摘要:

The continuum theory is used to study a polar electrooptic effect in a nematic insulator which is caused by applying an electric field normal to the sample planes having different director anchoring strengths. In the low field limit the magnitude of the linear electrooptical effect is studied as a function of anchoring strengths, director tilt at the boundaries, magnitude of flexoelectric constants and an applied magnetic field. The effect becomes stronger with increasing flexoelectricity or a stabilizing magnetic field and becomes weaker with a destabilizing magnetic field, pretilt of the initial director away from the homogeneous orientation or increase in sample thickness. As in the case of the Fréedericksz threshold, the magnitude of the applied voltage at which the director field orients along the sample normal is found to depend on the sign of the voltage as a consequence of unequal anchoring strengths and flexoelectricity. Under the rigid anchoring hypothesis the mathematical model is extended to study magnetic field induced bistability. It is found that the bistability width as well as the associated optical properties of the nematic sample can be profoundly influenced by the application of an electric field.

ISSN:0267-8292    

DOI:10.1080/02678299308027290

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report the synthesis, characterization and structural studies of the higher homologues ofN(4-n-pentyloxybenzylidene)4-n-alkylaniline compounds, viz. 5O.12, 5O.14 and 5O.16 by thermal microscopy, differential scanning calorimetry and X-ray diffraction. The compounds exhibit N–SA–SB, N–SA–SBand N–SAphase sequence variants, respectively. The X-ray results reveal the existence of an interdigitated smectic A phase in 5O.16 and smectic B phase in 5O.14. Results are discussed in light of the available data on thenO.mseries

ISSN:0267-8292    

DOI:10.1080/02678299308027291

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A new chiral organic acid, 2(S)-[2(S)-methylbutoxy]propionic acid (2), was synthesized from ethyl lactate and (S)-1-iodo-2-methylbutane. The acid2containing two centres of chirality was used as a building block for preparing ferroelectric liquid crystals. Three series of chiral compounds were synthesized to study the influence of the molecular structures on the formation of liquid crystal phases. They are 4-{2(S)-[2(S)-methylbutoxy]propionyloxy}phenyl 4-(4-n-alkoxyphenyl) (series B), and 4-{2(S)-[2(S)-methylbutoxy]propionyloxy}phenyl 4-(4-n-alkoxyphenyl) benzoate (series B), and 4-{2(S)-[2(S)-methylbutoxy] propionyloxy}-4′-biphenyl 4-n-alkoxy benzoate (series C). Only one member of series A, 4-{2(S)-[2(S)-methylbutoxy]propionyloxy}phenyl 4-n-octyloxybenzoate (5c), exhibits a monotropic SAphase. Then-alkoxy chain length at the biphenyl ring of series B has a remarkable efect on the S*Cphase formation. An enantiotropic S*Cphase can only be observed when then-alkoxy chain is longer than then-octyloxy group. Then-heptyloxy andn-octyloxy homologues exhibit a monotropic S*Cphase. However, all members of series C possess an enantiotropic S*Cphase. The occurrence of the phase sequence C-S*C-SA-I or C-S*C-N*-I depends on the position of the biphenyl ring. The S* phase temperature range in series B and C is up to 70°C.

ISSN:0267-8292    

DOI:10.1080/02678299308027292

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A viscoelastic model, composed of the Ericksen and Landau-de Gennes nematic continuum theories, is used to study numerically the relaxation phenomena after cessation of simple shear flow for a model rigid rod uniaxial nematic polymer. This model predicts that under certain conditions the relaxation of stored molecular and coupling elastic free energies due to periodic fluctuations in the scalar order parameter results in a transient periodic distortion of the director field. These conditions are that: (1) the ratio of the wavelength scales of the initial periodic spatial variation in the scalar order parameterksto the initial periodic planar director orientation fluctuationkφ (i.e.kS/kφ) and the amplitude of the initialSspatial variation exceed certain minimum values, and (2)kφ is not zero. It is shown that the wavelength selection mechanism is controlled by the director reorientation-induced backflows. The digitized optical patterns of the transient periodic director field show transient periodic optical patterns similar to the transient banded texture nematic polymers exhibit after cessation of shear flow when observed between crossed polars. The numerical results and digitized optical patterns replicate frequently reported experimental observations.

ISSN:0267-8292    

DOI:10.1080/02678299308027293

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Transition temperatures and smectic B mesophase ranges are examined for a number of homologous series, and some general rules are proposed that relate the smectic B phase behaviour to molecular structure. A mechanical model is developed for the crystal and hexatic smectic B phases, based on the dynamics of rigid or elastic rotors, which suggests two possible types of behaviour. It is found that such predicted behaviour is observed for particular series, but in general the smectic B phase characteristics of homologous series may be between the limits established for the model.

ISSN:0267-8292    

DOI:10.1080/02678299308027294

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Molecular dynamics simulations of cyclic siloxane-based liquid crystals offer new insights into the conformational flexibility of these materials. Interdigitation between the cholesteryl-4′-allyloxybenzoate and biphenyl-4′-allyloxybenzoate mesogens pendant on the cyclic siloxane ring is observed in the simulated structures. All molecular models considered viz. disc, cone, and cylinder, display a large conformational flexibility, which is important regarding the liquid crystalline phase behavior. The disc molecular model exhibits the largest flexibility as indicated by mean dihedral angles and their range for certain principal torsions, evaluated from the molecular dynamics simulations. Results from the dynamics simulations of cylinder molecular pairs indicate a significant amount of conformational flexibility in the siloxane rings. The degree of interdigitation between mesogens is dependent on the flexibility of the siloxane rings, as shown by calculations for a fixed ring system resulting in less interdigitation, also reflected in calculated X-ray scattering sections along the starting molecular direction. Weaker molecular transforms were observed for the non-fixed system due to a lack of boundary conditions. In general, the qualitative agreement of the starting structure's reflections and those shown by the experimental data is encouraging.

ISSN:0267-8292    

DOI:10.1080/02678299308027295

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The binary mixture sodium decylsulphate/water exhibits a hexagonal structure for a degree of hydration of about 50 per cent by weight. In this structure the amphiphiles build infinitely long cylinders which are organized on a 2D lattice of space group p6m. If, in such a mixture, sodium decylsulphate is progressively substituted by decanol, the hexagonal structure transforms itself successively into two rectangular structures. In these rectangular structures the amphiphiles also build cylinders, but these are organized on 2D lattices with space groups cmm and pgg, respectively. This evolution of cylindrical structures in the ternary mixture is also marked by a growth of the mean normal section of the cylinders. An analysis of these results is proposed, in which the growth is accompanied by a change in the normal section, from circular for the hexagonal structure to non-circular for the rectangular structures; this change occurs abruptly at the hexagonal-rectangular transition and is stabilized by an important fluctuation of the relative concentrations of the two amphiphiles within the cylinders.

ISSN:0267-8292    

DOI:10.1080/02678299308027296

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The synthesis, characterization, mesomorphism and ferroelectric behaviour of ten new chiral dimeric compounds are reported. These compounds are non-symmetric dimers consisting of two mesogenic units joined by two different flexible spacers derived fromS(−)-lactic acid. The mesogenic units contain ester and thioester groups. The effects of the length of the flexible spacer and the structure of the mesogenic units on the mesomorphism of the compounds are investigated. These compounds are ferroelectric and the spontaneous polarizations of some were measured, giving values ofc.10nC cm−2.

ISSN:0267-8292    

DOI:10.1080/02678299308027297

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The observation of polymeric nematic schlieren textures with singularities of high strength (|S| > 1) is reported. The polymers studied are of the type with two dimensional mesogenic units. Inversion walls were also observed in the liquid crystal state of these polymers.

ISSN:0267-8292    

DOI:10.1080/02678299308027298

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0267-8292    

DOI:10.1080/02678299308027289

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor