Smectic H phase

FIGURE 21 Ordered tilted smectic phases (a) smectic F phase (b) smectic H phase. The smectic 1 phase has the same structure as smectic F but another tilt direction (not along the prism face but along the prism edge). 

An excellent example of counterion influence is the quite different thermal behavior of double-chain l-methyl-3,5-bis(n-hexadecyloxycarbonyl)pyridinium ion in crystals with iodide or chloride as counterion [4]. The iodide salt revealed three phase transitions solid crystalline-solid crystalline at —326 K, solid crystalline-liquid crystalline at —358 K, and liquid crystalline-isotropic liquid at —378 K. The X-ray diffraction pattern of the liquid crystalline phase could be best rationalized in terms of a smectic-H phase. The chloride anion could be unfavorable for liquid crystalline behavior because of its smaller ionic radius relative to the iodide anion. Less shielding of the positive charges of the pyridinium rings by the chloride counterion leads to increased electrostatic repulsion between headgroups. 

This region has been divided into two subphases, L and S. The L phase differs from the L2 phase in the direction of tilt. Molecules tilt toward their nearest neighbors in L2 and toward next nearest neighbors in L (a smectic F phase). The S phase comprises the higher-ir and lower-T part of L2. This phase is characterized by smectic H or a tilted herringbone structure and there are two molecules (of different orientation) in the unit cell. Another phase having a different tilt direction, L, can appear between the L2 and L 2 phases. A new phase has been identified in the L 2 domain. It is probably a smectic L structure of different azimuthal tilt than L2 [185]. 

The smectic A phase is a liquid in two dimensions, i.e. in tire layer planes, but behaves elastically as a solid in the remaining direction. However, tme long-range order in tliis one-dimensional solid is suppressed by logaritlimic growth of tliennal layer fluctuations, an effect known as tire Landau-Peierls instability [H, 12 and 13]... 

Gorecka E, Nakata M, Mieczkowski J, Takanishi Y, Ishikawa K, Watanabe J, Takezoe H, Eichhorm SH, Swager TM (2000) Induced antiferroelectric smectic-CA phase by doping ferroelectric-C phase with bent-shaped molecules. Phys Rev Lett 85 2526-2529... 

Kajitani T, Masu H, Kohmoto S, Yamamoto M, Yamaguchi K, Kishikawa K (2005) Generation of a chiral mesophase by achiral molecules absolute chiral induction in the smectic C phase of 4-octyloxyphenyl 4-octyloxybenzoate. J Am Chem Soc 127 1124-1125... 

Miyachi K, Matsushima J, Ishikawa K, Takezoe H, Fukuda A (1995) Spontaneous polarization parallel to the tilt plane in the antiferroelectric chiral smectic-CA phase of liquid-crystals as observed by polarized infrared-spectroscopy. Phys Rev E 52 R2153-R2156... 

In the framework of irreversible thermodynamics (compare, for example, [31, 32]) the macroscopic variables of a system can be divided into those due to conservation laws (here mass density p, momentum density g = pv with the velocity field v and energy density e) and those reflecting a spontaneously broken continuous symmetry (here the layer displacement u characterizes the broken translational symmetry parallel to the layer normal). For a smectic A liquid crystal the director h of the underlying nematic order is assumed to be parallel to the layer normal p. So far, only in the vicinity of a nematic-smectic A phase transition has a finite angle between h and p been shown to be of physical interest [33],... 

On passing from the least ordered smectic A phase down to the more ordered crystal H and K phases the layer planes tend to sharpen up. In the smectic A and C phases the layers are therefore very diffuse, and can be thought of as one-dimensional density waves relative to the director. Thus, locally these two phases are very similar in structure to the nematic phase. 

A large number of low molecular weight smectogens have been prepared and studied by X-ray diffraction methods At this time relatively few smectic polymers have been prepared and identified, and the smectic D, G, and H phases have not, as yet, been reported in polymers. 

The local symmetry of smectic A phase is the same as that of the nematics, be., its point group is D h, while the symmetry of the smectic C phase is ( b/, (a ( 2 symmetry axis plus a reflection plane perpendicular to the axis). In addition, both smectic phases exhibit a one-dimension translational order. Owing to the difference in symmetry, the smectic phases show different optical properties. The smectic A phase is optically uniaxial, but the smectic C phase is optically biaxial. 

Bouwman W G and de Jeu W H 1992 3D XY behavior of a nematic-smectic A phase transition confirmation of the de Gennes model Phys.Rev.Lett 68 800-3... 

Chen J-H and Lubensky TCI 976 Landau-Ginzburg mean-field theory for the nematic to smectic C and nematic to smectic A phase transitions Phys.Rev. A 14 1202-7... 

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 mesogenic structure of a benzoic acid dimer has been introduced as a noncovalent cross-linker for polysiloxanes [79]. Polymer 57 exhibits a smectic C phase due to the dynamics of H-bonding. In contrast, mesomorphic order is locked in the solid state of poly[(4-acryloyl)benzoic acid] by polymerization in its mesophase [128]. No liquid-crystalline state is observed for this material because of the lack of flexibility of the structures. Main-chain-type polymeric liquid-crystal associates are formed from carboxyl-bifunctionalized aromatic compounds [129]. 

The interaction of carboxylic acids affects the molecular order of side-chain polymer liquid crystals [130]. Copolymerization of an acrylate containing a cyanobiphenyl unit through butyl group and acrylic acid leads to the induction of a smectic A phase. The intra- and intermolecular H-bonds of carboxylic groups stabilize layered structures. 

Compound 64 exhibits only a smectic C phase [148]. In this case, the aUcylamino group introduced into this structure induces the association of H-bonded molecular chains and a smectic C arrangement, as shown in Fig. 21. 

Fig. 7. Chemical structure of a biforked compound and its corresponding thermotropic behaviour. K is the crystalline phase. Sc the lamellar smectic C phase, 4>h the colmnnar mesophase and 1 the isotropic phase...

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