Cholesteric polymer

DJ Broer, J Lub, and GN Mol, Wide-band reflective polarizers from cholesteric polymer networks with a pitch gradient, Nature, 378 467-469, 1995. 

The best known low molar mass substances forming cholesteric phases are esters of cholesterol, which are responsible for the name cholesteric phase. Therefore it was straightforward to attach these derivatives to a polymer in order to obtain cholesteric polymers. These experiments, however, failed because in all cases not cholesteric but isotropic, smectic or crystalline polymers resulted 22) (refer also to Shibaev and Plate, this issue). 

Another possibility to obtain cholesteric phases is well established for l-l.c. s. Nematic phases can be converted into cholesteric phases by the addition of chiral molecules, which must not necessarily have a mesogenic chemical constitution (induced cholesteric phases). With increasing amount of the chiral derivative an increasing helical twist is induced. This principle can also be applied to obtain cholesteric polymers 81 S3) in form of... 

The selective reflection of circularly polarized light on radiation of normal light is also exhibited by the cholesteric polymers. Like the 1-l.c. systems, the Grandjean-texture is formed spontaneously, if the polymer is sandwiched between glass plates as shown in Fig. 23. The measurements of indicate no difference to low molar mass systems. In Fig. 24 VHT) is shown for the induced cholesteric polymers, whose birefringence was discussed above (refer to Table 8, copolymers No. 4, m = 6). The different curves refer to different mole fractions of the chiral comonomer. With... 

Actually, these considerations are confirmed by experiments 82). The systems investigated are shown in Table 8, No. 4, which are induced cholesteric polymer systems. The nematogenic host molecules of benzoic acid phenyl esters are linked via spacers of different length m (m = 3, 4, 5, 6) to the polymer backbone. The polymers are converted to polymers having a cholesteric phase by the chiral cholesteryl derivative, which is also linked to the polymer backbone (copolymer). 

While for nematic polymers the statistical distribution of the centers of gravity of the mesogenic side chains is compatible with a more or less statistical main chain conformation, for smectic polymers a three dimensional coil conformation is no longer consistent with the layered structure of the mesogenic side chains. The backbone has to be restricted in its conformation, which will cause a more pronounced interaction between the main chain and the anisotropically ordered mesogenic side chains, compared to nematic and cholesteric polymers. 

With these properties a wide field of application is revealed As the l.c. side chain polymers can be orientated in the l.c. state by an electric or magnetic field, it is possible to store any information obtained in the l.c. state by cooling the liquid crystalline polymer down to the glassy state. Obvious applications are e.g. optical filters or reflectors, prepared for linearly or circularly polarized light by cholesteric polymers. Furthermore the glassy polymers can serve as anisotropic matrices for dissolved molecules. 

Consequently, as in the case of nematic LC polymers (see Chap. 4.2), apparently, a greater extent of ordering is typical for cholesteric polymers than for their low-molecular analogs. The enthalpy of melting AHcl of polymeric cholesteric is close to that of nematic LC polymers. 

Strzelecki and coworkersfound that the use of (+) 3-methyladipic acid in the synthesis of liquid crystal polyesters led to a cholesteric polymer. They prepared a homopolymer and a series of copolyesters of the following structures ... 

H. Narisawa, R. Kishi, and M. Sisido, Photocontrol of orientation ofdichroic dyes in cholesteric polymer films, Macromol Chem. Phys. 196, 1419-1430 (1995). 

Osipov, M.A. Molecular theory of cholesteric polymers. In Liquid Crystalline and Mesophoric Polymers, Shibev, V.P., Lam, L., Eds. Springer-Verlag New York, 1994 1-25. 

FIGURE 6.19 Shibaev s ternary photochromic cholesteric polymer. 

The more concentrated cholesteric polymers in solution have small pitch. No satisfactory theory has been presented to describe the relationship between... 

High viscosity is the main drawback for cholesteric liquid crystalline polymers in applications. However, cholesteric polymers have their advantages. They may exhibit the memory effect. These polymers make processing and handling easily. Therefore, cholesteric liquid crystalline polymers have become the new materials for optical filters, temperature indicators, etc. The Merck product with the trade name Transmax is actually associated with the cholesteric liquid crystalline polymers. 

The polysiloxane cholesteric polymers doped by diphenyl ketone were used as an optical storage material (Pinsl et al., 1987). The display was based on the convertible photo-reaction. The information was written by a semiconductor laser of wavelength A = 800 nm. The storage density of 1.33 x 109 bytes/cm2 was achieved. The image can be kept up to two or three months. 

D. Medvedeva, A. Bobrovsky, N. Boiko, V. Shibaev, I. Zavarzin, M. Kalik, M. Krayushkin, A combination of selective light reflection and fluorescence modulation in a cholesteric polymer matrix. Macromol. Rapid Commun. 26, 177-182 (2005)... 

In addition to the homopolymers also copolymers were studied, in which the comonomers were characterized by spacer groups of different length. The combinations studied were n = 2/6 n = 6/12 and n 2/12. Smectic polymers were observed by copolymerization of monomers with n = 2/6 and n = 6/12, whereas cholesteric polymers were obtained by copolymerization of monomers with n = 2/12, if the composition was approximately 1 1. Apparently large differences in spacer... 

The expected preference of a statistical distribution of the mesogenic side chain has so far been only realized by the copolymerization in which n = 2 n.p = 12 and only if the monomer ratio is 1 1. Other compositions also led to smectic phases. The best proof for its cholesteric structure is its selective reflection of left-circular polarized light in the visible which indicates a left-handed cholesteric helix of the polymer. The structure has been established also by x-ray investigations (8). This copolymer is the first enantiotropic cholesteric polymer. 

CHOLESTERIC POLYMERS WITH MESOGENIC SIDE GROUPS... 

The alternatively black and white retardation lines are connected to the cholesteric pitch or more precisely to the anisotropy of polarizability of the macromolecules and to their orientation relative to li t propagation. Figure 2 illustrates how in the case of the same polymer, these finger print patterns are modified by dilution of the cholesteric polymer in a simple solvent. A quantitative measurement of the disappearance of the texture at the order-disorder transition temperature can be obtained by recording the light... 

Reflective Bistable Cholesteric/Polymer Dispersion Display... 

Kim, Y.C., et al. 1999. Reflective bistable cholesteric/polymer dispersion display with a black polypyrrole electrode cast from the solution. Mol CrystLiq Cryst 327 157. 

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