Isotropic Phase Of Liquid Crystals Research Articles

Electro-Optic Kerr Response in Optically Isotropic Liquid Crystal Phases

The results of an experimental investigation of the temperature and wavelength dependence of the Kerr constant (K) of mixtures with an increasing amount of chiral dopant in an isotropic liquid crystal phase are reported. The material was composed of a nematic liquid crystal (5CB) and a chiral dopant (CE2), which formed non-polymer-stabilized liquid crystalline blue phases with an exceptionally large value of K∼2 × 10−9 mV−2. The measurements were performed on liquid and blue phases at several concentrations covering a range of temperatures and using three wavelengths: 532 nm, 589 nm and 633 nm. The work focused on changes caused by concentration and their impact on the increase in the value of K, and it was found that in the case of the 5CB/CE2 mixture these changes were significant and quite systematic with temperature and wavelength. It is shown that the dispersion relation based on the single-band birefringence model described K well in isotropic liquid crystal phases at all of the measured concentrations. In an isotropic fluid, both temperature-dependent parameters in the dispersion relation had a simple linear form and, therefore, the K-surface could be described by only four constants. In the blue phase, the expression reproducing the temperature variation of K depended on concentration, which could vary from being almost linear to quasi-linear and could be represented well by an inverse exponential analytic expression.

Acousto- and acousto-optic effects in liquid crystals: An overview

An overview of the following three aspects of the acoustical and related optical behavior of liquid crystals, namely i) ultrasonic studies, ii) acousto-optical effects, and iii) surface wave induced optical effects have been discussed. The temperature dependent ultrasound velocity has been used for the evaluation of the adiabatic compressibility (kS) and the acoustic impedance (Z). A correlation between thermodynamic functions and thermo-elastic properties facilitated the evaluation of the specific heat at constant volume (CV), the ratio of specific heats (g) and the isothermal compressibility (kT) across liquid crystal-isotropic phase transition. Acousto-optical effect in liquid crystals has been described in terms of molecular reorientation related to flows and turbulences due to acoustic vibration and radiation pressure in the medium. One important aspect of the acousto-optical effects is the surface wave induced optical effects. The change in transparency of a normal layer of liquid crystal under crossed polarizers are discussed which is interpreted as a consequence of the rotation of the optic axis. In addition, the acoustic emission occurring from homeotropically-aligned liquid crystals irradiated by surface acoustic waves are also discussed.

From Hidden Shear‐Elasticity to Opto‐ and Thermo‐Elastic Properties in Confined Polymer Melts and Viscous Liquids

AbstractThe newly identified low frequency elasticity of mesoscopic liquids opens the route to the identification of novel dynamic liquid properties. Two experimental examples illustrating the impact of the liquid elasticity in optics and thermodynamics are presented. It is revealed that at mesoscopic scale, liquids are able to convert the mechanical energy in optical and thermal effects. The fluids are submitted to a low frequency (∼Hz) mechanical shear strain (external stress), thus excluding a coupling with molecular time scales. The dynamic response is probed using two in situ methods accessing in real time optical response of the isotropic phase of liquid crystals and probing the thermal response upon shear deformation. In both cases, a dynamic response is identified: the isotropic phase of liquid crystals exhibits a low frequency dynamic birefringence while ordinary liquids exhibit cold and hot thermal waves synchronous to the low frequency excitation.

Determining the Kerr constant in optically isotropic liquid crystals.

A new scheme is investigated for evaluating the temperature dependence and dispersion relation of the Kerr constant (K) of an optically isotropic medium in isotropic and blue phases (BPs) liquid crystals. The scheme employs the measurement of the component of the transmitted light intensity of double modulated frequency using the modified in-plane-switching cell geometry (based on metallic film electrodes). It overcomes to a large extent the problem of a nonuniform electric field, employs relatively small driving voltages, and allows K to be measured directly. It is shown that the dispersion relation based on the single-band birefringence model describes well both blue and isotropic liquid crystal phases. It is found that the experimental data indicate that the temperature-dependent coefficients in this relation have a simple linear form in the isotropic phase, which allows a general model for the temperature and wavelength dependence of the Kerr constant in the isotropic liquid crystal phase to be formulated. In the BPs the temperature dependence of the experimental data deviate from the simple linear trend, but follow well an inverse exponential form.

Probing a microviscosity change at the nematic-isotropic liquid crystal phase transition by a ratiometric flapping fluorophore.

Understanding the microviscosity of soft condensed matter is important to clarify the mechanisms of chemical, physical or biological events occurring at the nanoscale. Here, we report that flapping fluorophores (FLAP) can serve as microviscosity probes capable of detecting small changes. By the ratiometric fluorescence analysis, one of the FLAP probes detects a macroscopic viscosity change of a few cP, occurring at the thermal phase transition of a nematic liquid crystal. We discuss the impact of the chemical structure on the detection capability, and the orientation of the FLAP molecules in the ground and excited states. This work contributes to experimentally providing a molecular picture of liquid crystals, which are often viewed as a continuum.

Polymer stabilized isotropic phase liquid crystals with large Kerr constant

ABSTRACT In this paper, we reported a polymer stabilised isotropic phase liquid crystal (PSILC). The influences of different content of chiral dopant and polymer monomer on PSILC were studied. Adding the proper proportion and total content of polymer monomer TMPTA and C12A into liquid crystal with 1.0 wt.% chiral dopant R5011, and UV photo-polymerisation at the isotropic phase, we obtain the Kerr constant greater than 10 nm/V2, hysteresis less than 2.0% and fast response in PSILC.

Acousto- and acousto-optic effects in liquid crystals: An overview

An overview of the following three aspects of the acoustical and related optical behavior of liquid crystals, namely i) ultrasonic studies, ii) acousto-optical effects, and iii) surface wave induced optical effects have been discussed. The temperature dependent ultrasound velocity has been used for the evaluation of the adiabatic compressibility (kS) and the acoustic impedance (Z). A correlation between thermodynamic functions and thermo-elastic properties facilitated the evaluation of the specific heat at constant volume (CV), the ratio of specific heats (g) and the isothermal compressibility (kT) across liquid crystal-isotropic phase transition. Acousto-optical effect in liquid crystals has been described in terms of molecular reorientation related to flows and turbulences due to acoustic vibration and radiation pressure in the medium. One important aspect of the acousto-optical effects is the surface wave induced optical effects. The change in transparency of a normal layer of liquid crystal under crossed polarizers are discussed which is interpreted as a consequence of the rotation of the optic axis. In addition, the acoustic emission occurring from homeotropically-aligned liquid crystals irradiated by surface acoustic waves are also discussed.

Manipulation of liquid crystal alignment through interfacial modification of photoresponsive polyimides remotely actuated by dual wavelength linearly polarized light

High-quality alignment of liquid crystals (LCs) for large area display has been highly demanding for display market. To achieve a uniform and stable LC alignment, a photoresponsive azo-polyimide with two different substitution ratio of azo-dye moiety in the side chain of the polyimide has been successfully synthesized and characterized. Depending on the ratio of azo-dye substitutions, the solubility and coating nature of polyimides vary greatly and initially show random planar or homeotropic alignment of LCs. In situ photoalignment process has been performed to achieve a homogeneous planar alignment of LCs from the initial random planar state and a uniform pretilt state from the initial homeotropic cells using linearly polarized visible light (LPVL) irradiation in the isotropic phase of LCs. Furthermore, obtained alignment states have been stabilized using unpolarized UV light for RM polymerization in the nematic phase of LCs. Hence, the dual wavelength photoalignment approach has been successfully implemented to obtain the different stable alignments of LCs. Also, the results conclude that the tilt angle varies by dose of LPVL irradiation and gets significantly altered due to change in reactive mesogen (RM) concentrations. The surface investigations reveal a contrasting nature for the planar and vertically aligned substrates before and after RM stabilization.

Orientational order induced by a polymer network in the isotropic phase of liquid crystal.

We studied the paranematic ordering induced by a polymer network in the isotropic phase of a liquid crystal (LC) that occurs in polymer-stabilized cells with bend configuration of the LC director (π cells) fabricated via photopolymerization of photoreactive monomer RM 82 added in small concentrations (3-5 wt %) to a nematic LC [4-cyano-4'-pentylbiphenyl (5CB)] when low voltage was applied across the cell. The polymer network formed in the nematic phase of the LC consists of fine fibrils that are aligned along the LC director and thus mirror the bend deformation of the LC at the time of polymerization. When heated to temperatures above the nematic-to-isotropic (N-I) phase transition such highly ordered polymer network anchors LC molecules providing ordering of the LC around the fibrils which results in unusually high optical retardation of the cell, R_{cell}. We present a theoretical model that relates R_{cell} to the degree of order of the fibrils, the anchoring energy of the LC molecules on the surface of the polymer fibrils, and the fibril radius r_{0}. Fitting of the experimental R_{cell}(T) curves with the developed model reveals correlation of r_{0} with the nematic correlation length ξ_{0} which characterizes penetration of the nematic order in the isotropic phase of the LC. Accepting ξ_{0} as a material constant of about 1 nm leads to a very small radius of the fibrils, r_{0}∼1nm, which is also supported by other reported experimental data. High optical retardation and fast electro-optical response of the cells at the temperatures deep into the isotropic phase point toward the enhancement of the polymer-induced paranematic order by a well-oriented layer of LC molecules that are absorbed on the surface of fibrils. Application of high voltage at the isotropic phase temperatures results in high variations of the optical retardation of the cells. Characteristic on and off response times were about 10-100 μs, independent of the cell gap. Combination of large voltage-driven changes of the optical retardation occurring in the low-viscosity isotropic state with switching times that are at least two orders of magnitude shorter than the typical relaxation times of the cells operating in the nematic phase make such polymer-stabilized π cells very promising for application in fast electro-optical switches and light modulators.

Dual Light and Temperature Responsive Micrometer-Sized Structural Color Actuators.

Externally induced color- and shape-changes in micrometer-sized objects are of great interest in novel application fields such as optofluidics and microrobotics. In this work, light and temperature responsive micrometer-sized structural color actuators based on cholesteric liquid-crystalline (CLC) polymer particles are presented. The particles are synthesized by suspension polymerization using a reactive CLC monomer mixture having a light responsive azobenzene dye. The particles exhibit anisotropic spot-like and arc-like reflective colored domains ranging from red to blue. Electron microscopy reveals a multidirectional asymmetric arrangement of the cholesteric layers in the particles and numerical simulations elucidate the anisotropic optical properties. Upon light exposure, the particles show reversible asymmetric shape deformations combined with structural color changes. When the temperature is increased above the liquid crystal-isotropic phase transition temperature of the particles, the deformation is followed by a reduction or disappearance of the reflection. Such dual light and temperature responsive structural color actuators are interesting for a variety of micrometer-sized devices.

Large Electro‐Optic Kerr Effect in Ionic Liquid Crystals: Connecting Features of Liquid Crystals and Polyelectrolytes

The electro-optic Kerr effect in simple dipolar fluids such as nitrobenzene has been widely applied in electro-optical phase modulators and light shutters. In 2005, the discovery of the large Kerr effect in liquid-crystalline blue phases (Y. Hisakado et al., Adv. Mater. 2005, 17, 96-98.) gave new directions to the search for advanced Kerr effect materials. Even though the Kerr effect is present in all transparent and optically isotropic media, it is well known that the effect can be anomalously large in complex fluids, namely in the isotropic phase of liquid crystals or in polyelectrolyte solutions. Herein, it is shown that the Kerr effect in the isotropic phase of ionic liquid crystals combines the effective counterion polarization mechanism found in polyelectrolytes and the unique pretransitional growth of the Kerr constant found in the isotropic phase of nematic liquid crystals. Maximum Kerr constants in the order of several 10-11 m V-2 (ten times higher than the Kerr constant of the toxic nitrobenzene and less temperature sensitive than Kerr constants of nematic liquid crystals) make ionic liquid crystals attractive as new class of functional materials in low-speed Kerr effect applications.

Exploring “Dormant” Opto-Mechanical Properties of the Isotropic Phase of Liquid Crystals and Revealing Hidden Elasticity of (Ordinary) Liquids

There is little literature on the flow properties of the isotropic phase of liquid crystalline fluids. However, this phase is an ideal tool to bridge the physics of liquid crystals with those of (ordinary) fluids. Optical and mechanical studies are presented, demonstrating that away from any phase transition, the isotropic phase of liquid crystalline molecules (LCs) and liquid crystalline polymers (LCPs) can work as an optical oscillator in response to low-frequency mechanical excitation, establishing the elastic origin of the flow birefringence and “visualizing” the very existence of the elastic nature of the liquid state. Additionally, mimicking the excellent anchoring ability of liquid crystals, an alternative rheological protocol optimizing the fluid/substrate interfaces is presented to access the low-frequency shear elasticity in various one-component liquids and salt-free aqueous solutions.

Impact of diffusion limited aggregates of impurities on nematic ordering

Abstract We study the impact of random bond-type disorder on two-dimensional (2D) orientational ordering of nematic liquid crystal (LC) configurations. The lattice Lebwohl–Lasher pseudospin model is used to model orientational ordering perturbed by frozen-in rod-like impurities of concentration p exhibiting the isotropic orientational probability distribution. The impurities are either (i) randomly spatially distributed or (ii) form diffusion limited aggregation (DLA)-type patterns characterized by the fractal dimensions d f , where we consider cases d f ∼ 1.7 and d f ∼ 1.9 . The degree of orientational ordering is quantified in terms of the orientational pair correlation function G ( r ) . Simulations reveal that the DLA pattern imposed disorder has a significantly weaker impact for a given concentration of impurities. Furthermore, if samples are quenched from the isotropic LC phase, then the fractal dimension is relatively strongly imprinted on quantitative characteristics of G ( r ) .

Controlling the volatility of the written optical state in electrochromic DNA liquid crystals.

Liquid crystals are widely used in displays for portable electronic information display. To broaden their scope for other applications like smart windows and tags, new material properties such as polarizer-free operation and tunable memory of a written state become important. Here, we describe an anhydrous nanoDNA–surfactant thermotropic liquid crystal system, which exhibits distinctive electrically controlled optical absorption, and temperature-dependent memory. In the liquid crystal isotropic phase, electric field-induced colouration and bleaching have a switching time of seconds. Upon transition to the smectic liquid crystal phase, optical memory of the written state is observed for many hours without applied voltage. The reorientation of the DNA–surfactant lamellar layers plays an important role in preventing colour decay. Thereby, the volatility of optoelectronic state can be controlled simply by changing the phase of the material. This research may pave the way for developing a new generation of DNA-based, phase-modulated, photoelectronic devices.

Synthesis and characterization of novel ABA-type azobenzene-containing tri-block copolymers from telechelic polystyrene

The telechelic polystyrene (tPS) was used as a macro-reversible addition fragmentation chain transfer (RAFT) agent to prepare the ABA-type tri-block copolymers (TBCs), poly{4-[4-(11-acryloyloxyundecyloxy)phenylazo]benzonitrile}-b-polystyrene-b-poly{4-[4-(11-acryloyloxyundecyloxy)phenylazo]benzonitrile} [poly(AUPAB)-b-PS-b-poly(AUPAB)] with 4-[4-(11-acryloyloxyundecyloxy)phenylazo]benzonitrile (AUPAB) as a monomer by the RAFT polymerization process. The poly(AUPAB) was used as a reference to probe the role of polystyrene block in poly(AUPAB)-b-PS-b-poly(AUPAB) macromolecular chain. The TBCs were characterized by spectral techniques, thermal analysis, and polarizing optical microscopic studies and compared the results with individual blocks, poly(AUPAB), and tPS. The TBCs exhibited higher thermal stability and liquid crystal–isotropic phase transition peak temperature (TLC-I), and glass transition temperature (Tg) values compared to the poly(AUPAB). TBC TLC-I value was dependent on the AUPAB wt% in macromolecular chain. Furthermore, the acquired results by optical microscopy suggest that after incorporation of the polystyrene block in acrylate-based pendant azobenzene polymer chain resulted in lower domain size of focal conic texture, which is characteristic of smectic-A liquid crystalline phase compared to the homopolymer. The photophysical properties of the TBCs were similar to those of the poly(AUPAB). Microphase-separated nano-segregation was observed in annealed thin films of TBCs.

Electrically tunable refractive index in the dark conglomerate phase of a bent-core liquid crystal

Here we report an electrically tunable refractive index observed in an isotropic liquid crystal phase known as the dark conglomerate (DC) phase. This unusual change in the refractive index which has not been reported before in the DC phase of other bent-core liquid crystals occurs because of a series of electric-field-driven transformations that take place in the DC phase of the studied bent-core liquid crystal. These transformations give rise to a decrease in the refractive index of the system, when an electric field is applied across the device, and no change in the birefringence is seen during such behavior. The electro-optic phenomenon is described in detail and the possibility of exploiting this for a number of liquid crystal based device applications is discussed.

Specific features of the freedericksz transition in nematic-cholesteric droplets: The effect of the liquid crystal-isotropic phase boundary

Quasi-static orientation transitions in nematic-cholesteric liquid crystal (LC) droplets occurring in an isotropic environment in a constant electric field have been studied. It is shown that transformation of the initial homeotropic orientation of the director field in these LC droplets has a non-threshold character, which is related to the presence of a small cholesteric additive. As the field increases, this dopant leads to continuous growth in the effective size of a deformation field from the LC-isotropic phase interface.

Hidden solidlike properties in the isotropic phase of the 8CB liquid crystal

Novel dynamic experiments have enabled the identification of a macroscopic solidlike response in the isotropic phase of a low molecular weight liquid crystal, 4,4'-n-octylcyanobiphenyl (8CB). This unknown property indicates that the low frequency shear elasticity identified in the isotropic phase of liquid crystal polymers is not reminiscent from the glass transition but reveals likely a generic property of the liquid state. The comparison to high molecular weight liquid crystals indicates, however, that the shear modulus is much enhanced when the liquid crystal moieties are attached to a polymer chain. The macroscopic length scales probed (0.050-0.100 mm) exclude wall-induced effects.

Feasibility study of novel fast-switching liquid crystal displays using electric birefringence effect

Both theoretical and experimental studies were made on electric birefringence effect of nematic liquid crystals in isotropic phase with respect to its applicability to novel LCDs. It was confirmed that response times are <1 ms, and a critical point exists where electro-optic hysteresis vanishes. Substrate surface treatments were also found to improve the electro-optic characteristics. The results are promising for both projection and direct view display applications featuring fast switching and high image quality.

Mid-infrared photothermal heterodyne spectroscopy in a liquid crystal using a quantum cascade laser.

We report a technique to measure the mid-infrared photothermal response induced by a tunable quantum cascade laser in the neat liquid crystal 4-octyl-4'-cyanobiphenyl (8CB), without any intercalated dye. Heterodyne detection using a Ti:sapphire laser of the response in the solid, smectic, nematic and isotropic liquid crystal phases allows direct detection of a weak mid-infrared normal mode absorption using an inexpensive photodetector. At high pump power in the nematic phase, we observe an interesting peak splitting in the photothermal response. Tunable lasers that can access still stronger modes will facilitate photothermal heterodyne mid-infrared vibrational spectroscopy.