摘要:

The orientational ordering of a series of ferroelectric liquid crystals 4'-[(2S, 3S)-3-methyl- 2-halopentanoyloxy]-4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates have been studied by two-dimensional carbon-13 separated local field spectroscopy combined with off-magic-angle spinning. All carbons are well-resolved in the carbon-13 NMR spectrum so that carbon-proton dipolar coupling constants for all carbons in the liquid crystal molecule can be determined, giving detailed segmental orienta- tional ordering information. (The order parameters of the biphenyl core segments and each carbon-proton bond in the aliphatic chains were measured as a function of temperature.) The results show that the substitution of different halogens on the chiral chain affects the carbon- proton bond orientational ordering of the entire chiral chain, while the orientational ordering of the rest of the liquid crystal molecule (core and achiral chain) remains unchanged. For the fluoro-substituted liquid crystals, carbon-fluorine dipolar couplings are also observed. At the SmA-to-SmC* transition, there is a discontinuous change in the magnitudes of all C-H bond order parameters. However, the aliphatic ordering relative to the core ordering is unchanged, suggesting that there is a negligible change in the mesogen conformation and the environment of the aliphatic chain at this transition, i.e. there is no evidence for substantial 'chiral interactions' in the SmC* phase.

ISSN:0267-8292    

DOI:10.1080/026782998207433

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Polymerization and phase behaviour of a branched liquid crystalline bismethacrylate in a ferroelectric liquid crystal (FLC) were characterized. Addition of the monomer increases the temperature range of the smectic A phase, and, at relatively low concentrations of monomer, the temperature range increases to more than 10 times that observed in the neat FLC. Other phases such as the smectic C* phase are no longer exhibited as the monomer interferes with the inherent tilt of the FLC molecules. After polymerization, the polymer network phase separates and the phase transition temperatures return to values close to those of the FLC. The monomer also shows a high degree of orientational order before polymerization, which is retained to a large extent after polymerization. The order in the polymer network results in considerable birefringence at temperatures well above the clearing point of the pure FLC. This behaviour is induced by the order of the polymer network and interactions of the FLC molecules with the network which prevent a fraction of the FLC molecules from exhibiting typical phase behaviour. In the formation of the branched liquid crystalline bismethacrylates/FLC composites, a steady increase is observed in the polymerization rate as the polymerization temperature increases and the order of the FLC phase decreases, a behaviour significantly different from that observed in other monomer/FLC mixtures for which the polymerization rate increases as the order of the FLC phase increases. Additionally, at appropriate polymerization temperatures around the clearing point, polymerization driven endothermic phase transitions may be observed.

ISSN:0267-8292    

DOI:10.1080/026782998207442

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A group of liquid crystal materials which contain the novel methylene-1,4-dihydropyridine substructure were synthesized and their mesogenic properties examined. Three main classes of liquid crystal compounds which differ in the structure of the aromatic core group (phenyl, azobenzene and diphenylacetylene) attached to the nitrogen of the 1,4-dihydropyridine group were studied. The synthesis of the methylene-1,4-dihydropyridine group was accomplished in excellent yield by a Knoevenagel condensation of a 4-pyridone intermediate with an active methylene compound. The liquid crystal materials prepared thus far which contain this methylene-1,4-dihydropyridine structure all possess broad enantiotropic smectic A phases and one example also possesses a tilted smectic C phase. These mesogens may possess useful properties such as high birefringence.

ISSN:0267-8292    

DOI:10.1080/026782998207451

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An homologous series of 6- O - n -alkyl- alpha -D-galactopyranoses has been prepared. The length of the terminal chains has been varied systematically and the effect on the liquid crystal transition temperatures studied. Most homologues of the series exhibit enantiotropic smectic A* phases. X-ray analysis indicates a lamellar structure for the smectic A* phase with hydrogen-bonded carbohydrate cores at the layer centre, either with no interdigitation of the tilted terminal alkyl chains but with a high degree of chain melting, or with some degree of chain intercalation. The 6- O - n -alkyl- alpha -D-galactopyranoses possess clearing points at higher temperatures than those of the corresponding n -alkyl alpha -D-galactopyranosides. The introduction of a higher degree of hydrogen bonding by the replacement of the oxygen atom in the ether linkage between the chain and the carbohydrate ring by an amide linkage leads to higher transition temperatures. The dependence of the liquid crystalline behaviour on the position of the same alkyl substituent and the nature of the sugar in the pyranose form, as well as on the anomeric configuration of the liquid crystalline carbohydrates with four hydroxy groups, is reported.

ISSN:0267-8292    

DOI:10.1080/026782998207460

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We report second-harmonic generation in samples with a pyroelectric liquid crystal polymer (PLCP) prepared from the ferroelectric liquid crystalline acrylate monomer 4-[(R)-(-)-2-(10-acryloyloxydecyl)oxy]-3-nitrophenyl 4-[4-(11-acryloyloxyundecyloxy)phenyl]- benzoate (A2c) by in situ photopolymerization. The relation between SHG efficiency, spontaneous polarization and amount of chiral substance in this and previously synthesized PLCP materials is discussed. Electro-optical properties of the monomeric and crosslinked A2c are investigated. In the crosslinked state, the molecular mobility is found to be very low, supporting the fact that the degree of polar order in the polymer is high. In contrast to the case of a poled electret NLO material, the polarization of this material is not 'frozen-in', but an intrinsic thermodynamic property.

ISSN:0267-8292    

DOI:10.1080/026782998207479

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Novel main chain poly(aryl ether ketone)s containing a lateral phenyl group were synthesized by nucleophilic substitution reactions of 4,4-biphenol and phenylhydroquinone with either 4,4-difluorobenzophenone or 1,4-bis(4-fluorobenzoyl)benzene and their thermotropic liquid crystalline properties were characterized by a variety of experimental techniques. Thermotropic liquid crystalline behaviour was observed in the copolymers containing 50 and 70 mol% biphenol. Melting (Tm) and isotropization (Ti) transitions both appeared on the DSC thermograms. A banded texture was formed after shearing the sample in the liquid crystalline nematic state. As expected, each of the copolymers had a relatively lower melting transition than the biphenol-based homopoly(aryl ether ketone)s because of the copolymerization effect of the crystal-disrupting monomer phenylhydroquinone.

ISSN:0267-8292    

DOI:10.1080/026782998207488

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Angular mesogenic structures of hydrogen-bonded supramolecules have been constructed from a 1:1 molar ratio of 3(or 4)-decyloxybenzoic acid, MA (or PA), mixing either with trans-4-alkoxy-3'(or 4')-stilbazoles, CnM (or CnP), or with their analogous N -oxides (CnMO and CnPO) to form the hydrogen-bonded (H-bonded) complexes CnPPA, CnPMA, CnMPA, CnPOPA, CnPOMA and CnMOPA. By controlling the relative bending position of the H-bonded complexes, the variety of supramolecules offer new molecular structures of different linearities. By means of these bending effects we are able to tune the shape of the molecular architecture and thus to modify molecular packing as well as mesogenic properties. New liquid crystalline properties are introduced by the nonlinear effects of molecular geometry, and powder X-ray diffraction (XRD) patterns have confirmed their novel molecular architectures.

ISSN:0267-8292    

DOI:10.1080/026782998207497

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0267-8292    

DOI:10.1080/026782998207505

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor