Calamitic liquid crystals—nematic and smectic mesophases

The fluid nematic phase has a low viscosity and nematic materials can be designed so that molecular orientation can be switched by an electric field the different optical properties of the two orientations enables display applications (see Chapter 13). 

Initially smectic phases can be subdivided into the true liquid crystal smectics and the 

When account is taken of the phenomenal number of organic compounds that have been prepared, and can possibly be prepared, then only a very small proportion of this total will exhibit any liquid crystal phases. The generation of liquid crystal phases is limited by both steric and polarity considerations, i.e., liqtrid crystal phases can only be exhibited by materials of specific molecular stractures. To be suitable for an application, not only does a material require the necessary molecular stmcture to generate a desired liqtrid crystal phase at the required temperatrrre over a specific temperatrrre range, but a material also needs to possess a suitable combination of physical properties for that 

Many questions now need answering, such as what type of stmctural units and combinations of stmctural units allow the generation of the nematic phase How do these differ from those required to generate smectic phases How do stmctural features vary from one type of smectic phase to another What stmctural features are responsible for the generation of a smectic phase where the constituent molecules are tilted (e.g., S )  

How do stmctural features affect the physical properties of liquid crystal materials What predictive powers does the synthetic chemist have to enable the accurate synthesis of a material with desired properties  

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Calamitic liquid crystals—nematic and smectic mesophases... 

Calamitic Liquid Crystals—Nematic and Smectic Mesophases 47... 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

Calamitic phases are essentially defined as a set of mesophases that occur between the breakdown of the long-range periodic, translational ordering of the crystal and the loss of long range orientational order at the transition to the isotropic liquid. In this context six mesophases exist the nematic phase and five smectic phases labelled A, B, C, F and I. Other closely related soft crystal phases, in which the molecules have long-range periodic order but are themselves rotationally disordered, also exist in concert with liquid crystal phases. This second set of mesophases are labelled E, J, G, H, and K, and are essentially the crystal versions of the... 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

The most important multicolour display technology in current use involves liquid crystal displays (LCDs), which overtook CRTs in popularity around the mid-2000s. Liquid crystals (LCs), commonly referred to as the fourth state of matter, are materials that are intermediate in character between the crystalline solid and liquid states.Unlike normal isotropic liquids in which the molecules essentially adopt a randomised orientation, liquid crystals show some time-averaged positional orientation of the molecules. In this sense, they resemble solid crystalline materials, although they retain most of the properties of liquids, notably the ability to flow. They are formed most commonly from molecules with rod-like geometry, which are referred to as calamitic. These molecules may orientate in various ways to form different types of LC phases [mesophases). There are three main mesophase types smectic, nematic and chiral nematic. In the smectic mesophase, as illustrated in Figure 11.1(a), the molecules are arranged in raft-like layers with their molecular axes parallel. These layers can pass over each other as the material flows. In the... 

FriedeP was the first who distinguished three main classes of liquid crystals, according to the different kind of orders in the mesophases nematic, smectic, and cholesteric. From the point of view of the geometrical shape of molecules, we divide the thermotropic LCs into calamitic phases (when the molecules are rodlike), sanidic phases (when the molecules are bricklike), and the discotic phases (when the molecules are disklike)... 

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