摘要:

Broadband dielectric measurements on a ferroelectric liquid crystal have been carried out. The alignment was planar and the cell and sample holder were the same in the whole frequency range. Five different relaxations have been obtained and characterized: soft mode, Goldstone mode, molecular reorientation around the transversal axis, molecular reorientation around the longitudinal axis and a ferroelectric domain mode. Strengths and frequencies of these modes have been obtained in the different phases for different bias fields.

ISSN:1058-725X    

DOI:10.1080/10587259508038668

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In the present work we have tried to test the theory of elastic constants of nematogens given by Priest, assuming Spherocylindrical shape of the molecules. The bend (K3) to splay (K1) elastic constant ratios of four closely related nematic liquid crystals, having cyclohexane carboxylate core, have been determined by us using Freedericksz transition in a magnetic field. The length and width of the molecules and the order parametersP2andP4, required for the theoretical calculations, have been estimated from our earlier X-ray studies on these compounds. The experimental values of the ratioK3/K1for three of these compounds agree quite well with those calculated from the Spherocylindrical model. The fourth nematogen, for which the agreement is poor, shows anomalous temperature variation of density and hence the rigid Spherocylindrical model is not probably applicable to it. We have also measuredK3/K1forn-pentyl cyanobiphenyl (5CB), and have shown that the X-ray experimental data implies thatP4is positive even near the nematic-isotropic transition temperature for 5CB, which is at variance with some experimental Raman scattering results. Finally, theK3/K1ratios for pentyloxy cyanobiphenyl (5OCB) at different temperatures obtained experimentally by Bradshawet al. have been compared with those calculated from Spherocylindrical model using our X-ray data.

ISSN:1058-725X    

DOI:10.1080/10587259508038669

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The molecular field theory of the Frederiks transition has been developed based on the Lebwohl-Lasher nematogen model and localized mean field approximation. We found that it needs five order parameters to generally describe the nematic system with boundaries. For the splay type of cell configuration, two of these order parameters are zeros. The theoretically calculated results are further confirmed by corresponding Monte Carlo simulation using Lebwohl-Lasher nematogen model as the prototype of the liquid crystal and the agreement is quite good considering the simplicity of the theory. The experimental law and the predictions of classical anisotropic continuum media theory are also reproduced by the present molecular field theory. Therefore, the results obtained in this paper lend a support to the general validity of the Lebwohl-Lasher nematogen model for the theoretical studies of small molecular liquid crystals.

ISSN:1058-725X    

DOI:10.1080/10587259508038670

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A method is described for the measurement of the rotational viscosityy1in nematic liquid crystal cell assemblies. Theoretical formulas to determiney1from transient current are developed by considering both director rotation and conduction of impurity ions. The pretilt angle and the product of the drift mobility and density of the ions are also obtained with the method. Experimental results for 4-pentyl-4′-cyano biphenyl cells with or without polyimide alignment layers are reported.

ISSN:1058-725X    

DOI:10.1080/10587259508038671

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Pretilt angles of various nematic liquid crystals (NLCs) aligned on the cured organic solvent soluble polyimide alignment film containing fluorine atoms have been measured. The relationship between obtained pretilt angles and physical properties of NLCs shows that the pretilt angle of NLCs increases linearly with the perpendicular component of permittivity of NLCs. This phenomenon suggests that the lateral substituents in NLC affects the generation of pretilt angles by the anisotropic-excluded-volume effect.

ISSN:1058-725X    

DOI:10.1080/10587259508038672

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The author reports for the first time that a critical feature arising from the fluctuations of the order parameter in the smectic-A <—> nematic (Sm-A <—>N) phase transitions of 4-cyano-4′-n-octylbiphenyl (8CB) and 4-cyano-4′-n-nonanylbiphenyl (9CB) can be clearly observed from nuclear magnetic resonance (NMR)13CT1(spin-lattice) andT2(spin-spin) relaxation times.

ISSN:1058-725X    

DOI:10.1080/10587259508038673

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We have combined the two cationsN-hexadecylpyridinium and the dicyclohexyl-18-crown-6 complex of sodium with the three anions bromide,p-toluenesulphonate and tetraphenylborate, into six different salts, and measured their conductivity in the smectic A liquid crystal, S3. This allows us to extract mobilities for the five individual ions.

ISSN:1058-725X    

DOI:10.1080/10587259508038674

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The increasing of pretilt angles of liquid crystals (LCs) in active-matrix twisted nematic liquid crystal displays (AM-TN-LCDs) is one of the most important subjects to achieve high contrast and defect-free AM-TN-LCDs. We have developed the new generation method of the pretilt angle by using newly synthesized organic-solvent-soluble polyimides (PIs) containing two types of diamine. These PIs are useful in the production process of AM-TN-LCDs due to their low temperature process, say 180°C. The maximum pretilt angle of 2.8° for an NLC, ZLI-1565, is obtained using the PI containing two types of diamine with a mole fraction of 50/50. This phenomenon can be explained on the basis of the micro-topography of the PI surfaces and, in particular, the obtained pretilt angles may be attributed to the relative bulkiness of diamines used in the PIs.

ISSN:1058-725X    

DOI:10.1080/10587259508038675

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Two-dimensional Nuclear Magnetic Resonance (NMR) spectroscopy is used to characterize polymer dispersed liquid crystals (PDLCs) in the solid-state. The wideline separation (WISE) pulse sequence is used to correlate structure and mobility in a PDLC. The PDLC is 40% E7 as 0.4 μ droplets in an Epon 828 matrix. The WISE pulse sequence generates1H wideline spectra in one dimension for each high resolution solid state13C resonance in the other dimension. For each13C resonance, the width of the1H line shape reflects the mobility of the protons in the vicinity of the respective13C. Spin diffusion is incorporated into the WISE pulse sequence to determine the relative spatial positions of components with different mobilities. The aliphatic portions of E7 exhibit extremely narrow1H line widths of a few hundred hertz. Therefore, the aliphatic pendant groups of E7 undergo pseudo-random isotropic motions with rates above 1 MHz. The aromatic portions of E7 exhibit pronounced spinning sidebands in the1H dimension. Therefore, the aromatic core of E7 undergoes anisotropic motions; specifically, ring flipping of the phenyl rings about their para axes. This ring flipping also occurs with rates exceeding 1 MHz.

ISSN:1058-725X    

DOI:10.1080/10587259508038676

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Data for the temperature dependence of the pretilt angles for nematic liquid crystal mixtures on a polyimide surface are compared with different theories. It is found that our recently proposed population distribution model can explain all the observations well, when we assume a Boltzmann distribution between the different states. The only exception is one sample with azoxybenzenes, which undergo chemical changes.

ISSN:1058-725X    

DOI:10.1080/10587259508038677

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor