FLC Mixture Research Articles

Influence of carbon nanotubes on the dielectric and electro-optical properties of a proto-type ferroelectric mixture used in display devices

Doping of different concentrations of single and multi-walled carbon nanotubes (CNTs) is found to influence considerably the dielectric and electro-optical properties of a room temperature FLC mixture. However, the comparative change of such properties between the two types of CNT doped systems is marginal. Dispersion of CNTs produces not enough change in the molecular ordering as revealed by the small decrease in the phase transition temperatures and tilt angles. Spontaneous polarization is also found to decrease due to the screening of the molecular dipole moments by the π–π electron system of the CNTs. Both the dielectric increments and absorption strengths are found to decrease remarkably due to the trapping of ions by the CNTs. Trapping also decreases ac and dc conductivities and the local electric field which in turn fastens the switching time considerably making the CNT-doped systems more suitable for FLC-based applications.

Quantum Dot Doped Ferroelectric Liquid Crystal System: Investigation of Electro-Optical Parameters and Relaxation Behavior

The effect of QDs on the dielectric and electro-optical (E-O) properties of different ferroelectric liquid crystal matrices has been studied in the SmC* phase. A substantial change in the different parameters like tilt angle, spontaneous polarization and relative permittivity has been observed for the doped system. For the better understanding of the effect of QDs on FLC relaxation process another quantum dot CdSe has been dispersed in two other FLC systems 16/100 and DOBAMBC. The effect of QDs on the three FLC systems on relaxation behavior attributes to the alteration of tilt angle in different way in different doped system. This difference in relaxation behavior is observed due to the dissimilar composition and structure of FLC mixture and FLC molecule.

Modification of the electrical parameters of CNT-doped deformed-helix ferro-electric liquid crystals

Liquid crystals are useful for a wide range of applications due to their exceptional properties. Doping of liquid crystals with carbon nanotubes (CNTs), even at very low concentrations, produces a detectable effect on the liquid crystal (LC) properties that can be very attractive for various functions. In this study, an attempt was made to investigate the effect of CNTs on the electrical properties of a short-pitch and high-spontaneous-polarization ferro-electric LC mixture, FLC-6304, at different temperatures. The inclusion of the CNTs significantly reduced the polarization at temperatures well within the SmC* phase, but the effect was gradually reversed as the transition temperature was approached. The insertion of the CNTs also reduced the response time and the rotational viscosity of the FLC mixture, which is highly desirable in the LCD industry.

Electrooptic and Dielectric Studies in Cadmium Sulphide Nanorods/Ferroelectric Liquid Crystal Mixtures

We present the results based on the electrooptic and dielectric studies in cadmium sulphide (CdS) nanorods/ferroelectric liquid crystal mixtures. Doping of CdS nanorods increases the spontaneous polarization and response time, which due to large dipole-dipole interaction and increase in anchoring energies exists between nanorods and FLC molecules. Dielectric measurements revealed a decrease (~40% for 0.3% CdS in FLC) in permittivity and dielectric strength in doped sample cell than pure FLC mixture. A decrease in dc conductivity and relaxation frequency with doping concentration was also noticed. The preexponent factor and fractional exponent factor are found as predicated by existing theories.

Sub‐millisecond transient vertical chevron formation during the electroclinic effect in the smectic A* phase

X‐ray diffraction studies are carried out in order to probe the smectic layer structure in liquid crystal devices filled with FLC mixture SCE8 and AFLC mixture CS4001, at a temperature just above the SmA*–SmC* phase transtion. The data gathered are time‐resolved in synchronization with a bipolar voltage pulse applied across the device. The layers are observed to move dynamically and reversibly with voltage application and removal, giving evidence for temporary vertical chevron formation due to the electroclinic effect on a timescale consistent with this phenomenon.

A bent‐core dopant‐induced smectic A* twist state†

The phase behaviour of a commercial calamitic ferroelectric liquid crystal mixture, doped with different mesogenic and non‐mesogenic bent‐core molecules was investigated through polarising microscopy, optical measurements and quenched growth. A twisted smectic structure, similar but not equivalent to a twist grain boundary (TGB) phase, and absent in the neat FLC mixture, was verified. The twisted smectic state can only be observed on cooling and its stability depends on the rate of temperature decrease, which indicates a kinetically governed behaviour. Further, the growth dynamics of the low temperature uniform SmA* bookshelf structure is dominated by viscosity instead of free energy density, as would be expected for a true thermodynamic phase transition. The investigations signify the chiral induction capability of achiral, bent‐core dopant molecules and we believe that the observed behaviour represents the onset of TGB formation at very large pitch. It can thus give valuable information for the fundamental physical understanding of twist grain boundary phase formation. †. †This paper is dedicated to Sven T. Lagerwall on the occasion of his 70th birthday

Field-induced reorientation dynamics of a surface stabilized ferroelectric liquid crystal mixture

Time-resolved measurements of electro-optical response and Fourier-transform infrared absorption have been employed to study the submolecular motion of a surface stabilized ferroelectric liquid crystal (SSFLC) mixture during a field-induced reorientation process. All observed submolecular groups were found to rotate in unison about the layer normal in a steady d.c. field. In a transient situation, the FLC director can respond rapidly to a pulsed driving field. However, the orientation distribution was found to spread at first and then slowly converge to the new direction of the FLC director. When the field is switched off, all the core groups relax to a steady-state direction with varying relaxation times. The results reveal that submolecular fragments of different molecular species in the FLC mixture move correlatively in a steady d.c. field and during a field-induced reorientation process.

Spontaneous polarization and response time of polymer dispersed ferroelectric liquid crystal (PDFLC)

Results reported on the spontaneous polarization, switching time and rotational viscosity measurement of a FLC compound in neat phase and that in polymer matrix in the present paper. Polymer-dispersed ferroelectric liquid crystal film was formed by photopolymerization due to polymerization induced phase separation. A detailed study has been made on the spontaneous polarization, switching time at different temperatures and measuring field. It has been observed that the magnitude of spontaneous polarization of the PDFLC mixture is not in accordance with their mixing ratio. The switching time in PDFLC mixture is less than that in neat FLC mixture.

Biaxial dielectric properties of phenyl ester ferroelectric liquid crystal mixture

Dielectric and electro-optic properties of phenyl ester FLC mixture have been studied in detail. Dielectric relatation spectroscopy was used to study the molecular dynamics and dipolar ordering. In the SmA* phase, this is accompanied by a sharp increase in the dielectric strength Δes on approaching Tc. At such high frequency (2 MHz) Δe is usually negative in the SmC* and SmA* phase. In the N* phase, at lower frequencies, Δe had a weakly positive value. In the SmC* phase, δe is positive and its maximum value is less than 0.1. The dielectric biaxiality is discussed as an order parameter of the tilted smectic phase. It is verified that if the tilt angle increases according to θ ∝ (TC − T)β, the biaxiality increases according to δe ∝ (TC − T)2β. A critical exponent for θ and δe are 0.27 and 0.54, respectively. The rotational viscosity γθ for soft mode is of 10 Nsec/m2 in the SmA* phase. The activation energy for the rotational dynamics of Goldstone mode is estimated to be 1.4 eV from an Arrhenius plot. Optical response time is of about 50 μs at room temperature in SmC* phase, and is on the order of 10 μs in the N* phase and almost independent of the applied field.

Design of an Ultrashort Directional Coupler With an SSFLC Coupling Layer

A design of a vertical directional coupler, operating at the wavelength λ= 632.8 nm, is presented. The coupler consists of a layer of the Hoechst ferroelectric liquid crystal mixture FELIX-M4851–025, homogeneously aligned and sandwiched between two single mode ion-exchanged BK7 glass waveguides. The design of the directional coupler was carried out by using the beam propagation method, taking into account also teflon alignment layer and ITO electrode contributions. Given the FLC mixture refractive indices, n o =1.611 and n o = 1.466 at λ= 632.8 nm and T= 25°C, calculations show that less than 60 μm coupling length, with an extinction ratio higher than 21 dB can be obtained by using single mode waveguides with a refractive index change at the surface Δn= 0.013, an FLC layer 2 μm thick, and cone axis tilted by about 19° from the propagation direction of the lightwave.

