Side Chain Polymeric Liquid Crystals

There has also been more recent work on the application of these types of materials in dynamic holographic recording, and this will be covered in section 5.4.3.3. 

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Fig. 27 a and b. Schematic representation of the molecular structure of a side chain polymeric liquid crystals b polymer model membranes studied by 2H NMR... 

The side chain polymeric liquid crystal (as host) is doped with NLO active small molecules (as guest). In the matrix of side chain polymeric liquid crystals, the NLO active molecules are aligned along the director. The system is then poled and processed. Meredish et al. (1982) adopted a side chain polymeric liquid crystal as the host and DANS as the dopant, as shown in Figure 6.32. The pole field is 1.3KV/mm. The resultant second NLO coefficient reached 6 x 10-9 esu. The glass temperature of the polymers is low (Tg < 50 °C) thus the orientation is not good enough. 

Figure 6.32. The host (side chain polymeric liquid crystal)—guest (DANS) system. (Modified from Meredish et al., 1982.)...

Griffin synthesized a side chain polymeric liquid crystal in which side group mesogens are NLO active themselves (Griffin et al., 1988), shown below... 

The material exhibits low transparency because its glass transition temperature Tg is low. Therefore, after the electric poling, the molecular orientation is not satisfactory and thus the sample scatters light. Recently, a side chain polymeric liquid crystal with high Tg was synthesized and the performance got improved. Mclulloch and Beiley (1991) achieved 33 of 19 x 10-7 esu (A = 1.31 xm). 

The orientation of side chain polymeric liquid crystals after the poling is not ideal and the defects appear often. Two techniques were proposed to deal with these problems ... 

Shibaev et al. (1984) first synthesized a ferroelectric side chain polymeric liquid crystal. In the following years a lot of liquid crystalline polymers of such kind were synthesized. In early research studies techniques used to understand polymers and whether they showed the liquid crystal phase were limited so that the conclusion was ambiguous. It was only in 1988 when Uchida et al. (1988) measured the spontaneous polarization and the tilt angle that people became convinced that this side chain polymer in the literature (Shibaev et al., 1984) is indeed a ferroelectric liquid crystal. 

Table 6.13. The phase transitions of side chain polymeric liquid crystals PI, P2 and P3. (From Sekiya et al., 1993.)...

The three side chain polymeric liquid crystals have approximately the same molecular weight, for PI Mn = 4,600 P2 4,700 and P3 4,900. 

It is observed that compared with the constituent small molecular mass liquid crystal monomers the side chain polymeric liquid crystals have a wider Sc temperature range, its upper limit going up while its lower limit goes down. 

Early studies on the use of liquid crystals as stationary phases focused on packed columns however as with other gas chromatographic applications, the technology has now almost exclusively shifted to capillary columns. Several reviews outlining the fundamentals and applications of liquid crystal stationary phases in both packed and capillary columns are available [444-449]. Stationary phases can be prepared from either monomeric or side-chain polymeric liquid crystals. Particularly relevant to the latter is the increased use of polysiloxane and poly-arylate backbones for the attachment of... 

In side-chain polymer liquid crystals (SCPLC), the mesogenic units are attached, by grafting or by polymerization of appropriate monomers, to the main chain. 

The recent work of Krigbaum on main-chain and Finkelmann, Ringsdorf, etc., on side-chain polymer liquid crystals has generated much interest in the potential of these systems for use in electro-optic devices. The combination of polymeric specific and monomeric liquid crystal specific properties leads to an interesting range of potential materials for new display devices. The majority of the research over the last five years has concentrated on synthesis and the establishment of the basic property-structure relationships. However in the last year or so papers have started appearing where the electro-optic properties of some of these materials have been examined." ... 

The study of blends of polymeric liquid crystals with low-molecular liquid crystals of known mesophase types, aiming at identification of polymeric mesophases, is at its very beginning there are only a few works concerning polymers with mesogenic groups in the main chain 67 69) and in the side chains as well 70 74). in view of the importance of such investigations, note that the principle of miscibility is thoroughly developed for low-molecular liquid crystals, whose molecules are similar in their sizes the justifiability of its application to the blends of polymers with low-molecular liquid crystals is not equally evident, as the molecular sizes of the components differ substantially. 

