Cholesteric polysiloxanes

Figure 6.47. A cholesteric polysiloxane copolymer applied in the optical information storage application.

This conclusion is confirmed by the data of Finkelmann and Rehage, who examined the optical properties of cholesteric polysiloxanes. These authors revealed that at equal compositions of copolymers the wavelength of selective reflection depends on the length of the spacer binding nematogenic units. 

The reported cholesteric polysiloxanes are characterized by typical reflection of incident light in accordance with the establishment of a right-handed helical structure. The pitch of the helix decreases with increasing temperature with a concurrent shift of the maximum of reflection wavelength from near-IR towards the visible range." ... 

Petn, A., Kummer, S., Anneser, H., Eeiner, E, and Btauchle, C. Photoinduced reorientation of cholesteric liquid crystalline polysiloxanes and applications in optical information storge and second harmonic generation. Ber. Bunsenges. Phys. Chem. 97, 1281 (1993). 

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. 

On the other hand, liquid crystalline polymers applied to optical information storage has attracted great attention. The liquid crystalline polymer is applied mainly in terms of the thermo-optical effect. The backbone of liquid crystalline polymer can be polysiloxane, polyacrylate, or polyesters. In order to enhance the absorption coefficient for the writing laser beam, the dyes may be either dissolved into the liquid crystalline polymer in the guest-host model or attached to the backbone of the liquid crystalline polymer to form a copolymer. The nematic, cholesteric and smectic liquid crystalline polymers are all be able to be utilized in optical information storage. 

Wang, J.-W. Meng, F.-B. Li, Y.-H. Zhang, B.-Y., Synthesis and Characterization of Side-Chain Cholesteric Liquid-Crystalline Polysiloxanes Containing Different Space Groups. J. Appl. Polym. Sci. 2009, 111, 2078-2084. 

In our own work, carried out under an extensive collaboration with Professor George Gray, F.R.S., and Drs David Lacey and Peter Gemmell of Hull University (UK), we have studied a number of nematic, cholesteric and smectic polymer mesophases with a view to exploiting their electro-optical properties. Some of the typical structures studied are given in Fig. 3. These are only a few of the structures now available and the reader is referred to ref. 14 for a more complete range of polysiloxane systems as well as to the recent reviews" which also cover the acrylate and methacrylate backbone systems. 

Fig. 4. Typical polysiloxane side-chain liquid crystal optical textures for (a) nematic, (b) cholesteric and (c) smectic A phases.

Figure 24. Biomesogenic structures a) (Bio)meso-gens displaying order-disorder distributions in CPK-presentation (left to right and top to bottom) hexa-n-alkanoyl-oxybenzene discoid - Chandrasekar s first non-rodlike liquid crystal [28 a, 51c] enantiomeric cholesteric estradiol- and estrone-derivatives [ 17 a, c, d, 26 f, 51 a, s, u] Reinitzer s cholesterolbenzoate [21, 22] - together with the acetate the foundation stones of liquid crystal history [21, 22] Kelker s MBBA -first liquid crystal fluid at ambient temperature [ 13 f, g] Gray s cyanobiphenyl nematics for electrooptic displays [25 a, 51 e] lyotropic lecithin membrane component [7 a, 14, 27 d, 52 a] and valinomycin-K -membrane carrier [7 a, 35] thermotropic cholesteryl-side-chain-modified polysiloxanes with the combination of flexible main-chain and side-chain spacers [51 a, h] thermotropic azoxybenzene polymers with flexible main-chain spacers [51a] thermotropic cya-...

Chien et al. [38] introduced a cyanohydrinester as chiral building block with the strong polar nitril group located at the chiral center. The polysiloxanes 19a-19c exhibit broad Sc phases, as shown in Table 6. With a spacer length of 11 methylene units, only a cholesteric phase occurs. No ferroelectric properties have been reported. For the monomer of 19b, spontaneous polarization of more than 500 nC/cm was measured. 

Several hundred LC polymers with mesogenic side groups which model the structure of nematic, smectic, and cholesteric liquid crystals have now been synthesized with these methods. Polymers of the acrylic and methacrylic series and polysiloxanes containing different types of widely distributed mesogenic... 

---