摘要:

Dielectric relaxation measurements in the frequency range from 1 Hz to 100 kHz on mixtures of a substituted polyacrylate and 4-n-hexyloxycyanobiphenyl have been carried out. In the mixtures the dielectric absorption properties which can be related to the individual compounds has been detected. The relaxation frequency of the low molecular compound is about 50 to 100 times higher in comparison with the δ-relaxation of the polymer but it exhibits nearly the same high activation energy. From the experimental data it can be concluded that there is a molecular disperse distribution of both compounds in which the dynamics is mainly determined by the excess component, the polymer.

ISSN:1044-1859    

DOI:10.1080/00268949008042704

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The influence of low molarity (10-50 mM) CaCl2solutions on the phases and transitions of dipalmitoylphosphatidylcholine was determined using calorimetry and x-ray diffraction. In all cases, a subgel phase was induced before characterization was initiated. The presence of 20-50 mM CaCl2was shown to have little influence on the pre- and main-phase transitions in DPPC bilayers. However, as the CaCl2solution concentration is increased, the enthalpy of the subgel to gel bilayer phase transition decreases until no calorimetric endotherm was observed for DPPC in 50 mM CaCl2. An observed broadening of the pre-transition for DPPC in 10 mM CaCl2can be correlated with a second order phase transition mechanism involving a continuous change in acyl chain packing with no change in mesophase spacing.

ISSN:1044-1859    

DOI:10.1080/00268949008042705

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We show the results on the planar alignment of chiral smectic liquid crystals made by mean of three different mechanical treatments. The orientational processes of these technics are explained on the basis of the orientation induced by a planar shearing of the samples. In addition we report on interesting realignment effects induced dynamically by the changing of the frequency of the sample thickness vibration.

ISSN:1044-1859    

DOI:10.1080/00268949008042706

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Triphenylene hexa-n-alkanoates (CmHAT) present thermotropic structural phase transitions in the solid state, before giving rise to columnar mesophases and to the isotropic state. A temperature-dependence study of the infrared and Raman spectra of C8HAT, C10HAT and C12HAT provides a description of the molecular conformations in the ordered phases and shows that the disordering process begins in the crystalline state. It implies angular deformations about the aliphatic carbon skeleton in the disorderd solid phases, while a very important conformational disorder appears at the crystal-mesophase transition, together with a change in the position of the ester plane with respect to the aromatic core. Dynamics of the alkanoates chains thus play an important part in the mechanism of the phase transitions.

ISSN:1044-1859    

DOI:10.1080/00268949008042707

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The elaboration of the Ericksen's result for the inequalities in nematics has confirmed the proportionality between the second-order splay-bend elastic constant K13and the difference between the elastic constants of bend K'33and splay K'11as well as the sign of K13in accordance with the relation 2K13= K'33- K'11obtained previously (Mol. Cryst. Liq. Cryst.,168, 7, 1989). On the basis of detailed theoretical calculations for the one-dimensional case it is demonstrated that this relation follows from the require- ment for a positive value of the elastic energy expressed with extremals and gradients of extremals when all possible symmetric and antisymmetric solutions are included.

ISSN:1044-1859    

DOI:10.1080/00268949008042708

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We present a combination of results from two seemingly unrelated studies undertaken on comb-like liquid crystal polymers in a low molecular weight (l.m.w.) mesomorphic solvent: rotational viscosity measurements and conformational study by X-ray scattering. The results deduced from the two experiments turn out to be intimately correlated. Moreover they provide consistent and complementary information. The explanation of viscoelastic results done in the framework of a hydrodynamic model due to F. Brochard admits the hypothesis of anisotropic macromolecules in ordered medium. The X-ray study provides us with the direction and the magnitude of the anisotropy

ISSN:1044-1859    

DOI:10.1080/00268949008042709

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Dipalmitoylphosphatidylcholine has been previously shown to have a biphasic response to the presence of ethanol-water solutions as a function of ethanol concentration (E.S. Rowe,Biochim. Biophys. Acta22(1985) 3299-3305). Subsequently, Simon and Mclntosh (Biochim. Biophys. Acta773(1984), 169-172) showed that DPPC bilayers formed an interdigitated multilamellar phase when the ethanol concentration was greater than 50 mg ethanol/ml water. We have used scanning calorimetry and real time x-ray diffraction to examine DPPC bilayers hydrated with 10 and 150 mg ethanol per ml water. When the lower concentration ethanol solution was present, DPPC bilayers formed subgel, gel, rippled, and liquid crystalline bilayer phases as the sample temperature was increased continuously. However, when the higher ethanol solution was used, DPPC bilayers formed subgel, gel (interdigitated), and liquid crystalline bilayer phases when driven by a temperature scan. In both cases, the subgel phase was not directly reversible upon cooling. Arguments are presented for the interpretation that both sub-gel phases consist of bilayers with interdigitated acyl chains. The transitions between the subgel and gel phases in both sample systems, and the pre-transition for DPPC in 150 mg ethanol/ml of water was shown to be second order while others proceeded via first order or two state mechanisms.

ISSN:1044-1859    

DOI:10.1080/00268949008042710

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Large-area displays and light valves can be made by dispersing nematic liquid-crystal droplets in a polymer matrix. To switch between scattering and transparent states of the polymer film, an electric field is used to alter the refractive index of the droplets by changing the orientation of their liquid- crystal directors. The film morphology—by which we mean the size, shape, and spatial distribution of droplets in the polymer matrix—is thought to have an important influence on its electro-optical properties, but definitive relationships have proved difficult to establish. This paper discusses the application of scanning electron microscopy (SEM) to characterize the film morphology. We have developed a means of sample preparation which allows us to obtain representative cross sections of the liquid-crystal droplets dispersed in the matrix. We then transform the statistical information obtained through digitization of the images, so that droplets are counted on a volume, rather than an area, basis. While a quantitative theory has not yet been developed, the histogram of droplet sizes should correlate with the scattering cross section of the film per unit volume, an important electro-optical parameter for many applications.

ISSN:1044-1859    

DOI:10.1080/00268949008042711

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Summary: The thermal properties of 4-Z-phenyl 4-(4-octyloxybenzoyloxy)benzoates (1), 4-octyloxy- phenyl 4-(4-Z-benzoyloxy) benzoates (2), 4-(4-octyloxybenzoyloxy) phenyl 4-Z-benzoates (3) and 4- octyloxyphenyl 4-(4-Z-phenoxycarbonyl)benzoates (4) have been examined, where the substituents (Z) are halogens, alkoxy, alkyl, formyl, acetyl, cyano, and nitro groups, etc. The smectic properties are strongly dependent on the orientation of the ester linkages. Compounds 1 and 4 having alkoxy and alkyl groups tend to show smectic A and C phases. Compounds 2 and 3 are intrinsically poor in smectic properties, and only a smectic C phase is formed by the long-alkoxy and alkyl derivatives. Throughout the compounds, the thermal stability of the smectic A phase is strongly dependent on the electrostatic nature of the substituents; that is, both electron-donating substituents such as the methoxy and methyl groups and electron-withdrawing ones such as the cyano and nitro groups, steeply lower the smectic A thermal stability, and the halogens, acetyl and formyl groups enhance it. The substituent effect on the smectic A thermal stability is correlated with the charges around the functional groups calculated by the MNDO method. The ratios of the smectic A-nematic transition temperature to the nematic-isotropic one show an interesting correlation with Hammett's constants, σp.

ISSN:1044-1859    

DOI:10.1080/00268949008042712

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The photoinduced phase transition behavior of liquid crystals (LCs) was investigated. The rate of the phase transition increased with increasing temperature and decreasing cell gap. The amount of isomerization of photoresponsive molecules to induce the phase transition was found to depend on the LC forming force and affinity of the photoresponsive molecule with the host mesogen. A larger amount of isomerization was required to induce the phase transition of LC matrix with higherTNIor ΔS. Mesogenic azobenzene derivatives induced the phase transition effectively. These results indicate that the interaction between photoresponsive molecules and matrix mesogens is an important factor affecting the isothermal phase transition behavior.

ISSN:1044-1859    

DOI:10.1080/00268949008042713

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor