Liquid crystal materials smectic phases

A recent observation has been that not all porphyrin LC phases are discotic. Thus 5,15-meso-substituted porphyrins have been prepared which show a variety of smectic phases that are not discotic. The porphyrin macrocycle could be acting to impose biaxial symmetry within the meso-phase, which makes the new porphyrins of great interest as new nematic liquid-crystal materials. 

Finally, the highly polar tetrazine core finds apphcation in the design of liquid-crystal materials. A new series of symmetrical 3,6-diphenyl-1,2,4,5-tetrazines 61 with four alkoxy tails was synthesized and their mesogenic properties were studied by differential scanning calorimetry and polarizing optical microscopy It was shown that they possess smectic-C phases (13MCLC34). 

The microstructure of an ER system definitely determines how this system behaves under an electric field. Figure 22 shows the shear stress of oclylcynaobiphenyl vs. the electric field at shear rate 329.5 s " and various temperatures. As indicated in the literature [70-73], Oclylcynaobiphenyl is a liquid crystal material, and has a phase transition from the smectic to the nematic phase at 306.72 K and from the nematic to the isotropic phase at 313.95 K. With the increase of temperature from 306.6 K to 312.8 K, oclylcynaobiphenyl may have the different structures marked as a to b [73]. The ER property of oclylcynaobiphenyl should depend on how the director is orientated in the fields, fhe shear stress passes through a maximum value when the liquid crystal material is in the smectic phase state. Once the material is in tlie nematic phase state, the ER effect becomes weak and saturates at the electric field strength above 0.7 kV/mm. 

On a macroscopic scale, the spontaneous polarization vector in the optically active phase spirals about an axis perpendicular to the smectic layers (Fig. 20), and sums to zero. This macroscopic cancellation of the polarization vectors can be avoided if the helical structure is unwound by surface forces, by an applied field, or by pitch compensation with an oppositely handed dopant. The surface stabilized ferroelectric liquid crystal display utilizes this structure and uses coupling between the electric field and the spontaneous polarization of the smectic C phase. The device uses a smectic C liquid crystal material in the so-called bookshelf structure shown in Fig. 21a. This device structure was fabricated by shearing thin (about 2 i,m) layers of liquid crystal in the... 

FIGURE 2.27 These cartoons illustrate the different scattering patterns that may be observed in an x-ray diffraction experiment on a liquid crystal material for (a) an unaligned smectic phase, (b) an aligned smectic A phase, and (c) an aligned nematic phase. 

The committee also organized the 4th International Conference on Ferroelectric Liquid Crystals in Tokyo in 1993. Many Japanese companies were very active in ferroelectric liquid crystals materials, especially a smectic C liquid crystal phase in which is growing globally against a background of the next nematic liquid crystal, and the meeting was a great success. 

Liquid crystals are wonderful materials. In addition to the solid crystalline and liquid phases, liquid crystals exhibit intermediate phases where they flow like liquids, yet possess some physical properties characteristic of crystals. Materials that exhibit such unusual phases are often called mesogerrs (i.e., they are mesogenic), and the various phases in which they could exist are termed mesophases. The well-known and widely studied ones are thermotropics, polymeries, and lyotrop-ics. As a function of temperatirre, or depending on the corrstituerrts, concentration, substituents, and so on, these liqirid crystals exist in many so-called mesophases— nematic, cholesteric, smectic, and ferroelectric. To understand the physical and optical properties of these materials, we will begin by looking into their constituent molecules. ... 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

Liquid crystals are materials that exist in a state that is intermediate between a liquid state and a solid. They are formed by anisotropic molecules, known as calamitic molecules, which are long and narrow, i.e. have a large length-to-breadth ratio. These rod shaped molecules orientate themselves in different ways as they change from the crystalline to the liquid state at different temperatures, existing in the smectic and nematic phases, as shown in Figure 5.1. 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Chiral molecules which form smectic liquid crystals are often capable of forming structures in which the electric dipoles associated with the molecules all point approximately in the same direction in a particular region but in which this direction rotates as one moves in a direction normal to the smectic planes. Such materials are rather misleadingly referred to as ferroelectric liquid crystals. The mechanism responsible for this effect is illustrated in Figure 7.3. The molecules tilt into a smectic-C phase due to their structure as illustrated. Dipoles associated with the molecules are supposed to point in a direction normal to the page. Thus, if the molecules all have the same handedness the dipoles all point in he same direction. This description is an oversimplification as the molecules rotate about their long axes but point preferentially in the manner indicated. This phenomenon has been successfully applied to... 

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