Dielectric properties, chiral

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

In the nematic phase the typical temperature dependence of dielectric constants and e are given in Fig. 2.4. Similar temperature curves are observed in the chiral nematic phase. The only difference comes from a specific definition of the main dielectric constants in the chiral nematic phase the components e h and e h are taken into consideration, denoting the dielectric properties parallel and perpendicular to the helical axis... 

Most recent studies [28,29] of the dielectric properties of the SmC phase have focussed on the ferroelectric chiral smectic C phase, because of its importance in applications. The molecular interpretation of the principal permittivities is contained in Eqs. (27), with appropriate correlation fac-... 

The theory of the dielectric properties of chiral smectic liquid crystals is far from complete, particularly with respect to a molecular statistical approach. Simple Landau theory [31 ] gives expressions for the contributions of soft modes (jj g) and Goldstone modes (Xo) to the low frequency permittivity as ... 

In the standard description of the dielectric properties of the chiral tilted smectics worked out by Carlssonet al. [152], four independent modes are predicted. In the smectic C the collective excitations are the soft mode and the Goldstone mode. In the SmA phase the only collective relaxation is the soft mode. Two high frequency modes are connected to noncollective fluctuations of the polarization predicted by the theory. These two modes become a single noncollective mode in the smectic A phase. There is no consensus [153] as yet as to whether these polarization modes really exist. Investigations of the temperature dependence of the relaxation frequency for the rotation around the long axis show that it is a single Cole-Cole relaxation on both sides of the phase transition between smectic A and smectic C [154]. The distribution parameter a of the Cole-Cole function is temperature-dependent and increases linearly (a=a-pT+bj) with temperature. The proportionality constant uj increases abruptly at the smectic A to SmC transition. This fact points to the complexity of the relaxations in the smectic C phase. 

Dielectric and Electrooptical Properties of a Chiral Liquid Crystalline Polymer... 

A wide variety of molecular properties can be accurately obtained with ADF. The time-dependent DFT implementation " yields UV/Vis spectra (singlet and triplet excitation energies, as well as oscillator strengths), frequency-dependent (hyper)polarizabilities (nonlinear optics), Raman intensities, and van der Waals dispersion coefficients. Rotatory strengths and optical rotatory dispersion (optical properties of chiral molecules ), as well as frequency-dependent dielectric functions for periodic structures, have been implemented as well. NMR chemical shifts and spin-spin couplingsESR (EPR) f-tensors, magnetic and electric hyperfme tensors are available, as well as more standard properties like IR frequencies and intensities, and multipole moments. Relativistic effects (ZORA and spin-orbit coupling) can be included for most properties. 

This is a chiral smectic A with symmetry Dqo. Its properties are similar to those of the achiral SmA. However, close to the transition to the smectic C phase, the chiral smectic A phase shows interesting pretransitional phenomena in the dielectric and electrooptical effects (the so-caUed soft dielectric mode and electroclinic effect). They will be discussed in Chapter 13. 

Let us simulate an appearance of the higher harmonics and optical properties of the cholesteric structure with the following parameters typical of chiral materials based on the well-known nematic mixture E7 helical pitch 0.4 pm, elastic modulus K22 = 5 X 10 dyn (or 5 pN) principal dielectric permittivity values Sn = 20,... 

In fact, a strong field smears the soft mode behaviour and the phase transition vanishes. All these results are in agreement with experiments on dielectric and electrooptical properties owed to the soft mode in the chiral SmA phase. 

In Section 12.3, we describe the synthesis and thermal properties of FLC polythiophene derivatives, where fluorine-containing chiral LC groups are substituted in the 3-position of the thiophene ring [59-66]. Then, we report the optical properties and temperature dependence of the dielectric constant of the FLC polythienylene derivatives. 

Figure 12.9 Temperature dependence of dielectric constant of P2 (100-5000 Hz). From H. Narihiro, X.-M. Dai, H. Goto, K. Akagi, H. Yokoyama, Synthesis and properties of polythienylene derivatives with fluorine-containing chiral liquid crystalline substituents (II), Mol. Cryst. Liq. Cryst., 365, 355-361 (2001), reprinted by permission of the publisher (Taylor Francis Ltd, http //www.tandfco.uk/journals)...

---