Anisotropic shear viscosity in nematic liquid crystals: new optical measurement method

We propose a new optical method and the experimental set-up for measuring the anisotropic shear viscosities of nematic liquid crystals (LCs). LC shear viscosities can be optimized to improve liquid crystal display (LCD) response times, e.g. in vertical aligned nematic (VAN) or bistable nematic displays (BND). In this case a strong back-flow effect essentially determines the LCD dynamic characteristics. A number of shear viscosity coefficients defines the LCD response time. The proposed method is based on the special type of a shear flow, namely, the decay flow, in the LC cell with suitably treated substrates instead of magnetic or electric field application. A linear regime of a quasi-stationary director motion induced by a pressure difference and a proper configuration of a LC cell produces decay flow conditions in the LC cell. We determine three principal shear viscosity coefficients by measuring relative time variations of the intensity of the light passed through LC cells. The shear viscosity coefficient measurements provide a new opportunity for the development of new LC mixtures with fast response times in VAN, BND and other important LCD types.     

Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow

The authors study shear flow in hybrid-aligned nematic cells under the action of an applied electric field by solving numerically a hydrodynamic model. The authors apply this model to a flow-aligning nematic liquid crystal (4'-n-pentyl-4-cyanobiphenyl) and obtain the director's configuration and the velocity profile at the stationary state. The authors calculate the local and apparent viscosities of the system and found that the competition between the shear flow and the electric field gives rise to an interesting non-Newtonian response with regions of shear thickening and thinning. The results also show an important electrorheological effect ranging from a value a bit larger than the Miesowicz viscosity etab [Nature (London) 17, 261 (1935)] for small electric fields and large shear flows to etac for large electric fields and small shear flows. The analysis of the first normal stress difference shows that for small negative shear rates, the force between the plates of the cell is attractive, while it is repulsive for all other values of shear rates. However, under the application of the electric field, one can modify the extent of the region of attraction. Finally, the authors have calculated the dragging forces on the plates of the cell and found that it is easier to shear in one direction than in the other.     

二氟亚甲氧基苯类液晶的合成与低温性能

合成了4个二氟亚甲氧基苯类液晶化合物,经过IR、1H NMR、13C NMR、19F NMR和MS谱图表明分子结构正确;并用动态流变仪测试分析了不同温度下的粘度,发现这些液晶不仅粘度低、对温度的依赖小,而且可以用作替代酯类的液晶溶剂。 如果用于液晶溶剂可以提高液晶器件的低温响应速度。     

WAXS studies of global molecular orientation induced in nematic liquid crystals by simple shear flow

Global molecular orientation function coefficients for the nematic liquid crystal 4-cyano 4- n -pentylbiphenyl (5CB) in shear flow are presented, being extracted from 2-dimensional Wide-Angle X-ray Scattering data. A linear increase in orientation parameter P2 is observed with a logarithmic increase in shear rate. It is proposed that this arises from an increased number of LC directors aligning to the shear axis. Upon cessation of shear flow, the anisotropy is seen to relax away completely, over a time scale which is inversely proportional to the previously applied shear rate.     

A viscometric study of the liquid crystalline phase of alkyloxybenzoic acids

The viscosities of three benzoic acid derivatives (p-n-heptyloxy-, p-n-decyloxy-, and p-n-dodecyloxy-) were measured on a unique viscometer of the class of CS-rheometer-viscometers with controlled shear stress over the whole temperature range of the liquid crystalline state. Shear rates were calculated and flow and viscosity curves constructed from the experimental shear stress values taking into account the Rabinovich-Moony correction. The smectic and nematic phases were characterized by non-Newton and Newton viscosities, respectively, in all the samples studied. The activation parameters of viscous flow were calculated for Newton viscosity. The results are discussed in terms of intermolecular interactions and structural peculiarities of liquid crystalline phases.     

Twist viscosities and flow alignment of biaxial nematic liquid crystal phases of a soft ellipsoid-string fluid studied by molecular dynamics simulation

We have calculated the twist viscosity and the alignment angle between the director and the stream lines in shear flow of a liquid crystal model system, which forms biaxial nematic liquid crystals, as functions of the density, from the Green-Kubo relations by equilibrium molecular dynamics simulation and by a nonequilibrium molecular dynamics algorithm, where a torque conjugate to the director angular velocity is applied to rotate the director. The model system consists of a soft ellipsoid-string fluid where the ellipsoids interact according a repulsive version of the Gay-Berne potential. Four different length-to-width-to-breadth ratios have been studied. On compression, this system forms discotic or calamitic uniaxial nematic phases depending on the dimensions of the molecules, and on further compression a biaxial nematic phase is formed. In the uniaxial nematic phase there is one twist viscosity and one alignment angle. In the biaxial nematic phase there are three twist viscosities and three alignment angles corresponding to the rotation around the various directors and the different alignments of the directors relative to the stream lines, respectively. It is found that the smallest twist viscosity arises by rotation around the director formed by the long axes, the second smallest one arises by rotation around the director formed by the normals of the broadsides, and the largest one by rotation around the remaining director. The first twist viscosity is rather independent of the density whereas the last two ones increase strongly with density. One finds that there is one stable director alignment relative to the streamlines, namely where the director formed by the long axes is almost parallel to the stream lines and where the director formed by the normals of the broadsides is almost parallel to the shear plane. The relative magnitudes of the components of the twist viscosities span a fairly wide interval so this model should be useful for parameterisation experimental data.     

Response mechanism of a vertical alignment mode driven by fringe-field switching

The response mechanism of a vertical alignment mode, driven by a fringe field, is investigated in detail using small-angle approximation. The flow effects can be ignored when using theoretical analysis. The period of the liquid crystal (LC) deformation in the transversal direction, instead of the lognitudinal direction, shows the cell gap effect on the response time in the LC layer's thickness. The authors' analytical results indicate that a liquid crystal display (LCD) mode with a small transversal period could provide a new method that gives a fast response.     

Dynamic mechanical properties of cyclohexane-based glass-forming liquid crystals and a linear side chain polymer analogue

A nematic and a cholesteric liquid crystal, both derived from trans-1,3,5-cyclohexanetri-carboxylic acid, and a linear side chain polymer analogue were characterized in terms of storage (G') and loss (G') moduli as functions of frequency and temperature. It was found that all three model compounds show a shear shinning flow behaviour with zero shear viscosities of the low molar mass systems significantly less than that of the polymer system at the same reduced temperature, T/T^g. With shift factors prescribed by the WLF equation, both the G' and G' data of all three compounds are adequately represented by master curves. Furthermore, within the framework of the stretched exponential model, the relaxation behaviours of the low molar mass systems are well described by a single Maxwell element, whereas the polymer system shows a relatively broad distribution of relaxation times. The observed viscoelastic properties suggest a relative ease of material processing of the low molar mass systems compared to the polymer analogue due to a lower zero shear viscosity and a single relaxation time characterizing the dynamics of response to a mechanical or thermal stimulus.     

Interfacial Interaction of Polymer/Liquid Crystal Molecules and Electrooptical Properties of their Composite Systems

Polymer/liquid crystal composite films were prepared from a solution of polymer and nematic liquid crystal (LC) by a solvent casting method. The phase-separated structure of the composite film was controlled by the solvent evaporation rate. The light-scattering profile of a poly(diisopropyl fumarate)/LC: 40/60 w/w solution during solvent evaporation exhibited a periodic structure, indicating that the phase-separated structure was formed by spinodal decomposition. The aggregation structure of the composite film was investigated with a scanning electron microscope (SEM). SEM observation of the composite film suggested the presence of periodicity and dual connectivity of polymer and LC phases. The faster the solvent was evaporated, the smaller the LC channel (domain) size in the composite film. The composite film, composed of poly(methyl methacrylate) (PMMA) and a nematic LC (E44) with a positive dielectric anisotropy, exhibited remarkable and reversible light-scattering-light-transmission switching, under the modulation of an ac electric field. The light-scattering state was dependent on such optical heterogeneities as spatial distribution of the nematic directors and/or mismatching in the refractive indices of the components. The electrooptical behavior of the composite film was strongly dependent on the LC channel (domain) size in the composite film. The transmittance increased and the rise and decay response times (τ_R and τ_D), decreased and increased, respectively, with an increase in the size of the LC channel (domain).

