Liquid mesogenic group

Penner T L, Schiidkraut J S, Ringsdorf H and Schuster A 1991 Oriented films from polymeric amphiphiles with mesogenic groups—Langmuir-Blodgett liquid-crystals/Macromo/ecu/es 24 1041-9... 

Fig. 29. Observed and calculated 2H NMR spectra for the mesogenic groups of a) the nematic (m = 2), b) the smectic (m = 6) liquid crystalline polymer in the glassy state, showing the line shape changes due to the freezing of the jump motion of the labelled phenyl ring. The exchange frequency corresponds to the centre of the distribution of correlation times. Note that the order parameters are different, S = 0.65 in the frozen nematic, and S = 0.85 in the frozen smectic system, respectively...

In summary the results of our 2H NMR investigation illustrate the spacer model for liquid crystalline polymers, indicating, however, that the decoupling of the mesogenic groups from the main chain, while effective, is not complete. 

Liquid crystalline main chain polymers with siloxane spacer groups were obtained by the hydrosilation of (Si—H) terminated polydimethylsiloxane oligomers and mesogenic groups with terminal double bonds as shown in Reaction Scheme XVII-(a). Reactions were usually carried out in THF with the Wacker Oil catalyst 255). Completion of the reactions was followed by the disappearance of the strong (Si—H) absorption band at 2140 cm-1 using IR spectroscopy. 

In this category, the units giving rise to the liquid-crystalline behavior can be in the backbone (as already discussed), in the side chains, or in both [172], Of considerable interest is the orientation of the mesogenic groups and the chain backbones to which they are attached [173,174], Frequently-studied backbones include siloxanes and acrylates [175,176], but a variety of other structures have also been studied, including amphiphilics [177]. 

Note 4 A liquid-crystal dimer with different mesogenic groups linked by a spacer is known as an asymmetric liquid-crystal dimer. 

Note 3 A MCPLC with cross-shaped mesogenic groups (b or g) is known as a cruciform (or star) polymer liquid crystal. 

Fig. 33. Examples of main-chain polymer liquid crystals I - mesogenic group, II - spacer.

Fig. 34. Examples of side-group polymer liquid crystals I - mesogenic group, II - spacer, III - backbone. The terminology side-group is used for (a), side-on fixed is used for (b), end-on fixed for (c) and side-chain for (d).

Liquid-crystalline polymer consisting of macromolecules in which mesogenic groups are incorporated both in the main-chain and in the side-groups. 

Fig. 36. Liquid-crystal dendrimers (a) with mesogenic groups in the whole volume of a macromolecule (b) with terminal mesogenic groups.

Nakajima et al. (1) prepared liquid crystalline polyrotaxane derivatives containing the mesogenic group 4-cyano-4 -hydroxybiphenyl attached to the a-cyclodextrin component of linear polyethylene glycol containing an a-cyclodextrin inclusion complex with an adamantane termini. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]-(ro-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a well-developed smectic liquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain liquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

The formation of macromolecular liquid crystals usually seems to be virtually complete. Also, when the mesogenic group is sufficiently mobile, there is only little supercooling of the transition to the liquid crystalline state. Plastic and condis crystals, on the other hand, are expected to behave more like the fully ordered, macromolecular crystals, i.e., partial crystallinity is expected in these cases. 

Schematically the arrangement of a macromolecule with side-chain mesogenic groups in the liquid crystalline state is shown in Fig. 13. Flexible spacers give the mesogenic group its mobility. It is of interest to note that in such liquid crystals all positional mobility of the mesogen is based on conformational motion of the flexible spacer and backbone. Restricting this mobility either prohibits ordering, or freezes the order into the glassy state.

Finkelmann, H., Happ, M., Portugal, M. and Ringsdorf, H. Liquid crystalline polymers with biphenyl-moieties as mesogenic group. Makromol. Chem. 179, 2541 (1978)... 

These examples indicate that the flexibility of the main chain, which is determined by its chemical constitution, directly influences the extent of the liquid crystalline state. With increasing rigidity of the main chain the motions of the mesogenic groups are more restricted. This can be noticed in an increase of the phase transformation temperature Tn. . 

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