Mesophase Phase Symmetry

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

The two tetracatenar complexes ( = 5, 12) exhibited a columnar phase, between 73 and 125 °C for the short-chain-length compound, and from room temperature up to 106 °C for the longer-chain derivative. The hexacatenar material (n = S) displayed an unidentified phase between 70 and 122 °C that was probably columnar due to the structural similarity with the previous compounds. These mesophase assignments were performed by microscopy only, and exact phase symmetry was not determined. 

In Eq. (1), only the relatively low C phase symmetry has been assumed. This is a very general requirement, which does not account for the nonpolarity and non-enantio-morphy of nematic phases. If the mesophase contains more symmetry elements, fewer invariants remain and the number of elastic parameters is reduced. In nonpolar systems such as cholesterics (D ), which are characterized by an additional two-fold symmetry axis perpendicular to the axis,, ATi 2... 

NMR spectroscopy can provide valuable information on the structure of mesophases. The most commonly used method for this is the analysis of line shapes in spectra of various nuclei, and, in particular, of H. For a large number of mesophases, NMR spectra are motionally averaged and the spectral line shape reflects the phase symmetry. Line shape analysis of NMR spectra has been employed successfully to investigate the molecular organization and tilt angle of smectic C phases, as well as the structure of columnar discotic phases and it has been shown to be more sensitive than optical techniques to some aspect of biaxial ordering. Moreover, this technique has been shown to be extremely helpful in the discrimination of lyotropic phase symmetry. As examples, NMR spectra of some lyotropic phases are reported in Figure 4. 

NMR is not the best method to identify thennotropic phases, because the spectmm is not directly related to the symmetry of the mesophase, and transitions between different smectic phases or between a smectic phase and the nematic phase do not usually lead to significant changes in the NMR spectmm [ ]. However, the nematic-isotropic transition is usually obvious from the discontinuous decrease in orientational order. NMR can, however,... 

It is further assumed that the mesophase layer consists of a material having progressively variable mechanical properties. In order to match the respective properties of the two main phases bounding the mesophase, a variable elastic modulus for the mesophase may be defined, which, for reasons of symmetry, depends only on the radial distance from the fiber-mesophase surface. In other words, it is assumed that the mesophase layer consists of a series of elementary peels, whose constant mechanical properties differ to each other by a quantity (small enough) defined by the law of variation of Ej(r). 

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

As in the case of LCP/conventional polymer blending, little data exists on the blending of LCPs of different inherent chain architecture or mesophase symmetry. Publications from the laboratories of Ringsdorf [80] and Finkelmann [81] show phase separation in blends of sidechain nematics with other similar polymers or small molecule analogs. It is now established that, in contrast to the behavior of low molecular weight LCs, LCPs are often immiscible. 

A similar though considerably less well-ordered lamellar product is obtained when ethanol or methanol are used instead of water under solvothermal conditions Figure lb shows the powder XRD diagram of a sample prepared in ethanol at 90 °C dm = 3.50 nm). Considerably different products, however, are obtained when alcohols are used at lower temperatures, i.e. under non-solvothermal conditions. Figures lc and Id show the diffraction patterns of two example products from syntheses in methanol at 25 °C and in ethanol at 10 °C, respectively. In both cases the XRD reflections can be attributed to two distinct phases. One of these has a hexagonal symmetry with a dm value of 1.88 nm this mesophase will be discussed in detail below. An additional broad reflection is found at a Bragg angle comparable to that of the 001... 

Lattermann found that the sixfold symmetry is not important for the presence of columnar mesophases in [14]-N4 when appropriate peripheral groups are chosen [105]. As substituents with one terminal alkyl (88a-d) chain and with three terminal alkyl chains (88e) are not able to induce liquid crystalline behavior in 88 (Table 5) [104], a substituent with two alkyl chains, namely 3,4-bis(alkyloxy) benzoyl, was chosen since it has been known to induce liquid crystalline phases in other systems. A mesophase (most likely Colh due to the texture) was observed between 96 and 132 °C for 90 (Scheme 44). 

In 5 (Fig. 4.20) steric hindrance in the peripheral region appears to be too high for formation of a liquid-crystalline phase. Mesophases were characterised by polarisation microscopy and X-ray diffraction. Presumably the LC properties cease as a result of segment mobility with increasing number of stilbene building blocks in principle, the number of conformers should double with each double bond although the maximum number of 2n (e.g. 221 for the third generation) is unattainable for symmetry reasons. 

A vast array of covalent molecules have been synthesised over the years in the search for LCs that show the useful cholesteric and ferroelectric smectic C phases, often on a trial and error basis ignoring the interactions between the molecules. The idea that one could think of the interactions between the molecules as a kind of molecular recognition came from the careful analysis of the conformations of molecules in the layers [77,78]. The arguments are based on the symmetry limitations of the angle formed by the alkyl chain and the phenyl benzoate moiety in the molecules that were the subject of this study. A molecular recognition site within the phase was used as the basis for these speculations , which have actually proved rather successful. The actual interactions between molecules are usually weak, but the formation of layers of aromatic and aliphatic units in these mesophases gives rise to their unique properties. 

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