摘要:

Several series of unsymmetrical hexa-alkoxytriphenylenes have been prepared. For almost every series a maximum in the Colh-I clearing point is found for the most symmetrical triphenylene, i.e. when all six alkoxy chains are of equal length. A similar trend is found for the melting points (Tm), although the effect is not so pronounced. A minimum length is required for formation of a columnar mesophase (Col hd). After this critical value has been reached for short alkoxy chains, much lower Colh-I temperatures are then observed for longer chains. Ordered hexagonal columnar phases required for efficient photoluminescence and electroluminescence have been found for several asymmetrical homologues with comparable TCol-I to those observed for the symmetrical homologues, but also with lower Tm. This may facilitate the preparation of mixtures of triphenylenes with the desired ordered hexagonal columnar phases with a TCol-I and a Tm below room temperature.

ISSN:0267-8292    

DOI:10.1080/026782998206443

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Three series of novel thermotropic metallomesogens have been synthesized and characterized using microscopy and DSC. Unexpected X-ray scattering is observed within all of the observed mesophases. Diffuse in-layer reflections are observed both at angles corresponding to approximately twice the molecular width and to the side-by-side separation of molecules within the nematic, SmA and SmC mesophases. Furthermore, the tilt angles within the SmC phases are observed to decrease as the terminal chain lengths decrease. Conoscopic observations show the SmC phases to be strongly biaxial as expected, but surprisingly, weak biaxiality is also observed within both the SmA and nematic mesophases. A model to explain the results is proposed, in which the cores remain orthogonal to the layers, whilst the overall molecular tilt necessary for SmC phase formation is provided by the contribution of the terminal alkyl chains to the overall molecular shape. IR spectroscopy is used to confirm the model.

ISSN:0267-8292    

DOI:10.1080/026782998206452

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The influence of temperature and impurities on the surface orientation of nematic 5CB on obliquely SiO evaporated glass plates has been studied. We observe on cooling a continuous anchoring transition from the planar alignment to the tilted one. The experimental data have been interpreted by using elastic models developed by Durand and coworkers in which the surface order parameter can vary in space, close to the surface.

ISSN:0267-8292    

DOI:10.1080/026782998206461

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An almost complete homologous series of the acyclic, open-chain 6- O - n -alkyl-D-galactitols has been prepared. Most homologues in this series exhibit an enantiotropic smectic A* phase. The liquid crystal transition temperatures of these open-chain carbohydrates are compared with those of the corresponding homologues in the cyclic form, i.e., with those of the 6- O - n alkyl-alpha-D-galactopyranoses. In general the carbohydrates in the open-chain form exhibit higher clearing points than those of the analogous cyclic pyranoses. However, the melting point of the galactitols is generally significantly lower than that of the corresponding galactopyranoses. This leads to a much wider temperature range of the smectic A* phase for the carbohydrates in the pyranose form. The lowest clearing point was found for related galactoses in the furanose form. Alternation was found for the clearing point of the 6-O-n alkyl-alpha-D-galactopyranoses and the 6- O - n -alkyl-D-galactitols. Powder X-ray diffraction studies indicate a non-intercalated structure for the crystalline state and an interdigitated arrangement in the smectic A* phase with a degree of chain melting for both series.

ISSN:0267-8292    

DOI:10.1080/026782998206470

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The ferric chloride mediated oxidative coupling of 3,3,4,4-tetrahexyloxybiphenyl with 2-hexyloxyanisole followed by demethylation gave 3,6,7,10,11-pentahexyloxy-2-hydroxyor triphenylene. Reaction with tert -butyldimethylsilyl-protected 6-bromohexanol, removal of the protecting group, and condensation with acryloyl chloride methacryloyl chloride gave acrylate and methacrylate monomers with hexamethylene 'spacers'. Alternatively, reaction of 3,6,7,10,11-pentahexyloxy-2-hydroxytriphenylene with 2-(2-chloroethoxy)ethanol followed by acryloyl chloride or methacryloyl chloride gave acrylate and methacrylate monomers with diethylenoxy 'spacers'. Both of the poly(acrylate) homopolymers and the poly(methacrylate) homopolymer with the diethylenoxy 'spacer' gave columnar liquid crystal phases. This is contrary to the current perception that the methacrylate polymer backbone is too inflexible to be incorporated in columnar phases.

ISSN:0267-8292    

DOI:10.1080/026782998206489

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The influence of spontaneous polarization on the smectic C*-smectic A* phase transition was studied theoretically by means of Landau theory. calculations that the transition temperature from a non-chiral to chiral smectic C phase is shifted due to the effect of bilinear and biquadratic couplings. A qualitative relationship has also been derived between spontaneous polarization and cell thickness which supports the experimental observations of the increase of spontaneous polarization with the increase of cell thickness for surface-stabilized ferroelectric liquid crystals. It was observed from the theoretical

ISSN:0267-8292    

DOI:10.1080/026782998206498

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, exhibit a rich smectic polymorphism for both symmetric and non-symmetric dimers which differ in the nature of the mesogenic groups. For example, smectic phases having monolayer, interdigitated and intercalated structures have been discovered. We have extended our studies of such systems to binary mixtures in an attempt to understand the origin of the different phase structures at the molecular level. The dimers studied include non-symmetric systems differing in the parity of the spacer and in the length of the terminal chains; for comparison we have also studied a mixture of symmetric dimers differing solely in the parity of the spacer. We have constructed the phase diagrams for the various mixtures and found that for certain systems the smectic phases exhibited by either one or both components can be destroyed. To investigate the local structure of the nematic phase for mixtures in which a smectic A phase is eliminated from the phase diagram we have determined their orientational order using NMR and ESR spectroscopy. To provide more direct information on the local structure an X-ray diffraction study was undertaken on certain of the mixtures.

ISSN:0267-8292    

DOI:10.1080/026782998206506

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Transition temperatures of binary mixtures of the two series, 4-hexadecyloxyphenyl-4- substituted benzoates (Ia-d) and 4-substituted phenyl-4-hexadecyloxybenzoates (IIa-d), have been determined on cooling and the phase diagrams compared with those previously obtained on heating. Tc values for the monotropes and the non-LC analogues have been evaluated by extrapolation. Examination of both series revealed that the square root of Tc is related to the polarizability anisotropy, Delta alphaX, of the CAr-X bond, except for the methoxy derivatives.

ISSN:0267-8292    

DOI:10.1080/026782998206515

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The influence of the molecular parameters of side-on fixed liquid crystal polymers (LCP), namely the spacer length, the aliphatic tail length and the proportion of mesogenic groups ('dilution' effect), on the mesomorphic, thermodynamic and structural properties of the nematic and smectic C phases has been studied. This shows in particular the stabilization of the SmC phase on increasing the terminal chain lengths. Moreover, this paper provides evidence for an original behaviour in the SmC organization of side-on LCP with a nonstructural monotonous evolution of the thermodynamic and properties with the 'dilution'. Finally, a comparison with side-end LCP can be established : the question of the participation of the polymer backbone in the smectic organization in side-on LCP is raised.

ISSN:0267-8292    

DOI:10.1080/026782998206524

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The influence of phototransformed molecules with chiralproperties changing on the absorption of light field on the light-induced Freedericksz transition threshold in a homeotropically oriented nematic cell is considered. It is shown that the appearance of the light-induced chiral molecules can decrease or increase the Freedericksz threshold value depending on the chirality sign of the phototransformed molecules and of the initial chiral dopant. Expressions for the threshold are obtained for circular and linear polarization of the incident light. The dependence of the threshold on the periodicity of the spatially modulated light intensity is estimated for large periods of modulation. The dependence of dopant threshold chirality on the director anchoring energy has been found.

ISSN:0267-8292    

DOI:10.1080/026782998206533

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor