Transition chiral nematics

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 introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

C stands for crystalline phase, N for nematic phase, N for chiral nematic, S , Sb etc. for smectic phases and I for isotropic liquid. The numbers in between are the transition temperatures in Kelvin. 

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. 

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]. 

One compound may exist in more than one liquid crystal form. The structure of cholestryl myristate changes from solid to smectic A at 71 °C, to chiral nematic at 79 °C, and finally to an isotropic liquid at 85 °C. Figure 16.10 shows the transitions in p-methoxybenzoic acid. Note that the order increases with increased pressure and lower temperature. Liquid crystal phases are more densely packed than the... 

Table 3.2 Transition temperatures (°C) for chiral nematic derivatives of cholesterol (12-14) and a tertiary mixture (30%I56%I14%. respectively) for a CNPC-LCD prototype ...

The material was chiral-nematic and had an apparent glass transition at 276 K and a clearing point at 325.2 K, as judged by DSC measurements. The clearing point was very sharp, the biphasic range being 0.5 K as determined by optical microscopy. 

Figure 2.9 Temperature-dependence of the axial ratio f = bx/d of Kuhn length bf to molecular diameter d for poly(hexylisocyanate) (PHIC) in toluene (O) and tetrahydrofuran ( ) and for hydroxypropylcellulose (HPC) in dimethylacetamide (A). Here is the transition temperature from the nematic or chiral nematic to the isotropic phase in the melt. The lines fit the expression

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

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

The analogues of the smectic A" and G" phases, the TGBA" and TGBG phases, have been detected at the chiral nematic to smectic A" and the chiral nematic or TGBA" to smectic G" transitions [16, 17]. Two helical forms of the TGB phases are possible, one where the helical structural arrangements of the blocks or sheets of the smectic phase are rational, i.e., the number of blocks required to form a 360° rotation of the helix is a whole number, in which case the phase is said to be commensurate , or alternatively where the number of... 

Looking back, the earliest written report concerning the possibility of having a smectic phase with twisted layers comes from the work of Wolfgang Ullrich Muller at the Technischen Universitat Berlin in 1974 [40], In his thesis entitled Verhalten cholesterischer Mesophasen unter dem Einfluss von Phasenum-wandlungen , Muller examined the mesophase behavior at the chiral nematic to smectic transition for various mixtures of cholesteryl oleoyl carbonate (COG), 5, and cholesteryl chloride (CC), 6. Muller came to the conclusion from his work that the smectic phase must have a helical structure and that the heli-axis must lie in the plane of the layers, and the structure must also have... 

When the terminal ester functionality in the phenyl propiolates is replaced with a carbonyl group, i.e., a keto function, TGB phases disappear altogether indicating that the increased size of the lateral dipole at terminus to the core has detrimental effects on the stability of TGB phases [35]. These small variations in structure show how delicate the stability of the TGB phase is, nevertheless, most chiral nematic to smectic phase transitions are accompanied by a TGB phase no matter how transient it is. 

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. 

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