Alignment in electric fields

Electric fields can be used to align permanent dipoles present in PLCs and, if the dipoles are rigidly linked to the mesogens, the latter can 

The rotation of the mesogen about the backbone chain is referred to as the 5 process. The mesogen director reverses its direction as a result of the d process. This process also occurs in MLCs and exhibits almost a single relaxation time with an activation energy of 50—70 kj mol [82, 83]. The d process is very narrow in one-comb polysiloxanes with 

Talrose et al. [85] reported that the response for the alignment of one-comb PLCs was faster at higher temperatures (nematics) and that the response time was proportional to the reciprocal of the square of the applied voltage, a trend which indeed was the same as for MLCs. Ringsdorf and Zentel [86] found that the response time decreased with increasing difference between the actual and glass transition temperatures. 

Polymers such as polysiloxanes with flexible backbones are therefore ideal for the purpose of achieving short response times [87-89]. 

The fastest response times reported have been of the order of 100 ms at temperatures close to the isotropization temperature or in the biphasic region [88-94]. For this reason, many researchers align their samples during a slow cooling through the isotropic-nematic phase transition. 

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Acetylene was also polymerized in the channels of AIPO4-5 crystals that were aligned in electric fields. ... 

Laminar Structures in Poly(y-benzyl-L-glutamate) Films Aligned in Electric Fields... 

Piezoelectric materials can enact deformation and mechanical forces in response to an applied voltage. Rather than undergoing a phase transformation, piezoelectric materials change shape when their electrical dipoles spontaneously align in electric fields, causing deformation of the crystal structure. 

As shown in the previous section. Sc elastomers can be well aligned in electric fields. These preparation techniques are limited to thin film geometries, while mechanical orientation offers the possibility to prepare uniformly oriented bulk samples. 

The first term is independent of the orientation of the director, while the second term has a dependence on the director s orientation. Thus, if Ae > 0,/e is minimized when the director is along the applied electric field. The director lines up normal to the electric field when Ae < 0. It should be noted that alignment in electric fields is often complicated by the presence of ionizable impinities in the liquid crystal and that electric field effects are complicated by the injection of supplementary carriers and chemical degradation at the electrodes. Therefore, they are not as easy to interpret as magnetic field effects. 

Molecules of nematic Hquid crystals also are aligned in flow fields which results in a viscosity that is lower than that of the isotropic Hquid the rod-shaped molecules easily stream past one another when oriented. Flow may be impeded if an electric or magnetic field is appHed to counter the flow orientation the viscosity then becomes an anisotropic property. 

Orientation polarization can occur in materials composed of molecules that have permanent electric dipole moments. The permanent dipoles tend to become aligned with the apphed electric field, but entropy and thermal effects tend to counter this alignment. Thus, orientation polarization is highly temperature-dependent, unlike the forms of induced polarization which are nearly temperature-independent. In electric fields of moderate intensity, the orientation polarization is proportional to the local electric field, as for the other forms of polarization... 

Pereira GG, Williams DRM (1999) Diblock copolymer melts in electric fields the transition from parallel to perpendicular alignment using a capacitor analogy. Macromolecules 32(24) 8115-8120... 

Independently, a highly interesting problem recently revived is that of the alignment of molecules in liquid crystals due to externally applied static and alternating electric fidds. The problem was first approached by Jezewski and Kast, and was developed by Carr, Helfrich, Wysocki et al. and many others, studying various aspects, such as dielectric loss in electric fields and anomalous alignment in the smectic phase and domains, the effect of an electric field on the temperature of mesomorphic-isomorphic phase tranritions of liquid crystals, electric... 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

In this section, we discuss the behavior of liquid crystal suspensions under the action of an external electric field. The behavior of colloidal suspensions in electric fields is of considerable technological interest with the so-called Electro-Rheological (ER) fluids [17, 18]. The main features of this behavior are now rather well understood. When an external field is applied, particles suspended in an isotropic fluid become polarized. Resultant dipole-dipole interactions between the particles lead to their chaining along the direction of the applied field. When suspended in a liquid crystal host, colloidal particles are also expected to be polarized upon the application of an electric field. However, new phenomena may take place because of the specific response of the liquid crystal. In this case, the external field is likely to alter the distortions of the liquid crystal alignment... 

Various attempts have been made to improve Born s simple treatment while still regarding the solvent as continuous. One way of doing this is to consider how the effective dielectric constant of a solvent varies in the neighborhood of an ion. The dielectric behavior of a liquid is related to the tendency of ions to orient themselves in an electric field. At very high field strengths the molecules become fully aligned in the field and there can be no further orientation. The dielectric constant then falls to a very low value of about 2. This effect is known as dielectric saturation. 

Large ions normally feature a complex distribution of charges and thus have nonzero electric quadrupole, octupole, and higher multipole moments. However, those produce zero torque in homogeneous fields and thus are not relevant to ion alignment in high-field IMS. 

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