Clearing chiral smectics

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Li and co-workers investigated derivatives of 43c,d [62]. Instead of the terminal heptyl chains, chiral (S)-2-methylbutyl or dodecyl chains were used. For the methylbutane side chain, a chiral smectic and a chiral nematic phase were observed and the clearing points were higher than in 43c,d due to the shorter chain while they were lower for the longer dodecyl side chains. It was also observed that the trans-compounds possess higher clearing temperatures compared to their c/.v-derivatives. 

The precursor 6 exhibits the enantiotropic nature of chiral nematic (N ), chiral smectic C (SmC ) and chiral smectic I (SmI ) phases. The shell-printed texture of the SmC phase and the rose-like texture of the SmI phase can be clearly seen in Figure 12.6. The thiophene monomers, M2 and M3, show enantiotropic N, SmA and SmC phases. The SmC phase is characteristic of ferroelectricity. The polymers show various mesophases. The phase transition temperatures are summarized in Table 12.4. PI shows an enantiotropic SmA phase. P2 shows enantiotropic SmA, SmC and SmB phases. The fan-shaped texture of the SmA phase and the striated fan-shaped texture of the SmC phase are shown in Figure 12.7. P3 shows an SmA phase in the heating process and SmA and SmX phases in the cooling process. XRD analysis suggests that the SmX phase of P3 might be a higher order smectic phase. 

There is one pecularity concerning the helix structure which should be mentioned. In contrast to the cholesteric helix, the chiral smectic C helix contaim a spontaneous bend in addition to the spontaneous twist (7]. This leads to a local Oexoelet tic polarization which adds up to the ferroelectric polarization (8]. Beresnev et al. (9] have claimed that the latter contribution clearly dominates, but the flexoelectric term may become significant tot materials with a short helix period p < I tun. 

Thisayukta J, Nakayama Y, Kawauchi S, Takezoe H, Watanabe J (2000) Distinct formation of a chiral smectic phase in achiral banana-shaped molecules with a central core based on a 2,7-dihydroxynaphthalene unit. J Am Chem Soc 122 7441-7448 Tschierske C, Dantlgraber G (2003) From antiferroelectricity to ferroelectricity in smectic mesophases formed by bent-core molecules. Pramana J Phys 61 455-481 Ungar G, Percec V, Zuber M (1992) Liquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on l-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R=lluoro, chloro and methyl and flexible spacers containing an odd number of methylene units. Macromolecules 25 75-80 Urayama K (2007) Issues in liquid crystal elastomers and gels. Macromolecules 40 2277-2288 Vorlander D (1908) About transparently clear, crystalline liquids. Rep Ger Chem Soc 41 2033-2052... 

From this discussion the clear similarity between the SmAPA and SmCsPA structures is easily seen. In addition, the suggestion of Brand et al.29 that a bilayer smectic with all anticlinic layer interfaces (the SmAPF) would produce an achiral ferroelectric smectic follows directly. The unanticipated tilt of the director in the tilt plane, leading to a chiral layer structure, seems to be a general response of the bent-core mesogens to the spontaneous nonpolar symmetry breaking occurring in these rigid dimer structures. 

The laterally appended dendrimer, 32, shown in Fig. 27, as expected exhibits a chiral nematic phase, with smectic mesophase formation being suppressed. The clearing point is almost 50 °C lower, whereas the melting point is only 25 °C lower in comparison to the terminally appended system. This demonstrates that lateral appendages of the mesogens causes disruption to the intermolecular packing, thereby destabilizing mesophase formation. The local structure of the chiral nematic phase is thus shown in Fig. 28. 

As this compound was one of the higher homologues in the series, and because we knew that the earlier homologues did not exhibit a chiral nematic phase, it was clear that the new phase also could not be a chiral nematic phase. In addition, it was clear from the formation of the defect structures seen in the microscope that the phase first formed from the isotropic liquid possessed a helix, see Plate 1, which had its heli-axis at right angles to the heli-axis in the lower temperature chiral ferroelectric smectic phase. This simple observation meant that if the phase was a lamellar smectic phase then the helix would have to be formed, inconceivably, in a direction parallel to the layers. Synthesis of the achiral variant confirmed that the phase formed first on cooling from the isotropic liquid was indeed a smectic A phase, and thus we immediately knew that we had found a smectic A phase where the helical macro structure formed in the planes of the layers, and thus the helix must... 

As the difference in the transition temperatures between the chiral and racemic forms of the same compound is found to be only of the order of 1 °C, it is clear that the transition is driven by intermolecular forces responsible for the tilted smectic phase and not by the dipole-dipole... 

The coefficient y is rotational viscosity of the director similar to coefficient yi for nematics. In fact, it does not include a factor of sin cp and, in the same temperature range, can be considerably larger than the viscosity ytp for the Gold-stone mode. This may be illustrated by Fig. 13.10 the temperature dependence of viscosities y and have been measured for a chiral mixture that shows the nematic, smectic A and smectic C phases [15]. The pyroelectric and electrooptic techniques were the most appropriate, respectively, for the measurements of ya and ytp describing the viscous relaxation of the amplitude and phase of the SmC order parameter. The result of measurements clearly shows that y is much larger than y and, in fact, corresponds to nematic viscosity yj. 

The thermotropic liquid crystal, 4,4 -diheptylazoxybenzene (HAS), exhibiting isotropic, nematic and smectic phases, has been studied through e NMR. The temperature dependence of e chemical shifts and spin-lattice relaxation times of the Xe gas dissolved in HAS showed clear signatures of the phase transitions. Theoretical models have been used to understand the influence of the different phases on the isotropic and anisotropic parts of the chemical shielding. From the studies it is also inferred that in the smectic phase, Xe atoms preferentially occupy interlayer spacings rather than the interiors. Bent-core or banana-shaped molecules display an array of novel chiral liquid crystalline phases. NMR studies on two of the banana core moieties have been analyzed using ab initio structure calculations and the steric inertial frame model. ... 

---