Polarising microscope

Hartshome NH, Stuart A. Crystals and the Polarising Microscope, 4th ed., Edward Arnold, London. 1970, pp. 308-318. 

Pleochroism-exhibits two different colours upon rotation of the stage of the polarising microscope... 

A domain can be of the order of m or even more. Several methods can be used to obtain pictures of the domains. Figure 9.18 is a photograph taken of a thin slice of barium titanate under a polarising microscope in which different domains can clearly be seen. Note the sharpness of the domain boundaries. 

Figure 3.17 Patterns of liquid crystalline phases of surfactants under the polarisation microscope (a) hexagonal phase the typical fan-like structure can be seen, (b) lamellar droplets with typical Maltese crosses and (c) lamellar phase.

These phase regions were formed in the water rich areas of the phase diagrams. The LC phases were identified using a polarising microscope and determined to be similar to an inverse middle phase observed in ternary systems. 

When a bicontinuous cubic lipid-water phase is mechanically fragmented in the presence of a liposomal dispersion or of certain micellar solutions e.g. bile salt solution), a dispersion can be formed with high kinetic stability. In the polarising microscope it is sometimes possible to see an outer birefringent layer with radial symmetry (showing an extinction cross like that exhibited by a liposome). However, the core of these structures is isotropic. Such dispersions are formed in ternary systems, in a region where the cubic phase coexists in equilibrium with water and the L(x phase. The dispersion is due to a localisation of the La phase outside cubic particles. The structure has been confirmed by electron microscopy by Landh and Buchheim [15], and is shown in Fig. 5.4. It is natural to term these novel structures "cubosomes". They are an example of supra self-assembly. 

The conclusion is that if different parts of the structure contain crystallites with their axes in different directions from each other the structure will be visible in the polarising microscope. In addition to all the usual measurements of size and shape that can be made with the optical microscope, it is possible to deduce the relative alignments of the axes of the crystallites within the structure from the relative brightness of its various parts. 

Some polymers, when they are suitably prepared in thin slices or as thin films, exhibit circular features when they are viewed in the optical microscope (fig. 3.13), whereas others show less regular patterns, depending on the polymer and the method of preparation of the sample. In order to see these features the polarising microscope with crossed polarisers (see section 2.8.1) is used. The circular features shown in fig. 3.13 are caused by spherical structures called spherulites which are a very important feature of polymer morphology, the subject of much of chapter 5, where the Maltese cross appearance seen in fig. 3.13 is explained. Each spherulite consists of an aggregate of crystallites arranged in a quite complicated but regular way. 

As the chain folds it must twist through 90° so that the chains are in correct crystal register (see fig. 4.17(b)) and, although the various sectors of the crystal grow by chain folding in different planes, there is no discontinuity of crystal structure across the sector boundaries, as is shown most simply by uniform extinction in the polarising microscope. 

Consider a spherulite observed between the crossed polariser and analyser in a polarising microscope (see fig. 5.14). Assume that the crystallites within the spherulite have a constant orientation with respect to the radius vector. The corresponding orientations of the indicatrices are then as shown in fig. 5.14(b) and, according to the principles of the polarising microscope explained in section 2.8.1, the Maltese cross will appear in the orientation shown in fig. 5.14(c). Even if the shorter axis of the indicatrix is parallel to the radius vector the orientation of the cross will not change. All that matters for the field to appear dark is that one of the principal axes of the indicatrix should be parallel to the axis of the polariser. 

Fig. 9.11. Polarisation microscope photographs of test displays composed of a patterned photoalignment layer (Staralign 2110 from Vantico) and a uniformly rubbed layer, filled with LC mixture MLC 7700 000 (Merck). The polarisers are crossed, (a) Stripe pattern with line widths of 10 and 3 pm, (b) checkerboard pattern of 10x10 and 100x100 pm.

The other method consists of measuring the phase difference y in the polarising microscope with the aid of a suitable compensating device. The birefringence is then given by yX... 

In the course of his very extensive studies of the soap boiling process many methods have been employed by him and his pupils analysis of the phases, observation of the temperature at which phases appear or disappear, investigation with the microscope and the polarising microscope, determination of the vapour pressure and observations with the dilatometer. With the... 

Isotropic solution (nigre lye). With these it must be noted that these solutions can be transformed into gels by addition of electrolytes, after which a separation into two layers can follow — the region of the coacervation of soap solutions studied later by Bungenberg de Jong and his pupils 2°. Middle soap, an anisotropic (polarising microscope) viscous solution which contains still rather a lot of water (about 50%). 

During the course of the pyrolysis of a petroleum-derived residue (pyrolysis tar, cat-cracker residue) or a filtered coal tar pitch it is possible to observe, under the polarisation microscope and at a certain temperature, the formation of anisotropic spherules, which grow as the reaction time lengthens and the temperature increases, coalesce and, at around 500 to 600 °C, are transformed into a semi-coke phase with marked anisotropy. Figure 13.2 shows photomicrographs of a filtered coal tar pitch pyrolyzed at 400 °C with the formation of spherulitic mesophases after reaction times of 2, 6,10 and 16 hours. 

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