Field-induced orientational order

Besides aligning liquid crystals, external electric fields can also change the orientational order and thus the electro-optical properties of liquid crystals. When the long molecular axis of a liquid crystal molecule, whose anisotropy of polarizability is positive, is parallel to the applied field, the potential of the molecule is low. Thus the applied field suppresses the thermal flue-mation and increases the order parameter. Now we discuss how the orientational order of a nematic liquid crystal changes with applied fields. Using the Landau-de Gennes theory, the free energy density of a liquid crystal in an electric field (when the liquid erystal director is parallel to the field) is [4] 

As discussed in Section 1.5.2, e// + 2cx = constant, namely, 3e// — 2As = constant and therefore // = (2/3)(A + constant). Equation (1.114) shows that the dielectric anisotropy Ae is a linear function of the order parameter S. At a temperature below the isotropic-nematic transition temperature and under zero applied field, when the order parameter is S , the dielectric anisotropy is (Ac)o. Approximately we have 

In the calculation of the order parameter by minimizing the free energy, the constant term can be neglected. The free energy density becomes 

Define the normalized field e = e E I2c, the normalized temperamre t = a T- T )k, and ft = btc. The normalized free energy density becomes 

The order parameter 5 as a function of the applied field E can be found by minimizing the free energy  

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Firouzi, A Schaefer, DH Tolbert, SH Stucky, GD Chmelka, BE, Magnetic-Field-Induced Orientational Ordering of Alkane Lyotropic Silicate-Surfactant Liquid Crystals, Journal of the American Chemical Society 119, 9466, 1997. 

The electric field-induced orientational order in a nematic liquid crystal is given by Equation (4.7). The liquid crystal has the parameter fi = 03. Numerically calculate the order parameter 5 as a function of the normalized temperature t in the region from -0.1 to 0.1 under various normalized electric fields e = 0.0, 030 121), 0121),... 

The anomalous Kerr effect near the isotropic-nematic transition can be qualitatively understood as follows. Approaching the I-N transition nematiclike aggregates form, and the sizes of the aggregates increase close to the transition. The growth of the Kerr constant can be calculated by taking into account the contribution of nematic aggregates to the orientation order parameter to the dielectric properties of the isotropic phase. This contribution was calculated in the frame of the Landau theory. Accordingly, the field induced orientation order is i... 

Three synthetic approaches to donor-acceptor-substituted conjugated molecules with enhanced orientability in electric fields, potentially applicable to the preparation of electro-optic polymers via electric field poling, are summarized. The three approaches are parallel attachment of chromophores to a common framework, embedding the chromophore in a zwitterion, and head-to-tail oligomerization of chromophores. The oligomerization method as well as the use of dyes as curing agents are briefly discussed in relation to the stability of electric field-induced polar order in polymer matrices. 

In the limit of the oriented gas model with a one-dimensional dipolar molecule and a two state model for the polarizability (30). the second order susceptibility X33(2) of a polymer film poled with field E is given by Equation 4 where N/V is the number density of dye molecules, the fs are the appropriate local field factors, i is the dipole moment, p is the molecular second order hyperpolarizability, and L3 is the third-order Langevin function describing the electric field induced polar order at poling temperature Tp - Tg. 

Electric-Field-Induced Orientation. Despite the fact that electric field orientation of liquid crystals is an important technology used, for example, in liquid crystal displays, there has been almost no work on the electric field alignment of LCTs. This may be because electric fields are effective only for thin films the high field strengths required, on the order of 10 V/cm or greater, can lead to dielectric breakdown in thick samples. Nevertheless, although bulk orientation cannot be obtained via electric fields, use of electric fields does provide a complement to the surface-induced techniques described above. 

First of all, due to a large molecular dipole /x, a field-induced polar order which is proportional to exp iiE/kBT) can be relatively high. This additional field orientation decreases the critical concentration for the formation of the nematic state by the value [240]... 

The concept of mechanical field induced orientation can easily be transferred to nematic elastomers with oblate chain conformation, i.e., side chain end-on elastomers with an even number of spacer atoms. In order to achieve a monodomain structure, a globally oblate chain conformation has to be established. This can be achieved by uniaxial compression or biaxial stretching of the polydomain elastomer which induces a uniform homeotropic alignment of the nematic director perpendicular to the film plane. Up to now, this orientaticMi technique has only been realized experimentally for chiral nematic elastomers [72]. 

As shown in previous sections, PR polymers with a low Tg, exhibit strong electric-field induced orientational effects that are responsible fi>r their high ciency. However, a drawback of these materials is the high electric field (30-100 V/pm) that needs to be >plied in order to orient these molecules efficiently. Another obvious... 

As the isoquinoline molecule reorients in the order listed above, the absorption of infrared radiation by the in-plane vibrational modes would be expected to increase, while that of the out-of-plane modes would be predicted to decrease (in accordance with the surface selection rule as described above). In the flat orientation there is no component of the dipole moment perpendicular to the surface for the in-plane modes, and under the surface selection rule these modes will not be able to absorb any of the incident radiation. However, as mentioned above, infrared active modes (and in some cases infrared forbidden transitions) can still be observed due to field-induced vibronic coupled infrared absorption (16-20). We have determined that this type of interaction is present in this particular system. 

Dopant orientation during and following electric field-induced poling can be studied continuously and in real time in order to examine the microenvironment surrounding the dopants in terms of the polymer relaxations and the applied corona field. In the results presented below, the SHG of 4-dimethylamino-4 -nitrostilbene (DANS) dispersed in polystyrene (PS) or poly(methyl methacrylate) (PMMA) matrices has been examined in corona poled films as a function of temperature in order to understand the effect of thermal conditions on the temporal stability of the dopant orientation. 

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