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'Liquid crystals' in keywords Facet   Publication Year 1994  [X]
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1Author    S. U. Rban, T. Briickert, A. W. ÜrflingerRequires cookie*
 Title    Dielectric Studies on Liquid Crystals under High Pressure: VI. Low Frequency Relaxation Process in the Nematic and Smectic A Phase of 4-n-Octyl-4'-Cyanobiphenyl (8 CB)  
 Abstract    Dielectric relaxation studies of 4-n-octyl-4'-cyanobiphenyl (8 CB) were performed in the pressure range 0 .1 -1 2 0 M Pa, the frequency range 1 k H z -1 3 M Hz and the temperature range 2 9 5-331 K. The dielectric behaviour of 8C B in the nematic phase is similar to that of 5CB, 6C B and 7CB. In particular, the activation enthalpy decreases with increasing pressure and the activation volume decreases with increasing temperature in the nematic phase. In the smectic A phase of 8CB both these parameters are markedly lower and show opposite pressure and temperature dependencies. We tentatively interpret these effects as a result of breaking of the dipole-dipole correlations which manifests itself differently in these liquid crystalline phases. 
  Reference    Z. Naturforsch. 49a, 552 (1994); received March 5 1994 
  Published    1994 
  Keywords    Dielectric relaxation, High pressure, Liquid crystals, Activation quantities 
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 TEI-XML for    default:Reihe_A/49/ZNA-1994-49a-0552.pdf 
 Identifier    ZNA-1994-49a-0552 
 Volume    49 
2Author    I. D. Ierking1, F. G. Ießelm Ann1, P. Zugenm, H. R2, H. Z. Aschke2, W. Kuczynski3Requires cookie*
 Title    The Origin of the Helical Twist Inversion in Single Component Cholesteric Liquid Crystals  
 Abstract    The cholesteric twist inversion by temperature variation o f a single com ponent thermotropic liquid crystalline system was experimentally investigated and is explained by the partial twisting power model, evaluating the temperature dependent pitch o f five different configurations of a benzoic acid biphenyl ester with two chiral centers. The temperature dependence o f the twisting power and the cholesteric pitch for several stereoisomeric com pounds can in first approximation be predicted by the partial twisting powers of the individual chiral centers determined from the partially racemic configurations. 
  Reference    Z. Naturforsch. 49a, 1081—1086 (1994); received August 12 1994 
  Published    1994 
  Keywords    Liquid crystal, Chirality, Cholesteric mesophase, Twist inversion 
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 TEI-XML for    default:Reihe_A/49/ZNA-1994-49a-1081.pdf 
 Identifier    ZNA-1994-49a-1081 
 Volume    49 
3Author    C. Forte, M. Geppi, C. A. VeraciniRequires cookie*
 Title    Study of the Molecular Dynamics from Deuterium Zeeman and Quadrupolar NMR Relaxation of 4,4'-di-n-Heptylazoxybenzene in the Nematic and Smectic A Phases  
 Abstract    The deuterium spin-lattice (T1Z) and quadrupolar (r iQ) relaxation times of the liquid crystal 4,4'-Heptyl-azoxybenzene (HAB), deuterated on the aromatic rings and the first methylene group of the chains, have been measured by means of the Jeener-Broekaert pulse sequence at two different frequencies (10 and 46.04 M Hz) and throughout the nematic and the smectic A phase. By applying suitable models, we have determined rotational and reorientational diffusion coefficients and evalu­ ated the contribution of the different m otions (internal, molecular and collective) to the relaxation. In particular we have found a non negligible contribution from order director fluctuations. A comparison between the Tlz and Ti Q measurements performed by means of Jeener-Broekaert and Wimperis pulse sequences is also shown. 
  Reference    Z. Naturforsch. 49a, 311 (1994); received August 16 1993 
  Published    1994 
  Keywords    Liquid crystal, Deuterium N M R, M olecular dynamics, Deuterium relaxation, Director fluctuation 
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 TEI-XML for    default:Reihe_A/49/ZNA-1994-49a-0311.pdf 
 Identifier    ZNA-1994-49a-0311 
 Volume    49