Liquid crystalline model compounds

Table 19.2 Thermal properties of liquid crystalline model compounds derived from BB and TA [25]...

Iki and Hori discussed the crystal structures of the compounds CPnOB and nOPCB in relation to the mesophase behaviour considering the dimer model proposed by Cladis et al. [99] for this kind of molecule in the liquid crystalline phase. 

The model compound produced from a BB unit exhibited liquid crystallinity at a lower temperature, while its liquid crystalline phase stability (Tt — Tn) was higher than the corresponding model compound made derived from a TA unit, as shown in Table 19.2 [25],... 

The reason for the lower liquid crystalline (LC) temperature of the BB model compound seems to be that the biphenyl unit of this compound was adopting a twisted structure in the LC state [26], Therefore, we prepared various polyarylates containing the BB unit and determined their thermal properties and the moduli of the as-spun fibers, as shown in Table 19.3. 

Pang, Li, and Barton indicated, based on viscosity data, that PAEs had a stiff structure in solution [43]. A PAE shows a Mark-Houwink constant as high as a= 1.92 [43], revealing that it has a very stiff structure, similar to rodlike poly-(pyridine-2,5-diyl) [65,66]. Liquid crystalline behavior of PAEs with a meso-gen has been reported [45]. The chemical properties of PAEs have also been investigated with oligomeric model compounds [67-76]. 

The present compounds generally displayed a peak only in the 960 cm l region. One of the methoxy-substituted (x=5) and two of the ethoxy-substituted (x=5 and 8) poly(esters) were not LC, neither were two of the ether model compounds and several poly(ethers). As the cis/trans ratio was small and relatively independent of substituent, there are two likely reasons for the limitations on liquid-crystallinity ... 

The ester models, as stated above, were all LC, but two of the ether model compounds, the methoxy- and ethoxy-substituted derivatives, were not, despite the fact that each of these mesogens yielded some liquid-crystallinity in the poly(ether) form. Therefore it seems that the polymers in this system tend to be "more liquid crystalline" than the related small molecules. This hypothesis is supported by the fact that Memeger(H) found liquid crystallinity in allhydrocarbon polymers incorporating the distyrylbenzene mesogen, even in cases where the cis/trans ratio of the unsaturations was as large as 0.3, while Campbell and McDonald (10) noted that iodine isomerization to the all-trans form was essential for the observation of an LC phase in the small-molecule derivatives which they prepared. 

There are several indications that a crystalline solid is the most appropriate state to model the protein interior (Chothia, 1984). The very fact that protein structures can be determined to high resolution by X-ray diffraction is indicative of the crystalline nature of the protein. Additionally, the packing density and volume properties of amino acid residues in proteins are characteristic of amino acid crystals (Richards, 1974, 1977). In spite of the apparent crystallinity of the protein interior, most model compound studies have investigated either the transfer of compounds from an organic liquid into water (see, for example, Nozaki and Tanford, 1971 Gill et al., 1976 Fauch-ere and Pliska, 1983), or the association of solute molecules in aqueous solution (see, for example, Schellman, 1955 Klotz and Franzen, 1962 Susi et al., 1964 Gill and Noll, 1972). Both these approaches tacitly assume a liquidlike protein interior. 

An important class of model compound study that we have not discussed here is the determination of transfer free energies mentioned above. Though the study of transfer of amino acids and their analogs from water into various organic compounds provides a wealth of information about various interactions, the current data base includes only values AG° (usually relative to glycine) and not values for AH0, AS0, and ACp, and thus is not suitable for the temperature-dependent information required within the context of this review. The thermodynamics from liquid hydrocarbon, crystalline cyclic dipeptide, and alkane gas dissolution are summarized in Table I. 

Further studies on the detailed structure of the polymers in this series are in progress and will be reported shortly. Synthesis of higher homologues of this series and low molecular weight model compounds is also being pursued in an effort to determine the role of the rigid and flexible units in influencing the formation of liquid crystalline phases. 

Abstract Recently, the isotropic phase of rod-like liquid crystalline compounds is advised as an experimental model system for studying complex glassy dynamics. One of unique phenomena occuring close to the glass... 

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