The electrooptical switching properties for the polymer/LC composite film should be influenced by miscibility between the polymer and the LC phases. The miscibility between both phases was evaluated from a distribution of relaxation time for interfacial polarization. The anchoring effect was also investigated by measuring the nonlinearity of the dielectric constant for the composite system.     

Oscillating Poiseuille flow in photo-aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10^-6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.     

Oscillating Poiseuille flow in photo‐aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10^?6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.     

Effect of polymer architecture on self‐diffusion of LC polymers

Two LC side‐group poly(methacrylates) were synthesized, and their melt dynamics were compared with each other and a third, main‐chain side‐group combined LC polymer. A new route was developed for the synthesis of the poly(methacrylate) polymers which readily converts relatively inexpensive perdeuteromethyl methacrylate to other methacrylate monomers. Self‐diffusion data was obtained through the use of forward recoil spectrometry, while modulus and viscosity data were measured using rotational rheometers in oscillatory shear. Diffusion coefficients and complex viscosity were compared to previous experiments on liquid crystal polymers of similar architecture to determine the effect of side‐group interdigitation and chain packing on center of mass movement. The decyl terminated LC side‐group polymer possessed an interdigitated smectic phase and a sharp discontinuity in the self‐diffusion behavior at the clearing transition. In contrast, the self‐diffusion behavior of the methyl terminated LC side‐group polymer, which possessed head‐to‐head side‐group packing, was seemingly unaffected by the smectic–nematic and nematic–isotropic phase transitions. The self‐diffusion coefficients of both polymers were relatively insensitive to the apparent glass transition. The presence of moderately fast sub‐T_g chain motion was supported by rheological measurements that provided further evidence of considerable molecular motion below T_g. The complex phase behavior of the combined main‐chain side‐group polymer heavily influenced both the self‐diffusion and rheological behavior. Differences between the self‐diffusion and viscosity data of the main‐chain side‐group polymer could be interpreted in terms of the defect structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 405–414, 1999     

Light scattering measurements of anisotropic viscoelastic coefficients of a main-chain polymer nematic liquid crystal

We report the first systematic measurements of elastic coefficients and viscosities that clearly demonstrate the distinction between rigid and semiflexible behaviour of a main-chain polymer nematic liquid crystal, a solution of poly-γ-benzyl glutamate (PBG) in mixed organic solvents. Quasi-elastic Rayleigh scattering studies show a crossover which occurs at a molecular chain length near the persistence length of PBG as the chain length increases. The results are in qualitatively good agreement with recent theoretical predictions for semiflexible chains. It is seen that bending distortions of an individual polymer play an important role in the fundamental nature of nematic elasticity and viscosity.     

Landau theory-based estimates for viscosity coefficients of uniaxial and biaxial nematic liquid crystals

Using Landau theory, it is shown that eight phenomenological parameters are needed to describe and distinguish the twelve viscosity coefficients of a biaxial nematic phase, or the five viscosity coefficients of a uniaxial nematic phase. The dependence of the coefficients on the macroscopic uniaxial and biaxial order parameters is established. Since these order parameters are determined by the anisotropies of the dielectric constant, we show that it should be possible to determine values for all eight of the phenomenological parameters of the theory from measurements of the temperature dependence of the five viscosities of a uniaxial phase.     

Rheology of a lyotropic mesophase through a stress-relaxation experiment

We report on the first rheological study of the structural relaxations in a nematic liquid crystalline phase. Linear dynamic and transient shear experiments were applied to a polydomain nematic phase of the CTAB/water system: a liquid crystalline mixture composed of 28 wt% CTAB at 35 degrees C. The decay of the shear modulus, G(t), was analyzed using the CONTIN inverse Laplace transform to obtain a distribution of relaxation times which were compared with ones from the usual fitting procedure based on a generalized Maxwell model. The behavior of the nematic lyotropic structure of the CTAB/water system is characterized by the presence of both slow and fast relaxation times. These were interpreted as being due to a progressive loss of the lyotropic domain orientation and to the breaking/reforming process of the cylindrical aggregates, respectively.     

The determination of the viscosity coefficients of nematic liquid crystals

This paper considers the flow generated by driving a sample of nematic liquid crystal through a rectangular capillary by application of a small pressure gradient in the presence of a large aligning magnetic field. A theoretical calculation based on the continuum theory of nematics is presented which makes some allowance for non-uniform alignment induced by flow, and allows a more accurate determination of the viscosities corresponding to the three principal configurations in the plane of shear.     

On the response of an ionic liquid to external perturbations and the calculation of shear viscosity

In this article, attention is directed to three related problems: (1) the response of the ionic liquid (IL) 1-hexyl-3-methylimidazolium chloride ([HMIM+][Cl-]) to different external perturbations, (2) the calculation of its shear viscosity, and (3) the investigation of the range of validity of linear response theory for these types of systems. For this purpose, we derive a set of equations linking bulk hydrodynamic predictions with microscopic simulations which are valid when linear response theory is applicable. As far as we are aware, this article reports results from the largest atomistic simulations ever performed on this liquid. Our study shows that even for systems with a box length as large as 0.03 mu the viscosities computed from perturbation frequencies compatible with this box size have not yet reached the bulk hydrodynamic limit. This is in sharp contrast with the case of other solvents such as water in which the hydrodynamic limit can be achieved by using perturbations on a length scale of typical molecular dynamics simulation box sizes. In order to achieve our goals, we comprehensively investigated how the IL relaxed upon weak external perturbations at different wavenumbers. We also studied the steady-state flow created by external shear acceleration fields. The short time behavior of instantaneous velocity profiles was compared with the results of linear response theory. The short time response appears to match the prediction from linear response theory, while the long time response deviates as the external field becomes stronger. From this study, the range on which a perturbation can be considered "weak" in the linear response sense can be established. The relaxation of initial velocity profiles was also examined and correlated to the decay of the transverse-current autocorrelation function. Even though none of our calculations reached the bulk hydrodynamic limit, we are able to make predictions for the shear viscosity of the bulk system at different temperatures which qualitatively agree with experimental data.     

A rheological and morphological study of a copolyester liquid crystal/polypropylene blend system

The effects of composition and shear rate on the rheology and morphology of blends of LC–3000, a thermotropic liquid crystalline polymer consisting of 60/40 of hydroxybenzoic acid and poly(ethylene terephthalate), with polypropylene were studied. It was found that the rheological properties depend in a complex manner on composition and applied shear. Both positive and negative deviations from the log-additivity rule were observed at low shear rates. Significant viscosity reduction was measured when the dispersed phase was a nematic TLCP. The accompanying microstructural transitions were characterized a posteriori, and it was found that the state of dispersion of the TLCP phase also influences the viscosity reduction phenomenon. A nematic, fibrillar TLCP phase shows a viscosity reduction of the order of fourfold with respect to the viscosity of the matrix. Another important finding was that the stability of these fibers would not be expected from work on other non-TLCP-containing immiscible blends. This suggests that the unique rheology of the TLCP minor phase is relevant to the formation of stable fibers. © 1996 John Wiley & Sons, Inc.     

New method for measuring polar anchoring energy of nematic liquid crystals

A new method for measuring a polar anchoring energy of nematic liquid crystals (LCs) is proposed. A variation of LC tilt angle on the surface with an applied electrical field was determined by a reflective method. The twisted LC cell configuration was selected to compensate a contribution of the induced birefringence in the reflective spectra. The electrical field controlled reflectance was used to analyse the potential form of the polar anchoring energy and to define the anchoring strength. The proposed method is applicable for 2–5 μm thick LC cells.