Conoscopy, biaxial nematics

Conoscopic studies suggest that this compound in fact exhibits a biaxial nematic phase although these experiments indicated only a small opticd biaxiality. Indeed, more recently, NMR spectroscopy using selectively deute-rated materials has shown that for other materials the assignment of a biaxial nematic phase on the basis of conoscopy alone is not necessarily reliable [118], This is discussed further in Sect, 3,4,... 

In 1980, the first justified claim for the discovery of a biaxial nematic phase was made for a lyotropic liquid crystal comprised of the ternary system of potassium laurate-l-decanol-D20 [4], In addition to a uniaxial phase (micelles of a bilayer structure), there were two further nematic phases. One of the phases was found to be uniaxial as well, probably corresponding to a phase with cylindrical micelles. Existing in a temperature range in between these two uniaxial nematic phases was a third phase which was found to be biaxial. The phases were classified by microscopic smdies as well as deuterium NMR measurements. Three years later, Galerne and Marcerou studied the same system by conoscopy, leading to a complete determination of the ordering tensor in all three nematic phases [5]. 

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],... 

Within the last few years, two different experimental techniques could successfully prove the existence of a biaxial nematic phase in liquid crystals based on bent-core mesogens. Madsen et al. [32] found indications for a phase biaxiaUty with polarization microscopy and conoscopy and also obtained additional quantitative evidence by means of deuterium NMR. Acharya and co-workers [33] used X-ray... 

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. 

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