Liquid, definition smectic

They found that an asymmetric carbon atom and a dipole moment in the molecular structure lead to a liquid crystal smectic C phase in which the molecular long axes are tilted from the layer normal of the smectic phase. Hence, ferroelectricity was expressed. The ferroelectric behavior definitely was confirmed by various measurements, including the D-E hysteresis [2]. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Note 4 The tilt direction varies in a random manner from layer to layer in conventional smectic C mesophases. However it can alternate from layer to layer, as in an antiferro-electric chiral smectic C mesophase (see Definition 5.9, Note 7) and in the smectic C mesophase formed by certain liquid crystal dimers with an odd-number of carbon atoms in the spacers. The recommended symbol for this type of mesophase is SmCa. 

Note 4 A thin sample of a smectic C phase with the layers parallel to the sample surfaces gives schlieren textures with centers that have four brushes. However, a smectic C phase formed by odd-membered liquid-crystal dimers (see Definition 2.11.2.9, Note 5) has schlieren textures with two or four brushes. 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

However, the intrinsic features of black films require new investigation techniques and, in some cases, new theories for the explanation of their stability, in particular for bilayer films. In the behaviour of the latter the short-range molecular interactions prove to be of major importance. The definition liquid is hardly valid for bilayers. They possess higher degree of ordering similar to that of smectic liquid crystals. 

Until now there was no obvious correlation found between the monomer structure and the resulting pol qner phase. No.theorr retical structural conditions were described which would result in a liquid crystalline polymer with a definite ordered phase e.g. with a nematic a smectic or a cholesteric phase as in conventional liquid crystals. Although previous examples have established (8 9) the existence of enantiotropic liquid crystalline side chain polymers additional considerations are in order for a systematic synthesis of such polymers. 

Liquid crystals appear to constitute a further step in the orientation of liquids. Although we are unable today to systematize finally the existing experimental material and the somewhat contradictory views of the authors working in this field, there appear to be two definite arrangements of particularly frequent occurrence, named by G. Friedel the smectic and the nematic state. 

The microstructure of an ER system definitely determines how this system behaves under an electric field. Figure 22 shows the shear stress of oclylcynaobiphenyl vs. the electric field at shear rate 329.5 s " and various temperatures. As indicated in the literature [70-73], Oclylcynaobiphenyl is a liquid crystal material, and has a phase transition from the smectic to the nematic phase at 306.72 K and from the nematic to the isotropic phase at 313.95 K. With the increase of temperature from 306.6 K to 312.8 K, oclylcynaobiphenyl may have the different structures marked as a to b [73]. The ER property of oclylcynaobiphenyl should depend on how the director is orientated in the fields, fhe shear stress passes through a maximum value when the liquid crystal material is in the smectic phase state. Once the material is in tlie nematic phase state, the ER effect becomes weak and saturates at the electric field strength above 0.7 kV/mm. 

We start with the microscopic definitions and discussion of the nematic and smectic order parameters and then proceed with some elementary information about anisotropic intermolecular interactions in liquid crystals. Then we discuss in more detail the main molecular theories of the nematic-isotropic phase transition and conclude with a consideration of molecular models for smectic A and smectic C phases. 

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