Molecular alignment birefringence

LCs are materials that undergo physical reorganization in which at least one of the rearranged structures involve molecular alignment along a preferred direction, causing the material to exhibit nonisotropic behavior and associated birefringent properties, i.e., molecular asymmetry. 

K. Hongladarom, W. R. Burghardt, S. G. Baek, S. Cementwala, and J. J. Magda, Molecular alignment of polymer liquid crystals in shear flows. I. Spectroscopic birefringence technique, steady-state orientation, and normal stress behavior in poly(benzyl glutamate) solutions, Macromolecules, 26, 772 (1993). 

The use of birefringence to determine the behavior of 5( 7) is a natural choice since the principal characteristic of the nematic phase is optical birefringence i.e., the refractive index differs for light polarized parallel (/ n) or perpendicular (%) to the axis of molecular alignment. Eor a nematic liquid crystal, the director n specifies this optical z axis and / n = and = Dg are called the extraordinary and ordinary refractive indices, respectively. In general, rig > rig and the difference is the refractive index anisotropy (birefringence)... 

For phase identification, therefore, it is essential to be familiar with the different birefringence textures each phase can exhibit. These textures depend on molecular alignment with respect to the optical axis and the polarizers. To illustrate this, we can consider the Schlieren texture of the nematic phase shown in Figure 2.22. 

This is because the polymer fibers and walls are formed in the molecular-aligned solution by oriented polymerization in the rubbing direction. Therefore, no birefringence of the formed polymer occurs under crossed Nicol polarizers, and a good contrast characteristic is easily obtained with suppressed black level in the off-state. 

Birefringence is a useful property to monitor orientation [1,12,38], Birefringence occurs as the molecular chains are aligned in the process of orientation the magnitude and sign of the effect are determined by the chemical nature of the polymer. 

As to infrared spectroscopy - and the same holds good for other spectral ranges -the orientational order is readily observable in form of dichroism Being related to the molecular shape, the molecular polarizability is anisotropic as well. By the alignment of the molecules this anisotropy is transferred to the sample, however damped due to the imperfect order as described by the order parameters. As a consequence, the dielectric function and furthermore the (complex) refractive index are anisotropic, so that eventually (linear) dichroism and birefringence occur. 

Orientational order of LCPs is conveniently described in terms of two differoit order parameters, characterizing the average orientation of the mesogenic units within a molecular domain (microorder) and the macroscopic alignment of the domains (see Fig. 3) [10,35]. The measurement of these parameters is important for relating macroscopic physical properties of LCPs to their molecular structure. Various methods have been used to measure order parameters in these systems, including the use of X-ray diffraction [128-130], birefringence [103, 104], and linear dichroism [104,131], but these methods can not essentially separate the two types of orientational order. 

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