ISSN:0267-8292    

DOI:10.1080/026782997209261

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The growth of the distortion of the director in an external field, and its relaxation on removal of the field are studied for nematic solutions of the rod-like poly(1,4-phenylene-2,6-benzobisthiazole), PBZT, over a range of concentration, polymer molecular weight and temperature. The twist viscosity and the order parameter are more weakly dependent on concentration than predicted for charged rod-like chains. This behaviour implies a short-range translational order in a direction orthogonal to the chain axis, induced by electrostatic interactions among the polycations on protonated PBZT. The relaxation gives a normal temperature dependence for the splay curvature elasticity, but that for the twist curvature is unexpectedly strong. This behaviour is interpreted as a weakening of the surface anchoring with increased temperature.

ISSN:0267-8292    

DOI:10.1080/026782997209270

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Non-mesomorphic solutes depress the normal nematic-isotropic transition temperature in liquid crystals. When non-mesomorphic solutes are added to a nematic liquid crystal, the nematic-isotropic transition temperature is depressed and a two phase region is formed due to the presence of impurities of the solutes. The present paper explains the formation of this two phase region by the Landau-de Gennes phenomenological theory, which agrees fairly well with the experimental observations. We also note that this two phase region indicates the tricritical behaviour of the nematic-isotropic phase transition and the phase diagram near the tricritical point is also obtained.

ISSN:0267-8292    

DOI:10.1080/026782997209289

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Electro-optical properties of nematics of the trimer series 4,4-bis-[omega-(4-cyanobiphenyl-4yloxy)alkyloxy]biphenyls have been investigated by the method of orientational elastic deformations in electric fields. The number n of CH2 groups in the oxyalkyl fragment was varied from n 2 to n 10. A strong odd-even effect was observed in the optical anisotropy Deltan and in the values of the threshold potential V0 for the trimer nematics. These effects are, respectively, caused by oscillations of the degree of inter- and intra-molecular orientational order when n is varied. The great difference in the dependences of Deltan and V0 on n is discussed and compared with data for dimers.

ISSN:0267-8292    

DOI:10.1080/026782997209298

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Light scattering from polymer dispersed liquid crystal (PDLC) material has been studied experimentally and by Monte Carlo simulation. Light scattering was measured as a function of both scattering angle and cell thickness. The cell thicknesses of practical interest are in an intermediate regime where neither single scattering nor light diffusion applies. Both the angular and the thickness dependence of the scattering intensity can be described accurately by a Monte Carlo simulation of multiple scattering from a homogeneous distribution of independent scatterers. The model smoothly interpolates between the single scattering limit for thin cells and the diffusion limit for thick cells. It can easily be extended to include any specific feature of a scattering display system.

ISSN:0267-8292    

DOI:10.1080/026782997209306

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The liquid crystalline behaviour of 1-(4-n-decyloxyphenyl)-3-(4-X-phenyl)propan-1,3-diones and their pyrazole and isoxazole derivatives has been studied by optical microscopy, DSC and X-ray techniques. The 4-substituents (X) were chosen to include a range of different polar and non-polar substituents: H, OCH3, Cl, Br and CN. A monotropic SA phase is observed for the beta-diketone derivative in which X CN and this is the first example of this phase found in a 1,3-diphenylpropan-1,3-dione derivative. The majority of the pyrazole and isoxazole compounds show SA phases. As regards the cyano-substituted compounds, X-ray diffraction studies on the mesophase show that the layer spacing is consistent with a partial bilayer SA mesophase. The first hyperpolarizabilities of the cyano-derivatives due to their push-pull structure have been measured by the EFISH method. Values for these compounds were found to be comparable to those for other conjugated CH3O-pi-CN systems.

ISSN:0267-8292    

DOI:10.1080/026782997209315

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A high resolution a.c. and relaxation calorimetric study has been carried out on heptyloxybenzylidene butylaniline (7O.4) in two silica aerogels with mass densities rho 0.08 and 0.17g cm -3. Bulk 7O.4 exhibits strongly first order N-I, N-SA and SC-CrG transitions as well as a mean-field second order SA-SC transition. The 7O.4/aerogel samples exhibit three first order transitions (N-I, N-SA, SA-crystal) that are appreciably shifted and broadened relative to bulk 7O.4. The SA-SC transition is not observed in either of the aerogel samples.

ISSN:0267-8292    

DOI:10.1080/026782997209324

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The light scattered by a comb polymer with a polyacrylamide main chain and biphenyl-based mesogenic units containing the substituent R CN in the biphenyl core is characterized by a stationary noise which has been measured between 70 C and 140 C in the frequency interval 5 10 - 2 nu 10Hz. This optical noise is analysed by means of a specific procedure introduced to clarify the role of random movements of segments of the main chains on the side chain fluctuations. The RMS amplitude of the random molecular movements, the relaxation times of the side chain and main chain fluctuations, and the damping coefficients governing their motion are investigated as functions of temperature. The results are compared with those recently obtained on a similar polymer characterized by R H. Common features and differences are brought into evidence and discussed in terms of the different mesophase structures exhibited by the two polymers in the temperature interval considered.

ISSN:0267-8292    

DOI:10.1080/026782997209333

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Polymer Dispersed Liquid Crystal (PDLC) films investigated in this work are obtained by phase separation induced by photopolymerization of a homogeneous ‘prepolymer/liquid crystal’ mixture. We show a close relation between the formulation/conditions of preparation of the materials, their morphology and their electro-optical properties. The effect of the mutual compatibility of the prepolymer and the liquid crystal on film performances is investigated using ‘model systems’ that consist of pure components. Finally, both SALS experiments and a Fourier Transform procedure lead to a simple model in which LC domains are separated by polymer walls.

ISSN:0267-8292    

DOI:10.1080/026782997209342

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The dynamics of the field-induced one-dimensional twist deformation in weakly anchored nematic layers was investigated by means of numerical simulation. The rise time constants and the decay time constants, characteristic for the onset and for the decay of the deformation, respectively, were determined. The time of delay, which had passed before the decay started, was revealed in the case of finite anchoring. For weaker anchoring, the time of delay and the decay time constant were longer. Surprisingly, the onset of the deformation was slower when the anchoring was weaker, provided that the ratio of the actual field strength to the threshold field strength was fixed.

ISSN:0267-8292    

DOI:10.1080/026782997209351

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor