Raman Scattering from Liquid Crystals

Abstract

The use of Raman scattering technique as a tool for studying local order in complex liquid crystal systems is illustrated by three examples, namely the locally anisotropic liquid L-phase, lipid membranes, and PDLC's.     

Molecular Ordering in Liquid Crystals from Incoherent Quasi-Elastic Neutron Scattering

The Elastic Incoherent Structure Factor (EISF) for neutron scattering in liquid crystals has been interpreted theoretically by evaluating the correlation functions for the molecular reorientation according to the diffusion model for highly ordered fluids. It is shown that the available experimental data are compatible with the orientational distribution function predicted by the Maier-Saupe model for uniaxial mesophases provided that the long time behaviour of the correlation functions within the time scale of the experiment be properly considered.     

Quasielastic Light Scattering from Orientational Fluctuations in a Cholesteric Liquid Crystal

Abstract

Examination of the Rayleigh line of a lyotropic cholesteric polypeptide liquid crystal by the techniques of quasielastic light scattering spectroscopy reveals the presence of two new purely dissipative low frequency light scattering modes of comparable intensity. Angular and electric field dependences of the deconvoluted spectra are consistent with the identification of the mechanism as due to twist and viscous-splay normal modes recently predicted theoretically. Analysis of the narrow mode on this basis yields a value of γ₁, the twist viscosity coefficient of the medium. The broader mode results in an estimate of the value of the ratio of the splay to bend elastic moduli, K ₁₁/K ₃₃ ～ 10^?5, which is shown to be in accord weith mean field theoretical calculations for a system of such highly elongated macromolecules.     

Light Scattering from Nematic Liquid Crystals in a Nonuniform Velocity Field

Fluctuations about a nonequilibrium steady state of a nematic liquid crystal subject to small, linear shear, are studied by means of a generalized Langevin method. We find a symmetric broadening of the director power spectrum and estimate the numerical size of the effect for a typical material.     

Neutron Scattering by Copolymers

Abstract

Neutron scattering is a very efficient tool for studying polymers in bulk as well as in solution. This is mainly due to the difference of scattering length between hydrogen and deuterium which allows the possibility of changing the contrast without too serious alteration in the conformation and thermodynamics of the systems.

In order to take full advantage of this possibility, one has to remember the general rules which allow the evaluation of the total scattering function S(q) as a function of the partial scattering functions S _pq (q) which describes the contributions of the interferences between scattering center of species p and q.^1.2 In a first part we shall briefly establish these rules for incompressible systems introducing what has been called by J. Koberstein³ the molecular contrast and the phase contrast.

In the second part of this talk we shall discuss the problem of copolymers. S(o) the intensity scattered at zero angle depends on the heterogeneity in composition of the sample,⁴ for homogeneous composition S(o) = o. This gives the possibility of studying trans-esterification in polyester, for example.

At finite q for a monodisperse system, S(q) is not zero and it will be shown how it depends on the length of the sequences and the architecture of the copolymers. If the number of blocks is large S _pq (q) is practically not affected by the total length of the polymer.⁵

The condition I ^?1(q) = o allows one to write the equation of a q dependent generalized spinodal and therefore to determine the parameters which govern the stability of a multiphase system.

In the last part we shall discuss the problem of the application of this formalism to networks and gels made of different types of units. It will be shown that the scattering is mainly sensitive to the local conformation and that the introduction of translational order between the units does not affect significantly the scattering curves but can explain the occurrence of secondary maxima which have been observed in polyurethane and ionomers.⁶     

Raman Scattering From TMTSF-Salts

Abstract

Experimental results for Raman scattering from TSeF, TMTSF-D12, (TMTSF)₂PF₆ and (TMTSF)₂AsF₆ excited with blue-green laser light at liquid nitrogen temperature are reported. A strong resonance effect for excitation of TMTSF with 4579 Å was observed and an assignment for the a_g modes was obtained by comparison with results from TTF. Methyl group modes were identified at 328 cm^?1, 472 cm^?1 and 916 cm^?1, respectively. Charging the TMTSF molecule resulted in a strong frequency shift of -122 cm^?1 and -76 cm^?1 for the a_gu₂ mode, respectively. A study of the temperature dependence of the scattering from molecular modes between 2 K and 300 K did not reveal any correlation with the phase transitions in the conducting compounds.     

Observation of Second Sound Using Light Scattering from a Free Surface Smectic A

A new technique for studying the second sound mode in smectic liquid crystals has been developed. Preliminary measurements for the smectic A phase of 4-n-octyloxy-4'-cyanobiphenyl (80CB) are described. We have developed a technique to prepare well aligned samples with one free surface. This free surface was electrically driven, and the resulting surface displacement was studied using an optical heterodyne technique. The elastic constant, B, and the effective viscosity, ν_eff, were determined from the spectrum of the scattered light.     

Advanced liquid crystal displays with supreme image qualities

‘LCD vs. OLED: who wins?’ is a heated debatable question. Each technology has its own pros and cons. We review recent advances in liquid crystal displays (LCDs) from three performance metrics: response time, contrast ratio, and viewing angle, which determine the final-perceived image quality. To enhance LCD performance, ultra-low viscosity materials, quantum dots, and new device structures have been explored, and their working mechanisms investigated. Another round of LCD innovation is around the corner.     

