Smectic phases

In this section we will discuss in some detail the application of X-ray diffraction and IR dichroism for the structure determination and identification of diverse LC phases. The general feature, revealed by X-ray diffraction (XRD), of all smectic phases is the set of sharp (OOn) Bragg peaks due to the periodicity of the layers [43]. The in-plane order is determined from the half-width of the inplane (hkO) peaks and varies from 2 to 3 intermolecular distances in smectics A and C to 6-30 intermolecular distances in the hexatic phase, which is characterized by six-fold symmetry in location of the in-plane diffuse maxima. The lamellar crystalline phases (smectics B, E, G, I) possess sharp in-plane diffraction peaks, indicating long-range periodicity within the layers. 

SmB SmC SmC SmCA SmCPA SmCPp SmCo, SmIA SmX UCST XRD Smectic B phase Smectic C phase (synclinic tilted smectic C phase) Chiral (synclinic tilted) smectic C phase Chiral anticlinic tilted (antiferroelectric switching) SmC phase Antiferroelectric switching polar smectic C phase Ferroelectric switching polar smectic C phase Chiral smectic C alpha phase Chiral antiferroelectric switching smectic I phase Smectic phase with unknown structure Upper critical solution temperature X-ray diffraction... 

The phase transition temperatures of compounds 7a and 7b are summarized in Table 2. Compound 7a shows a highly viscous smectic (denoted as Sml) phase, smectic (denoted as Sm2 and Sm3) phases with lower viscosity, and an SmA phase. On the other hand, compound 7b shows a highly viscous smectic (denoted as Sm4) phase and a nematic phase. 

In the crystalline version of smectic B, there is positional as well as bond-orientational order, and this ordering is called hexagonal, as opposed to hexatic. In the tilted version of the hexatic smectic B, there are two different directions the tilt has been found to take with respect to the hexagonal bond orientation, namely toward nearest and toward next nearest neighbors, producing the two different smectics I and F (see Fig. 10-27). Similarly, there are two different tilted crystalline hexagonal phases, smectics J and G. Even in smectic crystals, the molecules retain freedom to rotate cooperatively about theii axes, hence smectic ciystals are not as solid as most simple crystals. Nature is wonderfully creative in contriving... 

Abbreviations K = crystalline phase. D = columnar phase, Sh = smectic B phase. = smectic E phase. S.-N - nematic phase. I = isotropic fluid, Tj = thermal decomposition. aa and a/ refer to isomers of porphyrin. 

Smectic A and C phases are the most common smectic phases. They exist in several texture variants. Similar to the nematic phase, smectic A phase can exhibit the homogenous texture. The corresponding texture of the smectic C is the schlieren texture, In this texture, the layers are parallel to the sample surface. The schlieren indicates strong distortions of the director field [1]. 

Nematic Phase Solid" Phase Smectic" Phase Model for "Solid" Phase or "Smectic" Phase... 

Planar-to-Homeotropic Structure Transition under Electric Field in an A-phase Smectic, Appl. Phys. Lett. Vol. 27, No. 11, December 1, 1975, pp. 575-576. 

The experimental evidence led to the conclusion that the onset of the glass transition is accompanied by phase transformation in the stmcture of the as-spun CPE-1 fibers. This phase transformation is accomplished at 250° C. Only at this temperature the development of crystalline phases in CPE-1 becomes well pronounced (Figure 8d). What is the origin of this transition at 180-250°C, which is concurrent with the glass transition of copolyester (the temperature interval of the glass transition as detervined by DMR and DSC scans from 110 to 230°C) Let us analyze the experimental evidence obtained in terms of the hypothesis of a single-phase smectic LC stmcture in the as-spun CPE-1 fibers. 

Thermotropic liquid crystallinity is also presented by alkylphosphonium halides. The influence of covalently attaching hydroxymethylene to the methyl groups of methyl-tri-n-alkylphosphonium halides (the alkyl chains are decyl, tetradecyl, or octadecyl and the halide is chloride or bromide) or adding methanol as a solute to the salts on their liquid crystalline phase (smectic A2) has been studied by various experimental techniques, including NMR for determination of dipolar splittings. These systems can be used as hosts to determine solute structure from NMR spectroscopic measurements. ... 

Smectic Liquid Crystals. These liquid crystals have two-dimensional order they are the most perfect liquid crystals. The rodlike molecules form layers, and often there is some mobility of the molecules inside the layers, but these layers can also move relative to each other. Depending on the arrangement of the molecules in the smectic plates, several different smectic phases are known, and sometimes transitions can be observed between these smectic phases (smectic A, B, C, etc.). Most smectic liquid crystals become nematic at a certain temperature before changing into an isotropic liquid at the clearing point. 

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 ... 

The symmetry arguments for achiral or chiral smectic C phases can also be applied in similar ways to the other smectic and soft crystal smectic phases. For example, all of the tilted phases (smectics I and F, and crystal phases J, G, H, and K ) would have broken symmetries leading to polar noncentrosymmetric structures resulting in ferroelectric properties. Even orthogonal phases, such as the smectic A phase would have different symmetries for the chiral versus the achiral forms. For instance, the smectic A phase has symmetry, where-... 

I phases, smectics 16 Idemitsu polymer FLC 651 irainomethyl, spacers 846 iminopyridine complexes, metallomesogens 905 improper ferroelectrics 537 f improved Alt-Pleshko addressing technique (lAPT) 207... 

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