Intercalated Smectics

The structure of the B6 phase is also illustrated in Figure 8.17. This achiral, orthogonal smectic phase is one of the class of intercalated smectics wherein the layer spacing is actually about half the molecular length. This phase, which has full translational symmetry within the layers, has the same symmetry (D2h), and the same basic structure as the all-anticlinic SmCA phase. Intercalated SmCA phases of this type were first described in pioneering work of Watanabe in main-chain polyesters.37... 

Figure 8.18 Smectic dimer of Watanabe, possessing an odd number of methylene units in linking group. This material self-assembles into intercalated smectic structure very similar to B6 banana phase. As for B6 phase, this achiral phase is also neither ferroelectric nor antiferroelectric.

Note 1 The recommended subscript to designate an intercalated smectic mesophase is c Note 2 Intercalated smectic mesophases are commonly observed for liquid-crystal dimers. Note 3 At present intercalated smectic A (SmAc) and smectic C (SmCc) as well as intercalated crystal B (Be), G (Gc) and J (L) mesophases have been observed. 

Note 4 The local structure in the nematic mesophase of certain dimers exhibit an intercalated smectic mesophase. 

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... 

The initial interest in liquid crystal dimers was triggered, therefore, by the similarity of their behaviour to that of the semi-flexible main chain liquid crystal polymers. It soon became apparent, however, that the dimers are of significant fundamental interest in their own right and exhibit quite different behaviour to conventional low molar mass liquid crystals. These studies, for example, have resulted in the discovery of a new family of intercalated smectic phases. This review focuses upon the novel behaviour of dimers and how it may be understood at a molecular level. 

Fig. 11. The dependence of the transition temperatures on the number of methylene groups, n, in the flexible alkyl spacer for the CBOnO.lO series.[69] indicates interdigitated smectic A-isotropic transitions, nematic-isotropic transitions, A interdigitated smectic A-interdigitated smectic C transitions, interdigitated smectic A-nematic transitions, O intercalated smectic A-nematic transitions and A intercalated smectic A-intercalated smectic C transitions. The melting points have been omitted for the sake of clarity. SmAa Interdigitated smectic A phase SmCa interdigitated smectic C phase SmAc intercalated smectic A phase SmCca intercalated alternating smectic C phase N nematic I isotropic...

Fig. 18a-c. Sketches showing the formation hy non-symmetric dimers of a a monolayer and b an intercalated smectic A phase from an equimolar mixture of differing mesogenic units c the smectic A phase composed of horseshoe-like molecules... 

The SmAc phase was the first intercalated phase to be discovered [68] but examples of intercalated SmC and SmI phases and intercalated crystal B and J phases have since also been identified [69]. It is important to note, however, that tilted intercalated phases have only been observed for non-symmetric dimers containing long odd membered spacers and presumably such bent molecules experience great difficulties in packing efficiently into the intercalated network. Indeed, this packing difficulty may provide the driving force for the formation of the tilted intercalated smectic phases. 

Intercalated smectic phases have also been observed for symmetric liquid crystal dimers. Watanabe and his co-workers [13, 160-163] have described the properties of the a,m-bis(4,4 -butoxybiphenyl-carbonyloxy) alkanes. 

The even members of this series exhibit the intercalated smectic A phase while the odd members show only the intercalated alternating smectic C phase. In this respect the dimers are behaving in an identical fashion to the analogous polymers for which even membered spacers give rise to smectic A phases while... 

Figure 9. A sketch of the molecular organization in the intercalated smectic A phase exhibited by a non-symmetric dimer, CBOIOO.IO.

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]. 

C1/C2 chevrons, ferroelectric devices 644 cadmium selenium TFT address 233 Cano wedge, chiral nematics 347 f, 351 Canon technology, ferroelectric devices 648 capillaiy flow, shear viscosity 143 carbocyclic compounds, charge transfer systems 958 carbocyclic rings, smectogens 412 carbon atoms, intercalated smectics 808 carbon-carbon bonds, dimers 823 carbon-carbon double bonds, chiral smectics 498 carbonaceous phases 693 carbonyl connectors, antiferroelectrics 687 carbonyl groups... 

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