摘要:

The influence of the surface treatment on the anisotropic part of the surface tension of a lyotropic discotic micellar nematic liquid crystal doped with ferrofluid is analysed. The treatment involves Langmuir-Blodgett (LB) films deposited on the glass substrates. Textural observations and phase-shift measurements of the transmitted light were carried out. A phenomenological model which allows calculation of the director average orientation in the bulk and at the surfaces is used for interpreting the experimental data. Our results show that the average director alignment depends on the concentration of the magnetic grains of ferrofluid at the glass surfaces. On the contrary, it is practically independent of the nature of the molecule deposited on the glass substrate. According to the model, the flat glass surface, independently of the treatment or the absence of any treatment, stabilizes onto itself a lamellar layer (or bilayer) formed by the amphiphilic molecules of the liquid crystal. This layer screens the effect of the LB film (or of the pure untreated substrate) on the alignment of the lyotropic nematic mesophase.

ISSN:0267-8292    

DOI:10.1080/026782998206597

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A symmetric, low-molecular-mass liquid crystal based on the oxadiazole ring was synthesized and characterized. The symmetric position of the heterocyclic group in the centre of the liquid crystal, bis(4-hexyloxyphenyl)4,4-(1,3,4-oxadiazole-2,5-diyl)dicarboxylate, yields a very rigid and non-linear mesogenic core. Despite this non-linear structure, a broad liquid crystalline range with a smectic C and a smectic A phase was found. Conoscopic experiments on freely suspended films revealed the existence of two optical axes in the smectic A phase, indicating a phase symmetry anticipated for either a 'McMillan' biaxial smectic phase or a biaxial (achiral) ferroelectric smectic phase.

ISSN:0267-8292    

DOI:10.1080/026782998206605

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A temperature induced helix inversion in a single component system has been observed by investigating a lattice system of chiral molecules using Monte Carlo (MC) computer simulations. The molecules were considered to interact as derived by van der Meer et al. taking into account fourth-order terms describing an anharmonic interaction. On cooling from an isotropic system, a cholesteric phase with a temperature dependent pitch was obtained. The cholesteric phase changed its handedness through an infinite-pitch system as a function of temperature. The study of systems with a small chiral energy contributionnd a large pitch was enabled using self-determined boundary conditions.

ISSN:0267-8292    

DOI:10.1080/026782998206614

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Computer simulations on the display characteristics of polymeric-film-compensated homogeneous liquid crystal cells were performed. Both transmissive and reflective displays were investigated. Results indicate that the biaxial film-compensated reflective displays exhibit small wavelength dispersion, wide viewing angle, high contrast ratio and short response time. Their potential applications in reflective-type colour-sequential projection and direct-view displays are emphasized.

ISSN:0267-8292    

DOI:10.1080/026782998206623

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

New photoisomerizable chiral dopants have been studied. The dopants used were menthone derivatives, a chiral stilbene derivative and a nematic copolymer of a menthone derivative and a benzoyloxybenzonitrile derivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, without the formation of undesired by-products. Isomerization of the menthone derivatives, including the copolymer, induced a substantial decrease in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were limited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illuminating the mixture with UV light. The viewing angle dependence of irradiated regions of a 90 -twisted nematic cell was rotated 90 with respect to the viewing angle dependence of the non-irradiated regions. It is expected that this approach may be useful in the preparation of dual domain TN cells with a reduced viewing angle dependence.

ISSN:0267-8292    

DOI:10.1080/026782998206632

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The analogy between the field-induced and flow-induced deformations of nematic liquid crystals is summarized. The threshold onset of the deformation occurring under rigid boundary conditions, the unwinding of the helical axis in a chiral nematic material, and the saturation threshold appearing for weak anchoring are described as examples of analogous phenomena which take place both in external fields and during shear flow. The analogy has allowed prediction of a new threshold effect possible in a sheared, weakly anchored nematic. The description of this effect is supported by numerical calculations. Stability analysis was also performed.

ISSN:0267-8292    

DOI:10.1080/026782998206641

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Results are presented of the non-linear dielectric effect (NDE) as a function of temperature (T) in the isotropic phase on approaching the nematic phase for the liquid crystalline materials 4-cyano-4-hexylbiphenyl (6CB), 5-heptyl-2-(4-cyanophenyl)pyrimidine smectic A phase of 4-cyano-4-decylbiphenyl (10CB). For low measurement frequencies (f) the NDE 1(T) exhibits linear behaviour from the clearing temperature (T) up to about C TC50 K. Increase of f causes an increased deviation from this dependence near T C. The influence of the measurement frequency can be associated with the link between the system time-scale, and the measurement time-scale defined by the relaxation time of pretransitional processes (tau),(f1). A quantitative agreement with de Gennes model NDE 1(T) analysis and with the results of time- and frequency-resolved the relation derived from the Landauof Kerr effect studies is shown. A possible influence of the intensity of the weak measurement field on the pretransitional effect in the immediate vicinity of TC is discussed. Similiarities between pretransitional behaviour in the isotropic phase of the nematogens and in the homogeneous phase of a critical binary solution are considered. (HCPP) and the

ISSN:0267-8292    

DOI:10.1080/026782998206650

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A study of textural changes is presented concerning the nematic phase of 4- n-alkyloxybenzoic acids (in particular, 4-n-heptyl- and 4-n-octyl-oxybenzoic acids), by means of a statistical approach to the image data observed by polarized light microscopy (orthoscopic mode). A new image processing method is developed in order to detect with high sensitivity any structural change in the image frame. To do this, a set of parameters is introduced, charaterizing the observed textures. Such a set is a vector, working like a pathfinder strongly increasing the human eye or in general the sensor skilfulness to appreciate any change of the optical texture, both in space and in time. This is suitable for revealing smooth transitions, such as phase transitions between smectic and nematic phases (or between different smectic phases), or order transitions, like alignment transitions in poorly oriented nematic layers. In fact, by using this method for detecting the order transition between two nematic 'subphases' of 4-n-alkyloxybenzoic acids, the sensitivity turns out to be enhanced by a factor higher than 10 with respect to that for standard techniques. The new method allows us to define a characteristic size of the image texture: this concept is applied to analyse several image data for estimating the mean size of the domains appearing in the smectic and in the nematic phase of the compounds under study.

ISSN:0267-8292    

DOI:10.1080/026782998206669

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The surface memory effect (SME) storage of smectic C textures in the nematic temperature range has been investigated at various conducting and dielectric orienting coatings. The surface memorization for p-n-heptyl-,-octyl- and-nonyl-oxybenzoic acids (HOBA, OOBA, NOBA) consisting of dimer molecules is relatively strong and is observed only on conducting coatings (ITO, SiO/ITO) while dielectric surfaces depress the effect. p-n-Decyloxybenzoic acid (DOBA) exhibits a SME neither for conducting nor for dielectric coatings. The basic role of adsorption in the surface memorization is discussed in combination with the influence of surface polarization and of certain mechanical properties of the liquid crystal molecules. It is demonstrated that the erasure activation energy V hyperbolically decreases with increasing cell thickness d and reaches saturation values at d 250mum. Probably this dependence indicates the role of the dielectric state of the coatings in the surface memorization process. The problem of the possible biaxial character of the memorized smectic C-like texture in the temperature range of the uniaxial nematic phase is also discussed.

ISSN:0267-8292    

DOI:10.1080/026782998206678

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Chiral ester derivatives of terephthalic acid containing one, three, or five phenyl rings were prepared by using (S)-2-methylbutanol or (R)-2-chloropropanol as starting materials. The one-ring terephthalates did not exhibit any liquid crystalline phases, but one of them was used as a chiral dopant in ferroelectric mixtures. Elongation of the aromatic core structure to three and five phenyl rings stabilized the SmA phase, whereas a (monotropic) SmC* phase was detected in some three-ring and five-ring derivatives. In ferroelectric mixtures based on the terephthalates, spontaneous polarizations up to 340 nC cm2 were measured.

ISSN:0267-8292    

DOI:10.1080/026782998206687

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor