Smectic A —isotropic

Fig. 23a,b. Snapshots of configurations showing a the nematic-isotropic b the smectic A-isotropic interfaces. The Gihhs dividing surfaces are indicated hy the dashed lines... 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

Fig. 9-6. Phase diagram of ferrocenes 11. Melting point nematic/isotropic liquid transition smectic A/isotropic liquid transition A isotropic liquid/smectic A transition O isotropic liquid/nematic transition.

Fig. 2. The dependence of the melting points (O) and the nematic-isotropic transition temperatures ( ) on the nmnher of methylene units in the flexible spacer, n, for the BCBOn series [18]. Also shown are the nematic-isotropic transition temperatures ( ) and the smectic A-isotropic transition temperatures ( ) for the nOCB series...

Fig. 7. The dependence of the transition temperatures on the number of methylene groups, n, in the flexible spacer for the 4.0n0.4 series [37]. indicates smectic A-isotropic transitions, the nematic-isotropic transitions, O the melting points, smectic A-nematic transitions and A smectic B-smectic A transition. Cr Crystal SmA smectic A N nematic I isotropic...

Table 1. Maxima in Smectic A Isotropic Transition Temperature ( C) obtained by the contact method for each mixture shown. A number indicates a temperature higher...

Let us first consider dodecyl-cyanobyphenyl which shows two phase transitions at 48 °C (crystalline-semectic A) and at 58.5 °C (smectic A-isotropic). The analysis of the whole spectrum is reported in the original work [112] and we focus here only on the variation with temperature of the conformational structure of the dodecyl side chain which we hope to reveal with the study of the temperature-dependent vibrational spectrum in the 1420-1280 cm" range where defect modes are expected to occur. 

Figure 7. Dependence of the transition temperatures on the number of carbon atoms, m, in the terminal chain for the CB040.m series. indicates the inter-digitated smectic A-isotropic transition, the nematic-isotropic transitions, A interdigitated smectic A-nematic transitions and intercalated smectic A-ne-matic transitions. The melting points have been omitted for the sake of clarity. SraA, interdigitated smectic A SmA ., intercalated smectic A N, nematic I, isotropic.

A-nematic phase transition (SNT) is of second order, while the NIT is of first order. In Figure 2.16b, the smectic order parameter discontinuously drops to zero with increasing <)> and the SNT is of first order. In Figure 2.16c, the nematic phase disappears and we only have the first-order smectic A-isotropic phase transition (SIT). Owing to McMillan theory for a pure nematogen (<)> = 0), the SNT should be second order for Tsf /T j < 0.87 and first order for larger values of 7 /T j, where shows the SNT temperature of a pure nematogen. 

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