Dimers nematic-isotropic transition temperatures

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],... 

Fig. 8. The dependence of the nematic-isotropic transition temperature, T i, on the total number of atoms, n, in the spacer for the cyanobiphenyl dimers linked via ether (O) [18], alkyl ( ) [16] and carbonate groups ( ) [16, 53]...

Quite different behaviour is observed if we now compare the ether and carbonate linked series. The nematic-isotropic transition temperatures for the carbonates decrease essentially without alternation as the spacer length is increased while the entropies show a much reduced odd-even effect when compared to the ether linked series [16, 53]. It should be noted, however, that the nematic-isotropic entropies for the carbonate series are still several times larger than those typically observed for conventional low molar mass mesogens. Similar weak odd-even effects in the transitional properties on varying the length of the spacer have also been reported for other dimer series... 

Table 1. Nematic/isotropic transition temperatures Tni, K) and enthalpies kj mol ) for monomers, dimers and polymers of series I [12].

Figure 23. The dependence of the nematic-isotropic transition temperature on the number of atoms in the flexible spacer predicted for liquid crystal dimers with methylene ( ) and ether (O) linkages between the spacer and the mesogenic groups.

Fig. 15. The dependence of the transition temperatures on the length of the flexible spacer, n, for the laterally linked dimer series, the a,(u-his[2,5-his(4-n-octyloxy-henzoyloxy)-henz-amido]alkanes [125]. Melting points are denoted hy O, indicates nematic-isotropic transitions and smectic C-nematic transitions. N Nematic I isotropic Cr crystal SmC smectic C phase...

Tni is the nematic to isotropic transition temperature). According to experiment [14], the additivity law holds both for weak and strong polar mixtures of liquid crystal compounds. However, in the latter case the specific intermolecular interaction between mixture components must be taken into account. Indeed, if the structural units of the liquid crystal are not only molecules, but also dimers (Fig. 2.5), then the effective dipole moment of the molecule in (2.12) must be replaced to this one of dimer /Lia Ma 9 where is the dipole correlation factor [14]. For the compounds, shown in Fig. 2.5 with R = C4H9 the correlation factor g 0.5. 

The values of y were similarly obtained for dimer CBA- (n = 9,10) and trimer CBA-Tn (n = 9,10). These compounds exhibit the nematic LC phase over a limited temperature range, hampering an accurate estimation of y by the extrapolation from this phase. Accordingly the y values were estimated by method 1 only from higher-temperature phases i.e., y i values are estimated from the isotropic phase, and ycN values from the nematic phase [95]. The ytr values thus derived are all accommodated in Tables 2 and 3, respectively, for the NI and CN transitions. Thermal pressure coefficients of monomer liquid crystals such as 4-cyano-4 -alkylbiphenyls ( CB) and 4-cyano-4 -alkoxybiphenyls ( OCB) are available in the article by OrwoU et al. [112]. The y values applicable to the NI transition of these compounds are cited in Table 4 for comparison. As shown in these tables, use of the volume change A Vtr at the transition (column 4) leads to the estimate of the volume-dependent entropy ASy (column 5) according to Eq. 3. 

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