Large Positive Dielectric Anisotropy Research Articles

Fast-response halogenated 4-alkyl-4′′-cyano-p-terphenyls as dual frequency addressing nematics

Twenty-two compounds belonging to halogenated 4-alkyl-4′′-cyano-p-terphenyls have been synthesized and mesomorphic properties using polarizing optical microscope and differential scanning calorimetry have been determined. The birefringence of the compounds using prism coupling technique was determinated. The multicomponent eutectic mixture consisted of synthesised compounds was prepared and investigated. The prepared mixture exhibits very broad nematic phase range, over 140 °C, with the melting point below 0 °C. Dielectric studies revealed that prepared mixture is very useful for dual frequency adressing system, because it posses large positive dielectric anisotropy (above 22) and becomes negative above the relatively low cross-over frequency (14 kHz). The total response time of the prepared mixture measured at room temperature at low operating voltage (24 V) is short (6.5 ms) which is suitable for fast switching optical applications.

Dielectric investigation on some binary mixtures of hockey-stick-shaped liquid crystal and octyloxy-cyanobiphenyl

ABSTRACTWe report the measurement of temperature-dependent dielectric parameters in some binary liquid crystal mixtures comprising of a hockey-stick-shaped mesogen 4-(3-decyloxyphenyliminomethyl) phenyl-4-decyloxycinnamate (SF7) and calamitic compound 4′-octyloxy-4-cyanobiphenyl (8OCB). All the investigated mixtures possess a large positive dielectric anisotropy (Δε), although a noticeable reduction has been found by increasing the diverse-shaped dopant concentration. Investigation on the pretransitional behavior in the vicinity of isotropic to nematic (I–N) phase transition suggesting a tricritical character for all the studied mixtures. Parameterization of dielectric permittivity close to the nematic to smectic-A (N–Sm-A) phase transition exhibits non-universal values of the critical exponents describing a second-order nature of the transition. Systematic variation of critical exponents against dopant concentration and McMillan ratio reveals a well consistency with those obtained from the high-resolution optical birefringence measurements.

Cyano terminated tolane compounds for polymer dispersed liquid crystal application: relationship between cyano terminated tolane based molecular structures and electro-optical properties

ABSTRACTThe structures of the liquid crystal (LC) molecules have a key role in impacting the electro-optical performance of a polymer dispersed liquid crystal (PDLC) film. In this paper, the relationship between the LC molecular structures and the electro-optical properties of PDLC films is investigated based on an unexplored cyano-terminated tolane compounds (CTTCs) doped E8 LCs/UV polymers system. Due to the high polarity of CTTCs, LCs doped with the cyano-terminated tolane (CTT) molecules exhibit high birefringence and large positive dielectric anisotropy. On the whole, PDLC films doped with the CTT molecules exhibit a lower driving voltage than that doped with the pure E8. More excitingly, PDLC films based on CTT molecules with larger length-to-width ratio and longer conjugated system show higher contrast ratio (CR) and faster response time. Eventually, the mechanism of the effects of CTT-based molecular structures and the relationship between the electro-optical performance of PDLC films and CTT molecules are illustrated. This work paves a new way for optimising the electro-optical properties of PDLC films.

Investigation of thermo-electro-optic properties of some mixed nematic liquid crystals

ABSTRACTThe 6CB/8CB/8OCB liquid crystal mixture has been studied by Differential Scanning Calorimetry (DSC), polarised optic microscopy (POM), Semiconductor Characterization System, and Ultra-violet spectrophotometry (UV). DSC and POM results indicate that the 6CB/8CB/8OCB mixture exhibits liquid crystalline properties. The capacitance-voltage and conductance-voltage measurements were performed in the frequency range of 200-500 kHz and in the temperature range of 30oC–50oC. The 6CB/8CB/8OCB mixture showed an extremely large positive dielectric anisotropy. The molar absorptivity ϵ for the 6CB/8CB/8OCB mixture was calculated and found to be higher than the absorptivity values of the binary mixtures due to the alkyl chain length with H-aggregation.

Selectivity, Thermodynamic and Anisotropic Properties of Substituted Liquid-Crystal Cyanoazoxybenzenes as Stationary Phases for Gas Chromatography

Herein we discuss physical properties of 4-(ω-hydroxyalkoxy)-4’-cyanoazoxybenzene homologs. 1D and 2D correlation NMR spectroscopy (in particular, 1Ði?½, 15N-HMBC experiment) have allowed elucidation of structure of the prepared rod-like supramolecular cyanoazoxybenzenes. Mesomorphic properties of the compounds have been studied by means of polarization thermomicroscopy and differential scanning calorimetry. All the studied cyanoazoxybenzenes have revealed enantiotropic nematic mesomorphism over wide temperature range. Nematic mesophase of the eighth homolog has possessed large positive dielectric anisotropy. Introduction of small amounts of the prepared cyanoazoxybenzenes as additive has stabilized the mesophase and has increased the dielectric anisotropy of 4-pentyloxy-4’-cyanobiphenyl. Gas-liquid chromatography studies have shown that sorbents based on 4-(2-hydroxyethyloxy)-4’-cyanoazoxybenzene are highly selective towards various structural isomers; that cannot be achieved using conventional nematic liquid crystals. Thermodynamic evidence of specific interactions between the mesogen and the non-mesomorphic sorbate has been discovered.

Relaxation frequencies for flip-flop rotation of three- and four-ring compounds and dual frequency addressing mixtures with high birefringence

Relaxation frequencies fr characterising the molecular rotation around the short axes (flip-flop movement) of 45 three- and four-ring polar compounds with high birefringence (mainly cyclohexyl, phenyl and biphenyl tolanes and quaterphenyls terminated with F, OCF3, NCS group) have been measured. These compounds exhibit a large positive dielectric anisotropy Δε = ε║ − ε⊥ in the nematic phase and relatively low fr values (in the range of dozens kHz). The dependence of fr upon molecular structures and physical properties of systems (molecular rigid core length, polarity and viscosity) is discussed. Mixtures for dual frequencies addressing display with the crossover frequency fco of the order of ~1 kHz have been prepared by joining the above compounds with a mixture consisted of two, three and four fluoro-substituted in the lateral position tolanes and phenyltolanes showing negative dielectric anisotropy. It was stated that relaxation frequency is correlated mainly with their length and polarity.

Synthesis of Fluorosubstituted Three Ring Esters and Their Dielectric Properties

Laterally substituted by fluorine atoms 4-cyanophenyl 4-(4-pentylbenzoyloxy)-benzoates have been prepared and their phase transition temperatures, enthalpies and dielectric properties upon frequency were evaluated. The novel compounds exhibit a broad range of nematic mesophase, large positive dielectric anisotropy changing sign at low frequency. They are convenient components of LC mixtures for dual frequency addressing devices.

Dielectric and Conductivity Anisotropy in Liquid Crystalline Phases of Strongly Polar Thioesters

The main objective of this paper is to study dielectric properties of two compounds of acronyms 5OSCl and 6OSCl having strongly polar bond (C–Cl) at para position. Dielectric measurements were done in the frequency range from 40 Hz to 15 MHz. The dielectric spectra were measured for two principal alignments. For homeotropic alignment, the reorientation of molecule around the short molecular axis was observed. Both substances studied exhibit large positive dielectric anisotropy and the anisotropy of conductivity was obtained in smectic A as well as nematic phase.

Experimental Investigation on Pretilt Angle in Binary Liquid Crystal Composed of Highly Polar Molecules

We report the measurement of dielectric constants, birefringence, and pretilt angle in binary mixtures of nematic liquid crystals (CP7B and CCN-47). CP7B has a strong longitudinal dipole moment and exhibits a large positive dielectric anisotropy (Δε=ε|| -ε⊥>0) and CCN-47 has a strong transverse dipole moment and exhibits a large negative dielectric anisotropy. In the mixtures, ε|| increases with increasing CP7B concentration, whereas ε⊥ remains almost constant. We show that the pretilt angle increases with Δε and birefringence (Δn=ne-no) much rapidly than reported earlier. It is suggested that this effect may arise due the presence of strong permanent dipole moments in the molecules.

