Optical textures

Mixed- Isotropic or Anisotropic -variety of shapes or porosities, char wall thickness and/or optical texture. 

Note 4 At one time, a number of mesophases were identified as smectic on the basis of their optical textures, but they are in fact soft crystals characterised by very low yield stresses. Hence, these three-dimensionally ordered phases should no longer be called smectic mesophases. They are akin to plastic crystals with some elementary long-range order and are referred to by the letters E, J, G, H, and K. 

Note 1 Diselinations are responsible for some optical textures seen with a polarizing microscope, such as the schlieren texture formed by disclination lines in nearly vertical orientations, whose projections are seen as dark points with two or four emerging dark stripes or brushes (see Definition 4.9.2). 

Image of a liquid-crystal sample seen with a mieroseope, usually with crossed polarizers. Note An optical texture results from surfaee orientation of the directors at the boundaries of the sample and by defects formed in the sample. 

Note 1 The nematic liquid crystal must have a negative dielectric anisotropy (Af < 0), and a positive anisotropy (Aa > 0). The optical texture corresponding to the flow pattern consists of a set of regularly spaced, black and white stripes perpendicular to the initial direction of the director. These stripes are caused by the periodicity of the change in the refractive index for the extraordinary ray due to variations in the director orientation. 

Note 2 A liquid crystal in DSM has a complicated optical texture. 

As in the case of low-molecular liquid crystals the majority of information about the structure of LC polymers is obtained from their optical textures and X-ray diffraction data. Because of high viscosity of polymer melts, which results in retardation of all structural and relaxation processes it is quite difficult to obtain characteristic textures for LC polymers. As is noted by the majority of investigators smectic LC polymers form strongly birefringent films as well from solutions, as from melts11 27-... 

Smectic phases described up to date in the literature, are restricted mainly to A, B and C phases (SA, SB and Sc — respectively) for low-molecular liquid crystals at the same time, there are already reported around ten smectic phases. Identification of polymeric smectics according to their optical textures is, with rare exceptions, impossible, as their textures are quite alike. 

The main feature identifying a cholesteric mesophase in polymers is the presence of optical texture with selective circularly-polarized light reflection. This indicates the formation of 1-helical cholesteric structure in LC copolymers. The X-ray patterns of actually all cholesteric copolymers described (with the exclusion of polymers 3.1 and 4.1, Table 13) correspond to those of nematic and cholesteric low-molecular liquid crystals, which is manifested in a single diffuse reflex at wide scattering angles. At the same time, for copolymers 3.1 and 4.1 (Table 13) small angle reflexes were observed 123), that are usually missing in low-molecular cholesterics. 

Figure 5. Optical textures corresponding to Figures 4a and 4c fiber diagrams, a, Unaligned at 165 °C and b, fiber. Original magnification, 320 x.

LamIso, LamN, and LamSm phases, where the layers of the rod-like molecular cores are separated by the layers formed by the disordered semiperfluorinated lateral chains. In the LamSm and LamN phases there is orientational correlation of the aromatic cores in adjacent layers, leading to birefringence and typical optical textures. The absence of a long range positional correlation of the in-plane periodicity of the layers in the LamSm phase (LamSm/dis phase) indicates only a weak coupling of these layers by the layers of the fluid chains. 

Akopova reported liquid crystalline phthalocyanines 116ac (Scheme 63) with eight peripheral crown ethers devoid of alkyl chains [132]. Nonetheless, discotic nematic phases could be observed as derived from miscibility experiments with the discotic nematogen hexa(cyclohexanebenzoyloxy)triphenylene and optical textures. In the case of metal-free 116a, a phase width of 43 K was found. Complexation with Zn(II) (116b) decreased the phase range to 35 K and with... 

In situ X-ray examination of crystallizing polyethylene, at high temperature and pressure, then confirmed this proposal in detail, showing that the wide-angle diffraction pattern changed abruptly with the optical texture [ 10]. That corresponding to the spherulitic texture was of the usual orthorhombic form while the new intermediate phase had two-dimensional hexagonal symmetry, with an increased cross-sectional area per chain, but without... 

Fig. 1 The differing optical textures, between crossed polars, of linear polyethylene after crystallization from the melt at pressures close to the triple point, 0.3 GPa (a) the conventional spherulitic texture of the orthorhombic phase (b) the coarse lamellar texture formed as the hexagonal phase then transformed to orthorhombic during return to ambient temperature and pressure from [ 14]...

The origins and development of the concept of carbonaceous mesophase, as derived from discotic aromatic nematic liquid crystals, enable applications to be made to industrial processes. The world availability of pitch materials is such that there is an abundance of pitch which produces cokes of little commercial value. A major incentive for research into the Chemistry of mesophase formation is the commercial up-grading of such pitches and the development of specialized pitches. Structure in cokes is described in terms of optical texture. 

The importance of viscosity of pyrolyzing pitch in controlling size of optical texture is stressed. 

Figure 3. Optical micrograph of a coke surface showing an optical texture of fine-grained mosaics <1.5 ym diameter, OTI = 1.

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