摘要:

Over 50 variously substituted 5-alkyl-2-phenylpyrimidines have been synthesized. The effect of the presence of an additional olefinic double bond in the terminal position of the alkoxy chain or of a trans-1,4-disubstituted cyclohexane ring on the liquid crystal transition temperatures of these systems has been studied in detail. All of the phenyl-pyrimidines studied possess an optically active centre at the point of methyl branching of one of the terminal carbon chains. The dependence of the transition temperatures of these systems on the position of the chiral centre has also been investigated. The effect of chain length has also been studied for various homologous series of mesogens. Almost all of the 5-alkyl-2-phenylpyrimides prepared exhibit enantiotropic chiral smectic C and cholesteric mesophases, some at and just above room temperature.

ISSN:0267-8292    

DOI:10.1080/02678298808086575

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A variational principle based on the curvature energy of a two dimensional liquid crystal membrane is constructed to determine the equilibrium shape of a cell with given surface area and volume. A sequence of the forms, calculated numerically, displays the process of cell division.

ISSN:0267-8292    

DOI:10.1080/02678298808086576

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Par analogie avec les lois de la mécanique classique, nous décrivons la cinétique de polymérisation de la déoxyhémoglobine S (déoxy HbS) au moyen d'une équation différentielle du deuxième ordre non linéaire qui se réduit à une équation du type Riccati par un simple changement de variable: il est ainsi mis en évidence la notion de fréquence d'agrégation. De plus, la croissance des polymères est largement affectée par un ensemble de forces de frottement variable dans le temps selon une loi de type logistique (sigmoïde). Cette étude précise le caractère viscoélastique du milieu et débouche sur une loi, également de type logistique pour l'évolution dans le temps du coefficient de viscosité dynamique. Enfin, nous avons tracé en fonction du temps les variations de la vitesse d'agrégation en estimant les valeurs à partir des données expérimentales des courbes d'absorbance obtenues dans la littérature pour des solutions diluées de déoxyhémoglobine S en tampon phosphate concentré. Notre modèle fournit des données intéressantes pour toutes les phases de polymérisation de l'HbS. En particulier, dans le domaine de nucléation correspondant à une durée inférieure ou égale àtd, il conduit à des résultats qui recoupent ceux obtenus par ailleurs à partir du mécanisme de la double nucléation.

ISSN:0267-8292    

DOI:10.1080/02678298808086577

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Recently we reported a pressure-induced re-entrant cholesteric phase for ternary systems of non-polar cholesteryln-alkanoates. In these systems one component contains a shortern-alkyl chain than the other two. We now show that the former produces a positive excess volume in the smectic phase, which is probably responsible for the pressure-induced re-entrant phase behaviour. The maximum temperature of the cholesteric/smectic A phase boundary is found to decrease drastically with decrease of then-alkyl chain length of this particular component.

ISSN:0267-8292    

DOI:10.1080/02678298808086578

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

We consider a classical system of particles, consisting of two-dimensional unit vectors associated with a one-dimensional latticeuk|k∈Zand interacting via translationally invariant pair potential(s)heremis a positive integer andTmis a Tchebyshev polynomial of the first kindwhere ϕ are the angles defining the orientations of the plane rotators in an arbitrary reference frame. For the casem= 1, Fröhlichet al.have proved rigorously the existence of a ferromagnetically ordered phase at low but finite temperature; moreover, all the potential modelsWmgive the same partition function, and several mean values can be defined in an m-independent way. For example, whenm= 2, this entails the existence of nematic-like order. The system was characterized quantitatively by Monte Carlo simulation, and calculations were performed in the nematic representation (m= 2); simulation results suggest a second-order transition atT*c= (≡kT*c/ϵ) = 2.16 ± 0.01. Comparison with molecular field and spherical model treatments is also reported: the former, but not the latter, agrees reasonably well with the simulation results.

ISSN:0267-8292    

DOI:10.1080/02678298808086579

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Light scattering experiments have been performed on cholesteryl oleyl carbonate (COC) and its mixtures with a nematogen in order to investigate pretransition phenomena accompanying the transition from the isotropic to the blue phase. Estimates of the critical temperature for the flat-spiral (m= 2) mode have been obtained in each case. The critical temperatures approach the transition temperature,Tc, in the mixtures, probably due to an impurity-induced blurring of the phase transition. Measurements of the optical rotatory power have also been carried out on pure COC as well as its mixtures with the nematogen and possible origins for the pretransition phenomena observed are discussed.

ISSN:0267-8292    

DOI:10.1080/02678298808086580

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Information on molecular interactions that give rise to the stabilization of various ferroelectric, liquid-crystalline mesophases is important to the realization of their potential utility in a wide variety of optical devices. Understanding the roles that optical activity and optical purity play in the formation and properties of the various smectic phases is therefore of particular interest. In order to study this relationship three new liquid-crystalline materials were prepared, one as a racemic mixture, the other two as the optically active analogues. The optically active isomers appear to exhibit a different mesophase morphology from the racemate. The chiral compounds apparently possess two extra ferroelectric mesophases in comparison to the racemic mixture. The transitions to and from these phases have extremely small enthalpies. An attempt is made to explain the results for the chiral compounds in terms of differing dipolar couplings in the chiral ferroelectric phases. In the racemic mixture these interactions are compromised or scrambled by a loss of asymmetry thus destabilizing these extra phases.

ISSN:0267-8292    

DOI:10.1080/02678298808086581

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Several of the polymers reported for surface alignment of ferroelectric smectic C phases in the literature are tested with a ferroelectric room temperature mixture. The results for this material and the reported findings in the literature are compared to the known crystal structure of the polymers. It is found that polymers with triclinic or monoclinic crystals give good alignment and bistability for the smectic C phase. Other crystal structures or non-crystalline polymers give a poorer performance. The mechanisms for creating a highly crystalline polymer surface are discussed, and the epitaxial growth of smectic phases on the crystalline surface is shown to be in accord with experiment.

ISSN:0267-8292    

DOI:10.1080/02678298808086582

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Six new aromatic polyesters were synthesized by the phase-transfer catalytic polycondensation. The polymers obtained were characterized by infra-red, X-ray diffraction, D.S.C., depolarized light intensity and polarizing microscope. Poly(p,p'-hydroquinone-1,4-phenylene dioxyacetate) and poly(p,p'-hydroquinone-1,4,7-trioxaheptamethylene dibenzoate) were found to be thermotropic liquid crystals but related polyesters are not. The results are discussed on the basis of the thermodynamic data obtained.

ISSN:0267-8292    

DOI:10.1080/02678298808086583

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A series of unsymmetrical dimeric mesogens was prepared, members of which were composed of a 4′-pentyloxybiphenyl-4-carboxyl core and a 4′-substituted-benzylidene-4-oxyaniline core joined via an alklyene spacer. The structural variants were cyano, nitro and trifluoromethyl. Comparison of mesophase behaviour of the unsymmetrical dimers with that of corresponding monomeric ‘halves’ and also with that of corresponding symmetrical dimers indicated a tendency for the unsymmetrical dimers to form smectic phases although not as highly ordered as those of some of the monomers corresponding to a relevant core group.

ISSN:0267-8292    

DOI:10.1080/02678298808086584

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor