Lyotropic polymers, banded textures

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. 

The HPC lyotropic solutions are, regarding the ability to form band texture, among the most studied systems and thoroughly described in the literature [16, 18]. The formed periodic structures can be locked within the polymer after the solvent evaporatiOTi, if the evaporation time is shorter than the relaxation time t, and the topography can be tuned by control of the processing conditions [19]. In the case of the anisotropic aU-ceUulosic based composites this was attained due to the small thickness of the films (20-60 pm). 

The self-organizing nature of liquid crystal polymers is reflected in their complex flow behavior. The relaxation phenomenon of lyotropic polymer solution after shear cessation leads to band texture morphology that can be further induced to isotropic materials. Future research should focus on solvents influence on band size implicitly related to the induced pattern in polymers with different structures. Another aspect that could be explored is the imidization of patterned polyimide precursors and those conditions in which the texture is still maintained. 

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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