Nematic liquid-crystalline side-chain

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. 

Liquid crystals can be in the smectic, nematic, or isotropic states. In the smectic liquid crystalline state there is a long-range order in the direction of the long axis of the molecules. These molecules may be in single- or bilayer conformation, have molecular axis normal or tilted to the plane of the layer, and frozen or melted chains. In the nematic liquid crystalline state the molecules are aligned side by side but not in specific layers. The isotropic liquid crystalline state is more or less a liquid state, but where clusters with short-range order persist (Small, 1986, pp. 49-51). 

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. 

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. 

Akihiko M. Novel biaxial nematic phases of side-chain liquid crystalline polymers. J Chem Phys 2012 137 224906. 

Conoscopy is known to be prone to artifacts because of its sensitivity to the symmetry of the refractive index, which might be tampered due to surface effects and flow phenomena. By using deuterium NMR spectroscopy. Severing et al. [11] confirmed phase biaxiality in a polymeric liquid crystal similar to that studied earlier by Leube. To evaluate different parameters that bias the formation of a biaxial nematic phase and gain a more general picture of the phase biaxiality in nematic liquid crystalline polymers, the investigations were expanded to side-chain polymers of different chemical constitutions as well as to mixtures of polymers and low molar mass liquid crystals [12],... 

In this chapter an attempt is made to integrate the information available from the thermal data, textural phenomena, miscibility tests and x-ray diffraction patterns in a discussion of the specific features of the nematic, cholesteric and smectic phases exhibited by liquid crystalline main-chain and side-chain polymers. Results obtained by these methods on liquid... 

---