Chiral nematics phase transitions

Dong, X.M., Kimura, T., Revol, J.-F., Gray, D.G. Effects of ionic strength on the isotropic—chiral nematic phase transition of suspensions of cellulose crystallites. Langmuir 12, 2076-2082 (1996)... 

X. M. Dong, T. Kimura, J-F. Revol, and D. G. Gray, Effects of Ionic Strength on the Isotropic-Chiral Nematic Phase Transition of Suspensions of Cellulose Crystallites. Langmuir 12, 2076-2082(1996). 

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. 

Finally, dispersions of MWCNT in chiral nematic liquid crystals were studied as well. These experiments suggested no change in the helical twisting characteristics of the chiral nematic phase. However, the MWCNTs were thought to disrupt the translational order in the SmA phase (decrease of the SmA-N phase transition) yet follow the twist of the nematic director in the chiral nematic phase [498]. 

In contrast, compound 53 exhibits only one enantiotropic transition by DSC, which occurs between a chiral nematic phase and the isotropic liquid, in... 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

It is worth mentioning that another type of tricarbonylchromium complexes derived from cholesteryl 4-alkoxybenzoate ligands have been reported to show broad chiral nematic phases [118]. The complexes are unsymmetrical, and this may the be the reason of the important decrease of both the melting and clearing transition temperatures as compared to the metal-free ligand. 

Note Phases are shown as k = eiystal phase N = nematie phase N = chiral nematic phase i = isotropic phase. The temperature of the transition from one to another is represented by the number between the phases on heating. That is, k to N at 106°C and N to i at 117°C. 

The phototuning of BPs can also be fabricated in a pure material system [147]. Das et al. reported a light-induced stable blue phase in photoresponsive diphen-ylbutadiene based mesogen 37. This compound was found to exhibit SmA and N during heating. When the temperature was kept at 118 °C, the photoisomerization induced an isothermal phase transition from SmA to N. Photoirradiation of the SmA film held at a higher temperature (124 °C) for 100 s resulted in transition to a phase with a characteristic classical BP texture showing in Fig. 5.30. The BP was thermodynamically stable and could be maintained at this state for several hours. The characteristic sharp reflection bands compared to the rather broad reflection bands observed for the chiral nematic phase confirmed the formation of BP. The photoinduced formation of the BP exhibited a reflection centered at 510 nm. Subsequent irradiation led to the blue shift to 480 nm in the reflection band. 

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