Smectic A mesophase

The effect of lateral methyl groups in the spacer on the phase behavior has been studied in several polybibenzoates [18,19] derived from poly(tetramethy]ene p,p bibenzoate), P4MB. The branched polymers display transition temperatures significantly lower than P4MB. Moreover, the substituents have a clear effect on the kind of mesophase formed. Thus, P4MB displays a smectic A mesophase, while the lateral methyl groups... 

In this section we will report on the crystal structure analyses of mesogenic 2,5-diphenyl pyrimidines. The crystal structure of 5-phenyl-2-(4 -n-butoxy-phenyl)-pyrimidine (5-PBuPP) and 2-phenyl-5-(4 -n-pentoxyphenyl)-pyrimi-dine (2-PPePP) were determined by Winter et al. [83, 84]. Compound 5-PBuPP forms a monotropic nematic phase, whereas compound 2-PPePP exhibits a smectic A mesophase within a wide temperature range. The chemical structure of the mesogenic 2,5-diphenyl pyrimidines is shown in Fig. 13. 

In the smectic mesophases the molecules are oriented, as in a nematic mesophase, with their principal axis roughly parallel to the director, but they are also defining layers. These layers can be perpendicular to the director, as in the smectic A mesophase (SmA), or tilted, as in the smectic C (SmC). The SmA and SmC mesophases are the less ordered and more common smectic mesophases. Other less common types of smectic mesophases are known, which differ in the degree or kind of molecular ordering within and between the layers [2]. 

Baars et al. functionalized polypropylene imine) dendrimers (generation 1, 3, 5) with pentaoxycyanobiphenyl and decyloxycyanobiphenyl mesogens [132], All dendrimers were found to exhibit smectic A mesophases. The SA-layer... 

Figure 16.10 Conformational changes of polypropylene imine) dendrimers in LC materials from spherical to (a) cylindrical, in hexagonal columnar mesophase [130], and (b) ellipsoid, in smectic A mesophase [132]...

Note Two types of short-range smectic-like structures are possible. One is analogous to a smectic A mesophase where the molecules tend to lie along a layer normal and the other is like a smectic C mesophase where the molecules tend to be oblique with respect to a layer normal. See Fig. 2 for illustrations of the molecular arrangements in the smectic A type structure and the smectic C type structure. 

Smectic mesophase involving a parallel arrangement of the molecules within layers in which the long axes of the molecules tend to be perpendicular to the layer planes and the molecular centers of mass have no long-range positional order parallel to the layer planes. Note 1 See Fig. 5 for the molecular organization in a smectic A mesophase Note 2 Each layer approximates to a true two-dimensional liquid. The system is optically uniaxial and the optic axis, Z, is normal to the layer planes. 

Note 4 The structure of a smectic A mesophase is characterised by the symbol Dooh in the Schoenflies notation (oo, 2 in the International System). 

Note 5 The lyotropic equivalent of a smectic A mesophase is known as a lamellar mesophase where layers of amphiphilic molecules are separated by layers of solvent, normally water, or by oil in an inverse lamellar mesophase. 

Note 3 The tensorial properties of a biaxial mesophase have biaxial symmetry unlike the uniaxial symmetries of, for example, the nematic and smectic A mesophases. 

Smectic A mesophase composed of board-like molecules with the longer and the shorter molecular axes orientationally ordered. 

Defect-stabilised mesophase created when a smectic A mesophase is subjected to a twist or bend distortion. 

Note 3 Two TGBA structures are possible in one, the number of blocks corresponding to a rotation of the layer normal by 2ti is an integer, while in the other, it is a non-integer. Note 4 A TGBA is found in a phase diagram between smectic A and chiral nematic mesophases or between a smectic A mesophase and an isotropic phase. 

The same reaction carried out with amines leads to diaminocarbenes (Figure 7.12) [11]. Only complexes prepared from primary amines show smectic A mesophases (melting points from 113 to 137 °C), although they gradually decompose before the clearing points. 

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

In the non-amphiphilic smectic A mesophase (Figure 2b), the parallel, fairly rigid, lath-like molecules are grouped with orientational disorder (at right angles to their long axes) and end-to-end so that the molecules lie statistically normal to the sheets, constituting an optically uniaxial... 

Stabilization of the mesophase was observed as the degree of polymerization was increased. The Tg values of the poly(norbornene)-polymers were about 30 °C higher than those of the poly(butadiene) polymers. Both polymers showed similar isotropization temperatures, but they differed substantially in their liquid crystalline behaviors. Poly-(IX-n)s with a poly(norbornene) backbone exhibited textures typical of nematic mesophases, whereas the poly-(butadiene)-based polymers poly-(X-n) displayed textures representative of smectic A mesophases. The more flexible backbone of poly(butadiene) allowed a higher order of alignment of the mesogenic units, resulting in the more ordered liquid crystalline smectic A phase. 

Recently we studied several polymers of structure 36 in Table 1 that were identified by texture observation as being smectic X-ray diffraction patterns showed a spacing at approximately 5 A and others at 28, 31, and 29 A for the polymers with 9,10, and 12 methylene groups in the spacer, respectively. Such repeat lengths are not consistent with the presence of fully extended chains in the smectic A mesophase for these polymers. Instead, the polymer with a nonamethylene spacer appears to form a smectic C phase in which the polymer chain is tilted with respect to the layers, as suggested by the small interlayer distances. 

The complex with [Li+]/[PO] = 0.10 exhibits a crystalline melting transition followed by a smectic A mesophase. By increasing the salt concentration as in the case of complexes with [Li+]/[PO] = 0.15 0.30, the smectic A phase is suppressed instead, they exhibit a hexagonal columnar mesophase as evidenced by X-ray scattering. The induction of ordered structure in the melt state of the rod—coil molecule by complexation is most probably due to enhanced microphase separation between hydrophobic blocks and polypropylene oxide) block caused by transformation from a dipolar medium to an ionic medium in polypropylene oxide) coil. 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

It is interesting to note that the mesomorphic behavior of these lanthanides carboxylates is much closer to the behavior of alkaline and alkaline earth soaps than to the transition metal soaps described above. However, it is less complex since only one smectic A mesophase is observed, although sometimes a second mesophase, which has not been identified by X-ray diffraction, is present for the short-chain homologs. Such a simple behavior has been attributed to the high carboxylate-to-metal ratio, which makes the ionic layer more rigid. 

Fig. 6.3 First-generation GLCs based on the core-pendant strategy accompanied by DSC data from second heating seans. In addition to the symbols defined in Fig. 6.2, Sa denotes smectic A mesophase. Used with permission [24]...

By comparison of the two series, which differ in the presence of a phenyl ring and oxadiazole moiety (36a—g) instead of an isoxazole ring (35a—g), the authors conclude that the presence of the 1,2,4-oxadiazole ring increases the polarizabihty of the molecule, favoring the formation of a smectic A mesophase (2008ARjC157). 

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