摘要:

The synthesis of side chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and (S)-2-methylbutyl 4-\[(4-oxybiphenyl-4-yl)carbonyloxy]-3-fluorobenzoate mesogenic side groups is presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction measurements reveal liquid crystalline properties for all synthesized monomers and polymers. All three precursor olefinic monomers reveal cholesteric and smectic A phases. The olefinic monomer which contains two oligooxyethylene units in the spacer is the only one which reveals a twist grain boundary A phase and a blue phase, besides the cholesteric and smectic A phases. All three polysiloxanes present enantiotropic smectic A and chiral smectic C phases. The mesomorphic behaviours of the monomers and polymers are compared with those of the corresponding monomers and polymers without the lateral fluoro substituent. The results seem to demonstrate that incorporating a lateral fluoro substituent in the mesogenic cores of the monomers affects not only the mesophase thermal stability, but also the nature of the mesophases formed. However, incorporating a lateral fluoro substituent in the mesogenic cores of the polymers affects only the thermal stability of the mesophases formed. The lateral fluoro substituent has a more profound effect on the mesomorphic behaviour for the monomers than that for the polymers.

ISSN:0267-8292    

DOI:10.1080/026782997208767

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The conformation of the backbone in the side chain liquid crystal polymer poly\[omega (4-methoxybiphenyl-4-yloxy)butyl methacrylate] has been studied in the smectic C and nematic phases. Small angle neutron scattering experiments were performed on mixtures of molecules with perdeuteriated backbones and unlabelled molecules. The polymer is found to adopt an oblate conformation in the smectic C phase. The components of the radius of gyration parallel and perpendicular to the director are determined as a function of temperature from Guinier plots of the SANS data. The radii of gyration do not vary across the smectic phase and are found to be Rg,||=(27+/-1)A, Rg, =(42+/-1)A. These results are compared with recent SANS results for other side chain liquid crystal polymers.

ISSN:0267-8292    

DOI:10.1080/026782997208776

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

By comparing the anchoring behaviour of (end-on) side chain polymer liquid crystals with that of the corresponding low molecular mass liquid crystals, we have studied the effect of the backbone on the anchoring of side chain polymer liquid crystals. We can distinguish two different effects: the loss of rotational freedom of the side groups and the interaction of the polymer chain with the surface. In the case of free surfaces, we can formulate a general rule stating that compounds with side groups ending with an aliphatic chain (at least four carbon atoms long) exhibit a homeotropic anchoring, and compounds with side groups ending with a polar group exhibit a planar anchoring.

ISSN:0267-8292    

DOI:10.1080/026782997208785

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The nematic liquid crystalline property of 4-benzoyloxybenzoic acid (BBA) was observed by polarizing microscopy and X-ray diffraction. The structure of the hydrogen-bonded BBA dimer was verified by FT-IR, and the axial ratio (l / d) of the rod-like molecule was calculated to be 5.8, which is large enough to exhibit a liquid crystalline phase. The mesophase was stable at 240 C. At higher temperatures, for example, 270 C, the trans -esterification side reaction occurred. The binary phase diagram of BBA and 4-acetoxybenzoic acid shows the eutectic temperature at 175 C.

ISSN:0267-8292    

DOI:10.1080/026782997208794

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The physical properties of a binary liquid crystal system were investigated, namely the transition temperatures, birefringence and viscoelastic constants measured by using thermal optical microscopy, differential scanning calorimetry and light scattering techniques with coherent laser light beams. These properties were sometimes found to exhibit linear dependence on the mole fractions of the components. Our results indicate that a required property is obtainable by the regulation of the molar ratio as if the length of the alkyl chain connected with a cyanobiphenyl core group were variable; i.e. the apparent chain length could be controlled.

ISSN:0267-8292    

DOI:10.1080/026782997208802

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The phase behaviour of cholesterol containing liquid crystalline (LC) copolymers of different compositions (copolymers A and B), and the phase behaviour and optical properties of their blends have been investigated. The phase diagram of the mixture consisting of two fractions A and B with low molecular masses (MW<104) corresponds to complete solubility of the components over the whole concentration range. The transmittance spectrum of all blends has one minimum, which lies between two peaks corresponding to the wave lengths of the initial samples A and B. In the case of the cholesteric copolymers with molecular masses above 104, the corresponding phase diagram exhibits a lower critical solution temperature (Tcrit). Below Tcrit only one peak of the selective reflection of light is observed (SRL). However, above Tcrit two separate bands of SRL, corresponding to the reflection bands for the individual initial copolymers A and B are observed. The process of appearance of the single or double bands is reversible and by varying the temperature one can obtain films with bimodal or unimodal curves of SRL.

ISSN:0267-8292    

DOI:10.1080/026782997208811

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Two new mesogenic series have been synthesized. They have the same main core which contains four aromatic rings, with two lateral substituents on the same side of one of the inner rings. One of the substituents is a 4-chlorobenzyloxy group and the second is an alkoxy chain. The positions of the lateral substituents are different in the two series. Despite the large aromatic branch, an enantiotropic nematic phase is obtained for most of the compounds. The orientational ordering behaviour of these new compounds has been investigated by C-13 NMR, and two isomeric compounds containing a lateral dodecyloxy chain were chosen for the study. The order parameters of the chain were obtained by a 2D C-13 NMR technique with variable angle spinning. The temperature dependence of the order parameters was determined using the C-13 chemical shifts with slow spinning of the sample parallel to the magnetic field. The first methylene fragment in each lateral substituent of each compound has a positive C-H bond order parameter, implying that this fragment adopts a cis conformation in the nematic phase. The two lateral substituents are folded back along the mesogenic core in opposite directions. The lateral chain is found to be roughly aligned along the molecular long axis imposed by the core, whereas the para -axis of the phenyl ring in the lateral aromatic branch makes a considerable angle with the molecular long axis. It was also found that the different patterns of substitution do not affect the position of the molecular long axis to any large extent.

ISSN:0267-8292    

DOI:10.1080/026782997208820

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A series of dialkyldimethylammonium bromides bearing a 4,4-oxynitrostilbene calamitic core at the end of one of the alkyl chains was synthesized. Its thermotropic liquid crystal behaviour was examined using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. An ordered smectic phase was identified in which the ionic ammonium bromide groups are arranged according to a square two-dimensional crystal lattice, and the oxynitrostilbene cores are tilted away from the layer normal by an angle of 54 degrees. This novel smectic structure is discussed in terms of structural compatibility of the smectic sublayers: the sublayers of the ionic groups and calamitic cores adapt their internal structure in order to become adequate for coherent superposition in a smectic structure.

ISSN:0267-8292    

DOI:10.1080/026782997208839

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The optical tensor configuration in a homeotropically aligned ferroelectric liquid crystal (FLC), SCE13 cell, is investigated by means of optical excitation of half leaky guided modes. A thin slab of FLC is confined between a high index pyramid and a low index substrate whose indices bound those of the liquid crystal. In this geometry there exists a small angle range over which a series of sharp resonant modes may propagate in the liquid crystal. Detecting the angular dependent reflectivity for plane polarized radiation and subsequently fitting the data by iteratively modelling from multilayer Fresnel theory, a full characterization of the tilt and twist profile throughout the cell is achieved. The temperature dependence of the tilt of the principal director, which is related to the smectic cone angle, and of the optical permittivity, as well as the pitch have been obtained. The tilt director profile across the cell is interpreted using a compressible continuum theory for SmC* liquid crystals which includes the possibility of variable cone angle and layer spacing.

ISSN:0267-8292    

DOI:10.1080/026782997208848

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A homologous series of dialkyldimethylammonium bromides bearing a cyano group at the end of one of the alkyl chains was synthesized. The cyanoalkyl group ranges from monomethylene to hexamethylene and the cyano-free chain from dodecyl to octadecyl. The thermotropic liquid crystal behaviour of the series was investigated using differential scanning calorimetry, polarizing microscopy, and X-ray diffraction. The molar volume of four selected compounds was determined as a function of temperature using dilatometry. The low-temperature crystal phases were found to have a lamellar structure in which the ammonium bromide groups are arranged within the layers in two distinct planes, alternately separated by single layers of upright, interdigitated alkyl chains and double layers of cyanoalkyl chains coupled through the cyano end groups. The high-temperature smectic A phases were found to display a similar structure regarding the layering of the molecules, in which however the alkyl and cyanoalkyl chains have a disordered conformation. The role of the cyano end groups is discussed by comparing this smectic A structure with the known ordered smectic T structure of the cyanofree dialkyldimethylammonium bromides.

ISSN:0267-8292    

DOI:10.1080/026782997208857

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor