Smectic A —isotropic 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...

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.

The liquid-crystal transition between smectic-A and nematic for some systems is an AT transition. Depending on the value of the MacMillan ratio, the ratio of the temperature of the smectic-A-nematic transition to that of the nematic-isotropic transition (which is Ising), the behaviour of such systems varies continuously from a k-type transition to a tricritical one (see section A2.5.91. Garland and Nounesis [34] reviewed these systems in 1994. 

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.

The heat of the clearing point transition ( 7-9 kj moK ) is typical for a smectic to isotropic transition, and two orders of magnitude higher than for Blue phase isotropic transitions. 

Fig. 5.2.3. Order parameters s and a, entropy 5 and specific heat versus reduced temperature for a = 0.6 showing a second order smectic A-nematic transition and a first order nematic-isotropic transition. (After McMillan. )...

The pre-smectic interaction is therefore observable already in the isotropic phase, although there is no direct phase transition from the isotropic to the smectic-A phase in 8CB. This makes the capillary condensation of the smectic-A phai in the gap between the surfaces impossible and consequently enables more detailed observation of the presmectic interaction. In the case when there is a direct isotropic-smectic-A phase transition, like for example in 12CB, a condensation phenomenon similar to nematic capillary condensation is observable. 

When the liquid crystal exhibits a direct isotropic to smectic-A phase transition, the capillary condensation of the smectic phase in a gap between the surfaces is possible, since the isotropic to smectic-A phase transition is a first order phase transition due to the coupling between the nematic and smectic order parameters [26]. 

The temperature dependence of the density (q) is practically linear, with the exception of jumps near phase transitions. The volume changes are 3-9%, 0.1-0.4%, and 0.01-0.2% for the crystal-mesophase, nematic-isotropic, and smectic A-nematic transitions, respectively. 

The Landau-de Gennes theory for the nematic isotropic transition can be extended to the smectic A-nematic transition. The order parameter for this transition is rl), the amplitude of the density wave describing the formation of layers in the smectic A phase. Since the difference between a value of rlr and -Irlrl only amonnts to a shift of one half layer spacing in the location of all the layers (and therefore no change in the free energy per nnit volume), the expansion in terms of powers of rlr can only contain even powers. Hence the free energy per unit volume in the smectic A phase can be written as... 

In the McMillan model, the smectic A-nematic transition can be continuous or discontinuous. If a is less than 0.7, then o decreases to zero continuously and S is continuous at the smectic A- nematic transition. If a is between 0.7 and 0.98, then a jumps to zero discontinuously and S has a small discontinuity at the smectic A-nematic transition. When a is greater than 0.98, the smectic phase transforms directly into the isotropic phase with discontinuities in both order parameters. So just as in the extended Landau-de Geimes theory for the smectic A phase, a tricritical point is predicted at a=0.7, which corresponds to a ratio in the smectic A—nematic transition temperature to the nematic-isotropic transition temperature of 0.87. A great deal of experimental work has been done on the smectic A-nematic transition, and the results seem to indicate that the tricritical point occurs when the ratio of the two transition temperatures is significantly larger than 0.87. 

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. 

The crystal-to-smectic, smectic-to-smectic and smectic-to-nematic and smectic-to-isotropic transitions are rarely observed for this class of polymers. The nematic-to-isotropic transition, is also not frequently observed because it usually occurs at such a temperature that thermal degradation precedes this transition. The phenomenon of observation of smectic (high order)-to-smectic (low order) and smectic-to-nematic transitions, known as polymesomor-phism, is common to low molar mass LCs (LMLCs). In contrast, LC... 

M24, which is 4-cyano-4 -octyloxybiphenyl having a smectic A nematic transition at 67.1 °C and a nematic isotropic transition at 79.8 °C [ASTM 2069-06 see Menczel (to be published)]... 

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