摘要:

Ferroelectric liquid crystalline polymers (FLCPs) with a low-moderate degree of polymerization were synthesized. These had a comb structure; the main chain was polyacrylate and the side chain consisted of a flexible spacer, a core and an optically active chiral end group. They exhibited electro-optic switching times ranging from a few milliseconds to a few seconds in the S*cphase. As the molecular weightMnincreased, the range of the S*cshifted to higher temperatures. At a given temperature, the switching time increased withMn.

ISSN:0267-8292    

DOI:10.1080/02678299308026439

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The energetic aspects of layer deformation in ferroelectric liquid crystal cells are discussed and the actual chevron shape is calculated in some situations. We emphasize two, in our view, essential energy contributions. One term considers the layer curvature. The other one refers to the variations in the distance between layers and the consequent changes of the smectic cone angle. In some simple cases we can determine optimal shapes of the chevron layer structure by analytical solutions, based on these two energy terms. In more complicated situations other contributions have to be considered and the chevron profiles are simulated numerically. The influence of the applied voltage and the choice of parameter values are studied.

ISSN:0267-8292    

DOI:10.1080/02678299308026440

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A detailed study of the dielectric and optical properties of the ferroelectric liquid crystal material (R)-4′-(3-methoxycarbonyl-2-propoxycarbonyl)phenyl 4-(4-(n-octyloxy)phenyl)benzoate (3MC2PCPOPB) has been carried out. It has been found that an anomalous temperature dependence of the dielectric constant in 3MC2PCPOPB is due to the antiferroelectric and ferrielectric properties. AT(temperature)-E(electric field) phase diagram has been obtained on the basis of the apparent tilt angle measurements. In a thin cell (< 3μm), both ferroelectric and antiferroelectric domains are simultaneously observed over a wide temperature range, and the complete antiferroelectric phase does not appear even at low temperature. A characteristic texture in which boundary focal conics are aligned parallel to a smectic layer has been observed. The movement of the zig-zag defect line caused by the application of the voltage is also observed.

ISSN:0267-8292    

DOI:10.1080/02678299308026441

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The synthesis, phase behaviour and spontaneous polarization of a new class of chiral LCs and chiral dopants for induced ferroelectric phases of general structure A, possessing a (2S)-2-hydroxymethyloxirane unit, and B, possessing a (2R)-2-oxirane carboxylic acid unit connected to mesogenic building blocks are described. One of these new compounds exhibits a S*cphase. A carbonyl group adjacent to the oxirane ring does not increase the spontaneous polarization which is in contrast to the results obtained for 2,3-disubstituted oxiranes. A comparison with analogous species containing a (2S)-2-hydroxymethyloxetane, a (2R)-2-hydroxymethylthiirane or a (2S,3R)-2-hydroxymethyloxirane unit is given.

ISSN:0267-8292    

DOI:10.1080/02678299308026442

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Hydrogen-bonded dimers of 4-alkoxy-4′-stilbazole homologues (CnOSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed withn= 1 and 2, and chiral smectic C phases were observed withn≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.

ISSN:0267-8292    

DOI:10.1080/02678299308026443

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The stability of a hydrogen-bonded complex built through inter-molecular hydrogen bonding between carboxylic acid and pyridine fragments has been examined using infrared spectroscopy. Infrared spectra as a function of temperature have been recorded for the 1:1 complex of 4-hexyloxybenzoic acid andtrans-4-propoxy-4′-stilbazole from the crystalline state to the isotropic state. A dependence of the stability of the hydrogen bond on molecular orientation is observed clearly in the infrared spectra. The spectra also suggest that the hydrogen bond is an unionized type with a double minimum potential energy.

ISSN:0267-8292    

DOI:10.1080/02678299308026444

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A new type of electrohydrodynamic instability originally reported in nematic liquid crystal mixtures with positive dielectric anisotropy and as moderately thick samples is further studied. The ability of homogeneously aligned nematics with positive dielectric anisotropy, in the presence of a magnetic field, to exhibit Williams domains as a threshold effect is numerically investigated. The variation of the threshold voltage for domain formation and dielectric alignment with dielectric anisotropy is calculated theoretically and compared with the experimental results as moderately thick and thin samples.

ISSN:0267-8292    

DOI:10.1080/02678299308026445

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We present the results of our investigations on the electrooptic properties of the smectic O and O* phases of m7tac (1-methylheptyl terephthalylidene-bis-4-aminocinnamate). At low voltages, we observed striking electrochromic effects and, in O* at high voltages, a field induced phase transition. Freely suspended thick films of m7tac in the O and O* phases are weakly biaxial. A framework is proposed to account for our observations.

ISSN:0267-8292    

DOI:10.1080/02678299308026446

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Polymer chain diffusion in the nematic mesophase was studied using a model main chain liquid crystalline (LC) polyether based on 2,2′-dimethyl-4,4′-dihydroxyazoxybenzene and mixed alkane spacers. A side chain LC polymethacrylate containing an azobenzene mesogenic group was also investigated. Tracer diffusion coefficients were determined as a function of temperature by an ion-beam depth profiling technique, forward recoil spectrometry. The results confirm that main chain LC polymer chain dynamics are dramatically affected by phase transitions and sample geometry. This behaviour is in marked contrast to the side chain LC polymer which exhibited no phase dependence on the part of the tracer diffusion coefficient.

ISSN:0267-8292    

DOI:10.1080/02678299308026447

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (SA1) or monolayer and partially bilayer (SAd) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A1phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ‒COCmH2m+ 1terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A1phase slowly changes to be more like the smectic Adphase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (SA1e(b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A1phase is rapidly transformed into the smectic Adphase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.

ISSN:0267-8292    

DOI:10.1080/02678299308026448

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor