Influence of the order parameter and of the director orientation on the surface tension of free nematic films

A radial hydrodynamic flow in the nematic phase of free, suspended cylindrical films of 4-n-heptyl- and decyloxybenzoic acid and in 4,4'-di-n-heptyl-oxyazoxybenzene has been observed. The flow starts about 7°C before the phase transition into the smectic C phase. Under the same experimental conditions such a hydrodynamic flow is not established in free nematic films of 4-n-heptyl-and octyloxy-4'-cyanobiphenyl, 4,4'-dimethoxyazoxybenzene and N-(4-ethoxybenzylidene)-4'-n-butylaniline after the completion of the transition from the isotropic liquid to the nematic phase. The observed hydrodynamic flow is explained by a non-linear temperature dependence of the surface tension.     

Electric field effects on director pattern and disclinations in free standing nematic films of 8CB

Studies of electric field-induced orientational effects are carried out on substrate-free nematic 8CB films, held between parallel electrodes and containing a peripheral-1/2 loop. The loop separates the birefringent boundary from the central homeotropic zone. Static and dynamic characteristics of the splay-bend layers contiguous to the electrodes are investigated by polarizing microscopy. A model based on director field symmetry is proposed to explain (a) the growth of the transition layers independently of the associated singularities and (b) the formation or otherwise of static solitons (Brochard-Leger walls). Linear dependence of the growth rate of transition layers on field strength is compared with a similar behaviour for line defects. Bifurcation of a soliton at elevated voltages into a pair of - 1/2 lines, through a non-pincement process, is described.     

Electric field effects on director pattern and disclinations in free standing nematic films of 8CB

Studies of electric field-induced orientational effects are carried out on substrate-free nematic 8CB films, held between parallel electrodes and containing a peripheral-1/2 loop. The loop separates the birefringent boundary from the central homeotropic zone. Static and dynamic characteristics of the splay-bend layers contiguous to the electrodes are investigated by polarizing microscopy. A model based on director field symmetry is proposed to explain (a) the growth of the transition layers independently of the associated singularities and (b) the formation or otherwise of static solitons (Brochard-Leger walls). Linear dependence of the growth rate of transition layers on field strength is compared with a similar behaviour for line defects. Bifurcation of a soliton at elevated voltages into a pair of - 1/2 lines, through a non-pincement process, is described.     

Influence of anchoring at a nematic cell surface on threshold spatially periodic reorientation of a director

We have analysed the influence of surface director anchoring in a planar flexoelectric nematic cell on the threshold spatially periodic reorientation of the director in an external dc electric field. By minimizing the free energy of the nematic cell we obtained the equations for a director and numerically solved them in the one elastic constant approximation. The dependences of the threshold electric field and the spatial period of director structure on the azimuthal and polar anchoring energy, as well as the flexoelectric parameters, are determined. It is shown that the domain of the flexoelectric parameter values, at which the spatially periodic reorientation of a director takes place, increases with decreasing azimuthal anchoring energy and increasing polar anchoring energy.     

Photoinduced gliding of the surface director in azo-dye doped nematic liquid crystals

We report photoinduced surface director gliding with azo dye-doped nematic liquid crystals. This gliding has the same characteristics as that induced by an applied magnetic field. Fast and slow dynamic regimes are observed, the latter fitting to a stretched exponential. In addition, the gliding demonstrates a 'sensitizing' effect for sequential measurements.     

Field quenching of director fluctuations in thin films of nematic liquid crystals

The continuum theory of director fluctuations in nematic liquid crystals is extended to take account of finite sample size which more accurately describes experiments on thin films. The effect of external fields on director fluctuations is considered for two cell geometries: (i) the field parallel to the director for materials of positive susceptibility anisotropy, and (ii) the field perpendicular to the director for materials of negative susceptibility anisotropy. In the latter geometry, quenching of fluctuations leads to induced biaxiality. Comparison with experimental results shows that even allowing for the finite size of the sample, there is still a significant disagreement between theory and experiment, especially from thin samples.     

