Comb-shaped mesogen

The details of the hydrodynamic behavior of comb-shaped mesogenic macromolecules are examined on the example of the polymers reported in Tables 3.2 and 3.3. 

Shibae%, V. R, Kostromin, S. G., and Ivanov, S. A. Photoregulation of the optical properties of comb-shaped polymers with mesogenic side groups and the problems of data recording. Po/ym. Scf. 39, 118 (1997). 

Shibaev VP, Kostromin SG, Ivanov SA. 1996. Comb shaped polymers with mesogenic side groups as electro and photooptical active media. In Shibaev VP, editor. Polymers as Electrooptical and Photo optieal Active Media. Berlin Springer Verlag. 

Figure 4 Schematic representation of macromolecules of different rigidity (a) flexible chains (b) rigid-rod LC polymers (c) main-chain LC polymers with mesogenic groups (d) side-chain LC polymers with mesogenic groups (comb-shaped LC polymers).

Further development of such studies resulted in the self-reinforced plastics based on LC polymers with main-chain mesogenic groups and laid a foundation for the use of comb-shaped LC polymers as new photoactive materials for optics, optoelectronics, photonics, holography, display technology, telecommrmications systems, and other no-less important areas of modem engineering. " ... 

Subsequently, this approach was verified and developed in numerous works performed in many laboratories across the world and has became a commonly accepted method for S)mth-esis of thermotropic LC polymers with side mesogenic groups, or comb-shaped LC polymers. " ... 

A special group of LC polymers is formed by LC ionomers comb-shaped copolymers of mesogen-containing monomer units with unsaturated acids, in which a part of the carboxyl groups is replaced with metal ions. ... 

Here, we will give attention only to the short consideration of the role of three essential stmctural components of comb-shaped polymers - a polymer backbone, a flexible spacer, and a mesogenic group. 

The existence of the backbone in LC polymers leads as a mle to a sharp improvement of the thermal stability of the mesophase due to inaease in isotropization temperature Tiso- Figure 16 shows a comparison of transition temperatures of low-molecular-mass alkoxycyanobiphenyls and different comb-shaped LC polymers with identical mesogenic groups. 

Figure 18 Kinetic curve of decrease in the light intensity (in crossed polarizers) of LC nematic polymer film on application of an electric field and scheme of mesogenic groups orientation of comb-shaped macromolecules.

Figire 21 Schematic representations of macromolecules of CL chiral photochromic comb-shaped polymers containing (1) mesogenic, (2) chiral, and (3) photochromic groups. 

To prepare comb-shaped silbutylidene polymers carrying mesogenic groups, 1-methyl-l-silacyclopent-3-enes and 1-phenyl-l-silacyclopent-3-enes containing phenoxy-, biphenyloxy, and ]3-naphthoxy groups linked to the silicon atom via... 

The possibility of synthesizing a large number of comb-shaped thermotropic polymers with a different structure of the main chain [31-35], different mesogenic fragments [36-41], and a different type of connection to the main chain provides broad opportunities for their investigation [1, 2, 19, 31, 43-50]. The presence of chemical groups which cause the formation of the liquid-crystalline phase in their side chains, while the main chain of the molecule can be flexible, is a basic structural feature of these comb-sh )ed mesogenic macromolecules. 

Hydrodynamic Properties of Mesogenic Comb-Shaped Polymers... 

TABLE 3.2. Hydrodynamic Characteristics of Mesogenic Comb-Shaped Macromolecules of the Polymethacrylic Series... 

The dependences of the intrinsic viscosities [Tl] on the molecular weight are shown in Fig. 3.2, and the parameters K, Kp in Mark-Kuhn equation (3.12) for a series of mesogenic comb-shaped polymers are reported in Tables 3.2 and 3.3. The nature of these dependences (a-b = 0.5) for polymers 2-5 in Table 3.2 and 1-3 in Table 3.3 are in agreement with the constancy of the experimental values of the hydrodynamic invariant Aq = TiQD([T ]M/100) r with respect to the fractions [14]. 

It follows from the data in Table 3.2 and Fig. 3.3 that the rigidity of the main chain increases with an increase in the length of the side chains for both the previously studied comb-shaped macromolecules and for the macromolecules of thermotropic polymers with mesogenic fragments in the side chains. The ob-... 

The high intramolecular ordering of the thermotropic mesogenic polymer is also reflected in the strong effect of the solvent on the size of the molecules—the value of the intrinsic viscosity [ti]— in different solvents with presCTvation of the molecular weight of the sample [58]. The intrinsic viscosities [t ] (at 21 C) of mesogenic comb-shaped polymers of the polymethacrylate series in different solvents are reported below ... 

A specific feature of the structure of thermotropic comb-shaped macromolecules—the compact structure of the molecular coil and the strongly interacting mesogenic side chains with comparatively low rigidity of the main chain (in comparison to the rigidity of the lyotropic macromolecules examined above, for example)—causes the essential dependence of the intrinsic viscosity on the temperature. 

The results of a study of the FBF of comb-shaped polymers of the polymethacrylate and polyacrylate series are reported in Tables 3.5 and 3.6. For comparison, the optical characteristics of comb-shaped molecules of similar structure without mesogenic units in the side chains are also reported in these tables. 

TABLE 3.8. Optical Properties of Comb-Shaped Macromolecules with Mesogenic Side Groups [49]... 

The combination of comparatively low equilibrium rigidity of the main chain and long side branches with mesogenic fragments is responsible for the unique electrooptical properties of comb-shaped macromolecules. 

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