Thermotropic polymers, banded textures

Above 110 °C, this arrangement becomes mobile, and a smectic C liquid-crystalline phase is entered. Samples cooled down from the isotropic melt (140 °C) show Schlieren and banded textures when viewed under crossed polarizers (Figure 8). These textures look similar to nematic Schlieren textures, but from the X-ray diffraction data it is clear that 12c forms a homeotropically oriented smectic C phase. In a nematic phase, the small-angle diffraction peak would be absent, and a broad scattering feature, a nematic streak , would be observed. Polymer 12c was the first example of a PPE derivative for which three states of matter, i.e. crystalline, thermotropic liquid crystalline, and a highly viscous isotropic liquid, were accessible [46]. 

Harrison, P. Navard, P. Cidade, M.T. Investigation of the band texture occurring in acetoxy-propyl cellulose thermotropic liquid crystalline polymer using rheo-optical, rheological, and light scattering techniques. Rheol. Acta 1999, 38 (6), 594-605. 

The pleated sheet can also be observed with optical polarization microscopy [119, 133]. Fig. 15 demonstrates the origin of the various banded patterns in the polarizing microscope, which are seen on a longitudinal section of a fiber. In the dark areas the chains are aligned parallel to the analyzer or polarizer direction. Such a banded texture is often observed in films and fibers of lyotropic and thermotropic polymers. 

Kwiatkowski Miroslaw, and Hinrichsen Grete. A new banded texture in sheared thermotropic liquid crystalline polymers. J. Mater. Sci. 25 no. 3 (1990) 1548-1550. 

Banded texture is generally observed in relaxed polymer liquid crystal solutions or melts after shearing or annealing of the melts of the thermotropic polymer liquid crystal. For the cholesteric liquid crystalline phase of cellulose derivatives in crosslinkable solvents, the banded texture can be fixed by crosslinking. When polymerizable solvents were used for the preparation of cholesteric liquid crystalline composites films, the... 

Banded textures are observed in different oriented main-chain liquid crystalline polymers, e.g. in Kevlar fibres and in other lyotropics and thermotropics. The reason for the banding is that molecules after shearing are oriented according to a serpentine. The average direction of the serpentine is parallel to the shear direction. The exact mechanism for the generation of the serpentine structure is not known. 

One of the most intriguing orientation phenomena seen in rigid-chain liquid crystal polymers is that of banding. The texture appears in samples which have been sheared and oriented in extensional flow, as contrasting bands normal to the direction of extensional flow. The phenomenon was observed in Kevlar fibres by Dobb et al It has since been reported in other lyotropic liquid crystal polymers " and in thermotropic random copolymers. In the latter studies a... 

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