| 1 | Author
| Christina Papenfußa, Jozsef Verhásb, Wolfgang Muschika | Requires cookie* | | Title
|  | | | Abstract
| A continuum theory of a biaxial nematic phase in liquid crystals is presented. The liquid crys talline ordering is described by a field of three unit vectors. Because these are macroscopic directors, the present approach is a generalization of the Ericksen-Leslie theory to biaxial nematics. The applied tools from nonequilibrium thermodynamics are classical Linear Irreversible Thermodynam ics and Gyarmati's variational principle. Onsager reci procity relations; Gyarmati's variational principle. A S im p lifie d T h e rm o d y n a m ic T h e o r y f o r B ia x ia l N e m a tic s | | |
Reference
| Z. Naturforsch. 50a, 795—804 (1995); received June 3 1994 | | |
Published
| 1995 | | |
Keywords
| Liquid crystals, biaxial nematics, Linear Irreversible Thermodynamics | | |
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| default:Reihe_A/50/ZNA-1995-50a-0795.pdf | | | Identifier
| ZNA-1995-50a-0795 | | | Volume
| 50 | |
2 | Author
| Ingo Dierking, Frank Gießelmann, Peter Zugenmaier | Requires cookie* | | Title
| A  | | | Abstract
| n O p tic a l M e th o d f o r th e D e te r m in a tio n o f th e C h o le s te r ic P itc h in L iq u id C r y s ta ls Preparation of a cholesteric liquid crystalline phase in planar orienting liquid crystal cells exhibits discontinuous colour changes with temperature when viewed between crossed polarizers. The trans mission spectra of such a system can be described by a modified de Vries equation, with the bire fringence dispersion included in a simplified single-band model. With a suitable choice of the cell gap as compared to the cholesteric pitch, the procedure can be used as a new method to determine the temperature dependence of the cholesteric pitch. | | |
Reference
| Z. Naturforsch. 50a, 589—594 (1995); received December 17 1994 | | |
Published
| 1995 | | |
Keywords
| Liquid crystal, cholesteric mesophase, pitch determination, birefringence dispersion, twist inversion | | |
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| default:Reihe_A/50/ZNA-1995-50a-0589.pdf | | | Identifier
| ZNA-1995-50a-0589 | | | Volume
| 50 | |
3 | Author
| T. Brückert, D. Büsing, A. Würflinger, S. Urban | Requires cookie* | | Title
|  | | | Abstract
| h e r m a l A n a ly s is (D T A) a n d D ie le c tr ic S tu d ie s o f 4 -(tra n s -4 -H e p ty l-C y c lo h e x y l) -b e n z o n itr ile (7 P C H) u n d e r H ig h P r e s s u r e With the aid of DTA the phase diagram of 4-(trans-4-heptyl-cyclohexyl)-benzonitrile, 7PCH, was redetermined. Dielectric studies on the static and complex permittivity have been performed on 7PCH in the pressure range 0.1 -220 MPa, the frequency range 1 kHz-13 MHz and the temperature range 290-380 K. For the slow relaxation process in the nematic phase, characterized by the relaxa tion time Tjj , we have calculated the activation volume, A* V u, of about 80 to 60 cm3/mol (decreasing with increasing temperature), and the activation enthalpy, A *Hn, of approximately 70 kJ/mol. The isochoric activation energy, A* Un, amounts to about 36 kJ/mol. The pressure dependence of A * Hu is less pronounced compared with previous findings for 5PCH and 7CB. Estimates of t 0 (extrapo lated from the relaxation time in the isotropic phase) allow the calculation of the retardation factor gu = Tn/t0 that is analysed in terms of theories of Martin-Meier-Saupe and Coffey in order to evalu ate the nematic potential q. Using data for the order parameter 5 from NMR studies it was possible to determine the strength of the interaction potential in the nematic phase and to check the relation q~S which is valid at p = const., but not at V= const. The results are compared with recent findings for other homologous series. | | |
Reference
| Z. Naturforsch. 50a, 977—983 (1995); received August 25 1995 | | |
Published
| 1995 | | |
Keywords
| Dielectric Relaxation, High Pressure, Liquid Crystals, Activation Parameters, Phase Transitions | | |
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| default:Reihe_A/50/ZNA-1995-50a-0977.pdf | | | Identifier
| ZNA-1995-50a-0977 | | | Volume
| 50 | |
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