Melting chiral nematics

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

The history of liquid crystals started with the pioneer works of Reinitzer and Lehmann (the latter constructed a heating stage for his microscope) at the end of the nineteenth century. Reinitzer was studying cholesteryl benzoate and found that this compound has two different melting points and undergoes some unexpected color changes when it passes from one phase to another [1]. In fact, he was observing a chiral nematic liquid crystal. 

Figure 2.1 A schematic representation of the melting of crystalline cholesteryl benzoate (I) at 145.5 °C to form a chiral nematic (cholesteric) phase, which in turn forms the isotropic liquid on further heating to 178.5 °C. ...

Using appropriate melting point apparatus, characterize the melting behavior of the purified solid. Cholesteryl nonanoate exhibits a chiral nematic, more commonly known as a cholesteric, liquid crystalline phase around 85°C and melts to the isotropic liquid around 93 C. The cholesteryl nonanoate first melts to form a smectic phase around 75°C and with further heating transforms to the cholesteric phase around 85°C. 

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

The laterally appended dendrimer, 32, shown in Fig. 27, as expected exhibits a chiral nematic phase, with smectic mesophase formation being suppressed. The clearing point is almost 50 °C lower, whereas the melting point is only 25 °C lower in comparison to the terminally appended system. This demonstrates that lateral appendages of the mesogens causes disruption to the intermolecular packing, thereby destabilizing mesophase formation. The local structure of the chiral nematic phase is thus shown in Fig. 28. 

Yelamaggad et al reported the more elaborate acetylide complexes 164 (n = 3, 4, 5, 7), all of which showed a chiral nematic phase due to the presence of the chiral cholesteryl moiety. For all complexes, decomposition occurred somewhere above 200 °C—the lowest melting point was seen for the complexes with the two heptamethylene spacers. 

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. 

Only a few solvents are known to dissolve cellulose completely, and solid cellulose decomposes before melting. Therefore, it is difficult to study the mesophase behavior of cellulose. Chanzy et al. [32] reported lyotropic mesophases of cellulose in a mixture of jV-methyl-morpholine-Af-oxide and water (20-50%), but were unable to determine the nature of the mesophase. Lyotropic cholesteric mesophase formation in highly concentrated mixtures of cellulose in trifluoroa-cetic acid + chlorinated-alkane solvent [33] and in ammonia/ammonium thiocyanate solutions [34] has been studied, and although poor textures were obtained in the polarizing microscope, high optical rotatory power has been measured in an optical rotation (ORD) experiment, which could be fitted to the de Vries equation [Eq. (3)] for selective reflection beyond the visible wavelength region and was taken as proof of a lyotropic chiral nematic phase. 

Fenchenko studied free induction decays and transverse relaxation in entangled polymer melts. He considered both the effects of the dipolar interactions between spins in different polymer chains and within an isolated segment along s single chain. Sebastiao and co-workers presented a unifying model for molecular dynamics and NMR relaxation for chiral and non-chiral nematic liquid crystals. The model included molecular rotations/ reorientations, translational self-diffusion as well as collective motions. For the chiral nematic phase, an additional relaxation mechanism was proposed, associated with rotations induced by translational diffusion along the helical axis. The model was applied to interpret experimental data, to which we return below. 

In addition to depressing the liquid ciystal phase stability and often reducing melting points, a branched chain that introduces chirality increases viscosity. A high viscosity is not a problem in the non-display, thermochromic applications of chiral nematic materials. The steric effect of a chiral centre is well illustrated by difluoroterphenyl 5. 

The nematic phase is the simplest mesophase. It is described by the director n and the orientational order parameter S. In the nematic phase, the mesogens are arranged in such a way that their long axes lie preferentially in one direction (long-range orientational order). The long-range orientational order is responsible for the characteristic differences with respect to their isotropic melt. If the constituent compound is racemic, it is possible to form another phase from the enantiomericaUy pure compound. This is chiral nematic. 

The best known of the chiral liquid crystal phases is the cholesteric phase or chiral nematic (N ). Here an asterisk is used to indicate a chiral phase. The cholesteric phase (Figure 2.10) was the first liquid crystal to be discovered by Reinitzer in 1888. Reinitzer observed pure cholesterol benzoate under the microscope and noticed two apparent melting points solid crystal to the phase that is now known as cholesteric and then a second melting point to the isotropic liquid phase. Cholesterol is a chiral molecule. 

Many other structural possibilities for the terminal chains can have a great influence on melting point, transition temperatures and mesophase morphology. For example, branched chains are common, usually for the purpose of generating a chiral centre for chiral nematic or chiral smectic C liquid ciystals. The effect of the branch is to broaden the molecules and hence the transition temperatures are usually depressed significantly, often with the largest reduction in the smectic phase stability, but melting points are often not so much affected (compare compounds 29 and 30) [32]. 

Tg = glass transition temperature, M = different columnar phases, in part their structures are as yet unknown, = monotropic phase transition "decomposition, % parent radial pentayne (R = H) with this particular R is non-thermomesomorphic, see its melting point in [17,44,56,60], two crystalline modifications, Et = ethyl group, "this transition temperature is reversible and was obtained on cooling the isotropic melt, chiral molecule carrying five (= R) (S)-2-methylbutyloxy, respectively one (= R ) or five (= R) (S)-3,7-dimethyloctyloxy substituents, diirai nematic discotic (N d) phase, data given in opposite order assigning inverse j ase sequences. 

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