POLARISING MICROSCOPY

In 5 (Fig. 4.20) steric hindrance in the peripheral region appears to be too high for formation of a liquid-crystalline phase. Mesophases were characterised by polarisation microscopy and X-ray diffraction. Presumably the LC properties cease as a result of segment mobility with increasing number of stilbene building blocks in principle, the number of conformers should double with each double bond although the maximum number of 2n (e.g. 221 for the third generation) is unattainable for symmetry reasons. 

The most common and easily applicable method of characterising liquid crystalline mesophases is polarisation microscopy. In this method, thin samples of the surfactant solution are viewed under a microscope between crossed polarisation filters. Due to optical anisotropy of liquid crystals they are birefringent. Hence, they give rise to a brightness in the microscope and show patterns that are very characteristic for the specific phases examples are shown in Figure 3.17. 

The purified Pc was vacuum deposited by thermal evaporation onto the AlO, films at different substrate temperatures from -17 °C to 93 °C. The deposition rate of Pc was 5-7 nm/min (except for sample D, 1 nm/min). The shutter was closed during the heating of the evaporation cell in order to avoid deposition of low mass impurities, degassing from the Pc evaporation cell, onto the samples. The evaporation temperature of the cell was between 260 °C and 310 °C. From optical polarisation microscopy we derived that all Pc films were polycrystalline with no preferential lateral orientation. The thicknesses of the Pc films <7pc afm were determined from the averaged height AFM profiles and ranged from 90 nm to 155 nm (see Table 8.1). The film thicknesses of the Pc films could be verified independently by X-ray diffraction (XRD) measurements which are described below in detail. 

Investigations by means of DSC and polarisation microscopy as well as temperature dependent MAXS- and WAXS-measure-ments resulted in the existence of sanidic liquid crystallinity from pentamer upwards. 

The smectic mesophase is more ordered than the nematic phase and furthermore, whereas only one nematic phase exists, the smectic phase exhibits polymorphism (see Figure 3. 2), i.e., there are many different types of smectic phases. As for the nematic phase, the smectic phases can be identified by optical polarising microscopy (see Chapter 9). In 1917, Grandjean was studying (by microscopy) a sample of smectic liquid crystal (later classified as smectic A) which showed stepped edges, indicating that the smectic phase was lamellar in natnre. The lamellar nature of smectic phase allows various combinations of molecular correlations both within the layers and between the layers , each of which constitutes a different type of smectic mesophase. 

Blue phases are so ealled because when first discovered they appeared blue when viewedby eye as thin films. However, the blue phases of many compounds exhibit other colours such as red and green when viewed by optical polarising microscopy, especially when using reflectance conditions. The generation of blue phases by chiral materials is quite cottunon and compound 5 is an example. 

Some liquid crystal phases can be identified quite simply by using just one technique. However, to be more certain of the hquid crystal phase type, several different techniques are often employed. The most widely used technique of liquid crystal phase identification is optical polarising microscopy, which reveals that each different liquid crystal phase has a distinct optical texture. However, the identification of hquid crystal phases through optical polarising microscopy is often difficult and requires a lot of experience. 

Everyone working in the field of hquid crystals will at some point need to use optical polarising microscopy in the analysis of hquid crystals. Primarily, optical polarising microscopy enables the identificahon of the type of liqiud crystal and other mesophases from the ophcal textirre that is generated. However, the technique is also essential when... 

The nematic phase was described in Chapters 1, 3 and 4 and is exhibited by certain rod-like molecules and certain disc-like molecules. The nematic phase is the least ordered liquid crystal phase and it is usually very easy to identify a nematic phase by optical polarising microscopy. The high degree of disorder of the phase stmcture means that the nematic phase is very fluid and dust particles within the sample are seen to undergo intense Brownian motion. When the coverslip is displaced (a vital operation when identifying liquid crystal phases by optical polarising microscopy), it is easy to detect how fluid the phase actually is and the sample shirmners quite intensely at the time of impact. 

From an optical polarising microscopy point of view the S phase exhibits two... 

The elhpses and hyperbolae of the focal-conic fan texture appear as black lines in the texture because these are the areas where the sharp changes in the direction of the optic axis are foimd. The area of a sudden change in the direction of the optic axis is isotropic and so this region appears black when viewed using optical polarising microscopy. 

As discussed in Chapter 6, some chiral liquid crystal phases are somewhat special in terms of their physical properties. The special properties are due to the helical nature of the phase stracture, caused by the reduced symmetry of the corrstituerrt chiral molecules. However, a small quantity of a chiral material is all that is reqttired to convert an achiral Uqirid crystal phase into the chiral analogue. The hehcal rrature of certain chiral liquid crystal phases can be identified by optical polarising microscopy. 

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