Smectic L phase

An exception to the rule that lowering the Iciiiperaiure cutises transitions to phases with increased order sometimes occurs for polar compounds w hich form the smectic l. phase (a layered structure formed by molecular dimers). Decreasing the temperature cutises u transition front nematic to smectic. 1,.,. but a further lowering of the temperature produces J transition back to the nematic phase tcalled the reentrant nematic phase). Electric or magnetic fields also may induce mesomorphic phase transitions. 

Due to special symmetry conditions, the presence of chiral molecules in tilted smectic phases leads to the appearance of a spontaneous electric polarization within each smectic layer [1], [2], Magnitude and direction of the polarization are coupled to the magnitude and direction of the molecular tilt giving rise to imique properties of tilted as well as orthogonal smectic phases of chiral molecules in external electric fields. In this chapter, the basic properties of smectic-C and smectic- l phases of chiral low-molecular weight molecules are summarized, mainly from the viewpoint of physics. 

Quite often both phases, smectic- and C, appear in the same compound, smectic-C then being the low-temperature phase to smectic-. There are, however, many compounds possessing a smectic- l phase without imderlying smectic-C phase also compoimds showing a direct transition from smectic-C to nematic or even isotropic are known. The smectic- l-smectic-C phase transition is in the vast majority of compounds a second-order transition, a few substances showing first-order transitions are known [7], [8]. The transition can be described by a complex order parameter T ... 

L. The liquid-expanded, L phase is a two-dimensionally isotropic arrangement of amphiphiles. This is in the smectic A class of liquidlike in-plane structure. There is a continuing debate on how best to formulate an equation of state of the liquid-expanded monolayer. Such monolayers are fluid and coherent, yet the average intermolecular distance is much greater than for bulk liquids. A typical bulk liquid is perhaps 10% less dense than its corresponding solid state. 

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]. 

McMillan W L 1971 Simple molecular model for the smectic A phase of liquid crystals Phys.Rev A 4 1238-46... 

In 1986, Walz and Haase [148] presented the crystal structure of the mesogenic hydrocarbon compound l,2-bis-(4 -pentylcyclohexyl)ethane. The compound exhibits a smectic B phase over a remarkably broad range of temperature. To our knowledge, this is the only crystal structure determination of a mesogenic hydrocarbon compound up to now. Since this compound does not contain any polar groups, the arrangement in the crystalline state is... 

If Lp = Lh, the monolayer smectic Ai phase is formed. In the case that Lp > Lh, the layer spacing d exceeds the molecular length L, which corresponds to the formation of the smectic A phase (Fig. lla,b). In the opposite case, Lp < Lh, the layer spacing is less than L, and the flexible hydrocarbon chains just All the space which is determined by the fluorinated tails (Fig. 10c). 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

We designed and studied [27] a family of unsymmetrically l,l -disubstituted ferrocene derivatives obtained by combining the organic units A and B (above), used to prepare the families 13 and 14, respectively within the same molecular framework. Structures 25 (Fig. 9-21) led to remarkable mesomorphic properties. All derivatives exhibited liquid crystal properties. Compound 25 (n = 11) gave rise to an enantiotropic smectic A phase. Complex 25 (n = 12) showed an enantiotropic... 

In the smectic A phase, molecules tend to be perpendicular to the smectic layers. The layer thickness d is roughly the same as the molecular length l. The thickness of the layers in the case of liquid crystalline polymers is about the order of the monomer s length. But in the smectic C phase, the molecules in the layers are parallel and tilted in arrangement with respect to the normal of the layers by a tilt angle 0. The layer thickness of the smectic C phase is d = l cos 6. The ordering of the smectic A and C phases are both higher than the nematic phase so that they appear at a lower temperature than the nematics do. The smectic A phase appears first as the temperature decreases if a compound shows both the smectic A and C phases. 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

Plate 2. The contact regions between the (R)- and the (S)-l-methylheptyl 4 -(4-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates under a cover-slip on a microscope slide and between crossed polars (xlOO). The central region is where the two materials met and mixed. This region exhibits the normal focal-conic texture of the achiral smectic A phase. The left- and right-hand sides of the plate show the homeotropic and filamentary textures of the chiral TGBA phase... 

Fig. 25. The temperature dependence of the layer spacing for the dodecyl (C12) (filled circles), tetradecyl (C14) (open circles), and the hexadecyl (Cie) (triangles) homologues of the (R)-l-methylheptyl 4 -(4-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates (nPlM7 s). The inset shows scans through the layer peak for the tetradecyloxy homologue at temperatures within the smectic C phase (filled circles T = 82.8 °C), the TGBA phase (open circles T = 93.3 °C), and just into the isotropic liquid (triangles T = 97.4 °C)...

Fig. 40a-c. A schematic representation of two dimensional intensity maps (a) in the nematic phase, (b) in the TGB phase and (c) in the smectic C phase of 3-fluoro-4-[(R or S)-l-methylheptyoxy]-4 -(4-dodecyloxy-2,3-difluorohenzoyloxy)tolane... 

The Freedericksz transition in the nematic and smectic C phases of 3- -heptyl-6-(4- -hexyloxyphenyl)-l,2,4,5-tetrazine has been studied. In both phases, the threshold voltage and the switching times were measured. In the smectic C phase, two thresholds have been observed which can be explained by an asymmetric chevron structure <1996MI131>. 

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