Polymeric Cholesteric Liquid Crystals

It is possible to make the colour of a cholesteric LC independent of temperature by locking it covalently into a polymer matrix. This can be achieved by cross-linking parts of the sample at different temperatures or by quenching locally at temperatures below the glass transition temperature. 

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Optically active polymerizable liquid-crystal monomers, (I), providing polymeric cholesteric liquid-crystal films were prepared by Seki et al. (2) and used in optical applications. 

Matsui T, Ozaki M, Yoshino K (2004) Tunable photonic defect modes in a cholesteric liquid crystal induced by optical deformation of helix. Phys Rev E 69 061715 Yoshida H, Lee CH, Fujii A, Ozaki M (2007) Tunable chiral photonic defect modes in locally polymerized cholesteric liquid crystals. Mol Cryst Liq Cryst 477 255... 

Song MH, Park B, Shin K-C, Ohta T, Tsunoda Y, Hoshi H, Takanishi Y, Ishikawa K, Watanabe J, Nishimura S, Toyooka T, Zhu Z, Swager TM, Takezoe H (2004) Effect of phase retardation on defect-mode lasing in polymeric cholesteric liquid crystals. Adv Mater 16 779-783... 

Song, M. H. et al. 2004. Effect of phase retardation on defect-mode lasing in polymeric cholesteric liquid crystals. A(iv. Mater. 16 779-783. 

To form cholesteric liquid crystalline polymers, one either polymerizes cholesteric monomers or mixes low molecular mass cholesteric liquid crystals with polymers. In the latter case, two components may be mixed homogeneously or in such a way that the polymers act as a matrix while the small molecular mass cholesteric liquid crystals are in droplets, known as the polymer-dispersed liquid crystals (PDLC) (Doane et al., 1988) or the nematic curvilinear aligned phase (NCAP) (Fergason, 1985). In addition, there are many polymers in nature exhibiting the cholesteric phase such as PBLG, cellulose, DNA, etc. 

Similar through-space asymmetric polymerization from achiral mono-, di-, or tri-thiophenes and pyrrole monomers was also achieved by the use of cholesteric liquid crystals as an asymmetric reaction solvent [19]. As no reaction occurred between the molecules of the liquid crystal and the monomers, the chiral morphology of the polymers (which have no chiral substituent) is considered to derive from the asymmetry produced by the chiral liquid crystal medium during polymerization. Heat treatment of the polymer causes disaggregation and a loss of chirality, and polymers prepared in this way exhibit an exiton splitting signal in the circular dichroism spectra in the absorbance region of the polymeric backbone they also display a circular polarized luminescence. A representative example is shown in Scheme 8.2 [19]. 

The electrochemical asymmetric polymerization of achiral pyrrole and di(2-furyl)-2,5 substituted phenylenes within a cholesteric liquid crystal electrolyte solution has been also reported [20-22]. 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

PSCT reverse-mode light shutter - This is also made from a mixtore of cholesteric liquid crystal (Ae > 0) and a small amount of monomer. The pitch of the liquid crystal is a few microns (-3-15 pm). The mixture is placed into a cell with homogeneous alignment layers. The mixture is in the planar texture at zero field because of the alignment layers. The monomers are polymerized in the planar textore. The polymer network formed is parallel to the cell surface [43,53,54]. 

Suto S. Hydroxypropyl cellulose (crosslinked, cholesteric liquid crystal). In Salamone JC, ed. Polymeric Materials Encyclopedia, Vol 5. Boca Raton, FL CRC Press, 1996 3167-3171. 

Figure 5. The SEM picture shows the helicoidal morphology of RM 3 polymerized in the planar texture of a cholesteric liquid crystal.

The characteristic functionalities of naturally occurring polymers are, in most cases, related to their specific chiral structure. In nature, proteins, nucleic acids, and polysaccharides are constructed of readily available chiral monomers such as sugars and amino acids. Both natural and synthetic chiral polymers are finding application as chromatographic supports, polymeric reagents and catalysts, chiral membranes, and materials for preparation of cholesteric liquid crystal polymers (471,472). 

The second group of LC composites - encapsulated polymeric films are usually based on the cholesteric liquid crystals... 

Shibaev PV, Tang K, Genack AZ, Kopp V, Green MM (2002) Lasing from a stiff chain polymeric lyotropic cholesteric liquid crystal. Macromolecules 35 3022-3025... 

Asymmetric electrochemical polymerization of thieno[3,2-h]thiophene (1), 2,2 -bisthieno[3,2-h]thiophene (bis-TT, 48), and 4,7-bis-thieno[3,2-h]thiophen-2-yl-benzo[l,2,5]thiadiazole (TT-Btdaz, 113) [46], linked through the TT 2-positions, was carried out in cholesteric liquid crystals (CLCs) (Scheme 25) [47]. 

Some of the very early applications of CPL spectroscopy involved chiral polymeric systems. In particular, the CPL from achiral dye molecules dissolved in cholesteric liquid crystals has been used to probe chirality changes. CPL from chromophores attached to a chiral poly-amino acid may also be used to study exciton coupling between aromatic chromophores. In these polymeric systems, it is often observed that gijjjjj is quite large. In some cases it is so large, in fact, that detection using static quarter-wave plates is possible. 

Churchill D, Cartmell JV (1971b) Display device containing minute droplets of cholesteric liquid crystals in a substantially continuous polymeric matrix. US Patent 3,600,060 Cole KS, Cole RH (1941) Dispersion and absorption in dielectrics-I alternating current characteristics. J Chem Phys 9 341-351... 

The pitch and handedness of the cholesteric helix can be affected by several factors temperature, chemical stmcture of the side chain and degree of polymerization. It was observed that in films casted from HPC cholesteric liquid crystal phase the pitch increases (Charlet and Gray 1987) with temperature. 

Kawabata, K. Takeguchi, M. Goto, H. Optical activity of heteroaromatic conjugated polymer films prepared by asymmetric electrochemical polymerization in cholesteric liquid crystals Structural function for chiral induction. Macromolecules 2013,46, 2078-2091. 

The macromolecular nature provides an interesting feature of LC polymeric cholesterics, namely the possibility of obtaining monochromic films. Thus for polymeric liquid crystals the helix pitch is practically not altered with temperature below Tg, when a cholesteric phase is frozen in a glassy matrix (Fig. 23a). This implies that fast cooling of polymeric films from a mesomorphic state (shown with arrows) fixes their optical properties, which makes it possible to use them at ordinary temperatures as selective monochromic reflectors. On the other hand, such polymeric films display the extraordinary polarizing properties of cholesterics, i.e. the different absorption... 

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