Crystalline smectics

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

X-ray measurements have shown that the positional and orientational ordering in highly ordered crystalline smectic phases is more comparable with the ordering in the solid state than the structures of the fluid smectic phases and the nematic phase. Therefore, a comparison of the structures of crystals and highly ordered smectic phases is possible because of their closer similarities. Up to now, only the crystal structures of few mesogenic compounds exhibiting highly ordered smectic phases have been reported in... 

A polycondensation reaction of 4,4 -dihydroxybiphenyl, sebacic, and m-carboranedicarboxylic acids was reported to produce a carborane-containing polymer (142) (Fig. 78) that led to the production of a new columnar phase at elevated temperatures.157 This new phase was formed in addition to the crystalline and liquid crystalline smectic phases typically formed from only 4,4 -dihydroxybiphenyl and sebacic acids. 

In crystalline smectic phases the arrangement of the molecules within the layer is ordered in a hexagonal, orthorhombic or monoclinic fashion and the layers are correlated to each other. The in-plane order of the mesophase is indicated by the existence of sharp wide-angle reflection. The in-plane correlation is recorded by a small value of Full Width of Half Maximum (FWHM) and, in principle, by more than one wide-angle reflex. 

Stabilization of the mesophase was observed as the degree of polymerization was increased. The Tg values of the poly(norbornene)-polymers were about 30 °C higher than those of the poly(butadiene) polymers. Both polymers showed similar isotropization temperatures, but they differed substantially in their liquid crystalline behaviors. Poly-(IX-n)s with a poly(norbornene) backbone exhibited textures typical of nematic mesophases, whereas the poly-(butadiene)-based polymers poly-(X-n) displayed textures representative of smectic A mesophases. The more flexible backbone of poly(butadiene) allowed a higher order of alignment of the mesogenic units, resulting in the more ordered liquid crystalline smectic A phase. 

Within the family of untilted smectics, there is a hierarchy of phases with order ranging from that of smectic A, in which there is no positional order within each layer, to the crystalline smectics B and E, which have long-range positional order within each layer, namely hexagonal and orthorhombic for the B and E phases, respectively. This inplane order, represented by the dots within circles or ellipses of Fig. 10-27, propagates from layer to layer, producing fully three-dimensional crystalhne order. The family of tilted smectics includes similar ordered phases J, G, K, and H. 

The photodimerization of the anthracene (176) having a 9-dendritic substituent has been studied within the vaterite crystal of CaCOs (R = H),"" and in its liquid crystalline smectic phase (R = Me). The crystal phase of the CaCOs is... 

A new series of liquid crystalline smectic C polymers has been successfully synthesized using anionic polymerizations of optically active mesogenic methaciylate... 

Some binary mixtures of protic molecules have also been reported to form mesophases that are not present in either pure component. For instance, Lawrence observed mesophase formation in 1 2 molar ratios of cholesterol and 1-alkanols containing 12,13,14,16 and 18 carbon atoms [151]. Mixtures of 1-hexadecanol and dodecanoic acid exhibits a lamellar liquid crystalline (smectic) phase [152]. Intermolecular hydrogen bonding must be an important contributing factor to maintain the necessary aggregation for mesomorphism in these cases. For applications of this concept to PLCs, see Chapter 3. 

We begin this section with an example of the X-ray diffraction on the nematic, smectic A and crystalline smectic B phases. In Fig. 5.16 there is a series of X-ray photos of the same mesogenic compound at different temperatures. In this experiment, the material flow induced by the electric current aligns molecular axes in the nematic phase parallel to the field direction, which is horizontal, but in the SmA phase parallel to the smectic layers. Correspondingly diffraction patterns of the nematic and smectic phase considerably differ from each other. In the crystalline SmBcr phase the picture shows the six-fold rotation axis perpendicular to the figure plane. Below we shall discuss such pictures in detail, but let us begin with solid crystals. 

Fig. 5.16 X-ray diffractograms of p-anisalamino-cinnamic acid in different phases, nematic, smectic A and crystalline smectic...

J.R. Bruckner, J.H. Porada, D. Krueerke, S. Jagiella, D. Blunk, F. Giesselmann, In search of the lyotropic liquid crystalline smectic C phase, 24th International Liquid Crystal Conference, Mainz, Germany, (2012)... 

In terms of elastic or electromagnetic properties, if two of the three directions in a material are equivalent, the material is said to be uniaxial. The nematic and smectic A phases of liquid crystals are uniaxial, since all directions perpendicular to the director are equivalent and different from the direction of preferred orientational order. Solids with hexagonal, tetragonal, and trigonal symmetry are also uniaxial. If all three directions in a material are inequivalent, then the material is biaxial. The liquid crystalline smectic C phase is biaxial because one direction perpendictrlar to the director is in the plane of the layers while the other direction perpendictrlar to the director makes an angle equal to the tilt angle with the layers. Solids of orthorhombic, monoclinic, and triclinic symmetry are also biaxial. 

Interaction of long-chain n-alkylammonium halides with a halide of a divalent metal leads to the formation of salts of the general formula (RNH3)2MX4 with peculiar phase changes in the solid state [65]. Bis(n-alkylammonium)bromo zinc-ates with n = 10-16 display two reversible high entropy solid crystalline-solid crystalline phase transitions and a solid crystalline-liquid crystalline (smectic phase) transition up to at least 450 K. 

The crystalline smectic phases such as Se Sg and Sh if present occur just above the solid phase and usually below a smectic B, F, or I phase. 

Figure 3. Molar volume development in the crystalline, smectic B, smectic A, and isotropic phases within the homologous series of 4-bromo-iV-(4-n-alkyloxybenzyli-dene)anilines. Adapted from Seurin et al. [94],...

Each term in the expansion for p (z) in Eq. (9) corresponds to a pair of Bragg peaks in the diffraction pattern. For example, for 1=2, the term is a cosine wave of period z=a/2, which gives a pair of Bragg peaks at Q2= 4n/a= 2Qi. The intensity of this pair of Bragg peaks, relative to that which would be observed for a perfectly ordered (crystalline) smectic phase, is proportional to the square of the amplitude coefficient (X, and the same is true for all the higher order peaks. We thus see that the intensities of the various Bragg peaks are direct measures of the smectic order parameters. To a good approximation [7] we may write... 

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