Dielectric Behavior of Nematic Liquid Crystals under DC Electric Field

Dielectric behavior of nematic liquid crystals of p-methoxybenzylidene-p-n-butylaniline and p-ethoxybenzylidene-p-n-butylaniline have been investigated under DC electric field in relation to their optical behavior. Both dielectric permittivity and transmitted light intensity decrease at a threshold voltage of external DC electric field, and the latter reaches a constant low value at higher field, while dielectric permittivity increases via minimum to approach a constant value at higher field strength of about 2 × 10⁴ V/cm. Such a change of dielectric permittivity under DC electric field is characteristic of nematic and supercooled states, and exhibits large dependence on both temperature and sample thickness. This dielectrical observation is discussed in terms of dielectric anisotropy of the nematic liquids oriented to the field, and the disturbance of molecular orientation due to the dynamic scattering at higher DC electric field.     

Electric Field-Induced Phase Transitions in Electro-Optic Cell with Dielectrically Negative Chiral Nematic Liquid Crystal

Behaviour of a chiral nematic mixture with a negative dielectric anisotropy was studied between two electrodes with homeotropic anchoring in the presence of an a.c. electric field. By changing the sample thickness d and field strength E at constant cholesteric pitch p, the phase diagram in the parameter plane (d/p, E) was determined. Usefullness of this diagram for realization of positive contrast dichroic LC displays with nematic-cholesteric phase change is discussed.     

Modulated Microdomain Switching of Nematic Liquid Crystals

Abstract

Modulated microdomain switching of nematic liquid crystals have been investigated. This display mode used for light switching of micron-sized domains regions using a complicated electric field with special mesh electrode structures. Under He-Ne laser irradiation, contrast ratio was 22:1 for homeotropic alignment cell and transmittance change was no dependent of incident polarization condition. Under white light illumination, the contrast ratio was 4.3:1.     

The influence of the boundary conditions on the character of the phase transition nematic-smectic C

The possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic-smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N-Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N-S_C of the substances.     

Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

Electric Field Effects on Sensitized Fluorescence Emission From Nematic Liquid Crystal Cells

Remarkable electric field effects were observed with sensitized fluorescence intensities of a perylene derivative (BPDC), when rectangular AC waves (up to 10 V_0-p ) were applied to nematic liquid crystal cells containing a sensitizer (C307 or AHDA) and an acceptor (BPDC) as the guest molecules. Measurements of the fluorescence lifetime of AHDA indicated that the rates of energy transfer from AHDA to BPDC were independent of the electric fields. The electric field effects completely changed the sign when the intensity variation detected at vertical direction was compared with that at the horizontal direction with respect to the liquid crystal cell. The observed electric field effects on the sensitized fluorescence were consequently suggested to come from the orientation control of transition dipole moment of BPDC through the molecular alignment of the liquid crystal. The suggestion was confirmed by the measurement of dichroic absorption of the donor-acceptor systems.     

Determination of the Surface Tilt Angle by Attenuated Total Reflection

Thin SiO₂ layers prepared by oxidizing an organo-silane in an Ar/O₂ rf plasma discharge produce parallel alignment for nematic liquid crystals with essentially zero tilt. In addition, if the rf plasma system is arranged to produce an Ar/O₂ beam, uniform parallel alignment can now be obtained with the director in the substrate plane at an angle perpendicular to the plane of incidence of the rf plasma beam. We have studied the aligning properties of such films using the evanescent wave generated by attenuated total reflection (ATR) on a thin gold film when a surface electromagnetic wave is excited. Liquid crystal cells were constructed on a high index glass prism with a 500 Å gold film and a 200 Å aligning layer. By following the ATR minimum as a function of applied electric field, the tilt at the interface of the liquid crystal and the aligning layer was calculated.     

Influence of a.c. Electric Field on Infrared Absorption Spectra of Liquid Crystals and Determination of Orientational Order Parameter by Infrared Dichroism. II. Molecules with Weak Negative Dielectric Anisotropy

The influence of a.c. electric field (10 kHz) on the IR spectra of the nematic phase of six p-alkoxybenzylidene-p,n-butylanilines with weak negative dielectric anisotropy was studied and the temperature dependence of the orientational order parameter and threshold voltage have been investigated. The values of band elastic constants K₃₃ were calculated for MBBA and EBBA.     

Effect of Nitro Terminal Substituent on the Liquid Crystalline Characteristics of Ester Mesogens: p-Nitrophenyl-p'-n-Alkoxycinnamates

The homologous series p-nitrophenyl-p'-n-alkoxycinnamates is a medium melting homologous series; the first two members are non-mesogens and do not exhibit even monotropy. The nematic phase begins from the third derivative in monotropic condition; from the fourth member enantiotropic mesomorphism commences. Polymesomorphism begins from the sixth member with the appearance of smectic mesophase and ends at the seventh member. The rest of the homologues up to the octadecyl are purely smectogens. Thus the homologous series is predominantly smectogenic even though -NO₂ group is reported to be enhancing nematic character.^' Within the polymesomorphic region the nematic mesophase is homeotropic; however, it shows a clear threaded texture where it is the only mesophase shown. The N-I transitions do not exhibit the usual alternation in transitions which is a rare phenomenon. The N-I transition curve is unusually neither rising nor falling though a slight curve is seen; it merges with the S-I curve which shows a slight rising tendency as the alkyl chain length is increased. The S-N transition curve steeply rises and seems to be merging with the S-I curve. The mesomorphic range is good; the nature of the curve can predict the latent N-S or I-N transitions. All smectic orientations are of focal conic smectic A texture except in the case of sixth and seventh homologues where homeotropic nature prevails.     

Structure and electrical conduction of Sb2O3 thin films

Antimony trioxide (Sb₂O₃) thin films have been deposited onto glass substrates using thermal evaporation technique at room temperature. The structural feature and surface morphology were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Sandwich‐type structures were deposited with films thickness d = 0.55 μm using evaporated electrodes of silver. Current‐voltage (J‐U) characteristics have been measured at various fixed temperatures in the range 293‐473 K. In all cases, at low electric field (E <10⁴ V/cm), ohmic behavior is observed. However, at high electric field (E >10⁴ V/cm), non‐ohmic behavior is observed. An analysis of the experimental data indicates that in the range of high‐applied electric field, the dominant conduction mechanism is space charge limited currents (SCLC). Using the relevant SCLC theory, the carrier concentration, total trap concentration and the ratio of free charge to trapped charge have been calculated and correlated with changes in the structures of antimony trioxide thin films. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Phase Transition Behaviour of Binary Systems: Various Non-Nematic Solutes in Nematic Solvent (EBBA)

Phase transition behaviour of the systems of EBBA with methyl alcohol (MeOH), 1.2-dibromo-ethylene (DBE), o-xylene, m-xylene, and p-xylene has been studied. The phase diagrams for each system have been determined by the polarizing microscope. The slopes, β_n and B_i, of the nematic and the isotropic phase boundary lines in each system have been qualitatively described by the molecular field treatment proposed by Humphries. The order parameters for both cis-and trans-DBE in EBBA have been obtained from analyses of ¹H-NMR spectra. In the nematic phase, they decrease with increase in temperature. In the nematic/isotropic coexisting phase (N/I phase), the order parameter for CIS-DBE is almost independent of temperature. The proton spin-lattice relaxation times (T₁) in the MeOH/EBBA system have been measured by a pulse NM R technique. Little temperature dependence of T¹ has been found in the N/1 phase region.c     

The Internal Field Parameters for Liquid Crystals

This paper presents calculations of the internal field constants γ_e for four homologous liquid crystal materials in their nematic and crystalline phases. We find that for the nematic phase, the γ_e ～ S ₂ curve is nearly a straight line for each compound. When one extrapolates the straight line to S ₂ = 1, the corresponding γ_e value is nearly equal to γ∥, the crystalline internal field constant. The birefringence data used are those reported by Somashekar et al.⁴ The four compounds studied are:

(1) p(p'-ethoxyphenylazo)phenyl valerate;

(2) p(p'-ethoxyphenylazo)phenyl hexanote;

(3) p(p'-ethoxyphenylazo)phenyl heptanoate;

(4) p(p'-ethoxyphenylazo)phenyl undecylenate.     

