Mesogen, definition

Note 4 The pendant groups in these polymers have structures compatible with liquid-crystal formation, that is, they are mesogenic but not intrinsically mesomorphic. See the examples given in Definitions 2.10 2.11.2.1. 

Such a mobility does not exist in layered LC-main chain polymers (Fig. 1 IB). The mobility required for the definition and existence of a molten state results in LC-main-chain polymers exclusively from a gliding of chains along each other. Such a motion is only possible when the intermolecular forces between the mesogens are relatively weak. Therefore only smectic -A and smectic -C phases are true LC-phases. In contrast to small molecules smectic -B (and higher ordered smectic phases) are solid mesophases. The difference between a solid smectic mesophase and a smectic crystalline phase lies in the extent of the three dimensional order and is usually difficult to determine experimentally (see Sect. 7). 

There is some doubt about the relevance of aspect ratio considered in isolation and as applied just to the empirically identified rigid mesogen. It can only really be applied to in-chain (or nematic unidirectional phases) but this is not applicable to discotic phases [61]. The examples quoted above also make it clear that flexible elements can be included in the mesogen. Strict definition of the mesogen is therefore difficult. 

The BCBOn series are exclusively nematics and in many cases it was found that the smectic tendencies of dimers are very much less than those of the analogous conventional monomeric mesogens. This observation has been attributed to the increased molecular flexibility of the dimer [6]. It was clearly of considerable importance, therefore, to determine the influence of the flexible core on smectic behaviour in order to test this suggestion. The definitive study of this issue by Date et al. [37] involved the synthesis and characterisation of eleven homologous series of dimers all belonging to the general family of compounds, the a,co-bis[4-(4-alkylphenyliminometh-yl)phenoxy] alkanes,... 

FIGURE 7.10 Typical examples of the H-NMR spectra of CBA-w in the nematic LC state and the definition of the molecular axis (see the text) (a) n = 9 (b) n = 10. In the upper diagrams, the quadrupolar splittings (Av ) are assigned to the individual C-D bonds of the spacer. The dipolar ( >hd) quadrupolar splittings (AVo) of the mesogenic core (lower diagrams) are due to the deuteron substituted at the ortho position. (From Abe and Furuya [1988].)... 

In many ways, the greatest problem in the study of anisotropy in non-crystalline polymer systems in general is its definition. For example, what is meant by molecular orientation in a liquid crystal polymer in which just the side-groups are mesogenic It may imply the anisotropy in the trajectory of the molecular backbone or it could mean the alignment of the side-chains alone. In this case both measures are needed for a full description of the anisotropic molecular organization, but each measure requires careful definition. 

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