Spacer-chain parity

Fig. 5.21. (a) A schematic illustration of relative core orientations in members of a covalently connected pair of mesogenic cores for even and odd spacer-chain parities (all-tmns conformation) and (b) dimer (D ) versus monomer (M ) LC core order parameters in the nematic phase (from [62]). 

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

A semi-flexible main chain liquid crystal polymer is composed of mesogenic units separated by flexible spacers, normally alkyl chains [ 7]. These polymers are not only of interest for their application potential [8] they are also of major fundamental interest because of their unusual liquid crystalline properties. It is well known, for example, that the transitional behaviour of a semi-flexible main chain liquid crystal polymer shows a dramatic dependence on the length and parity of the flexible spacer linking the mesogenic units [9]. Other fascinating behaviour includes the observation of a nematic-nematic transition [ 10] and the occurrence of alternating smectic phases [11-15]. 

Thus on varying the length and parity of the flexible spacer in dimers the nematic-isotropic transition temperature exhibits a dramatic odd-even effect which attenuates as the length of the spacer is increased while the nematic-isotropic entropy also exhibits the same pronounced alternation but which appears not to attenuate as the spacer length is increased. Such behaviour is also observed for semi-flexible main chain liquid crystal polymers for example, Figs. 5 and 6 show the dependence of and ASni/R, respectively, on the length of the flexible spacer for the poly a,co-[4,4 -(2,2 -dimethylazoxy-phenyl)]alkandioates [9],... 

The distinct odd-even alternation in ASjsn is a character peculiar to the polymethylene (PM)-type spacers. A comparison between the two different types of spacer, BCBOn (designated as CBA-n in this paper) [25] and MBBE-x with the oxyethylene (OE)-type spacer [26, 27] is shown in Fig. 5, where the latent entropy ASjsfi/R is plotted against the number of crmstituent atoms ( ) of the spacer. For MBBE-x with the OE spacer, irrespective of the parity of n, the A5ni versus n plot tends to decrease monotonicaUy with some tiny bumps. The odd-even trend characteristic of the tetrahedrally bonded chain system is rapidly smeared out by the... 

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