ENHANCED FLEXOELECTRIC SWITCHING BEHAVIOUR IN THE CHIRAL NEMATIC PHASE

Abstract

In order to develop materials that exhibit enhanced flexoelectric switching in the chiral nematic phase we have identified mesogenic units that display inherently strong flexoelectric coupling capabilities. Here we examine the oxycyanobiphenyl (OCB) moiety: homologues from the nOCB series exhibit significant electro-optic switching effects when doped with a highly chiral additive. Here we have examined lower dielectric anisotropy materials, since they allow the flexoelectric response to be extended to high field amplitudes. We show that dielectric coupling strength can be low in symmetric bimesogenic molecules. The flexoelectric response of such a molecular structure is tested by doping a homologue from the series CBOnOCB with a chiral additive: very significantly we find that the optic axis is rotated through 2φ = 45° in < 50 μs on reversing the polarity of the field (amplitude E = ±6 V μm^?1). Subsequently we have synthesized room temperature chiral nematic materials that exhibit 2φ > 90° at E = 10 V μ^?1.     

Dielectric Studies of the Ordering Due to an External DC Electric Field in Nematic 4,4′-di-n-hexyloxyazoxybenzene

Studies of dielectric permittivity changes under the action of a DC electric field on nematic 4,4′-di-n-hexyloxyazoxybenzene (DHAB) have been performed. Observations of time dependent processes occurring during the application of a voltage were made for samples of 0.5–2 mm thickness as a function of the applied voltage. The dielectric permittivity changes which are related to changes in the ordering of the samples were interpreted in dielectric as well as in electrohydrodynamic terms. Three voltage regions were distinguished which are characterised by ordering due to dielectric orientation, hydrodynamic laminar flow and turbulent flow.     

The Electron Microscopic Approach to the Structure and Reactivities of Organic Solids

In this paper we continue investigations started in Refs. 1 and 2 studying the effect of a magnetic field on electrohydrodynamics of liquid crystalling complex ethers with dielectric constant anisotropy close to zero. The investigations were made with 100 μm thick layers having initial planar “first” structure of B and C mixtures described in Ref. 2.

Figure 1 shows dependences of optical layer transmittance on the intensity of the magnetic field (H) directed normal to bearing surfaces, and in the absence of an electric field in the absence of an electric field (curves I and II). For some threshold values H = H_th all the samples showed an increased transmission arising due to Fredericks transition.³     

Field and Frequency Effects in a Nematic Mixture of Negative and Positive Dielectric Anisotropy

The molecular alignment in the nematic liquid crystal mixture of p-ethoxy benzylidene p′-n-butyl aniline (EBBA) and p-ethoxy benzylidene p′-cyano aniline (EBCA) has been investigated by measuring the dielectric constant at 1 MHz. Detailed investigation has been carried out for a composition of 2.74%, of EBCA in EBBA for different electric and magnetic field strengths and at different frequencies of the applied electric field. Both parallel and perpendicular electric and magnetic field configurations have been employed. Temperature and frequency dependence of the dielectric anisotropy have been studied.

Two interesting features have been observed. I) In the crossed field configuration complete molecular alignment could be observed by suitably changing the electric field strength. The range of electric field strength required to change the molecular ordering, is frequency dependent. This range shows a minimum in the neighbourhood of 2 kHz. 2) It has also been observed that in the parallel field configuration minimum dielectric constant change is observed apparently independent of frequency and electric field strength. Detailed results of the field and frequency effects and of optical studies in this mixture are reported.     

