Dielectric Properties Of Liquid Crystals Research Articles

Influence of proteins on the dielectric characteristics of 5CB and 8CB liquid crystals

ABSTRACT In this paper, we study the influence of the protein Bovine Serum Albumin (BSA) on the dielectric properties of liquid crystals (LCs) such as 4-cyano-4-pentyl biphenyl (5CB) and Octyl-4-biphenylcarbonitrile (8CB). Studying the effect of proteins on the dielectric properties of LC is essential for both fundamental research and practical applications in material science, biomedical engineering, biotechnology, and optoelectronic devices. It helps understand the protein behaviour on the surface of LCs, which offers an opportunity to develop novel sensing platforms. A polarising optical microscope (POM) was used to examine the correlation between textures formed by LCs interacting with different concentrations of BSA. In the dielectric studies, dielectric parameters such as impedance (Z), real (Z’) and imaginary (Z’’) parts of the impedance, and the tangential loss (tan δ) were measured. The investigation involved analysing relaxation times using the collected data, revealing distinct behaviours between 5CB and 8CB liquid crystals. Bode and Nyquist’s plots were constructed and analysed to investigate the changes LCs undergo while interacting with BSA. An equivalent electrical circuit (EEC) was constructed using Bode and Nyquist data, from which circuit element values were successfully extracted. The results obtained from the POM studies and the interferences drawn from the EIS can help enhance the performance of protein-based liquid crystal devices.

Dielectric properties of liquid crystals

In the paper, dielectric properties of liquid crystals, especially the nematic phase, arediscussed. An outline of the development of theories describing these properties is presented. Thestatic theory of dielectric permittivity of anisotropic nematic liquid crystals on Onsager, Maier andMeier approach is discussed in details. In this view, anisotropic properties of dielectric permittivityof nematics as well parameters describing these properties are presented.Keywords: material science, liquid crystals, nematics, dielectric permittivity, electron polarisability,constant and induced dipole moment of molecule, vectors of electric field intensity, induction andpolarisation in nematic phase.

Calibration-free method to study temperature dependence of microwave dielectric properties of nematic liquid crystals

ABSTRACTThe present article shows an original transmission-reflection-based method to characterise the microwave dielectric properties of liquid crystals. This new method offering broadband capabilities of transmission-reflection method without the need for vector network analyser calibration. It is based on two measurements in the frequency range: one measurement is performed with the empty cell, the other with the cell filled with the material. The analysis of the two recorded scattering matrices allows to eliminate all error terms and to extract real and imaginary parts of the complex dielectric constant. In this paper, we first present this method, its validation in comparison to other characterisation methods. Secondly, we present an appropriate measuring cell which allows measurements in the frequency range 26 GHz-40 GHz which is also well-suited for temperature-dependent measurements. This method is used to perform dielectric microwave measurements on 5CB liquid crystal. Finally, we discuss the temperature-dependent microwave dielectric properties (permittivity, losses and dielectric anisotropy).

RECYCLING OF LIQUID CRYSTALS FROM E-WASTE

For several decades, Liquid Crystal Displays (LCDs) have been widely used in televisions, laptops, mobile phones, and other devices. Nowadays, liquid crystals (LCs) represent an important economic value of the recycling system of LCDs. The reuse of these organic molecules could become a profitable basis since it permits to preserve the value of these materials. In this context, the general objective of this work focuses on the recovery of LCs as well as on other valuable materials present in end-of-life LCDs. An orderly, manual LCD dismantling line is put into operation for differentiated recycling of electronic boards, cold cathode lamps that may contain mercury, polymers, metals, and other valuable materials. There is also an extraction line where LCD panels are opened and exposed to an ultrasonically activated organic solvent bath to recover LCs. The resulting solution contains LCs, solvent, organic and inorganic impurities. The LCs mixtures were purified and then characterized mainly by spectroscopic, chromatographic, and thermal techniques. A study of the influence of adding diamond nanoparticles at 0.05, 0.1 and 0.2 wt% to recycled LCs was also performed using dielectric spectroscopy. Dielectric properties of LCs were measured at room temperature, using an impedance analyzer in the frequency range from 0.1 to 106 Hz.

Areas of opportunity related to design of chemical and biological sensors based on liquid crystals

ABSTRACT The societal impact of liquid crystals (LCs) in electrooptical displays arrived after decades of research involving molecular-level design of LCs and their alignment layers, and elucidation of LC electrooptical phenomena at device scales. The anisotropic optical, mechanical and dielectric properties of LCs used in displays also make LCs remarkable amplifiers of their interactions with chemical and biological species, thus opening up the possibility that LCs may play an influential role in a data-driven society that depends on information coming from sensors. In this article, we describe ongoing efforts to design LC systems tailored for chemical and biological sensing, efforts that mirror the challenges and opportunities in LC design and alignment tackled several decades ago during development of LC electrooptical displays. Now, however, traditional design approaches based on structure–property relationships are being supplemented by data-driven methods such as machine learning. Recent studies also show that computational chemistry can greatly increase the rate of discovery of chemically responsive LC systems. Additionally, non-equilibrium states of LCs are being revealed to be useful for design of biological sensors and more complex autonomous systems that integrate self-regulated actuation along with sensing. These topics and others are addressed in this article with the aim of highlighting approaches and goals for future research that will realise the full potential of LC-based sensors.

Dielectric properties of liquid crystals from the thioester series with a terminal polar group

ABSTRACTThis paper presents the results of our studies of the phase transition behaviour and dielectric properties of three compounds from two series of thiobenzoates characterised by the presence of polar terminal groups. In two of the compounds (5OSCl and 6OSCl), the polar terminal substituent is −Cl, whereas the third compound (11OSNO2) has a terminal nitro group. The compound 5OSCl has an enantiotropic nematic phase N and monotropic smectic A phase and 6OSCl is characterised by the existence of two enantiotropic phases, the N and smectic A phases. The compound 11OSNO2 only exhibits an enantiotropic SmA phase. The permanent dipole moments of the studied molecules are located parallel to their long axes, which promotes the formation of SmA phase variants. The results of dielectric permittivity dispersion and absorption and spontaneous polarisation measurements allowed the SmA phase to be identified in the compounds we investigated.

Liquid crystal-based capacitive, electro-optical and dielectric biosensors for protein quantitation

ABSTRACT The electrical, electro-optical and dielectric properties of liquid crystals (LCs) are routinely manipulated in liquid-crystal display (LCD) devices, but their potential application in the development of biosensors is still in a nascent stage. In this review, utilising the electrical properties, electro-optical effect and dielectric anisotropy in LCs, we provide insights into several possible modes of label-free biodetection and describe how capacitance, electro-optical and dielectric measurements of various LCs assist in quantitative analysis of biomolecules. It is concluded that the electrically induced biosensing techniques proposed here provides new incentives for researchers to study the interaction between LCs and biomolecules and to resolve technical hurdles facing the development of LC-based biosensors.

Dielectric properties of liquid crystal formed by laterally fluorine-substituted banana-shaped molecules

ABSTRACT Dielectric spectroscopy in the frequency range from 10−2 to 107 Hz was applied to investigate the dynamics of the relaxation processes in a banana-shaped liquid crystal compound with fluorinated chain segments. The dielectric spectra indicate the presence of two relaxation processes and a conductivity contribution. The high-frequency process shows Arrhenius-like temperature dependence in the B4 and SmCP phases and can be assigned to the hindered rotation of the bent-shaped molecules around their long molecular axis. The low-frequency process can be related to the dynamics of the periodic superstructures in B4 phase and fluctuations of the modulated ferroelectric domains in the SmCP phase. In the B7 (or SmCG) phase, both processes show the nonlinear dependence of the relaxation frequencies especially close to the isotopic phase transition. The dielectric measurements confirm the existence of the SmCP – B7 (or SmCG) second order or weakly first-order phase transition.

Performance analysis and negative dielectric anisotropy behavior of nematic liquid crystal doped with newly synthesized 1-(p-chlorobenzyl)-5-nitrobenzimidazole cobalt complex

The study investigates electrical current and dielectric properties of liquid crystal (LC) mediums. The studied samples are pure E63 and di[1-(p-chlorobenzyl)-5-nitrobenzimidazole] cobalt dichloride doped E63.The combination of 1-(p-chlorobenzyl)-5-nitrobenzimidazole (1) with metal salt (CoCl2·6 H2O) was prepared stable solid new di[1-(p-chlorobenzyl)-5-nitrobenzimidazole] cobalt dichloride (2). Synthesized di[1-(p-chlorobenzyl)-5-nitrobenzimidazole] cobalt dichloride was identified by 1H NMR spectroscopic technique and elemental analysis. Frequency dependence of the dielectric properties and dielectric anisotropy of LC sampleswas investigated using the admittance spectroscopy method (C-V and G/ω-V) in the frequency range of 100 Hz-10 MHz at room temperature. E63 coded nematic crystal was used as pure liquid crystal. The doping material concentration is ensured 1 wt % in E63. Dielectric constant (ε'), dielectric loss (ε"), dielectric loss tangent (tanδ) and real and imaginary parts of electrical modulus (M'and M") of the LC samples calculated. Also, dielectric anisotropy (Δε) versus frequency plots were obtained for both samples.

Electrically Tunable Open-Stub Bandpass Filters Based on Nematic Liquid Crystals

Tunable filters are crucial in networks for microwave signal processing, but are being stymied for millimeter-wave applications, due to a lack of materials with suitable properties. This study presents liquid-crystal-based filters with tunable passband resonances at 30, 50, and 85 GHz. These filter devices additionally provide an interesting method to characterize the dielectric properties of liquid crystals (and solids and liquids in general) in the 1--1000 GHz frequency range. Such a method promotes better engineering of planar microwave devices, and the scientific investigation of materials, in this frequency range.

The Fe3O4 nanoparticle doping effect in liquid crystal on electrical and dielectric properties

Fe3O4 nanoparticles were doped into E63 coded liquid crystal (LC) with 0.1 wt% to investigate the effect of Fe3O4 on the electrical and dielectric properties of LC in a wide range of frequency, bia...

Effect of frequency on dielectric properties of liquid crystal doped with side-chain liquid crystalline polymer

The frequency and applied bias voltage dependence of the dielectric properties and dielectric anisotropy of liquid crystal (LC) doped with side-chain liquid crystalline polymer (SLCP) mixture have been investigated using the admittance spectroscopy method ( C– V and G/ω– V) in the frequency range of 10 kHz to 10 MHz at room temperature. The liquid crystal used in this experiment is E63. The doping material used in this study is SLCP and its concentration is ensured 1 wt % in E63. Dielectric constant ( ɛ′), dielectric loss ( ɛ), dielectric loss tangent (tan δ) and real and imaginary parts of electrical modulus ( M′ and M″) of the E63/SLCP mixture was also calculated. Moreover, dielectric anisotropy (Δ ɛ) as a function of frequency was obtained. Results show that the values of the all dielectric parameters are strong functions of frequency and applied bias voltage. After a critical frequency, dielectric anisotropy has negative values according to p/n type changing.

Dielectric properties of liquid crystals in polycapillary matrices

This paper reports on the results of investigations into the dielectric properties of liquid crystals embedded in polycapillary matrices and describes a technique for their measurement. It has been revealed that the chemical structure of the rigid core and the length of mobile alkyl groups of liquid-crystal molecules of the alkylcyanobiphenyl group substantially affect the equilibrium configuration of the liquid-crystal director in capillaries. The reorientation of liquid-crystal molecules embedded in capillaries in the nematic phase under the influence of an external magnetic field has been investigated.

