Comb nematics

Fig. 2.36. Three possible phases for comb nematics. (From Ref 284b.)...

In addition to their unusual rheological properties, the nematic phases of polymers, like those of simple compounds, can be oriented by the application of magnetic or electrical fields. These properties have been more fully examined for comb-type polymers with mesogenic side-chains than for polymers with the mesogenic groups in the main chain, since in the comb polymers it is possible to influence the side-chain orientation independently of the main-chain orientation. 

The synthesis of polymers capable of entering into a chiral nematic phase initially proved difficult as many of the acrylate and methacrylate comb-branch polymers to which a cholesterol unit was attached as a side chain tended to give a smectic phase. This was overcome by either copolymerizing the cholesterol-containing monomers with another potential mesogenic monomer, or by synthesizing mesogens with a chiral unit in the tail moiety. Examples of both types are shown as structures VIII and IX. 

In the latter system, there appears to be competition between alignment and thermal motion, so the best results were obtained when the poling was carried out close to the 298 K rather than at higher temperatnres the nematic to isotropic transition was = 373 K. More recent attempts have been made to improve the poled systems by incorporating the nonlinear, optically active molecnles into the polymer chain structure and comb-branch liquid crystalline polymers with R and the new group... 

The major advantage for the usage of LCPs as stationary phases for LC applications is that coating of the polymer on the silica gel is a simple process. However, also comb-shaped polymers prepared by octadecyl-acrylate and 3-mercaptopropyltrimethoxysilane as chain transfer agent can be immobilized on silica gel by bonding. It was shown that the telomer behaves as nematic material in the range of 42-47°C. The separation of geometrical isomers could be achieved. ... 

Pashkovskii, E. E., and Litvina, T. G., Temperature dependences of the twist viscosity coefficient for solutions of comb-like mesogenic polymer in a nematic solvent an estimation of the anisotropy and the rotational relaxation time of chains, J. Phys. II, 2, 1577-1587 (1992b). 

A final remark will concern the "glassy liquid crystal". All the textures observed for both linear and comb-like polymers in the nematic gi atggcag eagily e quenched and supercooled to room temperature 99999 Both the homeotropic alignment ggd tjie planar one remain fixed in the glassy state on cooling . From the dichroic ratio of the bands in the IR spectra of... 

The. first reliable results on the temperature dependence of the birefringence were reported by H. Finkelmann Results obtained on comb-like polymer VIII which exhibited a typical nematic... 

The activation energy of the acrylic comb-like polymers of smectic structure is about the same as that in nematic systems (Table 2). This is not typical for low molecular smectics which are oriented with more difficulty than nematics. The incorporation of methacrylic units increases values up to 160 kJ/mole for azomethine (polymer 2c) and 228 kJ/mole for cyandiphenyl (polymer 4b) derivatives. These data show that the activation energy depends not so much on the mesophase but primarily on the chemical structure and the mobility of polymer backbones taking part in the orientation process. 

The values quoted above for the domain order parameter refer to the alignment of the mesogenic groups. It is known, for instance, that the order parameter of the spacer methylene units in smectic one-comb PLCs is only 0.3-0.4 whereas the mesogenic group order parameter is close to 0.9 [30]. Finkelmann et al. [31] reported the following data on the order parameter of absorbing dichroic dyes, chemically bonded to a nematic, one-comb PLC with a siloxane backbone, close to the isotropization temperature Tjt 0.48 (T/Tj = 0.90) 0.40 (T/Tj = 0.95) 0.30 (T/Tj = 0.99) 0.2 T/T. = 1.0). 

Talrose et al. [85] reported that the response for the alignment of one-comb PLCs was faster at higher temperatures (nematics) and that the response time was proportional to the reciprocal of the square of the applied voltage, a trend which indeed was the same as for MLCs. Ringsdorf and Zentel [86] found that the response time decreased with increasing difference between the actual and glass transition temperatures. 

Figure 10.16 The order parameter of two oriented, nematic one-comb PLCs with different spacer groups as shown in the graph as a function of temperature. The samples were continuously heated and the order parameter was obtained by IR spectroscopy. (After data of Buerkle et al [128].)...

Figure 10.17 (a) The birefringence of an oriented one-comb PLC (polyacrylate obtained by photopolymerization of a surface aligned MLC) as a function of temperature. The various phases (K, crystalline S, smectic N, nematic and I, isotropic) are shown in the graph, (b) The order parameter of the same polymer (open symbol) in its nematic state as a function of reduced temperature, T/T., where Tj is the isotropization temperature. Data for the monomer (filled symbol) are shown in the same graph, ((a) and (b) from data of Broer et al [111].)... 

Figure 10,18 The temperature dependence of the order parameter for different moieties (explained in the graph) in a nematic, one-comb PLC with a polysiloxane backbone. (After data of Hempei et al. [130].)...

The molecular structure—macroscopic property connection is a vast subject we shall provide an example. Consider simple or one-row combs, subclass eO. Transition from a LC state such as nematic into isotropic Uquid, accomplished by a temperature increase, results in lowering the viscosity [32]— as usual and as expected. However, for longitudinal polymers, class a, similar isotropization results in a viscosity increase the rigid LC sequences were aligned in the LC state, but in the isotropic state all directions are equiprob-able, and the flow is more difficult. 

The field-induced reorientation of nematic polymers caused by the coupling of the electric field with their dielectric anisotropy was studied in a variety of papers [229, 231-238]. Unfortunately, only in a few papers (e.g., [237]) is a certain preliminary orientation of a polymer specified and we can speak of the true Frederiks transition with a well-defined threshold voltage. Nevertheless, the general opinion is that the Prank elastic moduli of both comb-like [232-235, 238] and linear-chain [236, 237] nematic polymers are of the same order of magnitude as of their low-molecular mass counterparts. 

The moduli were calculated from the threshold of the Frederiks transition ((4.9) induced by a magnetic (Ax > 0) and electric (Ae < 0)) field in homeotropically oriented liquid crystal layers. The same order of magnitude (10 -10 dyn), which is typical of conventional nematics, has been found for elastic moduli Kn and for other nematic polymers [233, 234]. Unwinding of the helical structure of chiral nematic polymers allowed the elastic constant K22 to be calculated K22 10" dyn for an arylic comb-like copolymer with cholesterol and cyanobiphenyl side-chair mesogens [229]). 

The results above show that the Frank moduli are determined mainly by the structure of mesogenic units which are similar for conventional nematics and thermotropic polymers (the situation changes considerably for the lyotropic solutions of long rod-like polymeric molecules, see the next section). On the other hand, the dynamics of reorientation are strongly influenced by the backbone. Field response and relaxation times depend dramatically on the molecular mass of a polymer though, in the first approximation, obey the same equations (4.30, 4.31). Figure 4.42 shows field-response times as a function of temperature for a comb-like acryl polymer H... 

FIGURE 4.42. Field-response times (at a fixed field strength) as functions of temperature for a comb-like acrylic nematic polymer with various molecular mass Mn = 6200 (1), 11,300 (2) [236]. 

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.

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