Orientation by external fields

Thus the dynamical structure factor has a very broad distribution of decay rates ranging from 0 to Dnk  

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Experimentally, the direction of a transition moment in a molecule can he evaluated by four methods (i) polarized spectra of single crystals, (ii) fluorescence or phosphorescence polarization, (iii) spectra of molecules embedded in stretched Aims, and (iv) spectra of molecules oriented by external fields. Only relative directions of the transition moment can be determined by means of the last three methods, whereas the polarized spectra of single crystals give the absolute direction of the moments if the crystal structure is known. The first method has been applied to the study of the electronic structure and spectra of several pyrimidine bases of nucleic... 

When a strong static electric field is applied across a medium, its dielectric and optical properties become anisotropic. When a low frequency analyzing electric field is used to probe the anisotropy, it is called the nonlinear dielectric effect (NLDE) or dielectric saturation (17). It is the low frequency analogue of the Kerr effect. The interactions which cause the NLDE are similar to those of EFLS. For a single flexible polar molecule, the external field will influence the molecule in two ways firstly, it will interact with the total dipole moment and orient it, secondly, it will perturb the equilibrium conformation of the molecule to favor the conformations with the larger dipole moment. Thus, the orientation by the field will cause a decrease while the polarization of the molecule will cause an... 

Up to 3/4 of the domains can be oriented by external electric field at elevated temperatures, thus obtaining a suitably high and permanent polarization. For example, several hours and 2kY/mm are required for this purpose in the case of BaTi03. The orientation is advantageously carried out during slow cooling down from a temperature above the Curie point. In this way, the domains become oriented during their formation and the state attained remains fixed at room temperature. 

Another important feature of the ferroics is the possibility to control the domain structure by external fields. This basic feature has been introduced by Aizu [4,5], who coined the term ferroic . He defined ferroic as a material, which has two or more orientation states in the absence of magnetic field, electric field, and mechanical stress, and can shift from one to another of these states by means of magnetic field, an electric field, a mechanical stress or a combination of these . It is obvious, that when orientation states (domains) are converted to each other under the action of external fields, the interfaces (domain walls) separating them are transferred accordingly. 

Mesogenic groups can be incorporated into polymeric systems [7]. This results in materials of novel features like main chain systems of extraordinary impact strength, side-chain systems with mesogens which can be switched in their orientation by external electric fields or—if chiral groups are attached to the mesogenic units—ferroelectric liquid crystalline polymers and elastomers. The dynamics of such systems depends in detail on its molecular architecture, i.e. especially the main chain polymer and its stiffness, the spacer molecules... 

Ong, H.L., Meyer, R.B., Hurd, A.J. Multistable orientation in a nematic liquid crystal cell induced by external field and interfacial interaction. J. Appl. Phys. 55, 2809-2815 (1984)... 

It is well known that the orientation of a nematic layer can be influenced by external magnetic or quasistatic fields too. Thus it can be expected that the orientational deformation caused by a light field can be controlled by external fields. The aim of the present letter is to report experimental results on such an effect. 

If LSCEs are prepared using one of the methods described in Sect. 4.1, a monodomain may be obtained with respect to the main director but not necessarily for the whole phase structure. In the case of Sc elastomers an orientation of the director by mechanical stretching or by external fields yields a polydomain with respect to the layer normal. The additional orientation steps that are necessary for a full orientation of this phase are outlined in Sect. 4.2.2. 

In the diagram below (Figure 11.4), the black arrows represent the electric field vector and the gray arrows show the electric force on the molecule. Even if a molecule does not form a permanent dipole, it can still be influenced by an electric field. In some cases, the field produces slight re-arrangement of electrons and protons in molecules such that an induced electric dipole results. While not as strong as permanent dipoles, orientation with external field still occurs. 

In addition to the above lamellar, columnar and optically isotropic phases, there are also lyotropic nematic phases, which usually involve mixtures of a charged amphiphilic, such as simple soap, with an alkanol (a weaker amphiphilic where the head group is an alcohol), together with water and a simple salt. They are termed nematic because, like thermotropic nematics, their optical axes are easily oriented by external magnetic fields. In contrast to thermotropic nematics, the basic units of lyotropic nematics are molecular aggregates with dimensions of about 2-10 nm. In lyotropics, the nematic phase is much less usual than in thermotropic liquid crystals. Lyotropic nematics... 

It has been reported that liquid crystalline polymers can be oriented mechanically (shear and stretching) or by external fields (electric, magnetic), but was unsatisfactory for optical films. 

This alignment giving rises to a low viscosity and makes the LCP phases easier in processing. Nematic mesophases has got fluidity characteristics very similar to that of isotropic liquids but they can be easily oriented by external source like electric or magnetic field. Aligned nematic phases have got very... 

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