Microscopic Description of the Dielectric Experiment of Some Liquid Crystals*

Dielectric properties measure the collective response of a charge-free system to an applied electric field, and are a probe of molecular polarizability, dipole moment and the internal field in the anisotropic media. Interactions between dipoles cannot be discounted in the molecular interpretation of the dielectric properties of liquid crystals. So, the theories for these properties are more complicated than for magnetic or optical properties. This paper presents comparison of the predictions of the Nordio–Rigatti–Segre theory with the results of the broadband dielectric studies of the dielectric relaxation processes of a few macroscopically oriented liquid crystals. Using the quantum chemical program MOPAC/7, with the AM1 Hamiltonian a few essential values have to be calculated in advance. Moreover a new procedure for the calculation of the order parameter has been proposed.

Dielectric properties of liquid crystals of the cyano derivative compounds with different fragments in the molecular core

This paper reports on the results of investigations into the influence of variations in the chemical composition of the aromatic core of cyano-containing molecules of liquid crystals on their dielectric properties in the frequency range 1–2000 MHz. It is shown that the dispersion of the longitudinal permittivity is adequately described by the sum of two Debye processes with different weighting factors and relaxation times. The frequency dependence of the transverse permittivity is well approximated by the Debye process with a continuous distribution of relaxation times in a specified range. It is established that the replacement of one benzene ring in the biphenyl core of the 5CB liquid-crystal molecule by a cyclohexane (or bicyclooctane) fragment leads to a considerable decrease in both relaxation times for the longitudinal permittivity, a change in the low-frequency limit of the relaxation time range for the transverse permittivity, and the evolution of the frequency dependence of the dielectric anisotropy.

Dielectric spectroscopy of liquid crystals in smectic, nematic, and isotropic phases confined in random porous media

The dielectric behavior of alkylcyanobiphenyls (5CB and 8CB) confined in porous matrices with randomly oriented, interconnected pores with two different mean pore sizes (1000 and 100 \AA{}) has been investigated by means of broadband dielectric spectroscopy in the frequency range from 1 mHz to 1.5 GHz. The confinement has a strong influence on the dielectric properties of liquid crystals (LCs), which resulted in the appearance of a low frequency relaxation process $(f<~$ 10 kHz) not present in bulk and a second new process due to the presence of surface layer at solid pore wall-LC interface. Bulklike relaxation processes due to the rotation of molecules around the short axis and due to tumbling of molecules are also observed. All observed relaxation processes are of non-Debye type. Other observed differences between bulk and confined behavior are as follows: (a) the relaxation processes in confined LCs are not frozen even at temperatures about 20 degrees below the bulk crystallization temperature; (b) in the temperature range corresponding to the anisotropic phase in pores, the temperature dependence of the relaxation times $(\ensuremath{\tau})$ of the process due to the rotation of molecules around the short axis is non-Arrhenius; (c) the retardation factor $g=\ensuremath{\tau}/{\ensuremath{\tau}}_{\mathrm{is}}$ is $\ensuremath{\simeq}1.5,$ whereas the typical value of $g$ in bulk nematic LCs is $\ensuremath{\simeq}4.$ At the nematic-isotropic phase transition in pores smooth and small changes in $\ensuremath{\tau}$ suggest that the "isotropic" phase of LCs in pores is not bulklike isotropic phase with complete disorder in molecular orientations, and some degree of orientational order still persists.

Application of Broad-Band Dielectric Spectroscopy for Investigations of Liquid Crystal - Porous Media Microcomposites

ABSTRACTWe applied ultra broad-band dielectric spectroscopy in the frequency range from 10–3 Hz to 109 Hz to investigate the effect of size, shape and volume fraction of the pores in the porous matrices on the dielectric properties of liquid crystals (LC) dispersed in these matrices. Measurements in such a broad frequency range make it possible to obtain detailed information on the important aspects of the electrical behavior of heterogeneous materials such as: conductivity, surface polarization, and influence of confinement on dynamics of molecular motion of polar molecules forming LC. We investigated alkylcyanobiphenyls in the isotropie, nematic and smectic phases dispersed in porous glasses (average pore sizes - 100 Å and 1000 Å) which have randomly oriented, interconnected pores, and anopore membranes (pore diameters - 200 Å and 2000 Å) with parallel cylindrical pores. Dispersion of LC resulted in qualitative changes of their dielectric properties. Analysis of broad-band dielectric spectra shows that in organic (LC) - inorganic (porous matrix) heterogeneous composites conductivity plays an important role at F <1 Hz. We observe the appearance of new dielectric modes: a very slow process with characteristic frequency ≃ (1 – 10) Hz and a second process in frequency range about (103 - 106) Hz. The slow process arises due to the relaxation of interfacial polarization at pore wall - LC interface. The origin of this could be due to absorption of ions at the interface. Another possibility is the preferential orientation of the permanent dipoles at pore surface. The second new mode is due to the hindered rotation of the molecules near the interface. Additionally we observed two bulk like modes due to the rotation of the molecules around their short and long axii which are modified.

Broad-Band Dielectric Spectroscopy of Liquid Crystals Confined in Random and Cylindrical Pores

ABSTRACTUsing dielectric spectroscopy in the frequency range 0.1 Hz-1.5 GHz, we investigated the influence of confinement on the dynamic properties of polar nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores as well as in parallel cylindrical pores with different, pores sizes. The confinement has a strong influence on the dielectric properties of LC which resulted in the appearance of a low frequency relaxational process (f≤ 10 KHz) absent in bulk and a strong modification of modes due to the molecular rotation around short axis and librational motion. The differences between bulk and confined behavior are: (a) - the dielectrically active modes in confined LC are not frozen even at temperatures about 20 degrees below the bulk crystallization temperature; (b) - in the temperature range corresponding to the anisotropie phase in pores,lnτ, where τ is the relaxation time corresponding to the molecular rotation around short axis, is not a linear function of 1/T and there is an evidence for smectic type order formation at sufficiently low T; (c) - the retardation factorg= τ/τisis ≃ 1.5, where as the typical value ofgin bulk nematic liquid crystals is ≃ 4; (d) - smooth and small changes inTat phase transition in pores suggest that the “isotropie” phase of LC in pores is not bulk like isotropie phase with complete disorder in molecular orientations, and some degree of orientation order still persists.

Dielectric Spectroscopy of Nematic Liquid Crystal Confined in Random Porous Matrices

ABSTRACTWe performed dielectric spectroscopy measurements to study dynamics of dielectrically active modes of nematic liquid crystals (LC) 5CB with polar molecules confined in silica porous glasses with average pore sizes of 1000 Å (volume fraction of pores 40%) and 100 Å (27%). In the nematic phase of bulk 5CB there is only one mechanism of dielectric relaxation: rotation of polar molecules around short molecular axis with relaxation time τ ∼10−8s. The spatial confinement and the existence of a highly developed interphase have a strong influence on dielectric properties of LC. We found that at temperatures about 30°C below the bulk melting point in both porous matrices the dielectric behavior of confined liquid crystal is very different from the behavior expected for solid state. The dielectric modes were not frozen in both porous matrices and we observed four well defined relaxational processes with relaxation times τ1 ∼ 10−1s, τ2 ∼10−5s, τ3 ∼10−8s (bulk like) and τ3 ∼10−9s. These four processes do not vanish even at temperatures corresponding to deep bulk isotropic phase. The relaxation time of the first process (slow) shows glass-like behavior in a wide temperature range below bulk melting point.

Near Neighbour Correlations and the Dielectric Properties of Liquid Crystals

Abstract It is demonstrated experimentally that there can be a reversal of the sign of the dielectric anisotropy (Δϵ) with decreasing temperature in a nematic. The results are explainable in terms of the Maier-Meier mean field theory and indicate that the occurrence of the smectic phase and the consequent dipole correlations within the smectic layers is not a necessary condition for the sign reversal of Δϵ.