PBLG, liquid crystal

Fig. 12. Plots of the reciprocal half-pitch vs. temperature for PBLG liquid crystals (O) in m-cresol ( ) in benzyl alcohol...

A similar thermally-induced inversion of the cholesteric sense was observed for the PBLG liquid crystal in benzyl alcohol. In this solution, a gel-like opaque phase coexists with the cholesteric phase at lower temperatures. The opaque phase disappears around 70 °C, where endothermic peaks are observed in the differential scanning calorimetry curve. The value of S below 70 °C remains constant, and then changes with temperature above 70 °C. The compensation occurs at about 103 °C, and the transition from biphasic phase to the isotropic phase is observed above 150 °C in this case. The results are summarized in Fig. 12, where the reciprocal of the half-pitch is plotted against temperature. The sign of 1/S is taken as positive when the cholesteric sense is the right-handed. 

Fig. 16. Temperature dependence of the helical twisting power of PBLG liquid crystals in dioxane-EDC mixed solvents, (c = 0.15 vol%) The contents (vol%) ov EDC in solvent mixtures (O) 0, ( ) 20, (A) 40, (A) 60, ( ) 80, and(H) 100...

Fig. 20. Temperature dependence of the helical twisting power of PBLG liquid crystals in EDC-TFA mixtures...

The dependence of the twisting power of PBLG liquid crystals on the solvent composition is well described by the extended Goossens theory, provided we use x as the molar fraction of one solvent component in a solvent mixtures. The twisting power in the dioxane-EDC system varies linearly with the solvent composition, and it can be fitted to Eq. (29). On the other hand, in the m-cresol-EDC system, the twisting power has a quadratic nature, as indicated by Eq. (28). 

In earlier publications3,6 have presented the results of our measurements of the bend and splay elastic constants of PBLG liquid crystals using the Frederik s magneto-optical distortion procedure. The central experimental necessity of this technique is the measurement of the change in the phase retardation of polarized light... 

Fig. 13. Tilted 2D spectrum of natural abundance 2H Q-COSY experiment of ( )-l-pentyn-ol dissolved in PBLG/chloroform chiral nematic liquid crystal. The spectrum was symmetrized prior to the tilting procedure. Note that all isotopomers of the mixture are clearly visualized. (Reproduced by permission of American Chemical Society.)...

Recently, experimental data of S for various liquid-crystalline polymer solutions have become available. Figure 12 illustrates the concentration dependence of S for three polymer liquid crystal systems (a) PBLG-DMF [92,93], (b) PHIC-toluene [94], and (c) poly(yne)-platinum polymer (PYPt)-trichloro-ethane (TCE) [33]. For systems (a) and (c), with the alignment made by... 

Fig. 9. Temperature dependence of the LCICD magnitude of the side chain phenyl band at 254 nm in the PBLG-dioxane (c = 20.0 %) liquid crystal...

A few non-amphiphilic molecules are able to show liquid crystallinity in solution at a certain range of concentration, such as PBLG, DNA, the tobacco mosaic virus, etc. They are of great molecular mass, very rigid, rod-like and have a very long anisotropic shape. They are typical macromolecules and are lyotropic liquid crystals. This class of liquid crystals does not aggregate to form sphere, column or laminar structures. These lyotropic systems depend on the properties of the solvent. They are one of major interest of this book and will be discussed in detail later. 

An experiment was carried out for PBLG to verify the theory. PBLG molecules in solution adopt an extended a-helical conformation so that they are rigid rods. In Figure 2.7, the critical volume fractions of PBLG in solvents (Flory, 1961) were depicted as a function of the axial ratio. The curve A is for 4> at which the liquid crystal phase starts to appear while the curve B is for 4> at which the isotropic phase completely disappears and the system becomes entirely a liquid crystal. The solid curve is the theoretical expectation while the dashed line is the experimental result. Both are in good agreement. 

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