The Structure of a Number of Nematogens

Leadbetter, A. J.; Richardson, Robert M.; Colling, CN

doi:10.1051/jphyscol:1975105

Cited by 234 publications

(175 citation statements)

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“…We note that such tendency for layering, known as ''cybotactic groups'', 20 is often observed even in rod-like molecules such as 8CB 21 in the nematic phase, but only in a narrow temperature range above a smectic phase. For this reason in the isotropic phase the calamitic materials essentially behave as Newtonian fluids with viscosities in the lower range of our measurement errors.…”

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confidence: 99%

Rheological properties of bent-core liquid crystals

et al. 2009

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We show that bent-core liquid crystalline materials exhibit non-Newtonian flow in their optically isotropic liquid phase. We conjecture that this behavior is due to the existence of nanostructured, fluctuating clusters composed of a few smectic-like layers. Shear alignment of these clusters explains the shear thinning observed in bent-core liquid crystals having either a nematic phase or nonmodulated smectic phase. By contrast, smectogens having a modulated smectic phase do not shear thin at low shear rates, but even show a slight shear thickening which may be due to entanglements of wormlike and/or helical clusters.

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confidence: 99%

Rheological properties of bent-core liquid crystals

et al. 2009

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“…Molecules with a CN or NCS terminal group exhibit the highest dielectric anisotropy; however, those with a CN group have much higher viscosity because their molecules exist in dimeric form as the most stable body. Dimers with the length L, which is 1.4 times larger than single molecule length l, can be noticed in small angle diffraction pattern measurements [67,68], see Figure 9. The molecules there are antiparallely correlated.…”

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High Birefringence Liquid Crystals

2013

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Liquid crystals, compounds and mixtures with positive dielectric anisotropies are reviewed. The mesogenic properties and physical chemical properties (viscosity, birefringence, refractive indices, dielectric anisotropy and elastic constants) of compounds being cyano, fluoro, isothiocyanato derivatives of biphenyl, terphenyl, quaterphenyl, tolane, phenyl tolane, phenyl ethynyl tolane, and biphenyl tolane are compared. The question of how to obtain liquid crystal with a broad range of nematic phases is discussed in detail. Influence of lateral substituent of different kinds of mesogenic and physicochemical properties is presented (demonstrated). Examples of mixtures with birefringence ∆n in the range of 0.2-0.5 are given.

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“…Consequently, there would be interactions between alkyl chains and/or a stacking of biphenyl groups. 7CB in its liquid and/or nematic liquid crystal phase has a similar structure to that of 8CB 2) . NMR and IR results 3,5) in the phase transition temperature regions suggest that the cyanobiphenyl and alkyl parts differently contribute to the formation of liquid crystal: relatively rigid biphenyl part in 4-n-alkyl-4'-cyanobiphenyl molecule largely contributes to the formation of liquid crystal phase, while flexible alkyl part also contributes to the formation of liquid crystal phase and to the determination of the type of the liquid crystal phases.…”

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confidence: 85%

“…It is noted that the temperature of intermediate region is close to the phase-transition temperature (the phase-transition temperature between the nematic liquid-crystal and isotropic liquid for 6CB is ca. 29 ) 2 , however, it is not likely that the rotational movements of nCB are specifically changed across the intermediate temperature region. Therefore, the relatively steep increase in their radii should be attributable to the enhancement of rotational fluctuations of nCB molecules, which lead to spherical-rigid rotation.…”

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Fluorescence Anisotropy and Rotational Diffusion of Two Kinds of 4-n-alkyl-4'-cyanobiphenyls in Glycerol

2007

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Absorption and fluorescence spectra, fluorescence lifetimes, and steady-state and time-resolved fluorescence anisotropies were measured for 4-methyl-4'-cynanobiphenyl (1CB) and 4-hexyl-4'-cynanobiphenyl (6CB) in glycerol at various temperatures. The energies and intensities of the absorption and emission spectra for 1CB and 6CB were almost equally observed, and the fluorescence lifetimes around 1.0 ns were also almost equally obtained and did not depend on temperature. However, the timeresolved fluorescence anisotropies measured in glycerol solution indicated that the molecules of 1CB rotate more rapidly than those of 6CB: the rigid-sphere radii for 1CB and 6CB obtained by using the pulsed exciting light were evaluated as 1.3 10 -10 m (1CB) and 1.8 10 -10 m (6CB) in the low temperature region (2-10 ), whereas those evaluated in the high temperature regions (20-60 for 1CB and 30-60 for 6CB) were 2.7 10 -10 m (1CB) and 3.6 10 -10 m (6CB). These radii evaluated in the high temperature region were in good agreement with those obtained by using steady-state exciting light, 2.9 10 -10 m (1CB) and 3.4 10 -10 m (6CB). It is noted that the increase of radius was observed across an intermediate temperature region (10-30 for 1CB or 10-40 for 6CB), and was more remarkable for 6CB than for 1CB. This implies that the movements of hexyl-chain are much more enhanced at high temperature, and are effectively reflected to the increase in radius of rotational diffusion. The changing phenomenon of movements of peripheral alkyl-chain in liquid-crystal molecules is important for the phase transition between liquid crystal and isotropic liquid phases: the changing to less vigorous movements of the alkylchain in 6CB is thought enough to contribute to the liquid-crystal formation of 6CB molecule.

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The Structure of a Number of Nematogens

Cited by 234 publications

References 0 publications

Rheological properties of bent-core liquid crystals

Rheological properties of bent-core liquid crystals

High Birefringence Liquid Crystals

Fluorescence Anisotropy and Rotational Diffusion of Two Kinds of 4-n-alkyl-4'-cyanobiphenyls in Glycerol

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