Cholesteric-smectic A transitions

The pitch of a cholesteric may vary quite rapidly with temperature, particularly if the phase diagram of the material contains a cholesteric to smectic A transition just below the temperature range of interest since the pitch in this case diverges to infinity as the smectic A phase is approached. Thus, the color of the selective reflection can be used a s a sensitive indicator of temperature. Material for this purpose is incorporated into plastic films with a black backing to absorb the unreflected polarization, or it is encapsulated to apply as a paint and used for a variety of medical, engineering, and consumer applications. 

There are of course many open questions and further possibilities in the field. Some specific points were emphasized in the text. It should be remarked that up to now most researchers concentrated on the nematic phase. Although there are still many important aspects to be investigated even in this phase, the study of other mesophases looks very promising as well. We called attention already to the problem of optical reorientation in the cholesteric and smectic C phases. Regarding thermal effects we remind that the interesting point about nematics is the nearly critical behaviour near the nematic - isotropic phase transition. Similar phenomena can be expected to take place at other second-order phase transitions such as the smectic A - smectic C or some of the nematic- smectic A transitions. 

The CD reflection spectra are quite sharp at all temperatures, and the reflection wavelength, corresponding to the optical pitch of the TChLC phase, increased progressively with temperature from 500 nm at 70 °C to 1,000 nm at 140 °C. It was considered that the positive sign of the CD reflection band indicated M screw sense helicity of the cholesteric phase. Very recently, a smectic A-cholesteric phase transition was also observed for PDMBS.348... 

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

Liquid crystals represent a transition between solid crystalline substances and isotropic liquids. On heating, mesophases are formed that have ordered structures which can be nematic, smectic or cholesteric. On further heating, the orientation is disturbed and the phases are converted into an isotropic liquid. The long structure of liquid crystals causes isomers with more drawn-out shapes to be readily dissolved in the ordered liquid crystal substrate ( mesophase ) thus yielding stronger sor-bat-sorbent interactions,... 

Fig. 5.2.5. Smectic A-nematic (or cholesteric) transition entropy versus ratio of transition temperatures Solid line is theoretical curve taken from fig.

Lubensky has shown that, in principle, the smectic A-cholesteric transition should always be of first order, in analogy with the behaviour of a superconductor in an external magnetic field. The relative shift of the... 

Fig. 5.5.3. Pitch versus temperature in cholesteryl nonanoate prior to the smectic A-cholesteric transition (74 °C) the crosses are values obtained from observations of the Grandjean-Cano walls and the circles from the wavelengths of maximum reflexion. (After Kassubek and Meier. >)...

The Goldstone mode in an achiral SmC tries to restore the symmetry of the smectic A phase Cooh —> Dooh by free rotation of the director along the conical surface with the smectic layer normal as a rotation axis. Thus, like chiral molecules convert a nematic into a cholesteric, they convert an achiral SmC into chiral SmC without any phase transition. In addition, mixing left (L)- and right (R)-handed additives results in a partial or complete compensation of the helical pitch both in cholesterics and chiral smectic C. For example, the L- and R- isomers of the same molecule taken in the equal amounts would give us a racemic mixture, that is achiral SmC without helicity and polarity. 

Fig. 9. Schematic dependence of the wavelength of selectively reflected light on temperature for a typical side chain polymer liquid crystal. Tsch denotes the cholesteric to smectic A phase transition and above this temperature the reflected colour changes markedly on heating. Rapid cooling of the material to below Tg stores the reflectance colour obtained during the heating cycle.

Like the APD mode, depending on the materials, the homogeneous texture of the smectic C phase with the stripe-shaped pattern can be formed by direct transition from the cholesteric phase or isotropic liquid phase without an applied electric field. However, most materials having the transition without a smectic A phase, do not exhibit such a homogeneous texture, but forms two kinds of layers at random. 

This acrylate exhibits a melting point at 59°C, becoming a birefringent melt that exhibits the characteristic features of a smectic A phase via microscopy. Near 100°C there is some indication that the polymer transforms to a cholesteric phase however, the isotropic melt phase appears at nearly the same temperature, with the polymer clearing at 108°C. These overlapping transitions result in the broadening of the endotherm, which appears between 97 and 120°C in Figure 7.5. 

Figures (a) Models for the smectic-B and hexagonal columnar phases of PyAG-18. (b) Variation in the cholesteric-to-columnar transition temperature with the polymerization degree. (Taken from J. Watanabe and Y. Takashina. Macromolecules 24 3423,1991. With permission.)...

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