Modern Polymer Spect..

Figure 2-18. Scheinatic representation
of the segmental
motions of the NLCP during
the switching process.
There is a strong correlation in the synchronous spectrum and no appreciable
asynchronicity in the asynchronous spectrum between the mesogen band (2230
cm-') and the v(CH2) bands (2926 and 2863 cm-I). The main part of the intensity
changes in the region of 2850-2950 cm-' is due to the spacer; thus, on the basis of
the 2D results we may draw the conclusion that the spacer and the mesogen reorient
simultaneously.
From a further detailed 2D analysis of selected absorption band intensities of the
corresponding power spectrum (16, 60, 611 it was finally concluded that only part of
the spacer of the NLCP takes part in the reorientation. This result is summarized
graphically in Figure 2- 18, where the orientational behavior of a NLCP-mesogen
during the switching process is symbolized schematically relative to the entire
polymeric structure, including the spacer and the main chain.
2.5.4 Nematic Liquid-Crystalline Guest-Host System in a
Switching Experiment
Generally, the phenomenon of dissolving and aligning a molecule or a group of
molecules, such as dyes or probes, for example, by a liquid crystal can be called the
guest-host phenomenon [65, 661. The host liquid crystal can be a single compound
or a multicomponent mixture. Depending on its structural geometry (e.g., elongated
or spherical) the guest molecule couples more or less significantly to the anisotropic
intermolecular interaction field of the liquid-crystalline host. The liquid-crystalline
solutions can be easily oriented by electric, magnetic, surface, or mechanical forces,
resulting in highly oriented solvent and solute molecules. This phenomenon provides
the basis for the application of liquid crystals as anisotropic solvents for
spectroscopic investigations of anisotropic molecular properties (67, 681. Actually,
in a broad sense. most of the commercially available liquid-crystal mixtures may be
regarded as being related to the guest-host effect, as they incorporate some nonmesomorphic
molecules to generate the desired electro-optical effects in the mixtures
(69-711. In the present section we present an example of how a guest molecule
is oriented anisotropically in a liquid-crystalline solution which is exposed to an