Dimeric structures, smectic phases

We saw in Sect. 3.1.1 that for a symmetric dimer containing terminal alkyl chains to exhibit smectic behaviour then the terminal chain length must be greater than half the spacer length. This empirical rule effectively eliminates the possibility that the dimers form an intercalated structure simply because the terminal chains can only be accommodated within such a structure if the total length of the two terminal chains is equal to or less than the length of the spacer (see Fig. 16a). This view is supported by X-ray diffraction studies which reveal that for the overwhelming majority of symmetric dimers the smectic phases have a monolayer structure (see Fig. 16b). 

In a model study aimed at elucidating the behavior of the polymers, Watanabe found that the dimer polymethylene diol diesters of type 7 (Figure 8.18) formed smectic phases.38 When the spacer between the two mesogenic units in the dimer had an odd number of methylene groups (diester 7 has nine methylenes in the spacer), then an intercalated tilted smectic structure with all anticlinic layer interfaces was formed. This structure is often termed SmC2 in the literature. As for the B6 phase, all of the layer interfaces in this structure are equivalent, and the X-ray layer spacing is less than half the fully... 

A liquid crystal dimer is composed of molecules containing two conventional mesogenic groups linked via a flexible spacer. These materials show quite different behaviour to conventional low molar mass liquid crystals and in particular their transitional behaviour exhibits a dramatic dependence on the length and parity of the flexible spacer. In this review a comprehensive overview of the relationships between molecular structure and liquid crystallinity in dimers is provided. This includes a description of the novel modulated and intercalated smectic phases exhibited by dimers. 

Keywords Liquid crystal dimers. Structure-property relationships. Modulated smectic phase. Intercalated smectic phase... 

Asymmetric dimeric liquid crystals with charge-transfer groups constitute good models for the mesomorphic self-organization of the related polymers discussed above. Their mesophase structures have been studied in some detail [34]. In this context, the formation of intercalated smectic phases of the A, C, and I type is especially noteworthy [34c]. However, the nature of the specific interactions between the unlike mesogenic groups, as well as the conformation of the spacer, are still to be explained [34d]. 

The nematic phase of all the compounds CBn is characterized by a coherence length of about 1.4 times the elongated structure of the molecule. Based on this behaviour local associations in form of dimers with cyano-phenyl interactions were postulated. For the smectic A phase a partial bilayer arrangement of the molecules (SAd) is most likely. But there are also example for the smectic A phase with a monolayer (Sai) or a bilayer (Sa2) arrangement of the molecules as well as a commensurate structure A large number of X-ray measurements were carried out in the liquid crystalline state to clear up the structural richness and variability (see Chap. 2, this Vol. [52]). 

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