摘要:

A side chain mesomorphic cooligomer presents a large blue phase temperature domain. The cooling down of the sample from BPI allows to quench coloured crystallites below the glass transition of this material. Observations by transmission electron microscopy on microtome cuts of these crystallites reveal the presence of mono- and bi-periodic domains corresponding to the respective quench of cholesteric and BPI phases at room temperature. The use of an atomic force microscopy technique directly on the cuts observed by electron microscopy, and the detection of the same textures by both methods reveal unambiguously that the TEM contrast originates from strong, “cut-induced”, surface relief. The nature of this corrugation is discussed using numerical simulations derived from a Ginzburg-Landau approach.

ISSN:1058-725X    

DOI:10.1080/10587259508033527

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The study of the kinetics of electric field-induced reorientation of liquid crystals is a well-established field. However, the microscopic mechanism of reorientation at a molecular level is not well understood. A means of studying the time course of reorientation is time-resolved Raman spectroscopy1. Each Raman peak relates to a different region of the molecule, thus by investigating the transient change in intensity of a given peak, information about the reorientation process of distinct molecular fragments can be gained. The method therefore has an advantage over methods such as polarised optical microscopy, which study the time response of the director only.

ISSN:1058-725X    

DOI:10.1080/10587259508033528

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Low-frequency dielectric properties of nematic liquid crystal (NLC) cells with and without polyimide alignment layers have been investigated with impedance spectroscopy. It is found that the dielectric behavior in the low frequency regime of NLC cells without polyimide alignment layers can be successfully explained in terms of the diffusive motion of impurity ions. The temperature dependences of the diffusion constant and the concentration of the impurity ions are determined from the dielectric behavior. NLC cells with polyimide alignment layers exhibit the interfacial polarization between the polyimide layers and NLC layer in the low frequency regime. The concentration of the impurity ions dissolved from polyimide layers to NLC layer is estimated.

ISSN:1058-725X    

DOI:10.1080/10587259508033529

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Dielectric relaxation studies are reported of several substances exhibiting the nematic (N) and smectic A (Sm A) polymorphism. From the relaxation times times τ∥characterizing molecular reorientations around the short axes the activation enthalpies ΔH∥for both phases were calculated. It was found that ΔH∥(N) is markedly larger than ΔH∥(SmA). The effect is interpreted on the basis of existing models of Sm A phase.

ISSN:1058-725X    

DOI:10.1080/10587259508033530

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Transient current technique is presented for measurement of rotational viscosity γ1and pretilt angle Θ°, in nematic liquid crystal display devices. Theoretical expressions for the determination of γ1and Θ0from transient current are derived by taking account of the conduction of impurity ions and director rotation. Experimental results for γ1and Θ0are compared with those with other conventional methods.

ISSN:1058-725X    

DOI:10.1080/10587259508033531

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The reorientation of molecules around their long axes should be influenced by the biaxiality of smectic phases with tilted molecules. Assuming a brownian reorientation process, the effect of a quadrupolar ordering on high-frequency dielectric spectra is investigated.

ISSN:1058-725X    

DOI:10.1080/10587259508033532

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We ask whether a model assuming a rotational diffusion of two molecular “halves”, consisting of rigidly coupled alkoxy groups plus benzene rings, works for interpretation of QNS data. For PAA such a model seems to work almost perfectly for both the normal and the methyl deuterated nematic substance. For PAP, however, it gives a much too intense quasielastic component in the normal nondeuterated substance indicating that there exist interconformational motions in ethoxy group which falls within the QNS observation window. When convoluting the scattering law of this motion with that of the molecular “halves” one obtains an almost perfect agreement with the QNS experimental data.

ISSN:1058-725X    

DOI:10.1080/10587259508033533

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The effects of uniaxial-biaxial phase transition on the spectral densities measured from the deuteron Zeeman (T1Z)and quadrupolar (T1Q) spin-lattice relaxation times are examined in two partially deuterated samples of 4-n-pentyloxybenzylidene-4‘-heptylaniline(50.7): one deuterated at the methine site (50.7-d1) and one with the aniline ring deuterated (50.7-d4). The deuterated sites are chosen so that their relaxation behaviours are not complicated by internal chain dynamics. It is found that the spectral densities show discontinuities at theSc- SBphase transition, but show no discontinuity at theSA- SCandSB- SGphase transitions. Both the smectic C and G phases are biaxial. The results are discussed in terms of molecular reorientation and director fluctuations. Preliminary data for the sample director oriented at 90° with the external magnetic field are reported in the B and G phases of 50.7-d1.

ISSN:1058-725X    

DOI:10.1080/10587259508033534

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Dielectric, electrooptic, DSC calorimetry studies and texture observations have been carried out for 4‘-[(S)-2 propoxypropoxy] phenyl 4-(4-decyloxy) benzoate in a wide temperature range. Dielectric spectra were taken in the frequency range from 10Hz to 10GHz. Electrooptic studies were performed on oriented SmC* phase. Texture observations and DSC calorimetry confirm the existence of a monotropic highly ordered S3phase. On the basis of TDS measurements and electrooptic studies this phase seems to be a highly ordered non-ferroelectric phase. Low frequency dielectric spectrum of the SmC* phase is complex and consists at least of two contributions which is typical of ferroelectric liquid crystals with high spontaneous polarization.

ISSN:1058-725X    

DOI:10.1080/10587259508033535

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The steady-state polarized fluorescence spectrum of an impure nematic liquid crystal has been investigated theoretically and experimentally. In the case of uniaxial fluorescent molecules the relationship has been established between the position of the centers of gravityVof the fluorescence bandsJ(V) and such factors as (i) the molecular electronic structure (the orientation of the absorption and emission dipole moments with respect to the molecular coordinate system), (ii) the orientational order parameters of the fluorescent molecules in their ground and excited electronic states, (iii) the characteristics of the rotational molecular dynamics and anisotropic probe-matrix interactions. The combinations of the data on the position and intensity of the polarized absorption and fluorescence bands have been used in order to determine the parameters characterizing the relaxation of the surrounding medium of the impurity over the lifetime of the latter in the excited state.

ISSN:1058-725X    

DOI:10.1080/10587259508033536

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor