摘要:

An approach to the synthesis of a new class of liquid crystalline (LC) compounds, dendrimers of regular structure with terminal mesogenic groups, was elaborated. LC dendrimers based on the carbosilane dendritic matrix of first generation were synthesized. Cyanobiphenyl, methoxyphenyl benzoate and cholesteryl groups were used as mesogenic fragments. Individuality and structure of all compounds obtained was proved by GPC together with1H- and29Si NMR methods. The mesomorphic behaviour and structure of the LC dendrimers synthesized were investigated. It is argued that different mesophases of the smectic type are realized in all cases. It is shown that the mesophase type of these compounds essentially depends on the chemical nature of the mesogenic groups.

ISSN:0267-8292    

DOI:10.1080/02678299608033789

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Recently we reported three homologous six-ring double-swallow-tailed compounds. For one homologue we observed the unusual phase sequence re-entrant smectic C-Dob-smectic C-nematic. For this reason, results for a more complete homologous series are now presented. It was found that the phase behaviour can be compared with some series of biforked compounds. The lower members of the series exhibit nematic and smectic C phases, whereas the long-chain homologues form columnar phases. For three members of the series with intermediate lengths of the terminal chains, lamellar (Sc) and columnar (Dob) phases occur for the same substance.

ISSN:0267-8292    

DOI:10.1080/02678299608033790

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

‘Reverse’ or ‘polymer ball’ polymer dispersed liquid crystal (PDLC) samples were prepared by a photopolymerization induced phase separation method. A detailed study of the effects of the sample preparation parameters, such as curing time, curing intensity, and liquid crystal concentration are reported. It was found that by adjusting these parameters, we were able to change the morphology of these ‘polymer ball’ PDLC samples and thus change their optical characteristics. Incomplete polymerization of the PDLC samples results in a higher threshold voltage and a lower ON state transmission. When the amount of monomer is too low, the shape of the resulting polymer ball becomes irregular, and the sample has a larger threshold voltage and a larger saturation voltage.

ISSN:0267-8292    

DOI:10.1080/02678299608033791

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Supramolecular ferroelectric liquid crystalline complexes have been obtained from 4-alkoxybenzoic acids and optically activetrans-4-substituted-4′-stilbazoles. Chiral smectic C phases are induced by the formation of supramolecular mesogenic structure through the selective intermolecular hydrogen bond between the achiral benzoic acids and the chiral non-mesogenic stilbazoles.

ISSN:0267-8292    

DOI:10.1080/02678299608033792

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This work concerns the dielectric relaxation study of a new compound containing a thiobenzoate group with a fluoro substituent, which presents a sequence of antiferro-, ferri-, ferroelectric and SCαphases. The polar character of these phases was confirmed by optical hysteresis loops. Three modes with a nearly monodispersive behaviour were identified: the soft mode, the Goldstone mode and a non-collective azimuthal mode. The soft mode softens above the SA- SCαtransition and triggers another mode within the SCαphase. Moreover, a collective anti-phase azimuthal mode appears and may be the origin of the onset of the antiferroelectric SCAphase. A strong influence of surface interactions on the thermal hysteresis and on the dielectric constant was evident in this work.

ISSN:0267-8292    

DOI:10.1080/02678299608033793

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The mesomorphic properties of five homologues of the 1,2,5,6,8,9,12,13-octa-alkyloxydibenzopyrene series (DBPn, wherenis the number of carbons in each of the side chains) and their charge transfer complexes with 2,4,7-trinitro-9-fluorenone (TNF) are described. The neat compounds exhibit a single mesophase which has been identified by X-ray diffractometry and optical microscopy as hexagonal columnar (Dhd). Two-dimensional unit cell parameters are reported for the various homologues as functions of temperature within the mesophase region. Deuterium NMR of labelled DBPnwas used to determine the nature and rate of the molecular reorientation in the mesophase. Addition of the electron acceptor TNF to the donor DBPnmesogens increases the stability range of the mesophases, with a maximum stability for the equimolar donor/acceptor mixtures. X-ray and deuterium NMR measurements on these charge transfer stabilized mesophases are also reported. Besides being thermally more stable, they are more highly ordered and even exhibit well defined intramolecular stacking order, classifying them as Dho. Under certain conditions, a diffraction peak at a position corresponding to twice the normal intracolumnar distance is also observed, indicating that the donor and acceptor molecules are stacked alternately in the columns. When alkanes are added to the 1:1 charge transfer complexes, a lyotropic nematic phase is induced. It is shown that these nematics are of the type Nc, with columnar structures as the mesogenic units.

ISSN:0267-8292    

DOI:10.1080/02678299608033794

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Several laterally connected twin mesogens incorporating rigidp-terphenyl units have been synthesized. Their liquid crystalline behaviour was investigated by polarizing microscopy and differential scanning calorimetry. If one compares the twins with their respective halves, the mesophase stabilizing influence of the lateral fixation becomes evident. It obviously pre-organizes the calamitic units and therefore stabilizes smectic phases. The mesophase behaviour of these twins depends strongly on the length and the structure of the spacer. Short as well as highly polar and rigid spacer units give rise to smectic phases with high transition temperatures. By increasing the spacer length the mesophase stability is drastically diminished. Dimesogenic 4,4″-didecyloxyterphenyl derivatives containing spacers consisting of two connecting atoms exhibit smectic C phases exclusively. Elongation of the spacer to three connecting atoms gives an SA-Scdimorphism, and in the case of compounds with longer spacers mostly the SAphase and in one case the nematic phase were found.

ISSN:0267-8292    

DOI:10.1080/02678299608033795

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

To continue the search for novel series of fluorinated ferroelectric liquid crystals, an additional two series of 4-[(S)-2-methylbutoxy]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] benzoates (C) and 4-(n-alkoxy)phenyl 4-[(4-(S)-2′-methylbutoxy-2,3,5,6-tetrafluorophenyl)] benzoates (D) have been synthesized. Polarizing microscopic textural observations and DSC measurements of the phase transitions of these novel compounds showed that compoundsCwere liquid crystals with a chiral nematic (N) phase and a monotropic chiral smectic C phase (Sc), and compoundsDexhibited a chiral nematic (N) phase.

ISSN:0267-8292    

DOI:10.1080/02678299608033796

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The 1,2-bis[3,4,5-tris(alkan-1-yloxy)benzamido] ethanes (DA-n, wherenis the number of carbons in the alkanyl group) form a high temperature hexagonal columnar (ϕh) liquid crystalline phase whenn' 10, 11, 12, 14, 16 and 18.DA-nwithn' 4, 5, 6, and 7 show at elevated temperatures a distorted hexagonal crystal phase. X-ray diffraction data support a model for the ϕhphase consisting of a 2-D hexagonal lattice of self-assembled supramolecular columns in which the melted outstretching alkyl chains ofDA-noccupy a space of constant density surrounding a rigid amide channel. A local ABAB stacking of layers along the column is proposed. Each layer contains twoDA-nmolecules. Layer B is rotated by 90[ddot] around the column axis relative to layer A. H-bonding between the amide groups is responsible for this mode of self-assembly.

ISSN:0267-8292    

DOI:10.1080/02678299608033797

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A series of 3-[ω-(4-cyanobiphenyl-4′-yloxy)alkyl] ethers of estradiol 17-[ω(4-cyanobiphenyl-4′-yloxy)alkanoates] with variable spacers has been prepared as chiral triplet liquid crystals. The compounds show very broad range cholesteric phases and are transformed into cholesteric glasses at room temperature. They exhibit odd-even effects for their cholesteric-isotropic transition temperatures and the associated entropy changes as a function of spacer length. The odd-even effects are observed upon changing the parity of both the ester and of the ether spacer. The best ordering is observed for compounds with an even number of methylene groups both in the ester and the ether spacer. In these cases the three mesogen units are oriented more or less parallel when the alkyl spacers are in the all-trans-conformations. Odd-even effects are also observed for the selective reflection wavelength of the planar cholesteric phase, depending on the parity of both spacers. For several compounds with short spacers the selective reflection wavelength increases strongly with temperature, whereas for other compounds this is almost temperature independent.

ISSN:0267-8292    

DOI:10.1080/02678299608033798

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor