Liquid crystalline polymers optical textures

These different contrast mechanisms can all be used to reveal the scale of liquid crystalline polymer microstructures. In specimens that exhibit a mosaic texture, and in those that contain predominantly planar defects, domain size is easily defined in terms of areas that uniformly show extinction between crossed polars. However, if the defects are predominantly linear, as in specimens that exhibit schlieren textures, such simple characterization of microstructural scale is no longer possible. Here it is more convenient to look at the length of disclination line per unit volume, which is equivalent to the number of lines intersecting unit area, and analogous to the dislocation density as defined for crystalline solids. Good contrast is essential in order to obtain an accurate count. Technologically, microstructural scale is of growing interest because of its relevance to processability, mechanical properties and optical transparency. 

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. 

Because of the additional translational order, the dislocations can exist in the cholesteric and smectic liquid crystals, which makes the texture of these liquid crystals even more complicated. Each liquid crystal phase shows characteristic textures and thus the optical texture becomes an important means to differentiate the phase of the liquid crystals. Liquid crystalline polymers have the same topologically stable defects as small molecular mass liquid crystals do, but the textures may be different due to the difference in the energetic stability of the same topological defects in both low molecular mass and polymeric liquid crystals (Kleman, 1991). In Chapter 3 we will discuss the textures in detail. 

Conoscopy provides an extremely sensitive method with which to determine the degree of biaxiality. By the early 1990 s, conoscopic measurements had already indicated the presence of phase biaxiality in a nematic side-on liquid crystalline side-chain polymer [9]. However, the method s sensitivity is also its weak point because surface effects may induce optical biaxiality in an actual uniaxial system. For this reason, deuterium NMR was used to confirm phase biaxiality in a liquid crystalline polymer system similar to the one investigated with conoscopy by Leube [11-13]. Due to the fairly high viscosity of the polymeric samples, the tilt experiment, employed by Yu and Saupe to show phase biaxiality in a lyotropic liquid crystal [4], was used. The results obtained in this way are in good agreement with observations of optical textures in a biaxial cholesteric copolymer [16], where phase biaxiality disturbs the smooth optical periodicity of the cholesteric phase structure. 

Except for the structure containing 100% of ferrocene unit, which decomposed before melting, all the organometallic copolymers exhibited birefringent melts. Nematic textures were identified by means of polarized optical microscopy and, in one case, by X-ray diffraction studies. For comparison purposes, a polymer without ferrocene unit was prepared, but showed no mesomorphism. The authors deduced that the ferrocene framework was contributing to the liquid crystallinity of the ferrocene-containing polymers. 

A second way to obtain liquid crystalline textures consists in heating up polymer samples, which display liquid crystalline phases, between glass slides or under the influence of external forces such as electric or magnetic fields. Textures will then form in a way similar to the case of low molecular weight liquid crystals. This was observed for the case of the polymers 1. On cooling down the textures can be frozen in since a glass transition occurs. Thus it becomes possible to keep a specific texture with interesting optical properties permanently. 

Moreover, in direct correspondence between structure, energy flow, and photophysics, pure PA exhibits minimal or no fluorescence, whereas functionalized acetylenes [206,207] or phenylacetylenes [208] can be highly emissive with some derivatives exhibiting photoluminescent yields that approach unity. Liquid crystallinity is often observed with the formation of various nematic or smectic phases. These can be easily inferred through polarized microscopy and the appearance of Schlieren and related mesomorphic textures. Functionalization with chiral-branched substituents [209,210] leads to optically active polymers. 

Figure 3.14 Optical microscopy photograph showing the smectic texture of liquid crystalline epoxy. Reprinted with permission from K. Sadagopan, D. Ratna and A.B. Samui, Journal of Polymer Science Part A Polymer Chemistry Edition, 2003, 41, 3375 2003, John Wiley and Sons Publishers...

Polymer P25/26 self-orders in solvent-cast films, with the hackhones parallel to the substrate and a strong solvent dependence of the degree of ordering (78). Spacings of 2.2-2.G nm are observed by x-ray diffraction, indicating interdigita-tion of the dendritic side chains. P25 6 also forms thermotropic nematic liquid crystalline phases. With optical microscopy, Schlieren textures are observed for thin films cast from solution. 

In fact the liquid-crystalline behavior of low molecular weight Py AG-18 appears to be much more complex [29]. Up to three types of textures were observed under the polarizing optical microscope for a 20 K sample of this polymer as a function of temperature (Figure 7). The low temperature lamellar... 

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