Liquid crystal polymers chiral nematic

FIGURE 11.17 Schematic representation of the cholesteric or chiral nematic phase, where p is the helix pitch length. 

The synthesis of polymers capable of entering into a chiral nematic phase initially proved difficult as many of the acrylate and methacrylate comb-branch polymers to which a cholesterol unit was attached as a side chain tended to give a smectic phase. This was overcome by either copolymerizing the cholesterol-containing monomers with another potential mesogenic monomer, or by synthesizing mesogens with a chiral unit in the tail moiety. Examples of both types are shown as structures VIII and IX. 

Similar to the other liquid crystal polymers described in Section 11.16, these materials offer the possibiUty of locking the chiral nematic phase into the glassy state by rapid supercooUng to temperatures below T. This leads to a preservation of the structure and, of course, the reflected color, thereby leading to the formation of stable, lightfast, monochromatic films, when suitable systems are used. 

Explain why the ratio of the density of the crystalline regions, p, to the density of the amorphons regions, p, is larger for nylon-6,6 than for isotactic polystyrene. 

Polyfvinyhdene flnoride) forms an orthorhombic nnit cell in which the angles between the b and c, a and c, and a and b axes (a, 3, and y, respectively) are a =P =y = 90°. The dimensions of the orthorhombic unit cell are a = 8.47 A, b = 4.70 A, and c = 2.56 A, and there are two monomer units per unit cell. Calculate the theoretical density of the crystal. The heat of fusion of a (polyethylene oxide) (PEO) sample was determined by DSC and found to be 6.70 kJ mol. By considering that the heat of fusion of 100% crystalline PEO is 8.29 kJ moT, estimate the degree of crystallinity of the sample. 

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Chiral disubstituted PEDOTs have recently been prepared for the first time via transetherification of 3,4-dimethoxythiophene monomers with chiral glycols followed by potentiodynamic oxidation.174 An alternative approach to optically active PEDOTs has also been recently described, which involves the electrochemical polymerization of the EDOT monomer in aqueous hydroxypropyl cellulose (HPC) as a polymer lyotropic liquid crystal to give a chiral PEDOT/HPC hybrid.175 The PEDOT prepared in this chiral nematic liquid crystal exhibited optically active electrochromism in that it could be electrochemically switched between a dark blue reduced state and a sky blue oxidized form that exhibited a different CD spectrum. 

The synthesis of conjugated polymers in chiral nematic liquid crystal fields has been reviewed [9]. 

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. 

K. Akagi, Helical polyacetylene synthesized in chiral nematic liquid crystal, in Handbook of Conducting Polymers, 3rd edn. Conjugated Polymers, ed. T. A. Skotheim, J. R. Reynolds, CRC Press, New York, 2007, Chapter 3, pp. 3-14. 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

It is also known that in side-chain LC polymers the copolymerization of optically active monomers with mesogenic monomers, in the same manner as the mixing of optically active compounds with nematic low molecular weight compounds, can induce the formation of a cholesteric mesophase. Therefore, it is expected that inclusion of chiral spacers in main chain liquid crystal polymers, which would be nematic... 

Blinov, L. M., Barberi, R., Kozlovsky, M. V., Lazarev, V. V., and de Santo, M. P. Optical anisotropy and four possible orientations of a nematic liquid crystal on the same film of a photochromic chiral smectic polymer. / Nonlinear Opt. Phys. Mat. 9, 1 (2000). 

As noted earlier form the work of Freidzon [41], side chain liquid crystal polymers derived from cholesterol can also apparently exhibit TGB phases. However, work on non-steroid systems also reveals that TGB phases can also be formed in typical chiral polymers. For example the polymethacrylate, structure 16, exhibits chiral nematic, TGBA , smectic A , and ferroelectric smectic phases. 

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