No abstract

If non-chiral smectic C host phases are doped with chiral guest molecules, helically structured Sc* phases are formed by symmetry breaking. If the chiral dopants bear transverse dipoles a spontaneous polarization, Ps, will occur in these so-called induced Sc* phases. In 'classical' dopants the chiral centre(s) as well as the transverse dipole(s) are situated in alkyl side groups (type I dopants). The tilt-angle, reduced polarization, P,=P,/si n 0 @=tilt angle), in these mixtures are independent of the host with a linear dependence on the guest mole fraction, xG. As a material parameter the polarization power, 6, = (dP0/axG),,, has been introduced and is discussed in terms of the molecular structure of the dopant. Deviations of PO(xG) from linearity can be considered by a local field correction. Novel chiral dopants bearing the chiral centre@) and the transverse dipole(s) in a rigid core (type II dopants) exhibit a significant dependence of the magnitude, and sometimes even of the sign, of Po on the nature of the host. The different behaviours of the two dopant types are discussed by means of a microscopic model and can be understood in terms of a hard core guest-host interaction which influences the rotational distribution of the dopant molecules with respect to their long molecular axes.In 1980 we showed that ferroelectricity can be observed not only in neat smectic C* phases (like DOBAMBC) but also in achiral smectic C host phases which are doped with chiral guest molecules bearing sufficiently large transverse dipoles.' We call these mixed systems 'induced smectic C* phases'. The guest molecules need not necessarily form mesophases by themselves. However, owing to the pronounced sensitivity of the Sc phase stability towards impurities, in the case of nonmesogenic chiral dopants the range of induced S,* phases is often restricted to mole fractions, xG, of the guest molecules up to 0.2 at most (for example, see the phase diagram shown in Fig. 12). Therefore, the most desirable dopants are those which form cholesteric or even Sc* phases by themselves. In the latter case, mixed systems of guest and host molecules over the whole range 0 < xG < 1 could be obtained (Fig. 2).It will be of great interest to determine whether the spontaneous polarization, P,, of induced Sc* phases will be determined only by the transverse dipoles of the chiral guest molecules, or whether the dipolar properties of the host molecules also contribute to the value of P,.3 In this paper 100 I 1 I , , , I , , , 1 90 80 t ./ I WCh 9 70 i2 60 50 1 1 " " " ' 1 1 1 1 0 0.05 0.1 0.15 XG Fig. 1 Phase diagram of a mixed system of the glucopyranoside S 262 as a non-mesogenic chiral dopant in the achiral smectic C host phase 8007298 A A 0 0.2 0.4 0.6 0.8 1 XG 20 Fig. 2 Phase diagram of a mesogenic oxirane W 46 as chiral dopant in the achiral host phase 800721we will show that the answer to this question depends significantly on the general molecular structure of the guest molecules. We will further investigate how the polarization depends on the mole ...

The phase ordering kinetics of various liquid crystalline materials from well known series (PCH, CCH, nO. m, BCH and CB) when turning from a disordered to an ordered phase induced by a rapid temperature quench have been investigated. The process of phase ordering can generally be described by the growth of a characteristic length L . The exponents n of the theoretically predicted universal growth law L ( t )~t n have been determined. In this paper we present in particular the results for the kinetics of phase formation which takes place under the mechanism of nucleus growth. As characteristic length we chose the diameter of the nuclei. We obtained values for n between 0´5 and 1 dependent on the depth of the temperature quench. These results for mainly spherical shaped nematic and smectic germs are discussed in terms of the Allen± Cahn theory for the movement of an antiphase boundary (APB). As a lower limiting value, 0´5 con® rms the classical theory, while 1 as an upper limiting value can be explained by the introduction of a volume driving force.

The orientation of nematic liquid crystals (LC) by monolayers of suitable amphiphilic compounds on solid substrates has been studied by systematic experiments using polarizing microscopy, magnetic Freedericksz‐transition, and contact angle measurements. Orientation is influenced mainly by the following parameters: 1. temperature; 2. physical properties of the substrate surface; 3. chemical properties of the substrate surface. Molecular models describing the orientating action of amphiphilic monolayers are developed and discussed in terms of the obtained quantitative results. Changes in LC orientation with film packing density and temperature can be understood as phase transitions of the system “amphiphilic monolayer + embedded LC”.

The action of solid surfaces coated with monomolecular films of amphiphilic compounds (especially lecithins) on adjacent liquid crystal phases resulting in an uniform homeotropic orientation has been investigated. The quality of orientation was found to be effected by molecular structure of the amphiphilics as well as the liquid crystals, the packing density of the film, and the temperature. The experimental results can be understood in terms of the following model: Lecithins form brush‐like films with the hydrophobic alkyl chains pointing off the glass surface. Under suitable conditions holes of molecular dimensions exist in the films into which nematogenic molecules can penetrate. By means of elastic interaction forces between these anchored molecules and the adjacent nematic bulk the homeotropization is induced.