摘要:

An analogy between liquid crystals and Langmuir-Blodgett films is briefly dlscussed. The application of the Stark-spectroscopy technique to investigation of various phenomena in Langmuir-Blodgett films is demonstrated. It is very useful for determination of the orientational distribution function of molecular dipoles including the case of superlattices, for study of the photo-induced charge transfer and the dynamic of the photo-induced dipole reorientation, for measuring both the local fields from external sources and from dipolar layers. A possibility for application of the same technique to liquid crystalline materials is outlined.

ISSN:1058-725X    

DOI:10.1080/10587259208037243

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A high-resolution differential ac free-standing film calorimeter has been constructed in our laboratories. We have employed this unique calorimeter to investigate the evolution of the temperature dependence of heat-capacity anomalies through the smectic-A-hexatic-B-crystal-E transition sequence of one liquid crystal compound as the film thickness is reduced from a few hundred to just three molecular layers. The existence of free surfaces in the free-standing liquid-crystal films has a very strong effect on the development of the molecular order through the smectic-A-hexatic-B transition and other unexpected results related to the crystal-E order.

ISSN:1058-725X    

DOI:10.1080/10587259208037244

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Through the example of nematic liquid crystals, which are the kind of crystalline phases in which surface effects have most extensively been studied, the principal aspects of the behaviour of liquid crystals at interfaces are presented. They can be separated in two categories. First, the presence of another phase in contact with the liquid crystal induces a perturbation of the liquid crystalline order. Second, the presence of a surface limiting the liquid crystalline phase has a macroscopic effect on the bulk of the phase: in absence of any other orienting field, the surface can impose the orientation of the whole phase. This orienting effect of surfaces on liquid crystals is the so-called anchoring phenomenon. These two aspects of surface effects are linked to each other as the induced anchoring direction depends on the structure of the perturbed region.

ISSN:1058-725X    

DOI:10.1080/10587259208037245

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

As first discussed by Pincus and Blinc, order fluctuations of the director in a nematic liquid crystal should lead to a characteristic square-root dependence of the nuclear magnetic relaxation times T1of the considered atoms on the Larmor frequencyv, i.e. T1-v1/2. The significance of this process is violently disputed in the literature, essentially because standard NMR spectrometers do not allow a broad frequency variation sufficient to separate different dispersion laws reliably. Using a newly developed field-cycling NMR spectrometer with a fast switchable 1.2 T detection field and fast Fourier transform data processing capabilities, it became possible to extend previous proton relaxation studies to selective frequency dependent deuteron T1measurements in the range between 500 Hz and 7 MHz for various deuterated nematogens (PAA-d6, PAA-d8, PAA-d14, MBBA-d6). Compared with previous proton results in the same frequency range, the relaxation dispersion of the deuterons at identical sites is relatively small in all studied samples, and unlike the proton T1the deuteron T1does not show a fully developed square-root regime. However, the process is clearly detectable at medium and low Larmor frequencies between about 5 kHz and 500 kHz, where the Pincus-Blinc model dominates the proton spin relaxation. Thus the first deuteron field-cycling measurements support the previous proton results that nematic order fluctuations are not visible by T1at standard high Larmor frequencies in the megahertz range.

ISSN:1058-725X    

DOI:10.1080/10587259208037246

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Different aspects of molecular dynamics study by optical methods have been discussed. The peculiarities of relaxation processes and problems of their investigation have been analysed.

ISSN:1058-725X    

DOI:10.1080/10587259208037247

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A general discussion is given of the application of density functional theory of liquids to liquid crystal problems, with particular emphasis to the onset of smectic phases. Hard particle systems have long been used as useful paradigms of liquid crystal behaviour. In this paper we also present recent work on the phase diagram of hard spherocylinders, using a non-local density functional theory of a kind which has been successful in the study of hard sphere thermodynamics. We also discuss the effect of electric quadrupolar forces on the phase diagram of Onsager-like systems.

ISSN:1058-725X    

DOI:10.1080/10587259208037248

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In this paper, we would like to discuss the problem of the modelisation of the structure and dynamical behaviour of liquids, more generally of dense fluid phases composed of and/or containing in solution, non-rigid molecules. More specifically, we shall focus on the problem of the interpretation of NMR data in anisotropic media, which is of direct relevance in this field of research, keeping in mind that the concepts and ideas presented here may be used for the interpretation of other kinds of (spectroscopic) data. In such a broad and difficult subject, there are many researchers and different schools of thought. We shall try here to present two different models that are used in this field, and which are based on two rather opposite pictures of a liquid, the solid-like picture and the gas-like picture. Emphasis will be particularly put on the former one, giving reasons why we believe it is (much) better than the other. This may upset several people who are supporters of the latter. For this we apologize, but we prefer the reader to draw his own conclusions.

ISSN:1058-725X    

DOI:10.1080/10587259208037249

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The azimuthal anchoring energy at the interface between the nematic liquid crystal pentylcyanobiphenyl (5CB) and an obliquely evaporated SiOxsubstrate is measured by using a reflectometric method. The time response of the director at the surface is investigated and two very different regimes are evidenced. The azimuthal anchoring energy coefficient exhibits an almost-critical behaviour near the clearing temperature and is almost independent on the thickness d of the SiOxlayer but it exhibits a sharp decrease for d ≤ dc≈ 25 Å. The anchoring function is found to depend on the azimuthal director angle ϕ as W(ϕ) = Wa/2 sin2(ϕ).

ISSN:1058-725X    

DOI:10.1080/10587259208037250

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In undercooled polymeric mesophases, being annealed at a temperature higher thanTg, nucleation and crystal growth processes take place. We investigate the ageing of comb-like polyacrilates both by calorimetry and non-linear ESR After annealing, the local transport properties of the polymers with quenched disorder exhibit relevant changes which are interpreted in terms of a decreased cooperativity of the amorphous part.

ISSN:1058-725X    

DOI:10.1080/10587259208037251

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

General properties of SO(3) - symmetric free- energy expansion for biaxial systems are studied. In particular, all invariants in powers of a traceless and symmetric quadrupole tensor order parameter and a vector order parameter are identified and their relation to possible local structures are found. A new class of polar, chiral biaxial phases are predicted.

ISSN:1058-725X    

DOI:10.1080/10587259208037252

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor