摘要:

A two-stage transient statistical investigation of the Freedericksz transition in Nematic Liquid Crystals (NLC) is presented. After applying an external field that exceeds the Freedericksz critical value, the director of the NLC relaxes from its unstable equilibrium state with the effect of noise. The statistical properties of this transient process can be characterized by the First Passage Time (FPT). We separate the whole process into two stages with the time when the distribution function of the director has its variance equal to the variance of the steady-state distribution at the critical point. The first stage is considered as an Ornstein-Uhlenbeck process and the second stage as a deterministic nonlinear transformation of the distribution function of the director. The explicit expression of the FPT distribution is analytically derived with this consideration. Monte Carlo simulations shown that the present theory describes the transient behavior of the NLC much better than the previous asymptotic approach. The mean, the variance and the skewness of our explicit FPT distribution function show excellent agreement with the numerical simulation.

ISSN:1058-725X    

DOI:10.1080/10587259808045643

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

N(p-n-dodecyloxybenzylidene)p-n-alkylanilines, viz., 120.mcompounds ofnO.mseries, form= 4, 5, 6, 8, 10, 12, 14 and 16 are synthesised and characterised by Polarised Thermal Microscopy (TM) and Differential Scanning Calorimetry (DSC). These compounds are found to exhibit 2D Bond-Orientationally (BO) ordered smectic-F liquid crystal phase with shift distortion along with orthogonal smectic-A, -B(cryst) phases. The intermediate members of the compounds of 120.mseries (m< 10) are found to exhibit orthogonal smectic phases along with the tilted phases, while the higher homologues (m> 10) show the tilted bond-orientational smectic phases only. The miscibility studies for the confirmation of smectic-F phase through binary (120.12 + 100.14) phase diagram, suggests the possibility for the occurrence of smectic-F phase down towards ambient temperatures. The smectic-G phase melting temperature,TFGis found to depress at lower and higher concentrations (x), while smectic-F phase clearing temperatureTIFremains constant with the concentration. The thermal span of smectic-F phase is found to depend on the chemical potential (represented by the mole fractionx) in the (T, x) phase diagram. The cubic fit in the vicinity of lower/higher concentrations, qualitatively infers the weak nature of Van der Walls' molecular forces for the growth of pseudo-hexagonal packed quasi-2-D BO orderedSFliquid crystal with a poor registry of positional correlations.

ISSN:1058-725X    

DOI:10.1080/10587259808045644

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The fluctuations of the director, the velocity and the temperature are investigatedviaquasistationary fluctuations theory [1], which has been applied to the linearized set of the nematodynamic equations near the threshold of the thermoconvective instability origin, when the nematic liquid crystal was subjected to temperature gradient. The densities of the spectral representation of the temperature, velocity and director fluctuations have been obtained. These results show a lorenzian behaviour of the mentioned variables near the threshold of the thermoconvection. The correlation functions dependence onz-component of wave vector in the constructed curves has been presented too.

ISSN:1058-725X    

DOI:10.1080/10587259808045645

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Four Thermotropic Liquid Crystalline (TLC) random copolyesters were prepared by direct polycondensation of two diols and oxaloyl chloride ino-dichlorobenzene solution. The structures of the repeating units of the synthesised polyesters were examined by FT-IR and PMR spectral data. The thermal transition temperatures were determined by differential scanning calorimetry. The LC behaviour of these polymers were studied by optical polarising microscopy in conjunction with WAXD studies. Three of these polyesters exhibited interesting biphase behaviour. The polyester containing 2,5-ditertiarybutylphenylene rings in the main chain exhibited discotic behaviour.

ISSN:1058-725X    

DOI:10.1080/10587259808045646

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Two mesogenic homologous series are synthesised, by fixing a rigid 4′-propiophenone group to 4-Hydroxy phenyl azo moiety. The hydroxy group is etherified and esterified byn-alkyl chains and 4-n-alkoxy benzoyl groups respectively. The etherified series (I)viz:4-n-alkoxy phenyl azo 4′-propiophenones exhibit nematic phase from C2to C4homologue; smectic A phase emerges from the C4derivative in the monotopic form, which becomes enantiotropic in the C5homologue and persists till the last C16derivative studied. The esterified series (II) namely: 4-(4′-n-alkoxy benzoyloxy) phenyl azo 4″-propiophenones shows only nematic phase in C1and C2homologues. The smectic phase emerges from C3derivative, together with nematic phase and the series exhibits polymesomorphism from C3to C12homologue. The last two homologous exhibit only smectic A phase. Both the homologous series are thermally very stable and exhibit a wide mesomorphic range. Their thermal stabilities and other characteristics are discussed.

ISSN:1058-725X    

DOI:10.1080/10587259808045647

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The liquid crystallinity of poly(3-dodecylthiophene) was studied systematically at selected temperatures with manually controlled shearing, using optical microscopy and differential scanning calorimetry, to determine the necessary conditions for the formation of liquid crystallinity. Poly(3-dodecylthiophene) did not show thermotropic liquid crystalline behavior under conditions which are usually used in the study of thermotropic liquid crystallinity. When a shear stress was applied to a solvent cast film, a Schileren texture was observedin situ. A finer texture was observed as the annealing temperature and time of the sample film were increased. There was no direct relationship between the melting behaviors in differential scanning calorimeter thermograms and the liquid crystalline behavior of poly(3-dodecylthiophene).

ISSN:1058-725X    

DOI:10.1080/10587259808045648

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The effect of doping aromatic compounds on the structure and physical properties of lecithin were studied using X-ray diffraction and broad-line1H-NMR methods and by the measurement of phase-transition behaviour using an optical method. To examine morphological change in the liposomal structures of doped egg-yolk lecithin systems, myelin figures were observed using a polarizing microscope. The lecithins used were L-α-dipalmitoylphosphatidylcholine (L-DPPC) and egg-yolk lecithin (egg-PC). Xylene and ephedrine were used as a hydrophobic aromatic dopant and a hydrophilic aromatic dopant, respectively. Xylene molecules were located between two ends of the hydrocarbon chains of lecithin molecules set toe-to-toe in the bilayer structure,i.e., the central part of the bilayer. In contrast, ephedrine molecules with water molecules were located between two polar head groups of the adjacent bilayers,i.e., the outer part of the bilayer. The doping of both aromatic compounds increased the stacking repeat distance of bilayer systems. The increase of the bilayer thickness due to the doping of aromatic compounds altered the thermotropic phase transition behaviour. The first phase transition point observed around 120°C for hydrated egg-PC shifted to a low temperature region, and the growing behaviour of myelin figures observed for these doped lecithin/water systems tended to grow in coiling and twisted structures.

ISSN:1058-725X    

DOI:10.1080/10587259808045649

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We report dielectric investigations on the fourth and fifth homologues of the series 1-nitro-2,3,6,7,10,11-hexaalkoxytriphenylene. A hexagonally ordered columnar mesophase (Dho) is exhibited by both the compounds but the fourth homologue shows in addition, a more ordered mesophase at lower temperatures. The dielectric constants observed are high compared to the values observed in other discotic materials. In the frequency range studied (1 Hz– 1 MHz) one relaxation mode is observed in theDhophase which can perhaps be correlated with a glass transition. A strong reduction in the magnitude of the relaxation frequency and an associated increase in the dielectric strength of the mode are observed at the transition fromDhoto the lower temperature phase. The data in the two-phase region of this first-order transition are also presented.

ISSN:1058-725X    

DOI:10.1080/10587259808045650

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Precision differential scanning calorimetric (DSC) analysis of the effect of doping cholesteryl nonanoate with the enantiomers of 4-(4-n-hexyloxybenzoyloxy)-isooctyl benzoate (R811 and S811) has been performed. Depressions of the enthalpies of both phase transitions, from crystalline to cholesteric and from cholesteric to isotropic phases, were found to be ∼ 1.5 times greater for the samples of cholesteryl nonanoate doped with left-handed S811. Depressions of the temperatures of phase transitions are also more pronounced in the case of S811. Thus, the left-handed enantiomer S811 affects the molecular packing of cholesteryl nonanoate much more strongly than its right-handed counterpart R811. Remarkably, the difference between the enthalpy of melting of samples containing only 1 wt% of either enantiomer reaches ∼ 20% of the enthalpy of fusion of pure cholesteryl nonanoate, indicating strong diastereomeric discrimination in the system. This calorimetric manifestation of diastereomer discrimination of S811 and R811 in cholesteryl nonanoate is in agreement with earlier observed nonsymmetrical effects of these enantiomers on the pitch of cholesteric liquid crystals; that is the left-handed S811 enantiomer constricts the pitch nine times more strongly than the right-handed R811 expands the helix.

ISSN:1058-725X    

DOI:10.1080/10587259808045651

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Liquid crystal (LC) waveguide claddings with different boundary couplings have been electrically switched. It is observed that the cut-off voltage of guided modes significantly decreases with decreasing the azimuthal surface anchoring energy of the LCs. An LC-covered planar lightwave modulator with low driving voltage of 12 volts is demonstrated by controlling the surface azimuthal anchoring energy of LC claddings using photoalignment technique.

ISSN:1058-725X    

DOI:10.1080/10587259808045652

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor