Smectic texture, birefringence

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-... 

Optical observations of mesophase textures have been previously discussed in detail It is to be stressed here that they are generally in excellent agreement with thermal analysis data and indicate the existence of smectic and nematic phases in samples 42 (w=2) through i (DP =4.1). On the contrary, polymer 43 7 CBPj =8.7) displayed a weakly birefringent, possibly nematic, texture. 

At temperatures above 232°C, we observe the colours of birefringent layers as previously, together with the threads characteristic of nematic phases. Below 213.5°C, the threads disappear and we observe the focal conic texture which typifies the smectic in Fig. 9.0c. In fact, certain regions of our preparation appear dark when viewed under crossed polarisers, whatever their disposition. Such an observation could only be possible if the optical properties of our smectic phase were the same as those of the uniaxial nematic described in the last section, and if this optical axis were parallel to the microscope axis. This implies that the smectic layers are parallel to the microscope slide in these regions of the preparation, and also that molecules are on average normal to the layers. Cooling further, at temperatures below 182.5°C, the dark regions become coloured and reveal a threadlike texture very similar to that of the nematic phases (see Fig. 9.10). 

The blue phases BP I and BP II, which appear in a narrow temperature range between the cholesteric and isotropic phase, are not birefringent but optically active. In the transmitted light of a polarizing microscope they look like a blue isotropic liquid. Sometimes small single crystals of the blue phase with distinct habit are seen. The polycrystalline blue phase platelet texture resembles the mosaic texture of the smectic B phase. 

Fig. 5.9. Polarizing-microscope images of the nematic schlieren texture (left) and the focal conic texture of a smectic A phase (right). The reader should not try to infer any three-dimensional character from these images. The subtle variation from light to dark is merely a two-dimensional mapping of the birefringence in the plane of the samples.

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