摘要:

The proton N.M.R. spectra of two partially deuteriated samples of the mesogen 4-methoxy-4′-cyanobiphenyl (1-OCB) have been obtained. The spectra were simplified by decoupling the interactions between protons and deuterons and analysed to yield sets of dipolar couplings,[Dtilde]ij. The[Dtilde]ijare used to investigate the angle between the two ring normals and the conformation of the methoxy group relative to the attached phenyl ring. If the potential governing rotation,V(), is assumed to have a deep minimum such that only structures with the minimum value of are appreciably populated, thenminis determined to be 30·4° ± 0·4°. However, adopting a continuous form forV() changesminto 36°. For the methoxy group the data are consistent with the potential governing rotation about the ring-oxygen bond being zero when the carbon atom is in the ring plane and infinite otherwise; in addition, we have investigated the consequences for the structure and ordering of adopting more realistic forms for this potential. The 24 structures of equal, minimum energy, which are generated by rotation about the inter ring bond, the phenyl-O bond and the O—CH3 bond, all have the same shape, and hence each of these conformations has the same ordering matrix, with principal elementsSzzandSxx—Syy. The magnitudes ofSxxandSxx—Syyhave been obtained, together with the orientation ofx, yandzwith respect to a frame fixed in the methoxylated ring, with the assumption that only these minimum energy structures are populated. The magnitudes of these principal order parameters are used to test the form of a potential of mean torque.

ISSN:0267-8292    

DOI:10.1080/02678299008029189

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We report the first systematic measurements of elastic coefficients and viscosities that clearly demonstrate the distinction between rigid and semiflexible behaviour of a main-chain polymer nematic liquid crystal, a solution of poly-γ-benzyl glutamate (PBG) in mixed organic solvents. Quasi-elastic Rayleigh scattering studies show a crossover which occurs at a molecular chain length near the persistence length of PBG as the chain length increases. The results are in qualitatively good agreement with recent theoretical predictions for semiflexible chains. It is seen that bending distortions of an individual polymer play an important role in the fundamental nature of nematic elasticity and viscosity.

ISSN:0267-8292    

DOI:10.1080/02678299008029190

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We show by simple model calculations that dipole-induced dipole and interchain interactions lead to aparallelalignment of molecules with strong longitudinal dipoles for intermolecular separations below a certain value. On the other hand an antiparallel alignment is favoured for larger separations because of dipole-dipole interactions. We incorporate this change in the intermolecular configuration, which naturally leads to two ‘lengths’ in the problem, in developing a simple McMillan type molecular theory of the double reentrance phenomenon in which the following phase sequence is observed on cooling the sample: isotropic-nematic-smectic Ad-reentrant nematic-smectic A1. The calculated properties including the phase diagram are in broad agreement with experimental trends.

ISSN:0267-8292    

DOI:10.1080/02678299008029191

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Novel optically active compounds derived from (R)-3-hydroxynonanoic acid as shown in a general formula (I)have been synthesized and their mesomorphic and ferroelectric properties have been investigated. The kind of mesophases and the transition temperatures depend strongly on the linkage groups and the substituents in the core. The compounds with an unsubstituted core show non-titled smectic mesophases (smecticAorB). The chiral smecticCphase appears in the compounds having a central ester linkage and a chloro-substituent as theA1orB1position and in the compound having a central -OCH2- linkage and a chloro-substituent at theA1position. The maximum spontaneous polarization (500 μC/m2) was observed in 4-dodecyloxy-3-chlorobenzoyl-oxyphenyl (R)-3-hydroxynonanoate. The thermal stability of each mesophase and the ferroelectric properties have been discussed from the structural point of view.

ISSN:0267-8292    

DOI:10.1080/02678299008029192

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

An analysis of an experimental technique which allows for a simultaneous determination of several parameters with special interest for ferroelectric liquid crystals is presented. These parameters are the spontaneous polarization, the rotational viscosity and the switching time. In addition, the susceptibility associated with the soft mode free of contributions related to the helicoidal structure can also be obtained. The experimental results for these parameters for the ferroelectric liquid crystal HDOBACEEC are reported. A comparison between the switching time values deduced from the rotational viscosity and those obtained by optical measurements is performed.

ISSN:0267-8292    

DOI:10.1080/02678299008029193

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The structure of rigid-chain polyesters and polyamides which possess flexible side chains consisting of methylene or 1,4,7-trioxaoctyl oxide units have been investigated by means of X-ray scattering. The X-ray studies were performed on fibres and monodomains. Additional information was gained from calorimetric, dilatometric and microscopic studies. The investigations revealed that these polymers display mesophases and that the structures of the mesophases differ from those observed for discotic and calamitic phases. They are characterized by the fact that in the ordered and disordered phases board-like molecules are stacked parallel on top of each other and that these stacks of molecules are oriented parallel to each other. In the least ordered phase the molecules are only oriented parallel to each other. The various modifications of the mesophases observed so far differ with respect to each other in terms of the order within the stacks as well as in terms of the spatial arrangements of the stacks. A characteristic feature is the frequent absence of any correlation between the long range order which exists along different principal axes of the structure. We have coined the term sanidic for this new type of mesophase.

ISSN:0267-8292    

DOI:10.1080/02678299008029194

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The torque effect of low twist cholesterics on capillary axial flow between coaxial cylinders has been studied as a function of flow rate and pitch. The motion of disclinations and textures as well as the flow-induced transformation into new textures were separately studied. A model of simultaneous translational flow (of Poiseuille type) and permeation describes the experimental relations in a quantitative way. Maximum permeation of about 2 per cent of total flow was found for a pitch being near the thickness of capillary layer.

ISSN:0267-8292    

DOI:10.1080/02678299008029195

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The behaviour of an anisotropic solution of hydroxypropylcellulose around an obstacle is investigated in shear and during relaxation. Experiments were carried out with an optical rheometer equipped with transparent cone-and-plate. The obstacle is a 200 micron glass sphere stuck on the plate. A typical Reynolds number past the obstacle is about 10−5. The flow of the anisotropic solution perturbed by the obstacle shows specific phenomena: distorted downstream streamlines, a very long wake behind the obstacle during shear which persists a long time after the ceasing shear, a change in the behaviour at very high shear rates and in particular, the appearance of a wake in front of the obstacle. To date there has not been any theoretical bases with which to explain these new findings. An interesting point is that the wake behind the obstacle is a good illustration of the problem of weldlines in injection moulding.

ISSN:0267-8292    

DOI:10.1080/02678299008029196

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The non-linear part of the electromechanical response of four S*Cliquid crystals were investigated. The second harmonic frequency vibrational spectra, the electric field, and the temperature dependences are presented. The experimental results are interpreted in terms of a microscopic process.

ISSN:0267-8292    

DOI:10.1080/02678299008029197

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We present five phase diagrams of binary systems in which one component is a bi-swallow tailed compound and the second component is a compound with pronounced nematogenic properties. The common feature of these binary systems is the induction of smectic A phases which have been investigated using X-ray. The smectic A induction seems largely to be the result of specific steric interactions between the components.

ISSN:0267-8292    

DOI:10.1080/02678299008029198

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor