Herbert Schmiedel

Generalized transmission ellipsometry for twisted biaxial dielectric media: application to chiral liquid crystals

We report on the application, for the first time to our knowledge, of spectroscopic generalized ellipsometry to liquid crystal materials. We have measured the three normalized elements of the Jones transmission matrix t at various sample temperatures within the spectral range from 340 to 1700 nm (0.73 to 3.65 eV) on thin cells filled with twisted nematic mixtures 4-cyano-4′-pentylbiphenyl (5CB) and 4-cyano-4′-(2 methyl)-butylbiphenyl (CB15) (Ref. 4). The Berreman 4 × 4 matrix for electromagnetic plane waves in a biaxial medium homogeneously twisted along the sample normal is derived and presented. Analytic expressions in the case of light propagation along the helical axis permit the calculation of the transmission and reflection coefficients simultaneously without numerical approximations. This solution is valid for any biaxial configuration of chiral liquid crystals, including the case of absorption. We have fully analyzed the measured Jones transmission matrix elements and obtained the geometrical sample properties and, as a function of the photon energy and the temperature, the refractive indices no, and ne of the chiral liquid crystals. We found that within the experimental error the main refractive indices of the mixtures 5CB and CB15 are those of pure 5CB at the same reduced temperatures. The handedness of the optical activity of the samples can be obtained immediately from the phase information of the Jones transmission matrix coefficients.

NMR studies of 27Al in decationated Y zeolites

Abstract Using stationary wide-line measurements, conclusions could be drawn concerning the electric field gradient of Al positions in decationated zeolites. Extra-lattice aluminum atoms in decationated zeolites could be detected after loading the activated zeolite with a solution of acetylacetone in ethanol. The amount of aluminum on extra-lattice positions increases from 5 to 50% going from 300°C shallow-bed pretreatment of an 84% NH4+ exchanged NaY zeolite to 500°C deep-bed pretreatment.

Low-field electrodynamics of high-Tc superconductors — theory and experiment

Abstract We have investigated, theoretically and experimentally, the penetration of magnetic fields into ceramic HTSC for fields below the lower critical field of the grains. In the framework of a critical state theory in the low-field electrodynamics of a random Josephson medium, expressions for the harmonics of the induction B(t) as a function of external parameters were derived for two asymptotic cases — the case of the weak field and the case of the thin sample. The behavior of the magnetic induction B was investigated in AC and DC fields in a wide range of temperature and field amplitudes. Rather good agreement between experimental and theoretical results has been found. From the experimental results the values of number of quantities characterizing the low-field electrodynamics of HTSC and their temperature dependences as well as an explicit expression for the critical current density j c (H, T) = j 0 (T)H 2 0 (T) H 2 +H 2 0 (T)) were derived.

Optics in Stratified Media—The Use of Optical Eigenmodes of Uniaxial Crystals in the 4 × 4-Matrix Formalism

Abstract A new method of evaluating the optical properties of stratified uniaxial media is proposed. Within Berremans 4 × 4 matrix formalism, the exact propagator of a homogeneous layer is used instead of the Taylor series expansion. The calculation of the transmission and reflection matrices for a given propagator is performed. Examples are given to demonstrate the usefulness of the proposed method.

Infrared dichroism investigations on the acyl chain ordering in lamellar structures:: I. Order parameter formalism and its application to polycrystalline stearic acid

We present the basic formulae for the analysis of IR dichroism data in terms of molecular order parameters of lamellar systems considering the special case of the Attenuated Total Reflection experiment on thick films. The infrared order parameter is expressed as the sum of five molecular order parameters which characterise the longitudinal and transverse ordering within the lamellae. The formulae are applied to polymethylene chains which are packed in a crystalline subcell. The information about transverse ordering is averaged out in the orthorhombic perpendicular packing mode of the hydrocarbon chains. The IR linear dichroism of vibrations of the methylene, methyl and carboxyl groups of polycrystalline stearic acid can be consistently explained by means of this formalism assuming a mixture of C- and E-type crystals. The . . mean tilt 358 and azimuth 408-508 of the polymethylene chain axes were determined from the IR order parameters of the split methylene bending and rocking modes. Mixtures of protiated and deuterated stearic acid were investigated to vary the degree of intermolecular vibrational coupling, and thus, to modulate the influence of Fermi resonance on the dichroism of the CH stretching bands. The disturbing effect of Fermi resonance is, however, small if the corresponding IR order 2 parameters are determined from the absorbance which is integrated over the range of the band maximum. The dichroism of the C-O stretching vibration depends strongly on the degree of intramolecular vibrational coupling with the methylene wagging modes. The systematic error of neglecting birefringence has been estimated and discussed. A method is presented which checks the validity of Harricks thick film approximation used for data analysis. q 1999 Elsevier Science B.V. All rights reserved.

MOLECULAR ORDERING IN MICROCONFINED LIQUID CRYSTALS : AN INFRARED LINEAR DICHROISM STUDY

Abstract The molecular arrangement of 5CB confined within the cylindrical pores of Anopore membranes was characterized by means of the IR-order parameter obtained from linear dichroism measurements of selected IR absorption bands. The treatment of the experimental data includes a local field correction extended to the twisted nematic configuration, yielding order parameters increased by about 30% in comparison with the uncorrected data. The nematic director of 5CB aligns along the pore axes, whereas in lecithin coated Anopore channels, the local nematic director is oriented approximately radially due to the perpendicular anchoring of the 5CB molecules at the pore wall. Doping of 5CB with the chiral agent CB15 yields local nematic directors tilted with respect to the pore axes. The average tilt angle increases up to about 40° at a fraction of CB15, x cb15 = 0.25 (w/w). These results are discussed in terms of the conical helicoidal and alternatively the radially twisted axial arrangement of the LC molecules...

Experimental evidence of a conic helical liquid crystalline structure in cylindrical microcavities

Abstract Optical activity has been measured for cholesteric (Ch), twisted nematic (N*) and ferroelectric (S*c) substances confined in cylindrical channels of anodic oxidized aluminium (Anopore membranes). 1H and 13CNMR studies of the adsorbed N* phase in combination with measurements of the rotary power suggest a conic helical director field in the pores, as in a common S*c structure, with the helical axes oriented along the cylindrical channels and the helicoidally ordered local directors on average being inclined at an angle θ < 90° to the cylinder axes. Supposing the validity of the de Vries formula, the sign and magnitude of the optical activity in Anopore indicate a change in birefringence and pitch with respect to the bulk sample.

Proton NMR signals of intra- and interpair dipolar order in liquid crystals

Abstract We demonstrate the existence of magnetic intrapair and interpair dipolar order in aligned liquid crystalline samples. The relaxation times of the corresponding dipolar signals are different and depend strongly on the system.

Incorporation of Thermotropic Liquid Crystals in Phospholipid Monolayers: Necessary Condition or Homeotropic Anchoring

Abstract We have studied the nionolayer phase behavior of various mixtures of dipalmitoylphosphatidylcholine (DPPC) with 4-cyano-4′-n-pentylbiphenyl (5CB) at the air/water interface by means of Langmuir technique and Brew-ster angle microscopy. The phase behavior is compared with the ability of 5CB to anchor homeotropically in liquid crystal cells (LC cells) whose surfaces were covered with DPPC monolayers of various molecular area using Langmuir-Blodgett technique. In the present investigations DPPC cells orient 5CB perfectly homeotropically, if the monolayers were transferred to, the display surfaces as tilted phases. In case of a transferred non-tilted phase thr orientation of bulk 5CB was no more homeotropical. We show that an cssential prerequisite for the homeotropic anchoring is the incorporation of 5CB in the orienting surface monolayer and thus the formation of new phases. Our conclusions are supported by DSC and x-ray experiments on fully hydrated DPPC hulk specimens (bilayer systems) mixed in v...

Transient Patterns in the Magnetic Reorientation of Low Molecular Mass Nematic Liquid Crystals

Abstract We present the results of optical investigations of transient patterns appearing during the reorientation of nematic LC in magnetic fields. The transition from a planar modulated director field to a metastable non-planar structure of twist-bend walls is observed. From the wavelengths and characteristic rise times of the investigated patterns, the validity of the linearized mode selection theory is confirmed and viscoelastic parameters are determined.