Modelling of director equilibrium states in a nematic cell with relief surface

We study two-dimensional equilibrium configurations of nematic liquid crystal (NLC) director in a cell bounded by two parallel surfaces. One surface is planar and the other one is spatially modulated. The relief of the modulated surface is described by a smooth periodic sine-like function. The orientation of NLC director easy axis is assumed to be homeotropic at one cell surface and planar at the other one. The NLC director anchoring with cell surfaces is assumed to be strong. We consider the case where disclination lines occur in the bulk of NLC above the extrema of the modulated surface. These disclination lines run along the crests and troughs of the surface relief. If the orientation of director at both bounding surfaces is of the same type, then NLC director field is continuous. For both configurations mentioned above (with defects and without defects), we obtain analytical expressions for director distribution in the bulk of NLC in the approximation of planar director deformations. Equilibrium distances from disclination lines to the spatially modulated surface are calculated when the defects occur. The dependences of these equilibrium distances on the period and depth of surface relief and the cell thickness are investigated in detail.     

On the refractive indices,polarizabilities and order parameter of a nematic discogenic mixture

Abstract

The phase diagram of a mixture of the discogens triphenylene hexa-n-heptanoate (HET7) and triphenylene hexa-(2-methyl-4-n-decyloxy)benzoate is presented. This mixture provides a more accessible nematic temperature range than was previously possible. The refractive indices of one mixture composition are measured and a rather small, negative birefringence found. The two-parameter Haller extrapolation method is used to calculate the molecular polarizability ratio Δα/α. This has a rather small magnitude compared with calamitic mesogens, although the individual polarizabilities are much larger, due to the greater molecular mass of these compounds. The calculated value of Δα/α is used to give the uniaxial orientational order parameter S, whose value is comparable to those of calamitic examples. Tabulated bond polarizability data are used to predict a value for Δα/α, which is found to be similar to the experimental result.     

Polar anchoring energy and order parameter at a nematic liquid crystal-wall interface

By measuring the integrated birefringence versus thickness of homogeneous and distorted nematic liquid crystal configurations, we have obtained the anchoring energy and the change of the order parameter at the orientating layer. The measurements were made for planar and tilted orientating SiO layers and for rubbed polyimide. The tilted orientating SiO layer shows the most significant decrease of the order parameter near the interface, which can be explained by its microscopically inhomogeneous alignment.     

Surface tension and surface free energy of polymers

The curent state of problems connected with the definition and experimental determination of surface free energy and surface tension of polymers is discussed. An analysis of the application of some equations based on classical and modern thermodynamics of polymer solutions shows that present theories need an essential improvement to fit experimental data. The Zisman concept of critical surface tension and Fowkers' hypothesis of additivity in the contribution of polar and dispersion forces to surface tension are criticized and a new approach to the problem is proposed.     

Optical rotation at the free surface of a nematic liquid crystal

When a small droplet of a nematic liquid crystal is placed on a horizontal glass plate in the presence of a magnetic field, the plane of polarization of light transmitted upward through the liquid crystal can be rotated. A defect usually forms preferring a direction perpendicular to the magnetic field and forming a diagonal of the droplet. This defect divides the free surface into regions giving rise to optical rotations that are clockwise and counterclockwise. It is suggested that the defect may be similar to a Helfrich splay-bend wall and the optical rotation in the regions near the defect may be explained by surface effects at the free surface of a liquid crystal.     

Biaxial order and a rotation of the minor director in the nematic phase of an organo-siloxane tetrapode by the electric field

Biaxiality in the nematic phase for a liquid crystalline tetrapode made up of organo-siloxanes mesogens is investigated using polarized infrared spectroscopy. An ordering of the minor director for the homeotropically aligned sample is found to depend on the amplitude of the in-plane electric field. On increasing the in-plane electric field, the minor director, lying initially along the rubbing direction, rotates to the direction of the applied field. The scalar order parameters of the second rank tensor are found to depend significantly on the strength of the electric field. A most significant increase is found in the nematic order parameter and in the parameter that characterizes the phase biaxiality.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4'-butylaniline (MBBA) and 4-n-pentyl-4'-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

Influence of hyperbranched polyesters on the surface tension of polyols

The influence of hyperbranched polyesters with different functional end groups on the surface tension of mixtures with an oligo(ester diol) was investigated. The temperature dependence of the surface tension of the pure components and of the mixtures was measured by a modified Wilhelmy balance technique. The results indicate that the surface tension of the pure hyperbranched polyesters strongly depends on the functionality of the end groups. The functionalization of the hydroxyl end groups by short alkyl chains (methyl, tert-butyl) reduced the surface tension depending on the degree of substitution. The surface tension of the mixtures with the hydroxyl-terminated hyperbranched polyester was slightly increased at higher concentrations of the hyperbranched polymer compared to the surface tension of the pure ester diol. On the other hand, the surface tension of mixtures could be considerably decreased using 1% of hyperbranched polyester polyols partially substituted with short alkyl chains. In that case, the modified hyperbranched polyesters act as surface active agents. On the molecular level, the enrichment of the modified hyperbranched polyester in the surface region was proven by X-ray photoelectron spectroscopy measurements.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Abstract

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4′-butylaniline (MBBA) and 4-n-pentyl-4′-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

Influence of silver nanoparticles on the order parameter of liquid crystalline polymers

New polymer nematic nanocomposites are prepared containing 1.43–4.64 wt % of silver nanoparticles whose mean dimensions are 2–4 nm. According to ²H NMR spectroscopic measurements, on increasing the content of metallic nanoparticles, the orientational order parameter S_zz of the nematic phase shown by the nanocomposites increases.     

Influence of silver nanoparticles on the order parameter of liquid crystalline polymers

New polymer nematic nanocomposites are prepared containing 1.43-4.64 wt % of silver nanoparticles whose mean dimensions are 2-4 nm. According to ²H NMR spectroscopic measurements, on increasing the content of metallic nanoparticles, the orientational order parameter S_zz of the nematic phase shown by the nanocomposites increases.     

Influence of the surface pretilt angle on spatially periodic deformations in nematic layers

The deformations induced by electric field in twisted or untwisted flexoelectric nematic layers can be homogeneous (i.e. one-dimensional) or spatially periodic (i.e. two-dimensional). The periodic deformations are undesirable from an applicative point of view since they destroy the homogeneous appearance of the area of an excited pixel of a display. They are particularly favoured when the nematic material possesses flexoelectric properties. In order to check whether the unwanted periodic deformations can be eliminated by means of suitable surface pretilt angle, the small deformations arising just above the corresponding threshold voltage were investigated numerically. The nematic materials exhibiting both weak and strong flexoelectricity were taken into account. The surface pretilt angles ranging from 0° to 30° were adopted. It was shown that the periodic patterns, arising in the case of planar surface alignment, disappear if sufficiently large surface pretilt angle is applied.     

Stable configurations in hybrid nematic cells in relation to thickness and surface order

The phase diagram of equilibrium configurations in thin hybrid nematic cells has been investigated in the framework of Landau-de Gennes theory extended to include weak surface anchoring. Surface interactions linear in the tensor order parameter were assumed and biaxiality was induced by the surfaces. It was found that in addition to the usual configuration where the director bends continuously from one plate to the other, there is also a possible configuration where bend does not occur, but where there is an eigenvalue exchange; i.e. the eigenvector of the tensor order parameter associated with the eigenvalue of the largest magnitude is different in different regions of the cell. Furthermore, for very small cell thickness a third possibility occurs: the eigenvector corresponding to the eigenvalue with the largest magnitude is uniform throughout the cell; it is either parallel or perpendicular to both plates depending upon the dominant surface interaction. Continuous transitions occur between the different configurations.     

Magnetic field effects on the crystal orientation and surface morphology of electrodeposited iron films

The effects of a magnetic field (5 T) on crystal orientation and surface morphology were investigated for iron films electrodeposited in ferrous aqueous solution. XRD measurements for the iron films showed that the preferred orientation parallel to the substrate was determined by the current density and not influenced by the magnetic field. By X-ray pole figure measurements, however, the crystal texture of the iron films electrodeposited at 10 mA cm^–2 and 30 mA cm^–2 was found to be controlled by the magnetic field. That is, the (110) planes were orientated in same direction of the magnetic field vector at angles of 30° and 35° to the direction normal to the substrate plane at 10 mA cm^–2 and 30 mA cm^–2, respectively. When the morphology was observed by SEM, iron grains at 30 mA cm^–2 changed from a triangular pyramid shape at 0 T to a complex star-like shape at 5 T.Presented at the 3rd International Symposium on Electrochemical Processing of Tailored Materials held at the 53rd Annual Meeting of the International Society of Electrochemistry, 15–20 September 2002, Düsseldorf, Germany