Molecular long axis

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

In the mixed crystal, the reactive molecules, which are related by a pseudocentre of symmetry, make a pair and are superimposed along the [Oil] direction without any displacement of the molecular long axis. The double bond on the pyridyl side in one molecule and the double bond on the... 

Fig. 15 Schematic molecular arrangements of (a) a normal /8-type crystal and (b) 2 OEt-2 OPr crystal viewed along the molecular long axis.

The monoclinic arrangement of a-6T is very similar to that of p-6V, as observed from Figs. 1.13(e) and (f). In a-6T the molecules are also planar and the molecular long axis is at an angle of 23.5° with the a-axis. The most salient difference is the more pronounced dimerized character in o -6T. 

The molecular mechanism of local segment rotations can be explained by the occurrence of twin reversals [33, 34] which are induced thermally due to the unusual course of the conformational potential [35]. Those twin reversals are torsion defects causing the helix conformation to change from left-handed to right-handed and vice versa. They are built into the PTFE helix without changing the direction of the molecular long axis (Fig. 23). Additionally, the long... 

Fig. 14.14 9,10-Bis(phenylethynyl)anthra-cene (21). (a) structure, showing molecular long axis, (b) columnar arrangement of molecules in the crystal, (c, top) detail showing...

Figure 2.6 The molecular structure of a typical calamitic, thermotropic nematic liquid crystal. The diameter of the molecule measured in the x-y plane is symmetrical about the z-axis due to very rapid rotation about the molecular long axis.

Figure 3.5 Schematic representation of the resultant transverse dipole moment across the molecular long axis of a 3, (S-disubstituted pyridazine derivative.

Table 3.14 Transition temperatures (°C), elastic constants fk/y, k22 kjj, 10 N), dielectric anisotropy ( e), dielectric constant measured perpendicular to the molecular long axis (e ), birefringence ( n), refractive index measured perpendicular to the director (noJ, rotational viscosity (y. Poise) and bulk viscosity (r, Poise) for tr ns-l-(4-cyanophe-nyl)-4-pentylcyclohexane (41), iTSins-l-(4-cyanophenyl)-4-[(E)-pent-l-enyl]cyclohexane (74) andtra.ns-l-(4-cyanophenyl)-4-[(E)-pent-3-enyI]cyclohexane (78) extrapolated to 100% at 22°...

The tetralins (181-183) collated in Table 3.23 are red liquid crystals with a transition moment orthogonal to the molecular long axis, see Figure 3.13. Since they are liquid crystalline themselves they are very soluble in nematic host mixtures. Such mixtures exhibit high negative dichroic ratios and can be used in White and Taylor GH-LCDs with positive contrast. Furthermore, mixtures containing an additional anthraquinone dye of positive dichroic ratio can be used to switch from one colour to another under the action of an electric field. 

Figure 5.6 Schematic representation of the smectic A phase (SmA) formed by calamitc, rod-like molecules. The director is parallel to the molecular long axis and normal to the plane of the layers.

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