Bipolar-homogeneous structural phase transition in nematic droplets formed in the polymer matrix in a magnetic field

The phase transition from the bipolar structure to the homogeneous structure in droplets formed from the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl in poly(vinyl butyral) in the presence of a magnetic field has been investigated. The phase transition is associated with the expansion of the regions with a tilted orientation of the director in the vicinity of defects (the disappearance of boojums) and with the alignment of the nematic director lines in the bulk of the droplet in the direction of the magnetic field. In the temperature range T = 24−34°C, cyclic mutual transformations between the bipolar configuration of the nematic director and the homogeneous structure, which have a period of ∼0.5–3.5 s and result from thermal fluctuations of the order parameter, are observed in droplets 3–15 μm in size after the forming field is switched off.     

Phase transitions from density and order parameter from polarisabilities in symmetric dimeric liquid crystals

As a part of our systematic studies on liquid crystal dimers, we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A exhibited by DLCs, α,ω-bis-(4-n-alkylaniline benzylidene-4?-oxy) alkanes. Further, the orientational order parameter in the nematic phase of these DLCs are estimated from the molecular polarisabilities calculated using the experimental refractive indices and density results. The molecular polarisabilities α_e and α_o are obtained for the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal. α_e and α_o calculated in this way are used to obtain Δα. The polarisability anisotropy in the perfect order (absolute K) is calculated semi-empirically using the δ-function model developed by Lippincott et al. and molecular vibration method. The values of polarisability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain α and Δα values considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence δn = (n_e-n_o) and this value of S is compared with those obtained from field models.     

A Study of Flow Alignment Instability During Rectilinear Oscillatory Shear of Nematics

Rectilinear oscillatory shearing of nematic liquid crystals in thin large-area (10 cm × 8 cm × 10 μm) cells has been studied in the range 20 to 100 Hz for two uniform initial alignments, one planar and parallel to the direction of oscillation and the other homeotropic. In the former case, for certain positive values of the viscosity coefficient α₃ a flow-alignment instability occurs above a critical amplitude A_c for the displacement of the movable upper plate, leading to formation of regular cm-wide bands of alternating 2 π and -2 π twist, running parallel to the alignment direction. The width of the bands decreases with decreasing temperature and increasing frequency, the total number present depending on the size of the cell. In homeotropic alignment, similar bands of lower twist form at an angle to the direction of oscillation, in this case both for certain positive values of α₃ and, in addition, for α₃ slightly negative. A theoretical expression for A _c in the cases with α₃ > O has been derived and substantially confirmed experimentally. The theory further predicts the existence of a remarkable “stability gap” where, although the expression for A_c is a minimum, there is no instability and formation of bands should not occur. By exploiting the divergence of α₃ at a smectic A phase, theory and experiment have been compared over a wide range of α₃ values, and the stability gap confirmed experimentally for several materials. In the case of “weak” smectic phases (i. e., showing actual or incipient re-entrant behavior) the expected divergence of A_c does not occur until significantly inside the smectic phase range. Extrapolation of 1/A² _c against temperature can be used to locate accurately in the temperature at which α₃ is zero. A variety of materials has been studied, revealing that α₃ may be positive in a rather wider range of materials than had previously been realized, and that when α₃ is positive it usually remains so over most or all of the nematic range.     

The Effect of Rhodamine 6G on Phase Transitions in Cholesteryl Ester Mixtures

Compensated cholesteric mixtures were the subject of a whole range of studies.^1-6 Due to their characteristic properties (the existence of a temperature at which compensation occurs and the dependence of the critical field on temperature), the orientation of guest molecules by compensated cholesteric hosts⁷ shows some particular features in comparison to the orienting mechanisms acting within nematic liquid crystals^8-15 or within chiral nematic mixture.^16,17 When the colour of sample varies due to the influence of the electric field and temperature the guest dye contributes to it as well the compensated cholesteric host.

In this paper we report the modification of compensated cholesteric mixture properties due to the introduction of Rhodamine 6G as guest.