摘要:

Small angle time-resolved X-ray diffraction was used to monitor the behaviour of the smectic layers during the electric field induced planar to homeotropic transition in a smectic A cell possessing a chevron layer geometry. The liquid crystal material used was S3, from Merck Ltd, and was sandwiched in a 15 mum parallel plate device. The main features of the transition are the cooperative rotation of layers and the creation of an asymmetric chevron structure during the early stages of switching. The time scale for the planar-to-homeotropic transition in the device is approximately 5 s, at a temperature of 3 C below the nematic-to-smectic A phase transition and for an applied electric field of 2 V mum -1 (rms).

ISSN:0267-8292    

DOI:10.1080/026782997207993

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The dynamics of different molecular modes in four antiferroelectric liquid crystal substances have been studied by a combination of spectroscopic methods.The fastest motion is the reorientation around the molecular long axis, here found in the low GHz range by time domain spectroscopy. The reorientation around the short axis has a characteristic frequency of about 10kHz and is detected by frequency domain spectroscopy in the homeotropic configuration. As for the collective excitations, the Goldstone and soft modes, characteristic of the ferroelectric phase, have counterparts in the antiferroelectric phase which appear very different. There are two characteristic peaks in the spectrum, one at high frequency, about 100kHz, the other at low frequency, about 10 kHz. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining different experimental techniques and different geometries it can be shown that neither is a soft mode, but both are collective modes of different character: the high frequency mode corresponds to fluctuations where molecules in neighbouring layers are moving in opposite phase, the low frequency mode to phase fluctuations in the helicoidal superstructure. In materials exhibiting a C* phase in addition to the C*a or C* gamma phases, an additional strong peak appears in at least one lower-lying phase adjacent to the C* phase. We show that this peak, which we call a hereditary peak, has nothing to do with the antiferroelectric or ferrielectric order, but is just the Goldstone peak from a coexisting C* phase. In the same way, a Goldstone mode peak from the C* gamma phase may appear in the underlying C* a phase. In a general way, narrow phases like C* gamma, being bounded by first order transitions on both sides (C* a -C* gamma -C*) are likely to show non-characteristic (hereditary) peaks from both adjacent phases.

ISSN:0267-8292    

DOI:10.1080/026782997208000

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The shear flow induced deformations of a nematic liquid crystal layer have been modelled numerically for the case of flow-aligning nematics. The director deviation from the plane of shear, which was predicted earlier for special surface orientation angles, has been confirmed. This deformation takes a form of director rotation about the axis perpendicular to the layer plane. As a result, transverse flow of the nematic arises. The rotation angle is close to pi at sufficiently strong shear stress, and the director is oriented at the usual flow alignment angle in a significant part of the layer. The director coming out of the shear plane should not be treated as a separate effect taking place during the flow, but rather as a way in which the usual flow-aligned structure is achieved.

ISSN:0267-8292    

DOI:10.1080/026782997208019

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Morphological, electro-optical and switching properties of polyester resin/nematic liquid crystal composite films have been studied for varying composition (10-40 wt% of LC), temperature (20-50 C), film thickness (10-75 mum) and UV curing time of the matrix (0.25-12 min). The PDLC films were formed by LC separation in a UV polymerization process of the thin layer of oligoester resin (liquid crystal mixture) between ITO coated glass plates. The electro-optical and response behaviour based on the electric field controlled light scattering of the composite films was observed. The results were interpreted in terms of effective anchoring strength at the interface of the polymer and liquid crystal depending either on the area fraction of the interface in the composite film (dependent on the size and shape of the liquid crystal droplets) or the stiffness and resistivity of the polyester resin changing in the course of the crosslinking polymerization.

ISSN:0267-8292    

DOI:10.1080/026782997208028

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A variety of N -acyl- beta -D-glucopyranosylamines, N -acyl- beta -D-mannopyranosylamines and 2-alkylamido-2-deoxy- alpha / beta -D-glucoses has been prepared regiospecifically and in good yield in a one step reaction of the appropriate acid chloride with D-mannosylamine, 2-amino-2-deoxyD-glucopyranose (prepared in one step from glucose and mannose according to literature methods) and D-glucosamine liberated in situ from the commercially available hydrochloride. The dependence of the liquid crystal transition temperatures and mesophase formation on the degree and nature of intermolecular hydrogen bonding has been studied by choosing two different carbohydrate cores and by attaching aliphatic terminal chains via an amide linkage in two different positions. Comparison is made with related compounds reported in the literature. Whereas the N -acyl- beta -D-mannopyranosylamines and N -acyl- beta-D-glucopyranosylamines possess wide-range smectic A* phases, the corresponding 2-alkylamido-2-deoxy- beta-Dglucopyranoses do not exhibit observable mesomorphism. Although some homologues of the compounds synthesized have already been reported in the literature, their liquid crystalline behaviour was not reported. These investigations confirm the general view that it is the number and type of hydrogen bonding groups on the carbohydrate moiety and the aliphatic chain length that primarily determine thermotropic mesophase behaviour, as well as the absolute values of the transition temperatures.

ISSN:0267-8292    

DOI:10.1080/026782997208037

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor