Rotational Viscosity Coefficient Research Articles

Effect of substituents on the properties of fluorine-containing polyphenyl liquid crystal compounds

In order to explore the influence of molecular structure on the properties of liquid crystal (LC) materials, the dielectric anisotropy, rotational viscosity coefficient and birefringence of different fluorine-containing polyphenyl LC compounds are measured experimentally. The effects of lateral substituents and terminal substituents are analyzed. The experimental results show that the substitution of fluorine substituents in lateral substituents or terminal substituents will lead to the decrease of rotational viscosity coefficient, whether it is fluorine-containing triphenyl LC compounds or fluorine-containing tetraphenyl LC compounds. The dielectric anisotropies of LC compounds (fluorine-containing tetraphenyl LC compounds ND-3 and ND-8, ND-4 and ND-9, and ND-1 and ND-10) in which the terminal isothiocyano substituents are substituted by fluorine substituents all show a downward trend, with the decreasing degrees of 41.3%, 20.7% and 19.2% respectively. In addition to the influence of the replacement of fluorine and isothiocyanato on the terminal substituents, the dielectric anisotropy is affected by different lateral substituents from large to small: fluorine, methyl, and ethyl. The lateral fluorine substitution impact of ND-3, with an increase of 9.4%, is the best among fluorine-containing tetraphenyl LC compounds when compared to the lateral ethyl substitution of ND-2. Besides, compared with -F, -NCS can effectively extend the π-electron conjugation length of the molecule and improve the birefringence of the LC. This research is of great help to improve the comprehensive properties of LC materials and provides data accumulation for the development and application of LC compounds.

Composite stability and electro-optical properties of carbon nanotubes/5CB-based nematic hybrid liquid crystals

ABSTRACT This study examined the impact of different concentrations of carbon nanotubes (CNTs) on five nematic hybrid liquid crystals (LCs) based on 5CB. Electro-optical properties of the composite were tested, and the interaction between CNTs and LCs was theoretically analysed by first principles. Experimental results showed the effect of CNTs on electro-optical properties of hybrid LCs depended on the groups contained in monomer LCs. LCs with polar groups at the end increased the dielectric anisotropy of the composite, while the impact of CNTs on hybrid LCs containing non-polar terminal monomers was the opposite. The same variation was reflected in splay elastic constants and clearing points. Meanwhile, dielectric anisotropy, splay elastic constant, and rotational viscosity coefficients all changed with CNTs doping concentrations, reflecting the order of binding stability between representative groups and CNTs, and the variation of threshold voltage and response time reflected the order of binding stability between LC molecules and CNTs, consistent with theoretical calculations. Conductivity tests showed the additive of CNTs increased the conductivity of the composite and revealed the impact of CNTs on threshold voltage. Dielectric tests showed the incorporation of CNTs increased dielectric real and imaginary parts in low-frequency regions and increased relaxation frequency in high-frequency regions.

Composite of DNA-Stabilized Multiwalled Carbon Nanotube and Nematic Liquid Crystal for Display Performance Optimization

We have prepared composites of a nematic liquid crystal (NLC) mixture to improve display performance by doping multiwalled carbon nanotubes (MWCNTs) and single-stranded-DNA-wrapped MWCNTs. Polarized optical microscopy images indicated the uniformity of the composite. Infrared absorption spectra showed there existed an interaction between MWCNTs and 4-pentyl-4′-cyanobiphenyl (5CB, a practical solvent-type monomer NLC) due to π–π stacking and charge transfer, indicating the anchoring of 5CB molecules on the MWCNT surface. The electro-optical property proved the incorporation of MWCNTs and DNA-wrapped MWCNTs reduced the threshold voltage and response time, thus following the changed splay elastic constant and rotational viscosity coefficient of LC, reflecting that DNA can improve the stability of MWCNTs in 5CB. Moreover, dielectric measurement showed the enhanced value to dielectric anisotropy and shift in relaxation frequency, including the changes of dielectric permittivity and loss, revealing the electrically “inertness” MWCNTs at high frequencies and the DNA-induced broadened frequency range of low dielectric loss. Then electrical conductivity tests further proved the cooperative orientation of MWCNTs with 5CB under the applied voltage and the contribution of MWCNTs’ high conducting along the long axis. This study revealed that MWCNT doping could improve the physical properties of NLCs, and DNA wrapping could pave a promising way toward stabilized MWCNT/NLC composites for display performance optimization.

A bistable ion-doped cholesteric liquid crystal smart window with a small amount of polymer

Bistable liquid crystal devices, which require electricity only at the switching processes of the two states, have advantages in energy saving and safety compared with monostable liquid crystal devices. Due to electrohydrodynamic and dielectric effects, the ion-doped cholesteric liquid crystal can switch between a focal conic state and a planar state with different frequencies of the driving voltage. However, the cholesteric liquid crystal that self-assembles from the dynamic scattering state always has a poor focal conic state, resulting in a low contrast ratio. This paper presents a bistable polymer-stabilized ion-doped cholesteric liquid crystal device. The introduction of a small amount of polymer network can greatly enhance the light scattering capabilities of the focal conic state, endowing the device with excellent electro-optic performance. The electro-optical properties of the device are investigated with the various content of monomer and chiral dopant, and rotational viscosity of liquid crystal. Liquid crystal with a low rotational viscosity coefficient is beneficial to the formation of a good polymer network. The pitch induced by chiral dopant impacts the formation process of the polymer network, which subsequently affects the electro-optical properties. The bistable device has a low driving voltage and a high contrast ratio, making it ideal for use in smart windows.

Temperature dependence of dynamic response of liquid crystal materials

Most properties of liquid crystal (LC) materials will change with the change of temperature, which makes the performance of liquid crystal display (LCD) and other devices change. However, at low temperature, the properties of LC will mutate with the decrease of temperature, which directly leads to the defects of LC devices. This paper makes use of the developed homemade temperature control device to realize the construction of high temperature to low temperature environment, using precision LCR meter (Agilent E4980A) and self-built optical path, combined with LC cell capacitance model and dual-cell model, the dynamic response properties of LC materials with temperature change law was explored. The results show that the decay time τ d of the LC with positive dielectric anisotropy increases from 12.04 ms to 159.7 ms, and the rotational viscosity coefficient γ 1 increases exponentially from 12.04 mPa·s to 1582.45 mPa·s. While τ d of the LC with negative dielectric anisotropy increases from 13.10 ms to 408.1 ms, γ 1 also increases exponentially from 162.73 mPa·s to 7903.69 mPa·s. The conclusions presented in this paper provide some guidance for the improvement of low temperature properties of LC.

Molecular Simulation Approaches to the Study of Thermotropic and Lyotropic Liquid Crystals

Over the last decade, the availability of computer time, together with new algorithms capable of exploiting parallel computer architectures, has opened up many possibilities in molecularly modelling liquid crystalline systems. This perspective article points to recent progress in modelling both thermotropic and lyotropic systems. For thermotropic nematics, the advent of improved molecular force fields can provide predictions for nematic clearing temperatures within a 10 K range. Such studies also provide valuable insights into the structure of more complex phases, where molecular organisation may be challenging to probe experimentally. Developments in coarse-grained models for thermotropics are discussed in the context of understanding the complex interplay of molecular packing, microphase separation and local interactions, and in developing methods for the calculation of material properties for thermotropics. We discuss progress towards the calculation of elastic constants, rotational viscosity coefficients, flexoelectric coefficients and helical twisting powers. The article also covers developments in modelling micelles, conventional lyotropic phases, lyotropic phase diagrams, and chromonic liquid crystals. For the latter, atomistic simulations have been particularly productive in clarifying the nature of the self-assembled aggregates in dilute solution. The development of effective coarse-grained models for chromonics is discussed in detail, including models that have demonstrated the formation of the chromonic N and M phases.

Low-temperature properties of nematic liquid crystal materials for display

ABSTRACT The orientation of Nematic liquid crystal (NLC) and order degree of LC molecules are easily affected by ambient temperature, so it is of great significance to explore the low-temperature properties of NLC materials to improve the low-temperature performance of LC devices. The dielectric and elastic constant of the NLC material 1M10500-000 under the high and low-temperature conditions were obtained by the dual-cell capacitance method, linear fitting method and numerical fitting, and were compared. The dynamic response of the NLC at different temperatures was tested, and the rotational viscosity was calculated. The experimental results show that the dielectric constant is still proportional to the order parameter at low temperature. The elastic constants k 11 and k 33 and the square of order parameter no longer conform to the proportional relationship at low temperature, and the lower the temperature, the greater the deviation of elastic constants from the proportional relationship. As the temperature decreases, the ratio of k 33/k 11 changes from 2.62 to 2.71, and the rotational viscosity coefficient changes from 5.66 mPa.s to 177.64 mPa.s. Finally, the transient hysteresis phenomenon at low temperature was proposed and explained by the residual direct current voltage.

Dynamic of orientation processes in liquid crystal systems of technical devices

The relaxation properties of nematic liquid crystals by the pulsed method of a fixed distance in a rotating magnetic field are investigated. The adequacy of the solution of the equation of motion of the nematic director in the conditions of asynchronous motion of the director and the magnetic field induction vector was experimentally confirmed. The values of the rotational viscosity coefficient and the orientation relaxation time that characterize the parameters of devices with a liquid-crystal working medium in a wide range of pressures and temperatures are calculated. In the framework of the free volume theory, the dependence of the rotational viscosity coefficient on temperature and pressure is analyzed.

Dynamic response evaluation of liquid crystal with negative dielectric anisotropy

ABSTRACT Rotational viscosity coefficient is an important material parameter affecting the dynamic response of liquid crystals. We evaluated the dynamic response of two series of mixed liquid crystals SCL17V33L00 and SCL17V28L00 with negative dielectric anisotropy on the basis of the accurate measurement of rotational viscosity. At the temperature of 25°C, 30°C and 35°C, the parallel and vertical dielectric constants of them were calculated from the dual-cell capacitance model and liquid crystal cell capacitance model. Then, the bend and splay elastic constants of them were determined by analysing the Fréedericksz transition threshold voltage in the vertically aligned nematic cell and comparing the liquid crystal layer capacitance–voltage curve through the finite-differential iteration method with experimental curve. At last, the rotational viscosity coefficients γ 1 of them were obtained by performing dynamic response simulation using Visual Fortran software. As the temperature increases, the rotational viscosity coefficient of them decreases. Although the rotational viscosity coefficient of the second mixed liquid crystal was relatively large under the same temperature, the viscoelastic ratio coefficient was small, and the figure-of-merit was large, thereby resulting in a relatively fast dynamic response.

Peculiarities in the Director Reorientation and the NMR Spectra Evolution in a Nematic Liquid Crystals under the Effect of Crossed Electric and Magnetic Fields

The illustrative description of the field-induced peculiarities of the director reorientation in the microsized nematic volumes under the effect of crossed magnetic B and electric E fields have been proposed. The most interesting feature of such configuration is that the nematic phase becomes unstable after applying the strong E . The theoretical analysis of the reorientational dynamics of the director field provides an evidence for the appearance of the spatially periodic patterns in response to applied large E directed at an angle α to B . The feature of this approach is that the periodic distortions arise spontaneously from a homogeneously aligned nematic sample that ultimately induces a faster response than in the uniform mode. The nonuniform rotational modes involve additional internal elastic distortions of the conservative nematic system and, as a result, these deformations decrease of the viscous contribution U vis to the total energy U of the nematic phase. In turn, that decreasing of U vis leads to decrease of the effective rotational viscosity coefficient γ eff ( α ) . That is, a lower value of γ eff ( α ) , which is less than one in the bulk nematic phase, gives the less relaxation time τ on ( α ) ∼ γ eff ( α ) , when α is bigger than the threshold value α th . The results obtained by Deuterium NMR spectroscopy confirm theoretically obtained dependencies of τ on ( α ) on α .

Accurate measurement of the twist elastic constant of liquid crystal by using capacitance method

ABSTRACTIn this study, the twist elastic constant (k22) of liquid crystals (LCs) was accurately measured using capacitance method. The constant can be obtained on the basis of accurate measurement of other LC parameters, such as parallel and vertical dielectric constants (ε// and ε⊥), splay and bend elastic constants (k11 and k33), and rotational viscosity coefficient (γ1). First, by using dual-cell capacitance method and an LC cell capacitance model to measure ε// and ε⊥, k11 and k33 can be obtained from the threshold voltage determined from the voltage–capacitance curve of the parallel-aligned nematic LC layer and the comparison between the experimental and theoretical results based on the Frank elastic theory, respectively. In addition, γ1 can be obtained from the measurement of the dynamic response in the parallel-aligned nematic cell. Finally, k22 can be accurately determined using the threshold voltage of the twisted nematic LC cell. By adopting the above method, the measured k22 for LC E7 was 6.7 × 10−12 N. The proposed method is more rigorous and yields a more accurate measurement result than the other methods reported in the literature.

Thermodynamic Scaling of Miesowicz Viscosity Coefficients via Molecular Dynamics Simulation

Thermodynamic scaling of Miesowicz viscosity coefficients was investigated via molecular dynamics simulation using the Gay–Berne potential. The Miesowicz viscosity coefficients, which were calculated using an established theoretical expression under different pressures, were rescaled onto a master curve with the scaling parameter γη. The parameter was found to be identical to the thermodynamic parameter Γ and scaling parameters reported for other dynamic properties, such as the relaxation time for flip-flop motion, the rotational diffusion constant, and the Leslie rotational viscosity coefficients.

Liquid film flow on a high speed rotary bell-cup atomizer

In a high-speed rotary bell-cup atomizer, which is mainly used in the automotive industry, atomization is achieved by disintegration of a thin liquid film at the bell-cup edge. To obtain the hydrodynamic behavior of the liquid film as it passes over the bell cup, the liquid-film formation was simulated using the volume of fluid method under various conditions of the rotational speed, flow rate, liquid viscosity, and surface tension coefficient, based on industrial painting conditions. The liquid supplied from the liquid supply nozzle to the bell-cup surface flowed radially upon rotation of the bell cup and formed a film, which reached steady-state. We clarified the flow field depended on the rotational speed, flow rate, and liquid viscosity, and was independent of the surface tension coefficient. Based on these results, an equation for the liquid film thickness was proposed. In addition, the unsteady behavior of film with flow fluctuation in liquid supply was also investigated; the fluctuations persisted at the edge of the bell-cup atomizer. The proposed equation can be applied to this case regardless of the normalized amplitude of the fluctuation up to 50%. Thus, in this condition, unsteady film flow may be described in the proposed equation, which assumes a steady flow.

Thermodynamic scaling of dynamic properties of liquid crystals: Verifying the scaling parameters using a molecular model

The thermodynamic scaling of molecular dynamic properties of rotation and thermodynamic parameters in a nematic phase was investigated by a molecular dynamic simulation using the Gay-Berne potential. A master curve for the relaxation time of flip-flop motion was obtained using thermodynamic scaling, and the dynamic property could be solely expressed as a function of TV(γτ) , where T and V are the temperature and volume, respectively. The scaling parameter γτ was in excellent agreement with the thermodynamic parameter Γ, which is the logarithm of the slope of a line plotted for the temperature and volume at constant P2. This line was fairly linear, and as good as the line for p-azoxyanisole or using the highly ordered small cluster model. The equivalence relation between Γ and γ(τ) was compared with results obtained from the highly ordered small cluster model. The possibility of adapting the molecular model for the thermodynamic scaling of other dynamic rotational properties was also explored. The rotational diffusion constant and rotational viscosity coefficients, which were calculated using established theoretical and experimental expressions, were rescaled onto master curves with the same scaling parameters. The simulation illustrates the universal nature of the equivalence relation for liquid crystals.

Optical, dielectric and visco-elastic properties of a few hockey stick-shaped liquid crystals with a lateral methyl group

Measurements of birefringence (Δn), dielectric permittivity (ε‖, ε⊥), elastic moduli (K11, K33) and rotational viscosity (γ1) have been carried out on the nematic (N) phase of a few hockey stick-shaped compounds 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-alkyloxycinnamates with a lateral methyl group inserted between the m-alkyloxy chain and the azomethine connecting group. Interestingly, a dual characteristic (i.e. partially calamitic like and partially bent-core like) is revealed in the N phase of these compounds. The SmCa–N transition is found to be of first order in nature while the SmCs–N transition is either second order or weakly first order. All the mesogens exhibit a temperature dependent inversion in the static dielectric anisotropy (Δε = ε‖ − ε⊥) from positive to negative values on entering the SmCa phase. Remarkably in the entire nematic range, the bend elastic modulus (K33) is substantially lower than the corresponding splay modulus (K11). The rotational viscosity coefficient (γ1) as obtained by extracting the relaxation time (τ0) values from two precise, independent probing methods, viz. the capacitive decay technique and the optical phase-decay-time measurement method, are slightly higher than those of many known calamitic systems. Moreover, the activation energy (Ea) calculated from the viscosity data is found to be considerably higher in the nematic phase than those obtained for conventional calamitics. The observed behaviours are accounted for by considering the intriguing shape-determined inter-molecular interactions and molecular associations appearing in the mesophase.

Rotational viscosity measurements of bent-core nematogens

Two simple but precise probing methods have been utilized to investigate the influence of structure, properties and molecular association on the director response time (τ0) and hence on the rotational viscosity (γ1) in the nematic phase of three 4-cyanoresorcinol bent-core derivatives (1/7, 1/9 and 1/10) having different terminal chain lengths. The results obtained from the two methods are in good agreement with a small deviation of about 10–15%. The γ1 values are more than an order of magnitude higher compared to the conventional calamitic nematics. Measurements of birefringence, dielectric anisotropy, splay and bend elastic constants and relaxation time of these compounds have also been carried out. For the lower homologues (1/7, 1/9) an inversion in dielectric anisotropy (Δe) is observed in the nematic phase, while the higher homologue (1/10) exhibits negative Δe throughout the entire mesomorphic range. The splay elastic constant (K11) is found to be comparatively greater than the bend elastic constant (K33); however, on approaching the low temperature CybC phase (a phase comprising elongated cybotactic clusters) K33 exhibits a small pretransitional divergence. For all three compounds the magnitude of the rotational viscosity coefficient (γ1) is found to exhibit a strong enhancement on approaching the CybC phase, while in the nematic phase activation energies are found to be nearly an order of magnitude higher than the normal calamitics. The observed behaviours are explained by considering the strong dipolar correlation of the bent-core molecules and the existence of the short range cybotactic clusters, their gradual growth and uniform alignment under the presence of strong surface anchoring.

Ferroelectric Switching of Thin Langmuir-Blodgett Films of P(VDF-TrFE) Copolymer: Comparison with Liquid Crystal Switching

The switching of polarization in ferroelectric polymer films is traditionally explained in terms of nucleation and motion of domain walls although such walls have never been observed in polymer ferroelectrics. As an alternative, the Landau-Ginzburg theory of the coercive field seems to be appropriate for ultrathin (1–5 monolayer) films. Data on the switching dynamics in the ultrathin films are very scarce. In this work the experiments on the polarization switching in the 20–240 nm thick films prepared by the Langmuir-Blodgett technique had been carried out using triangular voltages of low frequency in order to separate capacitive and polarization components of the current. Then the results have been compared with the data on polarization switching in liquid crystalline, antiferroelectric Langmuir-Blodgett films. There is a striking similarity in the time dependences of repolarization current for two materials. Due to this, we tentatively suggest a switching mechanism based on the field-induced rotation of the polarization vector. The latter allows one to estimate the rotational viscosity coefficient of the ferroelectric polymer responsible for the energy dissipation.

Orientational order parameter and rotational viscosity in C 60 doped E7 nematic liquid crystal

The reorientational order parameter dependent complex linear susceptibilities are derived based on the angular averaged linear response theory of the molecular polarizability, which includes the Vuks type equations of anisotropic refractive indices and absorbances. The polar angular absorbance at 633 nm wavelength is measured as a function of the concentration of fullerene C 60 in E7 nematic liquid crystals (NLCs), from which the order parameter is deduced. It is shown that the order parameter has a linear dependence of dopant concentration. We also present a simple optical transmission method by adopting the closed-aperture Z-scan geometry under the influence of switching on and off the dc electric field to determine the optical relaxation time and the rotational viscosity coefficient of C 60 doped E7 NLC. The experimental results are pretty well explained by using the Osipov–Terentjev theory of the rotational viscosity with the measured order parameter.

Viscoelastic model for polarization switching in soft matter organic ferroelectrics

Although the mechanism of polarization switching in crystalline ferroelectrics is well understood, there are serious difficulties in understanding the same phenomena in polymer ferroelectric films. On the other hand, a striking similarity has been observed between the latter and liquid-crystal ferroelectrics. To explain the results of our experiments on 20--200-nm-thick films of copolymer P(VDF70%-TrFE30%), a simple viscoelastic model is proposed, which seems to be quite general and can be applied to variety of organic ferroelectric materials. Our approach is based on extension to polymer and other ``soft matter'' ferroelectrics of the Ericksen-Leslie continuum dynamic theory well recognized in physics of liquid crystals. In soft systems the polarization switching through the viscoelastic deformation becomes more favorable than that described by the Landau-Ginzburg theory. The model operates with rotation of the polarization vector in the electric field and shows the time and field dependences of the polarization current in surprisingly good agreement with experiment. The latter allows one to extract the rotational viscosity coefficient responsible for the energy dissipation.

The director and molecular dynamics of the field-induced alignment of a Gay–Berne nematic phase: An isothermal-isobaric nonequilibrium molecular dynamics simulation study

Isothermal-isobaric molecular dynamics simulations have been performed for the generic Gay–Berne (GB) mesogen, GB(4.4, 20.0, 1, 1), to investigate director and molecular rotational motion during the field-induced alignment of a nematic. The alignment process for the director is discussed within the context of a hydrodynamic analysis based on the Ericksen–Leslie theory and this is found to predict the simulated behavior well. The dependence of the relaxation time for the alignment on the field strength is also in good accord with the theory. The rotational viscosity coefficient estimated from the simulation is smaller than that typically observed for real nematics and the possible reasons for this are discussed. However, the simulation results are found to follow not only the theory but also the experiments, at least qualitatively. No significant variation in the local and long-range structure of the nematic phase is found during the field-induced alignment process. In addition, we have explored the molecular dynamics in the nematic phase in the presence of the field using the first- and second-rank time autocorrelation functions. More importantly we are able to show that the director relaxation time is longer than that for molecular rotation. It is also possible to use the two orientational correlation times to explore the relationship between the rotational viscosity coefficient and the rotational diffusion constant. The diffusion constants determined from the orientational correlation times, based on the short-time expansion of the autocorrelation functions, are found to be significantly different. In consequence it is not possible to test, unambiguously, the relationship between the rotational viscosity coefficient and the rotational diffusion constant. However, it would seem that the second-rank rotational correlation time provides the most reliable route to the rotational viscosity coefficient.