摘要:

The synthesis and characterization of a new diastereomeric compound are reported; this exhibits a temperature induced inversion of the cholesteric twist. Measurements of the cholesteric pitch and handedness are presented and qualitatively discussed in terms of the nature of the cholesteric twist inversion in pure compounds. Preliminary measurements of properties of the ferroelectric S*c phase of this compound are also reported.

ISSN:0267-8292    

DOI:10.1080/02678299508036643

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The result of optical X-ray, DSC and magnetic susceptibility studies of new Co(II) metallomesogens are reported. The di-(5-substituted-salicylidene)ethylenediaminato cobalt(II) complexes exhibit broad range Saphases. A comparison with corresponding copper(II) and nickel(II) complexes leads to conclusions about how the properties are influenced by the metal atom. Cobalt(II) complexes of this kind form different modifications which are either active or inactive against oxygen. The conversion process (inactive-active) is monitored by time dependent X-ray diffractometric measurements.

ISSN:0267-8292    

DOI:10.1080/02678299508036644

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The alcohol, 2(S)-[2(S)-methylbutyloxy]propanol(1), containing two centres of chirality was used as a chiral building block to prepare two series of new FLCs. The first series, 4-{2(S)-[2(S)-methylbutyloxy]propoxy}phenyl 4-n-alkoxybenzoate(7–10), exhibits monotropic SC* and SAphases at or below room temperature. The second series, 4,4′-{2(S)-[2(S)-methylbutyl-butoxy]propoxy}biphenyl 4-n-alkoxybenzoate(11–15), possesses enantiotropic S*Cand N* phases. The S* phase temperature range is up to 37°C. The spontaneous polarization of 22–32nCcm−2was measured in the series of the FLCs11–15. The comparison experiments indicate that the dipoles of the two chiral centres in the alcohol1add in the SC* phase so that thePsvalue of the new FLCs are enhanced.

ISSN:0267-8292    

DOI:10.1080/02678299508036645

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A structural characterization of chiral side chain siloxanes with different average degrees of polymerization,DPn, was performed by X-ray diffraction experiments on powder and oriented fibre specimens. Polymers (DPn= 35) and oligomers (DPn= 4) contained the 4,4′-biphenylene unit with either an (S)-2-methylbutoxy(An, Bn)or an (S)-2-chloro-3-methylbutanoyloxy substituent(Cll). The spacer segment connected to the siloxane backbone had a variable number,n, of methylene groups (n= 5, 8, or 11). Independent of the spacer length and the chiral tail nature, the polysiloxanes underwent the same sequence of phases: C-SF1(or SI1)-SC1-SA1-I, whereas in the oligosiloxanes the sequence C-SB1-SA1-I (B11) or C-SF1-SC1-I (B5) occurred. The influence of the structure of the polysiloxanes on the formation of the smectic (tilted or orthogonal) mesophases was elucidated. The rather large number of reflections (three or four) detected in the X-ray patterns at low angles, allowed a drawing of the projection of the electron density profiles along the layer normal,p(z), and deduction of the most physically acceptable electron density profiles from among the numerous possibilities for each smectic phase. The electron density profiles were in agreement with monolayer smectic phases presenting a microphase separation between the siloxane backbones and the side chains, so constraining the polymer backbones within a thin layer.

ISSN:0267-8292    

DOI:10.1080/02678299508036646

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The thermotropism of 1-n-alkyl-(4-methyl and 4-tolyl)pyridinium bromides were compared for alkyl chain lengths ranging fromn= 12 to 22 carbons. A smectic A mesophase is present in both series for the longer chain compounds,n≥ 16, with the clearing temperature being similar for both series but increasing rapidly with chain length. The series with the elongated mesogen also possesses an ordered mesophase identified as smectic G. The transition between this mesophase and the SAor isotropic phase in the 4-tolyl series, and the transition to and from the crystalline phase in both series, are affected relatively little by the alkyl chain length. It seems that the SAmesophase is governed primarily by the amphiphilic character of the substances, whereas elongation of the ionic head group is responsible for the appearance of a more ordered mesophase at intermediate temperatures.

ISSN:0267-8292    

DOI:10.1080/02678299508036647

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The origin of spontaneous polarization in the ferroelectric smectic C* phase is investigated within a mean-field microscopic model which describes the coupling between the tilt of molecules from the normal to the smectic layers and the rotation of a molecule around its long axis. The mean-field potential is studied which takes into account a chiral polar and a non-chiral quadrupolar biasing of the rotation of molecules around the molecular long axes. Each molecule is characterized by three transverse molecular axes: the chiral axis which turns parallel to the macroscopic C2axis at small tilts, the polar axis in the direction of the transverse dipole moment and the quadrupolar axis which tends to be parallel to the C2axis at very large tilts. A numerical analysis of the model shows that there are four different types of spontaneous polarization dependent on the temperature, including the sign-reversal type. The influence of three microscopic parameters, i.e. two angles between the three characteristic axes and the relative strength of the chiral versus the non-chiral biasing, on the type of spontaneous polarization is investigated. The relationship between the microscopic and the equivalent Landau model is established and discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508036648

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Novel liquid crystalline 2-phenyl-1,3,4-thiadiazole based oligomers with three and four rigid aromatic units linked by a flexible central unit have been investigated by polarizing microscopy. The synthesis of these compounds and the influence of structural variations on the mesomorphic properties are described. The combination of suitable mesogenic moieties with appropriate central units leads to oligomers which exhibit Scphases.

ISSN:0267-8292    

DOI:10.1080/02678299508036649

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Molecular dynamics of side chain liquid crystalline polymers (LCP) and their components were studied using the technique of paramagnetic resonance. A cigar shape spin probe (COL) and a nearly spherical spin probe (TPL) were used to study the motions and order of the LCPs. Computer simulations of the observed spectra were performed. Both rotational correlation times and order parameters were extracted from these simulations. We found that LCPs containing 30 per cent and 50 per cent of mesogenic side chains had about the same viscosity as indicated by nearly equal tumbling times at the same temperature. In addition, the LCPs motion is considerably slower than that of the monomeric liquid crystal indicating that the spacer couples the motions of the side chains to those of the main chain. Rotations about axes perpendicular to the side chain are slowed more than rotations about an axis parallel to the side chain. DSC measurements were employed to study the phase transitions. The 30 and 50 per cent LCPs displayed first order NSAtransitions, but the 50 per cent LCPs transition was much weaker, in agreement with McMillan's theory which predicts a first order transition forTNS/TNI>0.87 (observed ratios are 0.98, 0.90 and 0.86 for 30, 50 and 100 per cent LCPs, respectively). The 30 per cent LCP has a very short nematic range so that the nematic order, which is not saturated at the NS transition, can couple with the smectic order. This was indicated by a sharp change in slope of the order parameter versus temperature plot as the smectic is entered. The LCPs studied formed a highly ordered glass when cooled in a 1 T field. If one could find a LCP with similar ordering properties whose glass temperature is well above room temperature, then one would have a useful binder for the manufacture of haze-free polymer dispersed liquid crystal displays.

ISSN:0267-8292    

DOI:10.1080/02678299508036650

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The interaction between morphological structure and phase behaviour of a LC side group block copolymer has been investigated using DSC, TEM and small angle X-ray diffraction. All samples of Polystyrene-block-2-(3-cholesteryloxycarbonyloxy)ethyl methacrylate (PS-b-PChEMA) show a phase separation between the two blocks. It was found that in the case of those samples where the liquid crystalline sub-phase is not continuous (spheres), only a nematic phase is seen, whereas in all samples in which there is a continuous liquid crystalline sub-phase, the smectic A phase of the homopolymer is formed. On the other hand, the block copolymer seems to stabilize the LC phase; no dependency of the clearing temperatures on the molecular weight of the LC blocks has been observed.

ISSN:0267-8292    

DOI:10.1080/02678299508036651

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The simultaneous influences of elastic stress and ionic fields on switching in ferroelectric liquid crystals are considered. Experiments are performed which show that the combined influences can be quite complex. When a reset pulse is applied, it can enhance or decrease the previous ionic field, as well as change the elastic state of the pixel so that when the reset and switching pulses are close together, the combination of these effects influences the result significantly. In some situations, the outcome from simple partial switching schemes cannot be explained from ionic and elastic effects; we suggest a surface switching effect in this case.

ISSN:0267-8292    

DOI:10.1080/02678299508036652

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor