Nematic liquid-crystalline side-chain polymer

It is perhaps appropriate to mention here that both nematic liquid crystalline side chain polymers with laterally attached mes-ogenic groups [16] and sanidic aromatic polyamides [17] have been shown to exhibit Nb phases. 

Until now there was no obvious correlation found between the monomer structure and the resulting pol qner phase. No.theorr retical structural conditions were described which would result in a liquid crystalline polymer with a definite ordered phase e.g. with a nematic a smectic or a cholesteric phase as in conventional liquid crystals. Although previous examples have established (8 9) the existence of enantiotropic liquid crystalline side chain polymers additional considerations are in order for a systematic synthesis of such polymers. 

Different types of liquid-crystalline side-chain polymers based on sUoxane backbones were synthesized by hydrosilylation reactions as described in Refs. [3] and [4]. The resulting nematic LC silicones have a broad chain length distribution. The length of the backbones are controlled by GC, H NMR and Si NMR. An example of an LC silicone used for the TCR films is shown in Fig. 1. Its degree of polymerization is about 14 and the phase transition temperatures measured by a differential scanning calorimeter are a glass transition temperature Tg of 18 °C and an isotropic transition temperature Tc of 68 °C. 

Severing, K., Saalwachter, K. Biaxial nematic phase in a thermotropic liquid-crystalline side-chain polymers. Phys. Rev. Lett. 92(14), 125501 (2004)... 

Weissflog and Demus [8] succeeded in synthesizing a new type of liquid crystalline material, in which the rotation of the molecules about their long molecular axis was hindered by the introduction of laterally attached substituents. However, experiments conducted on these liquid crystals proved the existence of conventional uniaxial nematic phases only. Beyond that, a further restriction of the rotation of the mesogens by connecting them via terminal spacers to a polymer backbone end-on polymers) resulted in uniaxial nematic and smectic phases. Consequently, the next step was the synthesis of a new class of liquid crystalline side-chain polymers, in which the mesogenic moiety was laterally attached to the polymer backbone. The idea was that this side-on connection should be successful in hindering the rotation... 

In a recent publication, these findings have been nicely confirmed by deuterium NMR investigations of a spin probe dissolved in the organo-siloxane tetrapodes [40], where the same sample-flip technique was applied as in [4, 11], In their study, the authors point out that the mode of stabilization of the biaxial nematic phase in tetrapodes and liquid-crystalline side-chain polymers, i.e., the lateral fixation of the mesogenic group, may in fact be similar. 

Conoscopy provides an extremely sensitive method with which to determine the degree of biaxiality. By the early 1990 s, conoscopic measurements had already indicated the presence of phase biaxiality in a nematic side-on liquid crystalline side-chain polymer [9]. However, the method s sensitivity is also its weak point because surface effects may induce optical biaxiality in an actual uniaxial system. For this reason, deuterium NMR was used to confirm phase biaxiality in a liquid crystalline polymer system similar to the one investigated with conoscopy by Leube [11-13]. Due to the fairly high viscosity of the polymeric samples, the tilt experiment, employed by Yu and Saupe to show phase biaxiality in a lyotropic liquid crystal [4], was used. The results obtained in this way are in good agreement with observations of optical textures in a biaxial cholesteric copolymer [16], where phase biaxiality disturbs the smooth optical periodicity of the cholesteric phase structure. 

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

Extensive studies on photochromic liquid-crystalline polymers have been made by Krongauz et al,2 Liquid-crystalline phases caused marked colour changes of poly(acrylates)98 and poly(siloxanes) substituted with spiropyran side chains upon UV irradiation owing to the aggregation of the photomerocyanines." In contrast, spirooxazines attached to liquid-crystalline polymer backbones displayed no aggregation and hence exhibited normal photochromism similar to that in solution. Fulgimides bound covalently to the side chains of nematic liquid-crystalline polymers also showed normal photochromism. 

An initial approach to supramolecular H-bonded mesogenic polymer complexes involves a polyacrylate with 4-oxybenzoic acid moieties via a hexamethylene spacer 29 [26]. The 1 1 complexation of the side chain of the polymer and stilbazole 3 n - 2) (nematic, 168-216 °C) results in the formation of an extended supramolecular mesogen in the side chain (Fig. 11). Side-chain polymer complex 30 exhibits a nematic phase up to 252 °C, which shows that a significantly stabilized mesophase is achieved by the complexation of two different components. Liquid-crystalline properties have been examined for the series of complexes formed between polyacrylates and trans-4-alkoxy-4 -stilbazoles [33, 78]. Figure 12 shows transition temperatures against the carbon number of the alkyl chain for the series of complexes 31 [33]. They exhibit thermally stable smectic liquid-crystalline phases. For example, smectic E, B, and A phases are observed until 192 °C after the glass transition at 38 °C for the complex with m = 6 [78a]. 

FIGURE 5.3 Schematic representation of (a) nematic phase and (b) smectic phase for main-chain liquid crystalline polymers, showing the director as the arrow. The relative ordering is the same for side-chain-polymer liquid crystals. 

The principle of using side chain liquid crystalline polymers as optical storage systems has been estabhshed. This has been demonstrated using a polymer film prepared from a side-chain polymer showing nematic hquid crystalline characteristics with the structure... 

Systematic investigations of the past years have proved that the structures of the liquid crystalline phases of liquid crystalline (l.c.) side chain polymers are similar to the structures of conventional lovi/ molar mass liquid crystals (1.1c). Nematic and cholesteric phases as vi/ell as S and S phases have been clearly identified The systematic realization of l.c. 

Polymer P25/26 self-orders in solvent-cast films, with the hackhones parallel to the substrate and a strong solvent dependence of the degree of ordering (78). Spacings of 2.2-2.G nm are observed by x-ray diffraction, indicating interdigita-tion of the dendritic side chains. P25 6 also forms thermotropic nematic liquid crystalline phases. With optical microscopy, Schlieren textures are observed for thin films cast from solution. 

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