New liquid crystalline tetrazine derivatives for guest-host displays

A number of new liquid crystalline tetrazine derivatives was synthesized. The electro-optical studies showed that the 3-n-alkyl-6-[4-(4-n-alkyl-cyclohexanoyloxy)phenyl]-1,2,4,5-tetrazines are excellent dyes for guest-host displays with positive colour contrast.     

Raman Scattering in Langasite-Family Crystals

Crystallography Reports - Raman scattering spectra have been investigated in La3Ga5SiO14 langasite crystals with Cr and Fe impurities and La3Ga5.25Ta0.25Si0.5O14 crystalline solid solution. It is...     

Light Scattering by Cholesteric Spherulites

Abstract

In this work, we analyze the scattering of light by the so-called spherulites or skyrmions in cholesteric liquid crystals. These are quasi-planar localized excitations of the director configuration. We compute the cross section of the polarization conversion for polarized incident light in the Born approximation, considering the anisotropic optical properties of the liquid crystal and the shape of the spherulite. We compare the results obtained by the analytical and the numerical skyrmion solutions found for several values of the model parameters. Finally, possible practical employments and theoretical extensions in the study of the phenomenon are foreseen.     

Scattering losses in binary borate glasses

The purpose of this paper is to examine the potential of three binary borate glasses; namely PbOB₂O₃, K₂OB₂O₃ and Li₂OB₂O₃ as candidates for fabrication of low optical loss and low cost fiber-glass wave-guides.The importance of ultrasonic measurements as the first step in a systematic search for a glass with low optical loss, is discussed. Results of ultrasonic measurements of PbOB₂O₃ system are then presented. Using these results and the published results for the K₂OB₂O₃ and Li₂OB₂O₃ systems, estimates of the magnitude of density fluctuations as a function of composition have been made for each system. Comparison with the previously published results on the K₂OSiO₂ system suggests that out of the three systems chosen, only 50 mole % Li₂O50 mole % B₂O₃ glass is a likely candidate for the production of low optical loss glass fibers.     

The Geometric Origin of Asymmetric Small Angle Neutron Scattering from an Elastically Bent Double Crystal Diffractometer

We have investigated the Small Angle Neutron Scattering (SANS) from a Double Crystal Diffractometer (DCD) in the (1, —1) setting using a focusing Si monochromator crystal in the symmetric (111) case and a defocusing analyser Si crystal in a completely asymmetric (111) case. We have found that SANS data from porous glassy carbon and Silica spheres are asymmetric about the Bragg Peak centre. We attribute this observation to a combination of factors, particularly geometrical effects, related to the angle of acceptance χ into the analyser crystal when a SANS sample is positioned between the crystal pair. A simple procedure for SANS data correction is outlined and shown to be effective in this case.     

Raman Scattering of Light in Diamond Microcrystals

Crystallography Reports - Raman spectra of monodisperse crystalline diamond powders have been studied in dependence of the sample sizes. Raman scattering was excited by a near-IR cw laser (λ0...     

Resonant Raman Scattering in the Polyacetylene System

The amplitude mode approach for describing the resonant raman scattering in dimerized chains is reviewed and applied to the polyacetylene system. trans-(CH)_x is disordered with a variable electron-phonon coupling constant. cis-(CH) _x is ordered with an extrinsic gap of ～ 5% of the full gap. A distribution of extrinsic gaps is shown to exist In trans-(CH) _x chains in partially isomerized polyacetylene.     

Light Scattering Experiments during Freezing and Melting

Anomalous light scattering has been observed at the solid-liquid interfaces of high purity systems during freezing as well as during melting. The model substances used are water, salol, cyclohexane and xenon. From light scattering experiments the existence of a layer of pre-ordered material at the solid-liquid interface is deduced. Light is scattered by long range fluctuations in this layer. The thickness of this layer is a few μM. The correlation length of the fluctuations in this layer has been determined to be ξ ≈ 260 nm. Dynamic and static light scattering experiments lead to the same correlation length. Xenon, salol and cyclohexane cover a range of a factor of 15 in viscosity at the melting temperature. Growth velocities have been varied by a factor of about 200. Thermal gradients at the solid-liquid interface have been varied in a wide range. No dependence of the correlation length on these parameters or on substance has been found in the experiments. The observations do not depend on the crystallographic orientation of the crystal surface nor on the orientation of the crystal surface relative to gravity. The interface-layer may be compared to the quasiliquid layer which has been found at crystal vapour interfaces at conditions close to the melting point. The existence of a pre-ordered layer in-between a crystal and its melt means that the crystal does not grow into the melt but into the pre-ordered layer.     

Searching for an Efficient Solution Reconstruction Algorithm in the Analysis of Small-Angle Scattering Data from Silicasol Solution

Crystallography Reports - The stabilities of reconstructing the solution to the problem of determining the volume particle size distributions are compared by the example of small-angle scattering...