摘要:

The synthesis of a series of bidentate Schiff 's bases; 3,5-bis(3,4,5-trialkoxybenzyloxy)phenyl2- N -(3-hydroxypropy)iminopropyl ketone, and their dicopper complexes are reported. The liquid crystalline behaviour of these dicopper complexes was studied by DSC, polarized optical microscopy and X-ray diffraction. The phases were characterized and identified as hexagonal columnar (D) phases, which are typical of disc-like molecules. h

ISSN:0267-8292    

DOI:10.1080/026782997208235

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The polymorphism of hexa- n -octanoate of rufigallol (RHO) was used to investigate the crystallization kinetics when starting from two different discotic mesophases, namely, a hightemperature enantiotropic mesophase and a low-temperature monotropic mesophase. In all cases, an Avrami exponent of n 3 was obtained, suggesting two-dimensional sporadic nucleation and growth. Although the same overall mechanism was predicted to operate for all crystallization temperatures investigated, the Avrami constant b has a temperaturedependence which is different for different starting columnar phases, indicating a strong influence on the rates of nucleation and of growth of the different molecular arrangements of the starting mesophases. Differential scanning calorimetry was used for the studies, supplemented by direct observations through optical polarizing microscopy.

ISSN:0267-8292    

DOI:10.1080/026782997208244

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Our earlier model of liquid crystals consisting of highly polar compounds in which the molecules can form either parallel or antiparallel pairs, has been extended. Depending on the model parameters the following results are obtained: (i) first order SmA-SmA which changes over to a continuous SmA-SmA regimes of the McMillan alpha parameter, (ii) a re-entrant nematic lake associated with the SmA-SmA boundary, and (iii) the merger of the re-entrant nematic lake with the nematic 1 d sea. The results are discussed in comparison with experiments and other theoretical models. transition 1 d evolution beyond a critical point in two 1 d

ISSN:0267-8292    

DOI:10.1080/026782997208253

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Heat capacities of the antiferroelectric liquid crystals 4-(1-methylheptyloxycarbonyl)phenyl4-octyloxybiphenyl-4-carboxylate (MHPOBC) and 4-(1-methylheptyloxycarbonyl)phenyl4-octylcarboxybiphenyl-4-carboxylate (MHPOCBC), have been measured with an adiabatic calorimeter between 350 and 460 K. MHPOBC showed three smectic subphases (ferrielectric C*, ferroelectric C* and a fancy phase C*) between antiferroelectric smectic C* and paraelectric gamma alpha smectic A, while MHPOCBC exhibited only one subphase (smectic C*). These phases are clearly discriminated by the existence of phase transitions. The enthalpies and entropies gained at the respective phase transitions were very small. A much larger phase transition from smectic A to isotropic liquid was also observed in both compounds. A alpha

ISSN:0267-8292    

DOI:10.1080/026782997208262

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The synthesis and properties of a new series of compounds having aromatic amide mesogenic cores are reported. Most of these new compounds are thermotropic mesogens. In consideration of the fact that aromatic amides form crystals of high melting point which is unfavourable for the formation of thermotropic liquid crystals, we make use of lateral substitution to decrease both the packing efficiency and the hydrogen bonding, so that the melting temperature of the aromatic amides is sufficiently depressed. The lateral substituent used in these new compounds is bromine. In order to investigate the influence on properties of the end groups, different alkoxy, alkyl and other groups are used at the two ends of the rod-like molecules. The two ends are either identical or different, with an electron-donating alkoxy as one end and the electron-accepting cyano group as the other. The results indicate that appropriate lateral and terminal substitution is essential for the aromatic amides to form thermotropic liquid crystals. The peculiar mesophase characterized by an X-ray diffraction pattern of a SmC phase, but a texture of a nematic phase is also noted.

ISSN:0267-8292    

DOI:10.1080/026782997208280

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A new series of 2-(4- n -alkoxyphenyl)-5-(4-methylphenyl)pyridines (CH C H C H NC HOC H, n 1-10) (APMPP), which are teraryl compounds containing a pyridine ring in n 2 m 1 the centre position of the rigid core, was synthesized and the phase transitions of the homologues were studied using DSC measurements, polarizing optical microscopy and miscibility tests with terephthalylidene-bis-4- n -pentylaniline (TBPA). Only a nematic phase was found for the shorter alkoxy homologues with n 4. The longer alkoxy homologues with n 4 showed the sequence of enantiotropic phase transitions CrG-SmF-SmC-SmA-N-I, while a monotropic CrH phase was observed for the n 5-10 homologues. Interestingly, the polymesomorphisms appear when n is larger than 4. 3 6 4 5 3 6 4

ISSN:0267-8292    

DOI:10.1080/026782997208299

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We present an experimental technique for the inflation of freely suspended films of thermotropic smectic liquid crystals to macroscopic bubbles. These bubbles have many properties in common with well known lyotropic soap bubbles, but on the other hand represent a completely different class of material with different physical properties and new phenomena. We describe the generation of such smectic bubbles and compare their physical properties with those of lyotropic soap bubbles. The measurement of inner pressure versus curvature is used for the determination of surface tensions of the smectic materials.

ISSN:0267-8292    

DOI:10.1080/026782997208307

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The phenomenon of anchoring in the nematic liquid crystal-amorphous substrate system is considered and model independent definitions of the surface nematic director, the surface tension and the anchoring energy coefficient are proposed. Then the Landau-de Gennes model of the system is studied for a specific choice of the surface parameters, which leads to a continuous homeotropic-conical anchoring transition. The free energy as a function of the director at a distance l from the surface is found. It is shown that its form is different in two regions of the temperature-distance plane separated by the line of a Freedericksz transition. The asymptotic behaviour of the free energy for large l and for infinitesimal deviations of the director from the anchoring direction is studied. It is found that the asymptotic formula holds also in the vicinity of the anchoring transition. Finally, the results of numerical studies of the Landau-de Gennes model are compared with the predictions of a simple phenomenological model.

ISSN:0267-8292    

DOI:10.1080/026782997208316

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A series of fluoro-substituted tolane derivatives: (R)-1-methylheptyl 3-fluoro-4-(3-fluoro-4- n alkoxybenzoyloxy)tolane-4-carboxylates is reported. Some members of this series exhibit the phase sequence: Cr-SmC*-TGBC-TGBA-BPI-BPII-BPIII-I. The blue phases, the TGBA and TGBC phases and the SmC* phase were characterized in detail by microscopic observation, differential scanning calorimetry, helical pitch measurements, X-ray structural analysis and electro-optical study. The blue phases directly next to the TGBA phase were shown to be a new type of blue phase exhibiting smectic ordering. A commensurate TGBC phase with constant number of slabs per pitch was observed.

ISSN:0267-8292    

DOI:10.1080/026782997208325

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We have studied the thermotropic and lyotropic phase behaviour of seven monoalkyl glycosides using polarizing microscopy, differential scanning calorimetry and X-ray diffraction. The phase behaviour of these non-ionic surfactants is compared with that of two previously studied monoalkyl glycosides \[1]. We have found that small modifications in chemical structure can lead to large changes in the phase behaviour. The results reveal the effect of changes in alkyl glycoside chemical structure on the phase behaviour, in particular, on the melting point, the clearing point, the solubility in water, and the extent of the lamellar and curved phases. Seven out of the nine surfactants were found to exhibit thermotropic mesomorphism. Furthermore, in the presence of water, three formed lamellar (L), type I cubic (Q) and type I hexagonal (H) phases, two formed lamellar and cubic phases, and two I I formed only lamellar phases. The cubic phase in each case was indexed, either from powderlike or monodomain samples, as spacegroup I a3 d. The lowest order low angle X-ray spacings were found to lie on single, continuous lines with varying water content across the L, Q and H phases, implying epitaxial relationships between these phases at the phase boundaries. I a a I

ISSN:0267-8292    

DOI:10.1080/026782997208334

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor