Thermotropic SmAs

Ferrocene was functionalized at the 1,1 -positions by a sugar moiety (1-amino-l-deoxy-D-sorbitol) and a long alkyl chain 44 in order to obtain lyotropic properties. The substituents, in particular the sugar framework, were selected in view of successful investigations performed with organic-type amphiphilic carbohydrate surfactants. Compound 44 showed both thermotropic and lyotropic mesophases. In the bulk, a SmA phase formed from 98 to 137 °C. When 44 was mixed with an excess of water, a fluid phase was obtained. For the thermotropic SmA phase, a r/-layer spacing... 

By plotting the intercepts d, l(J) versus the relative temperature T — Tq, the temperature dependence of a hypothetical bilayer thickness for 0 wt% of formamide is obtained (Fig. 5.27b). It exhibits the typical temperature dependence expected for a thermotropic SmA to SmC phase transition. The two kinks in the extrapolated data at r - Tc = -6 and —10 arc caused by the reduction of analyzable data points due to the crystallization of the samples with high solvent concentration. At the lamellar L to lyo-SmC phase transition the calculated layer spacing is at its... 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

As discussed m Section It).4.8, small-molecule thermotropic smectics, such as 8CB, show two flow alignment regimes under steady shearing (see the left half of Fig. 10-38 labeled SmA ). At high shear rates and high temperatures, 8CB orients preferentially in the a, or perpendicular, direction under steady shear, the same as is seen in block copolymers in region II. Recent theory indicates that layer fluctuations destabilize the c, ... 

On the basis of small angle X-ray scattering, it was shown that the metal alkanoate matrix in the mesophase consists of bilayers. In thermotropic ionic liquid aystals, an alternation exists of hydrophobic alkyl chains and conductive layers which contain metal cations and anionic carboxylic groups. In lyotropic ionic liquid aystals, there are metal cations, carboxylic groups, and water between the hydrophobic alkyl chains layers. As a result, metal alkanoates form an SmA structure [64]. 

For X-Y = 8-hydroxyquinolato ( = 0) and X-Y= 1,10-phenanthroline ( =1), the complexes were non-meso-morphic. However, when X-Y = acac (n = 0), a material with a monotropic SmA phase resulted (Cr 83 (SmA 68) I) and when X-Y = 2,2 -bipyridine (n= [BF4] counterion), a material with an enantiotropic nematic phase was produced (Cr 146 N 158 I) related complexes with PF6 or SbF6 anions were non-mesomorphic. These are further, rare examples of ionic materials showing a thermotropic nematic phase. 

There are very few structural studies of thermotropic side-chain polymers under shear flow. Recent work [54] describes the neutron scattering by the liquid crystalline polymethacrylate labelled PMAOC4H9 (Sect. 4.1) in the SmA phase. Typical liquid crystal behavior was observed in which the layers orient... 

The more obvious choice for a model compound corresponds to exactly one repeat unit of the polymer. The a-ethoxy-m-(4-n-alkoxy-4 -cyanobiphenyl)s exhibit nematic mesophases at n=4-9 and SmA mesophases at n = 8-ll, and therefore match the thermotropic behavior of the polymer better than the vinyl ether monomer. However the SmA mesophase is enantiotropic only at n = 11, and the nematic mesophase is monotropic at all of these spacer lengths. Compounds which take into account only the mesogen and spacer are also good, if not better, models of the polymers. In contrast to the ethyl ethers, all of the SmA and most of the nematic mesophases are enantiotropic, which means that the melting temperature mimics the relative temperature of the glass transition of the polymer backbone better. However, the nematic mesophase still appears at n = 6 -11, and the SmA mesophase doen t appear at n = 5-7, 10, 11. 

Figures. Structure of 1,2-dihydroxyalkanes (7, R = alkyl). Thermotropically, these diols exhibit layered phases of the SmA and SmB type [78].

The ethylene glycol derivatives exhibit their thermotropic mesophases only mono-tropically, see Table 4 interestingly, their mesomorphic phase range can be stabilized remarkably by addition of water. In the latter case, the clearing temperature increases whereas the transition temperature of the SmB-SmA transition decreases. The authors conclude, here, that the formation of hydrogen bonds between adjacent molecules is responsible for their mesophase formation [7 8]. 

In contrast to 9 which exhibits a SmA and a nematic mesophase on heating, the lyotropic 8 is thermotropically not liquid crystalline. After two solid-solid transitions the diol 8 melts at 94 °C directly into the isotropic liquid. The thermotropic data of both compounds are given in Table 5. 

Addition of water to the amphiphile 8 induces several lyomesophases. With growing water content N, SmA, and SmC mesophas-es are observed [83]. Fig. 10 shows the phase diagram of 8 with water [83]. The uptake of water into the liquid crystalline phase is limited to 10 to 20 water molecules per diol molecule. This is slightly more than for other diol amphiphiles where the thermotropic mesophases could be stabilized by addition of water and where the amount of water in the mesophases is also limied [85,86]. 

The a, 0-bisdiols 21 depicted in Fig. 22 are characteristic examples of this kind of amphiphile. Both ends of the molecules carry a hydrophilic, vicinal bisdiol unit. Except for the one with the shortest alkyl middle section (21a, n = 4), all these multiols are thermotropic liquid crystalline showing at least a SmC phase [80, 81, 108]. The thermotropic phase transition temperatures of members of this amphiphilic family are compiled in Table 12. Multimesomorphism is observed here for examples of which n > 11 their SmC phase is followed by a narrow (1-5 K broad) SmA phase on heating. The mesophase stability increases with growing length of the alkyl spacer. X-ray investigations show that in the low tempera-... 

According to Fig. 3, classical thermotropic smectic phases of amphotropic liquid crystals are (SmA ), colunmar hexagonal (Col ), bicontinuous cubic (Cub, i), or discontinuous cubic (Cubjis) [169]. All these meso-phases include a disclination surface between the hydrophilic and the lipophilic parts of the unordered molecules. This surface can be uncurved (SmA), curved in one direction (columnar), curved in two directions with the same sign (discontinuous cubic), or curved in two directions with opposite sign (bicontinuous cubic). 

In their paper the authors follow the nomenclature by Luzzati and indicate the observed me-sophases L or Lp , respectively. Here, however, to separate between lyotropic and thermotropic mesophases the indication was changed into SmA and SmC, respectively. 

Only a few examples of optically active ferrocene-containing thermotropic liquid crystals have been reported. Compounds (304) and (305) have been prepared by functionalizing ferrocene with a chiral fragment bearing an asymmetric carbon atom. Ferrocene derivative (304) showed an SmC phase, while for (305), SmC, SmA, TGBA, N, and blue phases (blue phases are referred to in Section 7.9.4.3.3) were obtained. " Ferrocene has also been substituted with a cholesterol derivative but the mesophases were not identified. ... 

Fig. 4.12 Two-dimensional diffraction patterns of an aligned sample of the thermotropic liquid crystalline material 5-(octyloxy)-2-(4-(octyloxy)phenyl)pyrlmidine a at 100 °C in the SmA phase and b at 50 °C in the SmC phase [23]...

The ratio determined for lamellar phase is v/al = l, and for the hexagonal phase is l/3predicted limits are relatively insensitive to the e5Kict values of v and a, but are strongly dependent upon the choice of 1. Thus for n-octyl p-o-glucopyranoside, shown in Fig. 3, heating from the crystal to the thermotropic liquid crystal state results in the formation of a lamellar phase (SmA ) as predicted by the modelling. 

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