摘要:

Global molecular orientation function coefficients for the nematic liquid crystal 4-cyano 4'-nn -pentylbiphenyl (5CB) in shear flow are presented, being extracted from 2-dimensional Wide-Angle X-ray Scattering data. A linear increase in orientation parameterP2is 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.

ISSN:0267-8292    

DOI:10.1080/026782997208046

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The thermotropic mesophase behaviour of an asymmetrically shaped and highly substituted tetrabenzotriazaporphyrin (TBTAP) derivative has been studied by optical microscopy, DSC and X-ray diffraction. The TBTAP macrocycle differs from the more common phthalocyanine ring system by the substitution of a methine for a nitrogen at one meso-position. The TBTAP core was substituted with a single heptadecyl chain at this meso-position and four neopentyl groups were statistically distributed around the peripheral benzene rings. In contrast to an earlier study of this system which described the structure of the mesophase as discotic lamellar, detailed X-ray diffraction study indicates that the TBTAP derivative forms a disordered hexagonal columnar mesophase (Colhd), with a weak tendency towards antiparallel orientation of neighbouring molecules observed in the form of a weak pseudo-centred rectangular packing.

ISSN:0267-8292    

DOI:10.1080/026782997208055

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The liquid crystalline properties of a series of 4-alkoxyphenyl β-D-glucopyranosides (methoxy to decyloxy and dodecyloxy) were studied using polarized light microscopy and differential scanning calorimetry. The compounds with the shortest alkoxy substituents are not liquid crystalline. The butoxy derivative displays a monotropic smectic A phase and the higher homologues display enantiotropic smectic A phases. The lyotropic behaviour was studied as a function of concentration and temperature. Hexagonal, cubic and lamellar phases were observed for compounds with alkoxy chains longer than butoxy. The nonyloxy derivative forms long ribbons in dilute solution as revealed by electron microscopy.

ISSN:0267-8292    

DOI:10.1080/026782997208064

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper explores further the predictions of the Lebwohl-Lasher model [1] for the description of order in a liquid crystalline system, especially in the region of the nematic- isotropic transition. The model is based on a lattice, each cell of which contains a director representing the long axis of a rod molecule. The energy of each director is determined by the relative orientations of its six nearest neighbours, and the probability of a director orientation being moved to another chosen at random depends on the Boltzmann function of the difference between the old and the new energies in accord with the normal Monte Carlo procedure. The validity of this model to describe this transition has been demonstrated in several previous studies [2–16] and its simplicity permits calculations over a statistically significant number of molecules. Preliminary studies of the model behaviour have been made below the transition temperature in order to investigate the influence of boundary conditions. The simulated P2and P2/P4quantities are compared both with experimental data and with the theory of Maier and Saupe [17]. The predictions of the model are analysed in the normal way using the Ornstein-Zernike expression for pairwise correlation functions, while this expression is modified in order to describe the short-range order which is superimposed on the background level of long-range order present in the nematic phase. The model's predictions of enthalpy changes across the transition are compared with calorimetric data from the literature [18]. The opportunity of working with a large model is taken to extend the Zhang plot to test for the presence of first order character within the transition. A structural description of the transition is proposed, based on the molecular director maps, and the identification of more ordered and less ordered regions achieved by the analysis of the distribution of local energies into two sub-distributions with widths in accord with the mean values of their energies. As the transition is approached from above, the isotropic melt structure is seen to contain nematic nuclei which increase in volume fraction with decreasing temperature. At the transition these nuclei appear to join to give a percolating phase having a single orientation across the model. With decreasing temperature within the nematic region, isolated regions of disorder become continually smaller with a corresponding increase in the overall order parameter (P2). This paper focuses particularly on the structural implications of the predicted nematicisotropic transition. It is recognized that aspects characterizing the physics of the transition can be obtained through averaging energy parameters over very large numbers of cycles, and using models as large as possible. While the 503 model presented here is particularly large by current standards, it is designed in this way to show a representative picture of the structure in the transition region. The aspect of the work reported, despite the availability of a large model, does not seek to generate better averages than previously reported, and the calculation of some such parameters in the earlier sections is to check that the model is behaving properly and in accord with results reported previously.

ISSN:0267-8292    

DOI:10.1080/026782997208073

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Flexoelectricity is a general and fundamental phenomenon in liquid crystals. It describes the linear coupling between an applied electric field and gradients in the director field. Whereas flexoelectricity has for decades been regarded as only of academic interest, we think it is time to point out its considerable application potential, for instance in the case of the flexoelectrooptic effect, and to urge a revival of interest in the subject. As a result of long-time neglect, published data on flexoelectric coefficients are scarce and inconsistent, even with regard to the sign of the reported effect. In this paper we critically review the possible definitions of flexocoefficients in order to propose an international standard. We point out that the absence of such a standard obstructs the understanding of the physical basis, microscopically as well as macroscopically, of the effect, and leads to the introduction of nonsensical concepts like ‘flexoelectric anisotropy’. Based on the only natural convention, we finally propose a simple method for measuring sign and magnitude of the effective flexoelectric coefficient which is the control parameter in electro-optic effects.

ISSN:0267-8292    

DOI:10.1080/026782997208082

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A low frequency relaxation mode 1 has been detected in ferroelectric phases of three materials. The relaxation frequency and the dielectric strength of this mode depend strongly on the sample thickness, but their temperature dependences are qualitatively different in the materials studied, reflecting the behaviour of the helicoidal structure of these materials. This mode has been attributed to a superposition of the Goldstone and thickness modes when the helicoidal structure exists and to the thickness mode only, when the helix is unwound. In addition, with one material, mode 2 has been detected at still lower frequencies, and this also exhibits a strong sample thickness dependence. It was attributed to fluctuations of the director field modified by a non-homogeneous ionic charge distribution across the sample. As both modes 1 and 2 are strongly sample thickness dependent, they do not represent bulk properties of the materials studied, but reflect the structure in real samples, which is determined by surface conditions.

ISSN:0267-8292    

DOI:10.1080/026782997208091

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We have studied the phase separation process in a mixture of a liquid crystal material and two different monomers undergoing a polymerization process. Phase equilibrium was theoretically analysed using a Van der Waals free energy density approach. Results indicate that the more slowly reacting monomer will be found at higher concentration at the droplet boundary in the fully cured polymer dispersed liquid crystal. Experimental evidence, which includes the observation of director configurations and the measurement of transmission in the on-state, is presented.

ISSN:0267-8292    

DOI:10.1080/026782997208109

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper investigates the stability of simple static orientation patterns in a sample of smectic liquid crystal confined to a cylindrical annulus, when a magnetic field is applied. Four different arrangements are considered, covering cases where the layer normal is everywhere either radial or axial, and the (orthogonal) magnetic field is either radial, azimuthal or axial. A classification is given of the threshold radii for mechanical instabilities, and of the threshold magnetic fields for Freedericksz transitions for these cases, with strong anchoring at the boundaries.

ISSN:0267-8292    

DOI:10.1080/026782997208118

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Small angle X-ray diffraction experiments show the emergence of a chevron structure on cooling from the nematic phase into the smectic A phase of the commercial mixture S3 (Merck Ltd, UK), in a display device. The chevron angle increases from 0° at the nematicsmectic transition temperature,TNS, to a maximum value of 8.5°, which is reached when the sample is 15°C belowTNS. Between 5°C and 15°C below the transition temperature a quasi bookshelf structure emerges, in addition to the prevailing chevron structure; such a structure has not been hitherto reported. There is no further resolvable change in the device structure on cooling lower thanTNS-15°C. The chevron structure is due to the combination of layer thinning and fixed surface layers, as confirmed by a comparison of layer spacing calculated from the Bragg angle equation and from the layer thinning equation (d=dnematiccosδ, where δ is the chevron angle).

ISSN:0267-8292    

DOI:10.1080/026782997208127

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A novel series of ferroelectric liquid crystals with a fluorinated asymmetric frame were synthesized by utilizing optically active (S)-2-, (S)-3-, (S)-4- and (S)-5-fluoroalkanols prepared from corresponding (R)-1,2-epoxyalkanes. Their mesomorphic and physical properties, such as spontaneous polarization, optic tilt angle and response time, were investigated systematically in a series of homologous compounds having the chiral centre at different positions on tails of various lengths. All the compounds exhibited the chiral smectic C phase in a wide range of temperatures and were found to possess a fast response time in spite of the small magnitude of the spontaneous polarization.

ISSN:0267-8292    

DOI:10.1080/026782997208136

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor