Spectroscopy Of Liquid Crystals Research Articles

The parasitic effects in high-frequency dielectric spectroscopy of liquid crystals – the review

ABSTRACT Dielectric (impedance) spectroscopy of liquid crystals is a powerful experimental technique. It is especially essential because liquid crystals in many applications are still controlled by the electric field. In this paper, the problems with the high frequency (>1 MHz) measurements in classical (in the shape of the flat capacitor) measuring cells are discussed. The procedure for overcoming measurement difficulties was presented and applied to the dielectric response of antiferroelectric, binary mixture. Additionally, the paper presents the first example of extending the measuring range of dielectric spectroscopy from 1 MHz to ~50 MHz by overcoming the parasitic effect related to resonance in measuring setup.

How to determine the limits of using Af n function for extraction of ITO cell contribution to dielectric spectra?

The “classical” way to estimate and extract the high frequency indium tin oxide (ITO) cell contribution to dielectric losses is to use a special shape of frequency function: [R. Dhar, An impedance model to improve the higher frequency limit of electrical measurements on the capacitor made from electrodes of finite resistances, Indian J. Pure Appl. Phys. 42 (2004), pp. 56–61; M.B. Pandey, R. Dhar, and R. Dąbrowski, Dielectric spectroscopy of a newly synthesized chlorinated analogue of MHPOBC antiferroelectric liquid crystals, Ferroelectrics 343 (2006), pp. 83–100]. This article explains why such a shape for estimating function is used and determines how fast relaxation can be investigated using this method. The shape can be derived from the model presented in [P. Perkowski Dielectric spectroscopy of liquid crystals. Theoretical model of ITO electrodes influence on dielectric measurements, Opto-Electron. Rev. 17 (2009), pp. 180–186]. This function can yield accurate results rather for slow relaxations investigated in ITO cells.

Nonlinear Raman spectroscopy of liquid crystals: orientational alignment and switching behaviour in a ferroelectric liquid crystal mixture

Nonlinear Raman spectroscopy of liquid crystals: orientational alignment and switching behaviour in a ferroelectric liquid crystal mixture

Nonlinear Raman spectroscopy of liquid crystals: orientational alignment and switching behaviour in a ferroelectric liquid crystal mixture

Picosecond inverse Raman spectroscopy has been employed to probe the alignment behaviour and switching characteristics of a 6 μm thick ferroelectric liquid crystal based on a host mixture of fluorinated phenyl biphenyl-carboxylates and a chiral dopant. Optical bistability is observed in the Raman signal on application of dc electric fields of opposite polarity. For particular polarities of the applied field, the Raman signals display a cos4 θ dependence on the angle of rotation around the beam direction. Reorientational rate constants of 300 μs and 590 μs are observed for the aromatic core at the high-voltage limit for the rise and decay of the 1600 cm−1 Raman signal on application of a switching ac electric field.

Photon Correlation Spectroscopy of Liquid Crystals Confined in Porous Matrices with Different Structure and Pore Size

ABSTRACTWe present the results of photon correlation spectroscopy investigations of the influence of confinement, interface, porous matrix structure, pore size and shape on the dynamic behavior of nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes). Investigations of LC in cylindrical pores together with studies in random porous matrices, makes it possible to separate the role of random structure and domain formation from the contributions due to existence of LC - solid pore wall interface and pure finite size effect in relaxation of order parameter or director fluctuations. In the temperature range below nematic - isotropie phase transition temperature we observed two overlapping relaxational processes which are satisfactorily described by the decay functionf(q,t) =a·exp(–t/τ1) + (1–a)·exp(–xz), wherex=ln(t/τ0)/ln(τ2/τ0) and τ0= 10−8s. For LC in 100 Å random pores the second term describing the slow process dominates, whereas for 200 Å and 2000 Å cylindrical pores as well as 1000 Å random pores the contribution from the first term (fast process) is more visible. Since the slow relaxational process which does not exist in the bulk LC and broad spectrum of relaxation times (10−6- 10)s appear not only for LC in random pores but in cylindrical as well, we conclude that differences in dynamical behavior of confined LC from that in the bulk are mainly due to the existence of the interface.

One- and Two-Dimensional ET-IR Time-Resolved Spectroscopy of Liquid Crystals

Abstract This paper reports the application of FT-IR time-resolved spectroscopy (TRS) to the study of liquid crystal reorientation dynamics. One- and Two-dimensional FT-IR TRS experiments carried out for nematic 5CB liquid crystals have revealed new information about sub-molecular mechanisms of the electric-field induced reorientational transition. The major conclusion derived from this study includes: (1) 1D FT-IR TRS with microsecond time resolution has been successful to follow the real-time transition dynamics of each individual functional group within the molecule; (2) 1D TRS experiments at high fields show that the 5CB molecules in the anchoring layer undergo fast reorientational switching motion; (3) 2D FT-IR TRS, capable of analyzing temporal correlations between motions of different sub-molecular segments, has shown that a flexible terminal chain of 5CB undergoes a fast local motion in addition to the rotational relaxation motion of the entire molecule.

Vibrational Spectroscopy of Liquid Crystals: IV. Infrared and Far Infrared Spectra of the MBBA and EBBA Liquid Crystals

Infrared spectra have been recorded for N(p-methoxy) benzylidene- (p-n-butyl) aniline (MBBA) and its ethoxy analog in the crystalline, mesomorphic and isotropic phases. From the spectra obtained fo...

Vibrational Spectroscopy of Liquid Crystals. 3. Very Low Frequency Raman Spectra of Some Polycrystalline Cholesteryl Esters

Abstract The very low frequency Stokes and anti-Stokes Raman spectra of polycrystalline cholesteryl chloride, benzoate, palmitate and stearate are reported. Contrary to some previous observations,1,2 significant variations in the observed frequencies between the various esters were found. It must be pointed out that vibrations of the molecule as a whole have to appear in addition to the lattice vibrations.