The discovered dependence of kinetic parameters of orientation processes on the degree of polymerization 44) is a consequence of the duplex nature of LC polymers — that is the presence of the main chain and of mesogenic side groups. This is why a correct juxtaposition of the kinetic characteristics of orientational processes of low-molecular and polymeric liquid crystals requires an explicit knowledge of the degree of polymerization of a corresponding polymer. 

A special class of polymeric liquid crystals is obtained by attaching rigid side chains to a flexible backbone. Inasmuch as it is the side chains that engage in formation of the liquid crystalline domains, these systems are more closely akin to low molecular liquid crystals. Polymeric liquid crystals of this type are discussed by Rehage and Finkelmann in Vol. 60 of this series. 

Side-chain type polymeric liquid crystal with side-on attachment Fig. 4.8 Basic types of liquid crystalline polymers... 

Thermotropic side-chain ionic liquid-crystalline polymers are particularly attractive when the aim is that of merging the liquid-crystalline characteristics of the low molecular weight mesogen side groups with the mechanical properties of the polymeric main chain. It is not surprising, then, that they attracted most of the research efforts in the polymeric ionic liquid crystals field. 

Noel, C. and Billard, J., 1978, Mol. Cryst. Liq. Cryst. (Lett.), 41, 269 Noel, C., 1985, in Polymeric Liquid Crystals, ed. A. Blumstein, New York Plenum. Noel, C., 1989, in Side Chain Liquid Crystal Polymers, ed. C.B. McArdle, Glasgow Blackie. 

The last volume is devoted to polymeric liquid crystals (with chapters about main-chain and side-group thermotropic liquid crystal polymers), amphiphilic liquid crystals, and natural polymers with liquid-crystalline properties. 

C. Noel and J. Virlet, "DSC, Miscibility and X-ray Studies of the Thermotropic Liquid Crystalline Polyesters with Aromatic Moieties and Flexible Spacers in the Main Chain", in "Liquid Crystals and Ordered Fluid, A. Griffin and A. Johnson eds., Plenvim Press, New-York, vol. 1+, 1+01 (198I+) Finkelmann, "Synthesis, Structure, and Properties of Liquid Crystalline Side Chain Polymers", in "Polymeric Liquid Crystals", A. Ciferri, W.R. Krigbaum and R.B. Meyer eds. Academic Press, New-York (I982)... 

The results of the present work demonstrate that the cholesteric mesophase of polymers having mesogenic side groups is, in many aspects, similar to that of low molecular cholesterics. The presence of the polymeric chain results, however, in the appearance of some specific features in the behavior of such systems. The possibility to quench the cholesteric structure by vitrification of polymers is worth emphasizing explicitly. The properties of polymeric cholesterics discussed above are far from covering the array of specific features particularly intrinsic to polymeric liquid crystals. The essential effect of molecular parameters on their structure and optical properties constitute one of these features. This problem will be treated in subsequent publications. 

The papers presented in this symposium give some indication of the wide variety of polymers which are now known to form liquid crystalline phases Polymeric liquid crystals are usually classified according to the mesophase structure e g., nematic, cholesteric, smectic A, etc ). However, these classes are quite broad For example, the cholesteric lyotropic phases formed by synthetic polypeptides in suitable solvents differ markedly from the cholesteric thermotropic phases formed from silicone polymers with cho-lesteryl ester side chains. In particular, the driving forces behind the formation of the mesophases are quite different for these two examples, being essentially due to chain stiffness in the first case and to anisotropic dispersion force interactions in the second case It may therefore be useful to classify polymeric liquid crystals according to the polymer chain structure ... 

Much of the current research on polymeric liquid crystals is directed towards thermotropic liquid crystals which are formed when certain polymer melts are cooled. Many of those polymers includes chemical groups similar to low-molecular-weight nematics either along the main chain or in the side chains. These materials are technologically interesting because of their ease of processing. Theoretically, these materials are expected to show curious behaviours which are a mixture of their... 

In general, polymeric liquid crystal materials are classified according to the dominant features of their backbone structure into backbone, side chain or... 

Polymeric liquid crystals can be classified into either of two types, thermotropic and lyotropic, according to their formation conditions, or three types, main-chain, side-chain and complex liquid crystalline polymer (MCLCP, SCLCP and CLCP,... 

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