Mesophases, characterisation

Note 1 There are several types of smectic mesophases, characterised by a variety of molecular arrangements within the layers. 

Smectic A and smectic C mesophases characterised by anti-parallel (SmA2, SmAj, and SmC2, SmCj) and random (SmAi and SmCi) alignments of the molecular dipoles within the layer thickness in Fig. 10. 

Columnar mesophase characterised by a hexagonal packing of the molecular columns. 

Columnar mesophase characterised by a liquid-like molecular order along the columns, in which the columns are arranged in a rectangular packing. 

Identification of liquid crystal phases —mesophase characterisation... 

Identification of Liquid Crystal Phases —Mesophase Characterisation 183... 

D. M. Riggs, The Characterisation andKinetic Mechanism of Mesophase Formation in High Molecular Weight Carbonaceous Materials, Ph.D. dissertation, Rensselaer Polytechnic Institute, Troy, N.Y., 1979. 

The liquid crystal properties of the complexes were characterised using polarised optical microscopy and showed a nematic phase for n = 4 and 6 and a SmA phase for n = 6, 8, 10 and 12. The mesophases were monotropic for n = 4 and 6 and enantiotropic for the others the progression from a nematic phase for shorter chain lengths to SmA at longer chain lengths is quite typical for simple, polar mesogens. 

Note 4 At one time, a number of mesophases were identified as smectic on the basis of their optical textures, but they are in fact soft crystals characterised by very low yield stresses. Hence, these three-dimensionally ordered phases should no longer be called smectic mesophases. They are akin to plastic crystals with some elementary long-range order and are referred to by the letters E, J, G, H, and K. 

Note 4 The structure of a smectic A mesophase is characterised by the symbol Dooh in the Schoenflies notation (oo, 2 in the International System). 

Note 2 A SmF mesophase is characterised by in-plane short-range positional correlations and weak or no interlayer positional correlations. 

Note 1 See Fig. 16 for an illustration of the molecular arrangement in a Nb mesophase. Note 2 From a crystallographic point of view, the biaxial nematic structure is characterised by the symbol D2h in the Schoenflies notation (2 m, m in International System). 

The characterisation of the hexagonal mesophase (i.e. of the samples with low amounts of the other phase) reveals a number of significant properties (i) The [Pg.561]

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. 

There are three main techniques used in characterisation of mesophases polarised optical hot-stage microscopy, differential scanning calorimetry (cf. Box 9.1) and small-angle X-ray scattering. 

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. 

It should be emphasised that the micellar structures themselves are still too small to be seen under an optical microscope but they form domains of uniform orientation that can be observed. Other methods for the characterisation of mesophases are scattering methods, e.g. neutron scattering, x-ray diffraction or rheology as pointed out in Section 3.6. For a more detailed description see the literature relevant to this subject [19]. 

If then we abandon the standard three-dimensional Euclidean perspective and adopt this non-Euclidean two-dimensional view, it can be seen that stable polymorphs are characterised by a global geometric constraint surface density 2"1, and a local constraint Gaussian curvature, . We shall see in Chapter 4 that this description is identical to one that accounts for the mesophase behaviour of lyotropic liquid crystals in amphiphile-water mixtures. 

The most common definition of a microemulsion characterises it as a thermodynamically stable, transparent, optically isotropic, freely flowing surfactant mixture, often containing co-surfactants (e.g. alcohol) and added salts [37]. We restrict the definition further to non-crystalline (disordered) aggregates, since crystalline isotropic phases are better considered as liquid crystalline mesophases. Indeed, the most succinct description of a microemulsion would involve its microstructure. However, this has proven to be a very equivocal issue. So much so that until very recently it was widely believed that microemulsions were devoid of microstructure hence the thermod)mamic definition. 

Hgure 4.33 Representation of the local structure of some chiral mesophases on heating. The matches describe the relative orientations of chiral molecules in space. As the temperature is raised, the system transforms from a crystalline phase (left) to a cholesteric phase (centre) characterised by a single twist, to a double-twist blue" phase (right). 

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