摘要:

We have studied the alignment of nematic liquid crystals on Electrically Poled Photopolymer (EPP) films. Polarization microscopy studies show different textures in the planar and Twisted Nematic (TN) cells that were made of an EPP substrate and a rubbed-polyimide (PI) substrate. The EPP films exhibit a small pretilt angle of 3 to 5 degrees depending on the cell configuration. In contrast to the small anisotropy of the EPP film compared to Linearly Photo Polymerized (LPP) films, the azimuthal anchoring strength of the EPP films is larger than that of LPP films and depends on the UV exposure time. The alignment of EPP films in the TN cells is stable for months whereas the alignment of the LPP films is destroyed within three days.

ISSN:1058-725X    

DOI:10.1080/10587250008023266

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Electroconvection in twisted nematics has been studied experimentally and theoretically. The frequency dependence of the threshold voltage and rolls period for the different twist angles as well as the Lifshitz point versus the twist angle have been measured. The experimental results agree well with the predictions of linear stability analysis. Various types of secondary instabilities were found above the roll instability depending on the twist angle. The transition from 1D structure to the 2D domain pattern was investigated.

ISSN:1058-725X    

DOI:10.1080/10587250008023267

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We study the role of defects in the flow properties of a thermotropic smectic (in Cano wedge geometry) by in situ deformation experiments in an optical polarizing microscope. The motion of the edge dislocation array is studied for imposed strain rates and their climb velocity is measured. At a stress close to 3000 dyn/ cm2a jerky climb of edges is observed implying the existence of screw dislocations which hinder the free motion of the former. At a stress close to 5000 dyn/ cm2a viscous motion of edges is evidenced for which we confirm the validity of the Orowan relation, established for solid crystals.

ISSN:1058-725X    

DOI:10.1080/10587250008023268

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The dependence of the cut-off frequency of the conduction mode electrohydrodynamic instabilities, exhibited by homogeneously aligned negative dielectric anisotropy nematic liquid crystals, on the conductivity of the nematic material, the thickness of the nematic layer and the intensity of an orienting magnetic field, is examined. It is found that for sufficiently high conductivity and/or large layer thickness, the cut-off frequency is proportional to the conductivity of the material, and independent of the intensity of the orienting field. Lowering the conductivity and/or the thickness of the nematic layer results in smaller values of the ratio of the cut-off frequency to conductivity, until a critical limit is reached beyond of which no conduction mode of instability has to be observed. It is found also that for given values of conductivity and thickness, the effect of the orienting magnetic field is to lower the cut-off frequency of the instability and, also, to increase the critical conductivity and thickness values.

ISSN:1058-725X    

DOI:10.1080/10587250008023269

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We analyse stable structures in the model where interactions between the nearest and the next nearest layers are taken into acount. Interactions can be noncompeting or competing. We find five different stable structures stable at different values of model parameters. Three of them are already known: the ferroelectric SmC*, the antiferroelectric SmCA* phase and by recent experiments confirmed helicoidal short-pitch SmCα* structure. Aditionally we find two structures, the structure with a primitive cell which extends over 2 smectic layers SmC2* and SmC4* the primitive cell cosists of 4 smectic layers.

ISSN:1058-725X    

DOI:10.1080/10587250008023270

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Dielectric measurements on two samples of banana-shaped molecules showing a B2and a B7phase were carried out. The low frequency dispersion in the B2phase is interpreted as a collective motion of ferroelectric clusters and the high frequency one as the reorientation of single molecules with a strong positive dipole correlation. In the B7phase a, soft mode-like“ behavior was detected pointing to an additional phase transition. AFM measurements support the existence of helical super-structures which probably appear in B7and in the low temperature phase Bx.

ISSN:1058-725X    

DOI:10.1080/10587250008023271

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We study the electric-field-induced transitions in antiferroelectric liquid crystals in a model that takes into account the intralayer elastic distortion, the nearest-neighbor interlayer interaction, the coupling of the spontaneous polarization with the applied field, the dielectric anisotropy, and the surface anchoring. We find that the field-induced phase sequence has a critical dependence on cell thickness. For moderately thin cells (thickness > 0.1 μm) with moderately strong anchoring (surface coupling ∼ 10−4J/m2), the system, in departing from the initial anticlinic alignment, first undergoes a continuous Fréederiksz transition, then a first-order surface boundary-layer transition, and finally a first-order or continuous transition to a complete synclinic ordering. Reducing the cell thickness to less than 0.1 μm will make the Fréedericksz transition and even the boundary-layer transition vanish.

ISSN:1058-725X    

DOI:10.1080/10587250008023272

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this paper we investigate the causes of poor alignment in antiferroelectric liquid crystal devices making use of spin coated and rubbed polyimide. We found that poor alignment is due to the deviation of the layer normal from the rubbing direction. By optimising the skew between the rubbing direction on opposite substrates we found that a high quality and stable alignment can be obtained. Quantitative experimental results showing a static extinction ratio higher than 3000:1 and dynamic contrast ratio higher than 2000:1 are presented.

ISSN:1058-725X    

DOI:10.1080/10587250008023273

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

2H-NMR spectroscopy is employed to investigate the orientational order, tilt angle and layers organization of the chiral smectic liquid crystal (-)-(S)-[4-(2-methylbutyloxy carbonyl) phenyl] 4-n-heptyl-biphenyl carboxylate-d8(MBHB-d8) in the magnetic field. The electrooptical behaviour of this material has also been investigated. The results from these two different techniques show the importance of molecular conformation for the electric and structural properties of this mesogen.

ISSN:1058-725X    

DOI:10.1080/10587250008023274

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A low-frequency (0.1 – 100 kHz) resonance mode interpreted as a texture one has been investigated on ferroelectric liquid crystals (FLC). Compounds and mixtures showing different types of the phase transition into the SmC* phase (I—SmC*, N*—SmC*, SmA—SmC*) have been examined in order to find the resonance properties in the ferroelectric phase. It has been shown that the origin of the resonance response (resonance mode) is caused by strong pretransitional order parameter fluctuations in both SmA and SmC* phases near the second order SmA—SmC* phase transition. The principal factor causing the resonance mode origination is the presence of an external mechanical field in the well aligned uniform bookshelf FLC texture. An interaction between the texture resonance mode and bending modes of glass cell plates and the synchronization effect have been investigated.

ISSN:1058-725X    

DOI:10.1080/10587250008023275

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor