ZfN: Search Results

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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	TEI-XML for	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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	TEI-XML for	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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	TEI-XML for	default:Reihe_A/50/ZNA-1995-50a-0977.pdf
	Identifier	ZNA-1995-50a-0977
	Volume	50