehned. Another indicator is the parallel orientation of the deposited chain with the PTFE substrate, which would be compatible with some form of milder interaction— but, of course, does not rule out hard epitaxy. One example of hard epitaxy, which also results in a tilted orientation of the chain axis relative to the PTFE substrate chain orientation, will be analyzed shortly. 

Ordered dialkoxy PPV derivative has been prepared by Yoshino et al. [491. oly(2 -nonoyloxy-1,4-phenylene vinylene) 27a forms a nematic liquid-crystalline phase upon melting. The material retains its order upon cooling to room temperature, and its band gap (2.08 eV) is measurably smaller than in an unoricnted sample. Oriented electroluminescence may be achieved by rubbing a thin fdin of the material to induce molecular orientation [50],... 

In some cases, chemical substituents can bring about unusual monotropic liquid crystalline phases which only exist upon heating or cooling. The diphenyl-diacetylenes are examples in this category [22]. Early theoretical connections between molecular electronic structure and orientational order... 

Photoisomerization of the azobenzene amphiphile was found to be strongly affected by molecular packing and orientation in the aqueous bilayer solutions. A rate constant of trans to cis isomerization was extremely faster in the liquid crystalline state than in the crystalline bilayer membrane [33]. Photoreaction of the aqueous bilayer membrane of CgAzoCioN+ Br was... 

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