Dipole reorientation, nematics

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

Anisotropic molecules show optically isotropic behavior in the bulk when they are disordered and randomly oriented, for instance in solutions or liquid crystal above the transition temperature. Under the influence of a strong beam, the induced dipole moment of the molecules feels a torque that tends to orient the molecule. The reorientation of the molecular dipoles induces a change in the refractive index. The typical values for molecular susceptibilities and the time-responses vary depending on the type of systems. For small anisotropic molecular systems, x 10 esu, with a time response 10 s. However, in the nematic phase, liquid crystal molecules are strongly correlated, resulting in much higher values, x 10 esu,... 

Before going into details of optical reorientation we survey the effects caused by static electric fields in nematic layers. (More precisely we consider quasi-static electric fields for which the complications arising from space-charges and fiexo-electricity can be neglected.) The interaction between the field and the nematic can be described by including an interaction term in the free-energy density of the liquid crystal. In the electric-dipole approximation this term is... 

In general, when a liquid crystal molecule is subject to an electric field the molecule will tend to become electrically polarized. This polarization creates a dipole in the molecule that will tend to align with the electric field. In the bulk nematic phase, aligned molecules will reorient together in response to the applied field. This bulk reorientation of the nematic director field is known as the Freedericksz transition. 

Consider first an anisometric molecule with the longitudinal p, and transversal p, permanent dipole moments in an isotropic phase. There are two relaxation modes mode 1, rotations of p, around the long axis, and mode 2, reorientation of p,. Figure 10-1. The mode 1 has a smaller relaxation time, Tj < Tj, because of the smaller moments of inertia involved. When this isotropic fluid is cooled down into the NEC phase, the dynamics is affected by the appearance of the nematic potential associated with the orientational order along the director n. The mode 1 remains almost the same as in the isotropic phase, and contributes to both the parallel and perpendicular components of dielectric polarization (determined with respect to n). Mode 2 is associated with small changes of the angle between p, and n it contributes to the parallel component of dielectric polarization. Mode 3 is associated with conical rotations of p, around the director (as the axis of the cone) it is effective when the applied electric field is perpendicular to n and contributes... 

Recently Urban and co-worko [15] have published dielectric measurements on l-(4-n-hexyl-bicyclo[2,2,2]octyl)-2-(3-fIuoro-4-methoxyphenyl)ethane. This compound has a much strongo perpoiclicular dipole moment i2 than pi. It is tho efore a suitable compound with which to study the influence of the nematic phase on motion reorientational about the molecular long axis. FIGURE 5 shows the Cole-Cole plots measured in the isotropic and nematic state. In the mesophase only a small absorption in the parallel direction with an increm Ain detected. 

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