Modern Polymer Spect..

2 Segmental Mobility of Liquid Crystals and
Liquid-Crystalline Polymers under External
Fields: Characterization by Fourier-tr ansform
Infrared Polarization Spectroscopy
H. W Siesler, I. Zebger, Ch. Kuliniia. S. Okretic, S. Shilov and U. Hoffnuinn
2.1 Introduction
Liquid crystals and maia-cliain/side-chain liquid-crystalline polymers have gained
scientific and technical importance due to their applications as display materials,
their exceptional mechanical and thermal properties, and their prospective applications
for optical information storage and nonlinear optics, respectively [ 1-13].
Among other aspects, the study of segmental mobility as a function of an external
(electric, electromagnetic, or mechanical) perturbation is of basic interest for the
understanding of the dynamics of molecular processes involved in technical applications
of liquid-crystalline materials.
Fourier-transform infrared (FTIR) polarization spectroscopy has proved an
extremely powerful technique to characterize the segmental mobility of specific
functionalities of a polymer or a liquid-crystal(1ine polymer) under the influence of
an external perturbation [ 14-23]. In a side-chain liquid-crystalline polymer
(SCLCP) aligned due to the influence of an external field, for example, the orientation
of the polymer backbone relative to the rigid mesogeii is strongly influenced by
the coupling via a more or less flexible spacer (Figure 2-1). Thus, the availability
of characteristic absorption bands for these individual functionalities will allow
a quantitative characterization of their relative alignment and contribute towards
a better understanding of their structure-property correlation. Based on this
knowledge, tailor-made systems can then be synthesized by a systematic variation
of partial structures.
In what follows, different phenomena will be discussed for selected classes of
liquid-crystalline compounds:
structure-dependent alignment of side-chain liquid-crystalline polyacrylates on
anisotropic surfaces
electric field-induced orientation and relaxation of a liquid crystal, a side-chain
liquid-crystalline polymer and a liquid-crystalline guest-host system
reorientation dynamics of a ferroelectric side-chain liquid-crystalline polymer in
a polarity-switched electric field
alignment of side-chain liquid-crystalline polyesters under laser irradiation
shear-induced orientation in chrial smectic liquid-crystalline copolysiloxanes