Bilayer structures, smectics

Similarly, N-allcylammonium [28] and alkylphosphonium [29] salts form lamellar phases with smectic bilayer structures. In both cases. X-ray scattering also showed the isotropic liquid not to be completely disordered and still displaying similar features to the mesophase. Buscio et al. [28] showed that in N-allcylammonium chlorides the feature was not only much broader than that observed in the mesophase but increased in width with decreasing chain length. 

We note that the bilayer smectic phase which may be formed in main-chain polymers with two odd numbered spacers of different length (Fig. 7), should also be polar even in an achiral system [68]. This bilayer structure belongs to the same polar symmetry group mm2 as the chevron structure depicted in Fig. 17b, and macroscopic polarization might exist in the tilt direction of molecules in the layer. From this point of view, the formation of two-dimensional structure of the type shown in Fig. 7, where the polarization directions in neighbouring areas have opposite signs, is a unique example of a two dimensional antiferroelectric structure. 

It is interesting to point out here that with all of the theoretical speculation in the literature about polar order (both ferroelectric and antiferroelectric) in bilayer chevron smectics, and about reflection symmetry breaking by formation of a helical structure in a smectic with anticlinic layer interfaces, the first actual LC structure proven to exhibit spontaneous reflection symmetry breaking, the SmCP structure, was never, to our knowledge, suggested prior to its discovery. 

Comparson of the transitions observed by differential scanning calorimetry in membranes of M. laidlawii and in water dispersions of the lipids from the membranes support the concept that most of the lipids exist as a smectic mesophase in the membranes. The evidence for a bilayer structure is straightforward in this case. Lipid transition temperatures are a function of fatty acid composition and correlate well with biological properties. The calorimeter possesses advantages over high resolution NMR for M. laidlawii, and perhaps in many other systems, because the data can be interpreted less ambiguously. In M. laidlawii membranes the bilayer appears to be compatible with the same physical properties observed in other membranes—a red-shifted ORD, lack of ft structure in the infrared, reversible dissociation by detergents, and poorly... 

Fig. 41 Schematic drawing of the bilayer structure of the smectic A phase of multipede 39. The 16 mesogenic units (cylinders and spheres which represent the cyano groups) per molecule are accommodated in the layers without the introduction of curvature in the packing of the mesogenic units together...

Figure 12.3 X-ray diffraction pattern of aligned PCBT and interdigitated bilayer structure of smectic A phase...

Monomer B5A exhibits two enantiotropic smectic mesomorphic phases (Se66Sb75I). The S phase does not crystallize even when cooled down to -20 "C. In this study, we will concentrate on the photopolymerization of B5A at room temperature (Sg phase) and the detailed structural characterization of the resulting polymeric network (subsequently called PB5A-RT). The synthesis and structural characterization for monomer B5A will be presented in a separate paper and only summarized results are presented here for comparison with its polymer. Monomer B5A in the smectic E phase has an orthorhombic structure with unit cell dimensions, a=6.609 A, b=7.704 A and c=51.92 A. It adopts a tilted molecular bilayer structure within the unit cell in the c direction. Within each smectic layer the molecules are tilted, making an angle of about SO with the smectic layer normal (or c direction). 

Figure 13. Bilayer structure.s of the tilted smectic C phase.

The nematic phase of all the compounds CBn is characterized by a coherence length of about 1.4 times the elongated structure of the molecule. Based on this behaviour local associations in form of dimers with cyano-phenyl interactions were postulated. For the smectic A phase a partial bilayer arrangement of the molecules (SAd) is most likely. But there are also example for the smectic A phase with a monolayer (Sai) or a bilayer (Sa2) arrangement of the molecules as well as a commensurate structure A large number of X-ray measurements were carried out in the liquid crystalline state to clear up the structural richness and variability (see Chap. 2, this Vol. [52]). 

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