摘要:

The liquid crystalline properties of a concentrated solution of poly{5,7-dodecadiyne-1,12-diol bis[(4-butoxycarbonyl)methyl)urethane]} (P-4-BCMU) -a soluble polydiacetylene- in chloroform, beyond the critical concentration, have been investigated. Optical microscopy shows the anisotropic liquid crystal solution of P-4-BCMU in chloroform to be a polymer nematic. Disclinations with strengths ofs= +½ and −½ are the most frequent type. Some disclinations withs= +1 were observed, but rarely disclinations withs= −1. Therefore, the liquid crystal texture usually displays a schlieren with two brushes in the vicinity of the singularities. The supramolecular structure observed by transmission electron microscopy shows a lamellar type morphology. As previously shown, the thickness of the lamellae is correlated to the molecular chain length. The chain length distribution leads to segregation with respect to molecular length and can be observed by thickness variations of both individual and subsequent lamellae. A kinetic study of disclination annihilation induced by isothermal relaxation in the liquid crystalline phase is presented. Possible mechanisms of the relation of the lamellar texture and the disclination annihilation are discussed. A stripe texture observed over large regions of the solid film is suggested to be a decoration effect due to internal stress as a result of solvent evaporation when the films are prepared by solution casting.

ISSN:0267-8292    

DOI:10.1080/02678299308027836

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We have computed the optical diffraction pattern for linearly polarized light incident normal to the twist axis (phase grating mode) of a magnetically doped cholesteric (ferrocholesteric). The intrinsic Faraday rotation of the magnetic grains results in extra orders of diffraction. Also we find diffraction for any azimuth of the incident vibration. Further, using the JonesNmatrices we have worked out the optical properties for light propagation along the twist axis on the very low wavelength side of the reflection band. We find that the medium behaves very differently from a normal cholesteric.

ISSN:0267-8292    

DOI:10.1080/02678299308027837

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Small-molecule liquid crystals show textures which are readily studied at low magnification in the optical polarizing microscope. In polymeric liquid crystals, however, the textures are often much finer, taxing the microscope's resolution. Nevertheless, studies of microstructure in such polymers have been made and it is apparent that they can differ widely both from small-molecule liquid crystals and, indeed, from polymer to polymer. This paper sets out to account for these variations by exploring the effect on microstructure of the marked differences between the magnitudes of the splay, twist and bend elastic constants which are a characteristic of many liquid crystalline polymers. We report a computer model which simulates the development of microstructure for different ratios of the elastic constants. When these are approximately equal, textures characteristic of small-molecule liquid crystals result, such as those involving escape into the third dimension with the degeneration of line defects into points. When the splay energy is high in comparison with bend and twist, as is the case for many thermotropic liquid crystalline copolyesters, escape does not occur and half integral disclination lines predominate. For simulations involving planar boundary conditions, layered microstructures result, frequently with little matching of the orientation from layer to layer. Within the layers the trajectory of the orienting units is sinuous. This simulated microstructure resembles textures observed in thermotropic copolyesters, studied both in this laboratory and elsewhere. The computer model uses a lattice approach which is similar in some respects to that developed by Lebwohl and Lasher. It should not be thought of as a molecular scale model, however, but rather as one based on assemblies of molecules which share a common director.

ISSN:0267-8292    

DOI:10.1080/02678299308027838

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Monogalactosyldiacylglycerol (MGDG) has been isolated fromVicia fabaleaves and distearoyl derivatives have been prepared by catalytic hydrogenation of the unsaturated galactolipid. The structure and the phase behaviour of 1,2-distearoyl MGDG in an excess of water were studied using a time-resolved X-ray diffraction method. The crystalline to liquid crystalline (Lc1→α) transition observed calorimetrically at 86° C was found to correspond to a lamellar crystalline to non-lamellar inverted hexagonal (H2) phase transformation. This transition appeared to be a two state process with no detectable intermediate states. In contrast cooling H2gives first Lα and then Lβ before reverting to Lc1. The transitions from the metastable lamellar gel (Lβ) phase to the lamellar liquid crystalline (Lα) state and from Lα to H2were reversible on cooling and reheating with a hysteresis of 4–5°C. A scheme consistent with the structural and calorimetric data obtained from MGDG is proposed. The similarities in the thermal and structural properties of MGDG and other non-bilayer forming lipids are discussed in terms of related functional roles of these lipid classes in biomembranes, in which they are found abundantly as natural constituents.

ISSN:0267-8292    

DOI:10.1080/02678299308027839

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Here, we carry out a theoretical analysis of thezscan experiments used to measure the non-linear optical properties of liquid crystals, with the aim of clarifying the mechanism responsible for the observed non-linearity in the nanosecond regime. We consider various orders of non-linear absorption and non-linear refraction in order to illustrate their effects on the resultingzscan curve. The shape of thezscan curve, in principle, contains information about the order of the non-linearity. Qualitative features of the experimentalzscan curves are consistent with a fifth order absorption and a third order refraction. However, given the level of disagreement between the calculated and experimental curves, it is not possible to make quantitative statements about the coeffients of the various orders. The magnitude of the refractive part of the non-linearity is consistent with laser heating due to linear absorption. The temporal profile of a two pulse experiment is also consistent with this mechanism. We suggest that the temperature dependence of the non-linear refraction might be explained in terms of a thermal mechanism by accounting for the temperature dependence of the rate of relaxation of the orientational order.

ISSN:0267-8292    

DOI:10.1080/02678299308027840

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A series of compounds containing a four-unit link, 4′-n-alkylphenyl-4-n-hexyloxycinnamates, has been synthesized, and their mesomorphic behaviour has been determined. The orientational ordering of two compounds in this series has been studied in detail by 2D carbon-13 nuclear magnetic resonance with variable angle sample spinning. The carbon-proton dipolar coupling constants were obtained and the order parameters were calculated for different molecular segments (two phenyl rings, the central link containing an ethylenic motif, and the methylene carbons of the alkoxy chain). The temperature dependence of the local order parameters of each fragment were determined using these data and C-13 chemical shifts which were measured as a function of temperature. In these non-rigid molecules, the ratio between the order parameters of the two phenyl rings was found to be temperature dependent. The ring bearing the double bond is more ordered at low temperatures compared to the ring bearing the short alkyl chain, but this behaviour is reversed near the clearing point. The position of the major axis of the rigid core was calculated from the order parameter of one ring and the order parameter of the ethylenic link. It was found that this major axis forms a constant angle with respect to theparaaxis of the phenyl ring connected to the ethylenic group. On the other hand, the angle of this major axis with respect to theparaaxis of the phenyl ring connected to the carboxylic group is temperature dependent, varying from 11° at T/TNI= 0·92 to 7° at T/TNI= 0·98.

ISSN:0267-8292    

DOI:10.1080/02678299308027841

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The dielectric response of the ferroelectric chiral smectic C phase in a bias electric field is studied theoretically within the Landau model using the constant amplitude approximation. It is argued that the response consists of two modes, one of which is related to the unwinding of the helix (the unwinding mode), whereas the other mode is associated with the distortion of the polarization distribution at constant pitch of the helix (the Goldstone mode). The relaxation frequency of the unwinding mode (fu), which is inversely proportional to the square of the sample dimension along the helical axis, is estimated to be of the order of 10−4Hz for a sample of lengthl≈ 1 mm along the helical axis. Consequently, in dielectric experiments performed usually at frequencies higher thanfu, only the Goldstone mode contribution is detected. In this way, we can explain the behaviour of the measured static dielectric susceptibility which goes to zero as the critical field is approached from below, although the model predicts that the total static dielectric susceptibility, being a sum of the Goldstone mode and the unwinding mode contribution, diverges at the critical field.

ISSN:0267-8292    

DOI:10.1080/02678299308027842

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Photosensitive cholesteric polysiloxanes, which contain an azo dye, were irradiated with linearly polarized light. The cholesteric samples were oriented in the Grandjean texture. Before irradiation they reflected circularly polarized light in the near infrared region. For perpendicular incidence, only one order of reflection was observed. Upon irradiation with linearly polarized light, which is absorbed by the azo dye, additional reflection bands appeared in the visible part of the spectrum. It turned out that the additional reflection is caused by a new Bragg type grating which shows higher reflection orders. The formation of the grating is based on the periodic deformation of the helical ordering of the molecules. The deformation is periodic, as due to photoselection, only dye molecules in equidistant layers with a suitable orientation absorb radiation. For low exposure, the grating reflects linearly polarized light. After continued irradiation, the reflection bands disappear almost completely. High birefringence, strongly dichroic dye absorption and the loss of the reflecting properties prove that a planar nematic texture has developed. The formation of this texture from the Grandjean texture is a new example for photoinduced rotational diffusion.

ISSN:0267-8292    

DOI:10.1080/02678299308027843

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:10.1080/02678299308027835

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor