Cholesteryl nonanoate transitions

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

The phase transitions of cholesteryl nonanoate have been studied with a new apparatus for thermal analytical microscopy. The enantiomer ratio of some chiral sulphoxides can be changed from racemic to a modest preference for one enantiomeric form by dissolution in a cholesteryl ester in its liquid-crystalline ( cholesteric ) state. 5,6-Epoxycholestan-3-yl p-nitrobenzoates exhibit liquid-crystal properties, but 5,6-diols and dibromides are inactive. ... 

Price, F. P. and Wendorff, J. H. Transitions in mesophase forming systems. Ill, Transformation kinetics and textural changes in cholesteryl nonanoate. J, Phys. Chem. 76, 276 (1972)... 

In the 1960s, there was already a forerunner of photochemically induced phase transition in LC (20). When mixtures of cholesteryl iodide and cholesteryl bromide with cholesteryl nonanoate were exposed to UV irradiation, the helical pitch of cholesteric LC changes as a result of photodecomposition of the halides. The reflected color shifts gradually to red with progression of photodecomposition. Pattern-wise imaging was demonstrated but the image was blurred within 15 min. since LC of small molecules is a viscous fluid. 

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. >)...

An electric field induced cholesteric-to-nematic transition is demonstrated in Fig. 4 for a partially compensated cholesteric mixture of cholesteryl chloride and cholesteryl nonanoate. The sample has been sandwiched between two glass plates in such a way that the helix axis is parallel to the glass surface. The distance between two adjacent dark (or bright) lines is therefore a measure for the pitch fo the cholesteric phase. It is clearly seen that the pitch increases with increasing field strength. At a critical field strength of = 10,000 V/cm the sample has become nematic with the director oriented parallel to the electric field. 

Figure 4. Electric field-induced cholesteric-to-nematic transition of a 1.8 1 by weight mixture of cholesteryl chloride and cholesteryl nonanoate. Viewed through crossed polarizers (magnification X 200). The helix axis was parallel and the electric field was perpendicular to the glass plates (separated by 20 jum). The mixture was nematic 5°C below the temperature of observation which was T = 40° C. a) o V/cm b) 3000 V/cm c) 6000 V/cm d) 10,000 V/cm.

Another typical example for solute-induced pitch shifts is the color change produced in (cholesteric) mixtures of cholesteryl derivatives upon addition of cis-or fram-isomers of azobenzenes or stilbenes. In a cholesteric mixture of cholesteryl chloride and cholesteryl nonanoate (70 mole percent) cis azobenzene produces a red shift, while the trans-isomer induces a blue shift [143]. For cholesteric mixtures well above the cholesteric to smectic transition the shift in the reflection... 

Figure 10. General overview for the reduced heat capacity per mole Cp/R for a temperature range covering all phase transitions involving the blue phases in cholesteryl nonanoate (CN) [56].

Figure 11. Detailed plot of the enthalpy near the phase transition from the blue phase III to the isotropic phase in cholesteryl nonanoate (CN). Note that for clarity a large linear background 140 R (T-T ) with Tg = 90.73°C, has been subtracted from the direct data. The two vertical dashed lines indicate the width of the two-phase region [56],...

Further specific volume measurements with chiral mesogens, cholesteric, and blue phases have been made [26-28, 159-161]. The influence of chiral dopants on C/A and C/N phase transitions has been studied [37, 162]. Earlier studies, particularly on cholesteric mesophases and their mixtures, have been published, for example, for cholesteryl acetate [163], cholesteryl myristate [53], cholesteryl nonanoate [54], cholesteryl... 

Adamski and Klimczyk analyzed cholesteryl pelargonate36) and caproate 37) liquid crystal to fully-ordered-crystal transitions over a temperature range of about 25 K. Again, the appearance of the fully ordered crystals was that of a spherulitic superstructure. The nucleation was time dependent, and the linear growth rate of the spherulites decreased with decreasing temperature by a factor 1/2 to 1/3, in contrast to the nonanoate and acetate. The Avrami exponent was close to 4 as judged from the measurement of the crystallized volume in the field of view under the microscope. 

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