Ordered Smectic Phases

X-ray measurements have shown that the positional and orientational ordering in highly ordered crystalline smectic phases is more comparable with the ordering in the solid state than the structures of the fluid smectic phases and the nematic phase. Therefore, a comparison of the structures of crystals and highly ordered smectic phases is possible because of their closer similarities. Up to now, only the crystal structures of few mesogenic compounds exhibiting highly ordered smectic phases have been reported in... 

This assertion is borne out by the small deviations from fluid-isotropiclike values of the photoproduct ratios found in the Norrish II reactions of the two alkyl perfluoroalkyl ketones 76 (with m, n = 7,8 and 9,10) in the (macroscopically) highly ordered smectic phases of F10H10 [273,274]. As... 

That is why here we describe two groups of polymers- those. forming an ordered smectic phase (of SB type) and polymers with mesogenic groups, for which the formation of crystalline structure is proposed (Table 5). Sometimes it is difficult to distinguish between crystalline and LC states of ordered smectic phases. Thus, the interpretation of data on crystalline phases of some polymers, listed in Table 5, is also possible from the viewpoint of smectic polymorphism. 

Brackets represent an extrapolated transition temperature several ordered smectic phases present. 

Some liquid crystals form one or more smectic phases. These display a variety of microscopic structures that are indicated by the letters A, B, C, and so forth. Figure 23.9c shows one of them, the smectic A structure the molecules continue to display net orientational ordering, but now, unlike in the nematic phase, the centers of the molecules also tend to lie in layers. Within each layer, however, these centers are distributed at random as in an ordinary liquid. TBBA enters the smectic A phase at 200°C, before undergoing transitions to two other more ordered smectic phases at lower temperatures. 

Some examples of pyrrolidinium-based ILCs have been reported in recent years, in all cases forming smectic phases, from low ordered SmA phases of de Vries type to highly ordered smectic phases of T type. These will be discussed in the following paragraphs. 

For application to thin film transistors (TFTs), which can operate electronic papers, high carrier mobility exceeding 0.01 cm2 V-1 s-1 is necessary. For this purpose, nematic semiconductors with low molecular order, resulting in relatively low carrier mobility, are not suitable. For application of thin films of semiconductors to TFTs, stabilization of the highly ordered smectic phases, which exhibit high carrier mobility, by photopolymerization may be desirable. Kreouzis et al. studied the carrier transport properties of photopolymer-izable phenylnaphthalene, diphenylbithiophene, and quaterthiophene derivatives having an oxetane moiety or l,4-pentadien-3-yloxy in their alkyl side chain (Fig. 21) [107,108],... 

Fig. 21 LC semiconductor containing an oxetane moiety, which retains highly ordered smectic phase around room temperature...

Such a mobility does not exist in layered LC-main chain polymers (Fig. 1 IB). The mobility required for the definition and existence of a molten state results in LC-main-chain polymers exclusively from a gliding of chains along each other. Such a motion is only possible when the intermolecular forces between the mesogens are relatively weak. Therefore only smectic -A and smectic -C phases are true LC-phases. In contrast to small molecules smectic -B (and higher ordered smectic phases) are solid mesophases. The difference between a solid smectic mesophase and a smectic crystalline phase lies in the extent of the three dimensional order and is usually difficult to determine experimentally (see Sect. 7). 

The use of chiral H-bonding components yields stable chiral phases by molecular association [35-37]. For example, a ferroelectric phase derived from a chiral smectic C phase can be induced by the self-assembly of nonferro-electric molecules [35-37]. Complex 11 (n = 8) exhibits a chiral smectic phase from 109 to 123 °C [35]. In this case, the H-bonded mesogen consists of nonmesomorphic chiral benzoic acid and a stilhazole that shows a very narrow temperature range of ordered smectic phases. The value of spontaneous polarization for 11 at 115 °C is 33.0 nC/cm. When the oxygen in the alkoxy... 

The interaction between phenol and pyridine has also been used to form mesomorphic complexes 18-20 [55-57]. A 1 1 mixture of 4-cyanophenol and a 4-alkoxy-4 -stilbazole shows a monotropic nematic phase, while the stilbazole exhibits a very narrow temperature range of ordered smectic phases [58] and 4-cyanophenol is nonmesogenic. Complexes 19, based on... 

It is generally observed that as longer spacer units are introduced, the Tg of the polymer is lowered by internal plasticization and the tendency for the more ordered smectic phase to develop is increased. A similar ordering effect is also encouraged by lengthening the alkyl tail unit (see Table 5.7). Both these phenomena reflect the known tendency for long side chain to order and eventually to crystallize, when sufficiently long, and this condition persists also in the liquid crystal state. 

X-ray measurements [7, 11, 13-15, 17, 19, 20] performed on combined LC polymers show that the LC phases are analogous to low molar mass liquid crystals, which also show nematic, smectic A, smectic C and higher ordered smectic phases at different temperatures (see Figure 8). 

Similarly, the two structural units of poly[ 8-[(4 -(2/ ,35)-2-fluoro-3-methyl-pentyloxycarbonyl)-3 -fluorophenyl-4"-phenoxy)octyl]vinyl ether)-co- 11-[(4 -(2/ ,3S)-2-fluoro-3-methylpentyloxycarbo-nyl)-3 -fluorophenyl-4"-phenox.y)unde-cyl]vinyl ether are isomorphic within both the SmC and SmA mesophases, but not the more ordered phases (Fig. 24 b) [139]. Therefore, copolymerization can be used to eliminate the crystalline and/or more ordered smectic phases of the corresponding homopoly mer(s). 

Compared to the corresponding mono-mesogenic copolysiloxane, the dimesogenic copolysiloxane depicted in Fig. 10 has nearly the same SmC -SmA and SmA-iso-tropic transition temperature. However, due to an increase of the local concentration of mesogens, the more ordered smectic phase SmX occurs at a higher temperature. 

SmX-SmC transition temperature, SmX high order smectic phase. 

Physical properties of the higher ordered smectic phases SmF, SmI, SmB, SmH etc. have not been investigated as yet. 

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