Conductance and dielectric anisotropy properties of 4-cyano-4′-hexylbiphenyl-salicylaldimine compound composite liquid crystal exhibiting large positive dielectric anisotropy

The electronic properties of 4-cyano-4′-hexylbiphenyl and salicylaldimine compound 1 liquid crystals have been investigated. The polarizing microscopy and differential scanning calorimetry results show that chiral salicylaldimine compound 1 exhibits enantiotropic smectic A* and SmC* mesophases. The novel 4- n-hexyl-4′-cyanobiphenyl/salicylaldimine compound 1 composite showed an extremely large positive dielectric anisotropy. The dielectric anisotropy of the LCs changes from positive type to negative type. The critical frequency f c values of dielectrical anisotropy for 4- n-hexyl-4′-cyanobiphenyl and salicylaldimine compound 1 LCs were found to be 630.50 and 813.01 kHz, respectively. The splay elastic constant K 11 values of the liquid crystals were calculated and the doping of salicylaldimine compound 1 into 4-cyano-4′-hexylbiphenyl liquid crystal increases the splay elastic constant of 6CB. The parallel conductivity σ ∥ values of the liquid crystals are higher than that of perpendicular conductivity σ ⊥ values and conductivity of the 4-cyano-4′-hexylbiphenyl decreases with doping of salicylaldimine compound 1.

Preorganization Effect of a Polar Supermolecule on Dielectric Anisotropy in a Nematic Liquid Crystalline Phase

We have prepared novel preorganized compounds in which 4-(4-fluorophenyl)phenyl and 4-(trifluoromethoxy)phenyl are connected via a rigid catechol unit or a flexible spacer and investigated their structural effects on the induced dielectric anisotropy in the mixture with a host material. The nonsymmetric bimesogenic compounds with the flexible spacer were found to be a dopant with negative dielectric anisotropy in host A composed of fluoro-substituted phenyl esters, whereas they were found to be a dopant with large positive dielectric anisotropy in host B (6OCB). The rigid U-shaped compound enhanced Δe in both hosts A and B. We discuss effects of host−guest interaction on conformational change of the guest molecule in the nematic liquid crystalline phase.

Dielectric study of nematic and induced smectic-A phases in binary mixtures of pentyloxy cynobiphenyl with three p-azoxybenzenes

Static dielectric properties of 18 binary mixtures of 4,4′-pentyloxy cyanobiphenyl (5OCB) with 4,4′-bis(alkoxy)azoxybenzenes (nOAB) for n = 5, 6 and 7 have been studied in detail. Seventeen mixtures show induced smectic-A (SmA) phase; a few mixtures exhibit nematic phase as well. Pure 5OCB has a large positive dielectric anisotropy while nOAB members have small but negative anisotropy. However, all the mixtures exhibit positive anisotropy. In the mixtures, except those with x 5OCB ≥ 0.85, the values of ε∥, ε⊥ and increase smoothly with temperature. The values of εiso are found to be higher than the values within the mesophases, with a jump at T NI or T SI. Effective values of the dipole moments in the mixtures are found to be less than the values observed in the pure compounds. This suggests that anti-parallel homo-dimers of 5OCB exist in the mixtures as observed in the pure compound. Moreover, at the onset of SmA phase, values of are found to decrease from those in the nematic phase. This implies that additional anti-ferroelectric type of ordering is developed in the induced SmA phase. In all the mixtures the anisotropy parameter Δε decreases with concentration, becomes minimum at x 5OCB = 0.5, and then increases quite fast. Kirkwood correlation factors have also been calculated to find the nature and extent of the molecular associations.

Synthesis and physical properties of novel liquid crystals containing pyranobenzopyrans as a core structure

The synthesis and physical properties of pyranobenzopyran ring system have been studied as a core structure of novel liquid crystal (LC) compounds. Substituted pyranobenzopyrans having an alkoxy group on the A ring (benzene ring) exhibited smectic and nematic phase, those having a fluoro-substituted phenyl group on the A ring exhibited wider range of smectic phase. When the latter are added to a base LC mixture (ZLI-1565, Merck), the clearing point was raised. Interestingly, pyranobenzopyrans having a trifluoromethyl phenyl group or a cyano group on the A ring showed an extremely large positive dielectric anisotropy.

Effect of a strong electric field on a nematogen: evidence for polar short range order

We present experimental studies on the effect of strong electric fields on the nematogen p-cyanophenyl p-n-heptyl benzoate which has the strongly polar cyano end group and a large positive dielectric anisotropy. We use a local temperature measurement to take into account heating effects and an electrical impedance analysis to determine both the dielectric constant (e∥) and the resistance (R) of the sample. We also measure the higher harmonic responses of the medium. The new results obtained in this study are: (i) a detailed temperature dependence of the terms which describe (a) the quenching of macroscopic thermal fluctuations of the nematic director and (b) the enhancement of the orientational order parameter due to Kerr effect, (ii) clear evidence for the critical divergence of susceptibility as reflected in the third harmonic signal, (iii) an unusual enhancement of the conductivity which shows a large peak just below the critical point Tc, (iv) a significant peak in the second harmonic signal at Tc and (v) evidence for a field induced nematic-nematic transition well inside the nematic range. We argue that the results (iii)-(v) indicate the presence of polar short range order in the medium and hence support a molecular model in which such an order has been proposed.

Electric-field-induced isotropic-nematic phase transition.

We measure optically the quadrupolar order S induced by a stabilizing electric field E in the isotropic phase of the nematogenic 4'-n-pentyl-4-cyanobiphenyl, with large positive dielectric anisotropy ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{a}}$\ensuremath{\sim}10. For high enough field and low enough temperature, a paranematic-nematic first-order transition is induced, with a critical point at ${\mathit{E}}_{\mathit{c}}$=1.41\ifmmode\times\else\texttimes\fi{}${10}^{5}$ V/cm, ${\mathit{T}}_{\mathit{c}}$=${\mathit{T}}_{\mathit{N}\mathit{I}}$+0.65 K, and ${\mathit{S}}_{\mathit{c}}$=0.15, where ${\mathit{T}}_{\mathit{N}\mathit{I}}$ is the nematic-isotropic phase-transition temperature.

Electric-field-induced change of the order parameter in a nematic liquid crystal.

We measure optically the increase of quadrupolar order \ensuremath{\Delta}S, induced by a stabilizing electric field E, in the nematic liquid crystal 4'-n-pentyl-4-cyanobiphenyl with large positive dielectric anisotropy ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{a}}$\ensuremath{\sim}10. \ensuremath{\Delta}S shows a linear and a quadratic dependence on E. Both effects are comparable for the largest field E\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{5}$ V/cm. The linear variation is interpreted as originating from the quenching of macroscopic orientational fluctuations, while the quadratic contribution is a superposition of the microscopic Kerr effect and of a saturation term due to the macroscopic effect.

Electrohydrodynamic instabilities in nematic liquid crystals with large positive dielectric anisotropy

Abstract The conditions for initiation of EHD instability at frequencies of 50–150 Hz and for DC for eight nematic materials having a positive dielectric anisotropy (λϵ > 3) have been experimentally studied. The dependences of the threshold voltage for the EHD instability on electrical conductivity, as well as on the translational and rotational viscosities of the LC materials have been found. For the measurements substances with strongly differing (from 10−9 to 5 × 10−7 Ω−1 m−1) electrical conductivities and profound temperature dependence of the viscosities were chosen. Analysis of the results based on the isotropic model of EHD instability shows good agreement between theory and experiment.

Novel Liquid Crystals Having a Large Dielectric Anisotropy

Abstract A homologous series of 4-cyanophenyl 4-(2,4-dialkoxybenzoyloxy)benzoates(from the pentyloxy to decyloxy homologs) shows a monotropic nematic phase, and the lower members show a large positive dielectric anisotropy in the mesophase.

Effect of the Pretilt Angle and Temperature on the Multiplexing Limits of Dual Frequency Addressed TN–LC Cell

We present below the results of (our studies on) dual frequency addressed twisted nematic liquid crystal (TN–LC) cells using recently developed Hoffmann La-Roche LC mixtures 3090 and 2851 which exhibit large positive and large negative dielectric anisotropy at low and high frequencies respectively. It is found that the surface alignment i.e., the tilt bias angle of LC molecules at the bounding surfaces and the temperature of the cell modify the electro-optic transmission characteristics significantly which in turn affect the multi-plexing limits. It is also shown that the cross over frequency `fc' can be easily determined from the threshold voltage variation studies with frequency.

The Physical Properties of the Cyanophenylcyclohexyl Ethanes (PECH)

Abstract The physical properties of a new homologous series of stable, low-melting nematic materials, the cyanophenylcyclohexyl ethanes (PECH) are reported and compared with those of other stable, nematic materials. A broad range mixture is reported with a low viscosity, medium birefringence and large positive dielectric anisotropy. This mixture shows a high order parameter which increases the contrast of displays using dissolved pleochroic dyes. A comparison is also made between the structural isomers of PECH and PCH which questions the validity of molecular theories of elastic constants.