Investigation of The Biased Rotation of Chiral Molecule Around its Long Molecular Axis in the Ferroelectric Liquid Crystal Mixture

Spontaneous polarization (Ps), tilt angle (θ), helical pitch (p) and densytometric (ρ) characteristics versus temperature and concentration of chiral dopant have been measured for FLC mixture. On the base of computer modelling the molecular dipole moment (μ), transverse (αt) and longitudinal (α1) molecular polarizabilities as well as angle (γ and Φ0) between the vector μ and main long n and short 1 molecular axes of inertia have been calculated. Knowing the molecular parameters μ, αt, αl, γ and Φ0 and state parameters θ, p, spontaneous polarization Ps has been calculated as the vector sum of the components of molecular dipole moment μ1 perpendicular to the tilt direction. To do it the Zeks1 rotational potential modified by us2 has been used. After comparing spontaneous polarization Ps taken from experimental and obtained from theoretical approximation the biased rotation parameter η has been calculated.

A construction for an analog grey scale SSFLC display

Abstract We introduce a promising low cost analog grey scale structure for SSFLC panels. The display under study consists of two glass substrates, one of which is covered as usual with ITO electrodes. The other substrate is covered with a high resistivity transparent film and with a pattern of thin, parallel metal conductors. Spacers can be formed by the photoresist stripes left on metal conductors with thickness controlled in spin coating process. The grey scale is produced due to variation of the FLC switching area beside the metal tracks according to the applied data voltage. The high resistivity coating leads to non-uniformity of the electric field which is responsible for domain creation and switching process growing gradually from the metal tracks. We also discuss addressing hysteresis and operation range problems and present our choice for FLC mixture, aligning layers and addressing waveforms. Finally we give addressing hysteresis measurements and preliminary results from our devices.

Comparison of modelling and experimental values of non-linear optical coefficients of SCE13(*)

Abstract Second order non-linear optical properties of tilted smectic liquid crystalline phases with chiral molecules have been recently measured 1,2. This paper describes experimental measurement of non-linear optical coefficients of a commercial FLC mixture, SCE13*3 and comparison with values derived from molecular modelling. Using a thin cell geometry and in-plane electrodes, Maker fringes were detected. From the SHG signal, the bulk coefficients were calculated to be deff = 0.0008 prnV-1, d21 = 0.0001 pmV-1, and d22 = 0.003 pmV-1 at 50°C. Molecular modelling of the chiral components of SCE13* was carried out using MM2 molecular mechanics and the semi-empirical MOPAC 5 and 6 packages. The d22 coefficient was calculated to be 0.0004 prnV-1 using simple geometric arguments, and was 0.0027 pmV-1 using an extended theory.

Studies of a Polymer Dispersed Ferroelectric Liquid Crystal

ABSTRACTFerrorlrtric liquid crystals (FLCs) have shown great potential for use in electro-optic and display technology due to theri inherently fast switching speeds. Recently within this area a great deal of attention has also been given to FLCs dispersed within a polymer networks. Adding the polymer may act to enhance certain electro-optic properties and will substantially increase the mechanical strenth on the FLC system. This study examines the effects of adding either a diacrylate monomer or a polymer network to a FLC mixture of known composition. The monomer depresses the phase transition temperatures to more orderd phases for both first and second order transitions and causes a marked decrease in the amount of liquid crystal which exhibits typical transitions behavior. During polymerization the network phase separates forming two cocontinuous phases and allows the liquid crystal transitions to return close to values seen in polymer systems. As a result of this decrease, the rotational viscosity decrease for these same samples. Maximum double bond conversions and polymerization rate maxima increase with monomer concentration until saturation on monomer in the liquid crystal is reached. The rate maxima then decreases as the monomer as the monomer must dissolute into the liquid crystal and diffuse to the reactive sites.

Naphthalene Base Ferroelectric Liquid Crystal and Its Electrooptical Properties

Abstract This paper reports the naphthalene base FLC mixture which provides an almost ideal bookshelf layer structure. The formation of the bookshelf layer structure may be governed by the naphthalene base FLC's particular phase sequence and the wide temperature range of SA phase. This paper also describe the layer tilt angle dependence of the electrooptic properties such as the polarization switchings and the memory capability.

Ferroelectric Liquid Crystals Having Various Cores. Effect of Core Structure on Physical Properties

Abstract Ferroelectric liquid crystals (FLC’s) having various cores were synthesized and the effect of the different core structures on the spontaneous polarization (Ps) values was investigated. The results indicate that the core structure has a great influence on the Ps value and introduction of a heterocycles such as pyrimidine and thiadiazole ring in the core increases the Ps value. Moreover, several compounds in this series are interesting as chiral dopants for FLC, because the mixtures containing them showed superior properties as FLC mixture.