摘要:

Freely-suspended liquid crystalline films of ethyl 4'-n-octyloxybiphenyl-4-carboxylate (28OBC) were prepared and transferred onto different substrates which enable detailed structural characterization. The structures of these thin film assemblies, which are only accessible in this way, were determined and compared with the crystal and molecular structure of 28OBC as formed by crystallization from toluene solution. The compound crystallizes in the monoclinic space group P21/c, a = 11.168(1) A, b = 7.595(2) A, c = 49.106(1) A, beta = 94.01(1) degree, Z = 8. The two symmetrically independent molecules of the asymmetric unit have been used as starting geometries for semi-empirical MO calculations. The difference between the experimentally observed and the optimized molecular structures is interpreted as the influence of the crystal field. The structures of the crystalline and E film phases have been investigated by SAXR and TED and the former has been found to be different from the bulk structure. The structural relationships between the different phases are discussed and a suggestion for the crystalline film structure is given as deduced from simulations of electron diffraction patterns.

ISSN:0267-8292    

DOI:10.1080/026782998205660

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Two binary mixtures, prepared from either 4-cyanophenyl 4-hexadecyloxybenzoate (Ib) or 4-nitrophenyl 4-hexadecyloxybenzoate (Ic), with 4-carboxyphenyl 4-hexadecyloxybenzoate (Ia), were thermally characterized to construct the phase diagrams for the two systems Ia/Ib and Ia/Ic. Due to differences in the smectic layering between the individual components of each system, phase separation took place either in the solid or in the mesophase. A mathematical relation was derived to calculate the composition of each phase in the solid state. Another two phase diagrams were constructed for binary mixtures of the linear dimer molecule, 4-hexadecyloxybenzoic acid (III) with either the acid Ia or its isomeric derivative, 4-hexadecyloxyphenyl 4-carboxybenzoate (II).

ISSN:0267-8292    

DOI:10.1080/026782998205679

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In this work, an atom-based molecular modelling technique is applied to determine the structures of smectic E, A, and C phases composed of phenyl ester mesogens. These mesogenic molecules are abbreviated as MDn21B. A computer search with molecular mechanics calculations is performed to identify the possible low energy configurations of two adjacent molecules. According to these results, the isolated molecules with their optimized structures approach dimer associations, and the favourable alignments are antiparallel. Depending on the lowest energy associations, ten different initial models for each phase, to simulate X-ray diffraction patterns, are developed by packing the dimers with an antiparallel association into periodic boxes. Also, relaxed models are obtained by applying cycles of energy minimization and molecular dynamics under (NVT) conditions at 500 K to these initial models. Molecular dynamics runs under (NPT) conditions are then performed on these relaxed models at a temperature of each smectic phase chosen to approach equilibrated structures in these phases. Simulations are also performed, and detailed molecular structures analysed, on the basis of these equilibrated structures. The simulated X-ray diffraction patterns for smectics E, A, and C are in good agreement with those obtained experimentally. The distributions of the dihedral angles at the bonds in the aromatic cores indicate that more fluctuations occur in the smectic A and C phases than in the smectic E. The average values of the aromatic core overlap, as calculated between neighbouring mesogens in the smectic phases, are in the range 4.78-5.91 A. These values are slightly higher than those found in experimental X-ray diffraction patterns at the position 2 theta 20. Total pair correlation functions have a similar appearance for smectics E and A. Also, these pair correlation functions are similar to the results for amorphous polymers, i.e. the total pair correlation functions lose their order outside a spherical shell with radius gamma greater than 5 A.

ISSN:0267-8292    

DOI:10.1080/026782998205688

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Mixtures of a photosensitive chiral dopant based on (—)-menthone with left-handed and righthanded cholesteric copolymers were prepared. The phase behaviour and optical properties of the mixtures prepared were studied. The action of UV radiation on planar oriented films of such systems was shown to induce dramatic changes in the maximum reflection wavelength as a result of E-Z isomerization of the dopant molecules. The kinetics of photoisomerization of such mixtures in solution and in the bulk were investigated at different temperatures. The above mixtures can be considered as promising and uprecedented materials for coloured data recording and storage.

ISSN:0267-8292    

DOI:10.1080/026782998205697

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The synthesis, characterization and mesomorphic properties of copper(II), palladium(II), vanadyl(IV) and iron(III) chloride complexes derived from salicylaldimine structures are reported. The copper and palladium complexes exhibited smectic A phases, however vanadyl and iron chloride complexes showed crystalline phase. The palladium complexes have lower melting and clearing points, and a wider temperature range of mesophase. This lowering in clearing points between M = Pd and M = Cu analogues was attributed to the weaker core-core interaction between palladium centres within layers in the mesophase. The crystal and molecular structures of bis[N-(3-hydroxypropyl)-4-octanoylsalicylaldiminato]copper(II) were determined by means of X-ray analysis. This complex crystallizes in the monoclinic space group p21/c, with a = 5.4908(23) A, b = 19.847(5) A, c = 16.636(4) A, beta = 96.87(3) degree, and Z = 2. The intramolecular separation of Cu-Cu atoms is 3.022(3)A. The structure shows that the molecular shape is nearly flat with two copper atoms lying 0.067(4)A above and below the plane of N1, O1a, O1 and O2 atoms.

ISSN:0267-8292    

DOI:10.1080/026782998205705

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In-plane switching (IPS) of liquid crystals showed advantageous voltage-holding ratio (VHR) characteristics so that liquid crystals with low resistivity could provide higher VHRs compared with the twisted nematic effect. This experimental result was obtained when electric fields were applied approximately parallel to the substrate plane using the IPS electro-optical effect. We found that the in-plane electric field generates supplementary capacities which support retention of an externally applied voltage over the liquid crystal layer during non-selected periods of the active matrix driving scheme, because the liquid crystal layer can be connected with an insulating layer, an orientation layer and even a substrate in parallel. Based on these advantageous VHR characteristics, liquid crystal materials suitable for the IPS effect were appropriately optimized. We propose evaluation parameters, derived from the physical switching principles of the liquid crystals, to obtain lower driving voltage and faster response speeds. These parameters are effective in optimizing the physical properties of liquid crystals without variation of the cell gap. We use the proposed evaluation parameters and the advantageous VHR characteristics to demonstrate the optimization approach and we suggest a novel possible use of liquid crystal materials with low resistivity which cannot be implemented conventionally. Finally, we prove that liquid crystals with low resistivity generate the Ir internal potential by the drift of ionic species.

ISSN:0267-8292    

DOI:10.1080/026782998205714

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

New liquid crystalline 1,3,5-triazines are presented that incorporate three mesogenic groups based on rod-like azobenzene moieties. The synthesis was carried out by reaction of cyanuric chloride with 4-alkoxy-4-aminoazobenzene derivatives. The mesomorphic behaviour was investigated by polarizing microscopy, differential scanning calorimetry and X-ray scattering. Besides a nematic and a SmA phase, the trisazomelamines form a higher ordered mesophase within a broad temperature range below the smectic A phase. With respect to the rod-like azobenzene sub-units, the low temperature phases display a smectic 'bilayer' structure. The molecules are antiparallel aligned within one layer. Furthermore, 1,3,5-triazines incorporating just two azobenzene groups are presented, these were prepared to investigate the influence of the number of rod-like sub-units linked to the triazine core on the mesomorphic properties.

ISSN:0267-8292    

DOI:10.1080/026782998205723

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A mean-field treatment is given of the off-lattice Krieger-James model of ordered fluids, which reduces to the more familiar Maier-Saupe liquid crystal (Heisenberg fluid) in the absence of ferromagnetic (nematic) interactions. As in the lattice version, isotropic, nematic and ferromagnetic nematic phases are found, but the nematic-ferronematic transition can either change order at a tricritical point, or terminate at a critical end point on the ferronematic-isotropic coexistence curve. In addition it is argued that the sequence of phase diagram topologies, as a function of the relative weights of ferromagnetic and nematic contributions to the free energy, should be similar to that obtained on varying the elongation of dipolar spheroids.

ISSN:0267-8292    

DOI:10.1080/026782998205732

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The splay orientational elastic constants for the nematic phases of a series of dimeric alpha,omega-bis(4-cyanobiphenyl-4-yloxy)alkanes have been determined by the method of Freedericksz threshold transitions in a magnetic field. A dramatic odd-even effect in the dependence of the elastic constant on the spacer length of the compounds investigated was revealed. It was shown that the splay elastic constant for the nematic dimers increases with increasing length of the molecules.

ISSN:0267-8292    

DOI:10.1080/026782998205741

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Miscibility phase diagrams of mixtures of side-on side chain liquid crystalline polymers (s-SCLCP) and low molar mass liquid crystals (E48 and E44) have been established by means of polarized optical microscopy and light scattering. E48 and E44 are cyanobiphenyl-based eutectic nematic liquid crystal (LC) mixtures with nematic-isotropic transition temperatures of 93 and 105 C, respectively. The phase diagram of the s-SCLCP/E48 system reveals the coexistence of an isotropic nematic region and a single nematic phase in order of descending temperature. The single nematic phase suggests that the pair is miscible in the nematic region. On the other hand, the s-SCLCP/E44 mixture shows liquid liquid and nematic nematic coexistence phases, suggestive of the immiscibility character of the pair. These nematic phase diagrams of the s-SCLCP/E48 and s-SCLCP/E44 have been analysed in the context of the combined Flory-Huggins (FH) free energy for isotropic mixing and the Maier-Saupe (MS) free energy for nematic ordering of the mesogens. This combined FH/MS theory is capable of predicting the observed nematic phase diagrams consisting of liquid liquid, liquid nematic, nematic nematic, and the pure nematic regions. The change of colour accompanying the appearance and disappearance of the inversion walls may be attributed to the temperature dependence of birefringence.

ISSN:0267-8292    

DOI:10.1080/026782998205750

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor