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High-resolution x-ray diffraction studies have been performed on the polar liquid-crystal binary mixture of octyl- and decyl-oxyphenyl nitrobenxoyloxy benzoate (47.4 mole% DB8ONO2 + 52.6% DB10ONO2). At temperatures below 127 °C this mixture exhibits the phase sequence (with decreasing temperature) smectic-Ad—reentrant nematic Nd—reentrant nematic N1—smectic-A1—tilted smectic antiphase smectic- [Ctilde] -smectic-A2—smectic-C2. The development of lateral fluid antiphase fluctuations from the partial bilayer fluctuations in the smectic-A1 phase have been probed and analyzed as well as the transition from the tilted smectic antiphase to the tilted bilayer smectic-C2 phase that is accompanied by the occurrence of an intermediate smectic-A2 phase.

The structural evolution of films of the liquid crystal mixture mx5112 in the Sm-A and Sm-C* phases has been studied using X-ray scattering techniques. Liquid crystal (LC) films spread on glass substrates with relief grating of 10um period are subjected to competing boundary conditions on opposite sides. On one side, the LC-air interface imposes a homeotropic boundary condition while on the other the glass substrate with grating generally imposes a homogeneous boundary condition. Near room temperature, the smectic layer normal is oriented parallel to the grating lines on the substrate. The measured smectic layer spacing reveals oscillations between a Sm-C* and a Sm-A like structures which depend on the film thickness as well as the distance from LC-air interface and is attributable to the difference in the molecular tilt imposed by a. the air-LC interface; b. the substrate; and c. the gratings' internal walls.

A longitudinal ferroelectric smectic phase lacks a reflection or mirror symmetry, which may be achieved with appropriate chemical design of molecules. Using x-ray diffraction, we have examined thre...

THE key requirement for a material to exhibit nonlinear optical (NLO) activity is the presence of non-centrosymmetric (polar) order, an attribute that is usually restricted to certain crystalline classes and ferroelectric liquid crystals1,2. NLO activity can also be obtained in some amorphous organic materials by applying an intense electric field (corona discharge) above the glass transition temperature, Tg, and subsequently quenching the field-induced polar orientation order3,4. Such materials are attractive for NLO device applications, as they promise lower costs and easier processibility than their crystalline organic and inorganic counterparts5. But field-induced polar order is not stable, and the eventual return to equilibrium (apolar) order results in a deterioration of NLO activity, particularly at temperatures near Tg (refs 6, 7). Here we show that this thermally activated decay of polar order can be circumvented by using a liquid crystal in which both mesogens (molecules that induce a liquid-crystal phase) and NLO-active chromophores are appended to macromolecular siloxane rings. We find that a shear-aligned melt of these composite macromolecules gives rise to a material with a monodomain lamellar superstructure that retains bistable, field-induced polar order above Tg. We attribute the thermal stability of these materials to an energetically favoured polar packing arrangement of the constituent macro-molecules.

The mechanism responsible for liquid crystal (LC) alignment on mechanically buffed or UV exposed polymer films is poorly understood. A comprehensive study of LC alignment on variously prepared substrates unequivocally shows that the anisotropy in the surface roughness of the substrate completely determines the direction of LC alignment. In all the cases studied, including those where an anchoring transition occurs with temperature, the LC director (re)aligns in the directions of low roughness.