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1.
T. Arun Kumar V.S.S. Sastry Ken Ishikawa Hideo Takezoe N.V. Madhusudana 《Liquid crystals》2013,40(8):971-979
We report experimental studies on defects in a nematic liquid crystal with negative dielectric anisotropy mounted in a cell with perfluoropolymer-coated surfaces. The sample exhibits a discontinuous anchoring transition from planar to homeotropic on cooling at zero or a small electric field, and above a cross-over voltage a continuous ‘inverse Freedericksz transition’, at which the director starts tilting in opposite directions at the two surfaces. Defects of strength ±1/2 are either annihilated or expelled when the director tilts. On the other hand, disclination lines of ±1 which end in partial point defects (boojums) at the surfaces in the planar alignment regime acquire point defects of strength ±1 at the midplane of the cell when the director tilts. At a low enough temperature, the homeotropic anchoring becomes strong, and an electric field above the Freedericksz threshold generates the usual umbilic defects, which follow the dynamic scaling laws found in earlier studies. 相似文献
2.
The term “liquid crystal” signifies a state of aggregation that is intermediate between the crystalline solid and the isotropic liquid. A substance in this state is strongly anisotropic in some of its properties and yet exhibits a certain degree of fluidity, which in some cases may be comparable to that of an ordinary liquid. A large number of organic compounds are now known to form liquid crystals. An essential requirement for mesomorphism to occur is that the molecules must be highly geometrically anisotropic. The transition to the intermediate state may be brought about by purely thermal processes (thermotropic mesomorphism) o r by the action of solvents (lyotropic mesomorphism). 相似文献
3.
Our earlier model of re-entrant liquid crystalline phases exhibited by highly polar compounds, in which the mutual orientation of near-neighbour molecules can change from an antiparallel to a parallel configuration, has been extended to include both nematic and smectic interactions. We show that, as the McMillan parameter alpha is decreased, the SmAd-SmA1 line goes over to the SmAd-N1Re line, finally becoming the NdRe-N1Re transition line, the latter ending in a critical point. This sequence is in agreement with the predictions of Prost's Landau model as well as with an experimental result. The phase sequence N-SmAd-NdRe-N1Re-SmA1 is obtained on cooling for a range of alphaA values. 相似文献
4.
Symmetrically N,N′-disubstituted ureas/thioureas were synthesized by heating amines or phenyl hydrazine and urea/thiourea on a preheated hot plate at 80–85°C under solvent-free conditions in the presence of a catalytic amount of ZnCl2 as a catalyst. The protocol has the advantages of not using toxic phosgene and other hazardous substrates or organic solvents. Increased reaction rate, good yield, and a simple workup procedure are involved. 相似文献
5.
The first stereoselective total synthesis of natural product, dendrodolide A (1) is described from readily available (R)-propylene oxide and 3-buten-1-ol as starting materials. The synthesis was achieved in 10 steps with an overall yield of 19.1%. The key steps involved in the synthesis are Jacobsen hydrolytic kinetic resolution, epoxide ring opening with 2-allyl-1, 3-dithiane, Yamaguchi esterification, and ring-closing metathesis (RCM). 相似文献
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7.
The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition
are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of
the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation
is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat
for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations.
Even the new mean field potential appears to be inadequate forz=3. 相似文献
8.
The hexane extract of dried fruits of Piper longum on fractionation afforded a new alkamide, isodihydropiperlonguminine and two phenyl propanoic acid derivatives. The structures of these compounds are established based on spectroscopic evidence and synthesis. 相似文献
9.
10.
Recently Pikin and Terent'ev have analysed the influence of the non-uniformity of the order parameter on the measured value of the surface cohesion energy in nematic liquid crystals (1988, Sov. Phys. Crystallogr., 33, 641). We argue that since the mathematical problem is ill-posed, the analysis has to be revised. In particular, quadratic terms in the second derivatives of the deformation angle and the order parameter should be taken into account. 相似文献