Modern Polymer Spect..

2.8 Conclusions 81
strain. As A0 is an experimentally determined parameter based on a uniaxial orientation
model whose value should represent the intensity of an absorption band
exclusive of orientation effects, the anomalous increase of the structural absorbance
during elongation-where the sample thickness actually decreases-points towards an
invalidity of the uniaxial syinmetry. T~us, the geometries a and b of Figure 2-52 can
be excluded on the basis that both would lead to a decrease of A" during elongation.
The occurrence of the structural model c at the inversion point of the order
parameter has first been proposed on the basis of small-angle X-ray measurements
by Finkelinann and Kiipfer [117]. Thus, the appearance of four diffraction maxima
have been explained in terms of a biaxial, planar orientation of the mesogens in the
film plane (Figure 2-52c). Depending on the inclination angle of the mesogens with
the stretching direction, such a transition state would lead to an order parameter
close to zero and could also explain the anomalous behavior of the structural
absorbance [36]. In conclusion, it could be shown that, during elongation of the
anisotropic LSCE network, the network structure changes from a uniaxial symmetry
perpendicular to the applied mechanical field (parallel to the orientation induced
in the second crosslinking step) via a planar-biaxial model (inversion of the order
parameter) to a system aligned in the stretching direction of the rheo-optical experiment
[36]. Simultaneously, the originally transparent polymer becomes opaque
at the inversion point of the order parameter and turns transparent again upon
reorientation of the mesogens into the machine direction.
2.8 Conclusions
The aim of this contribution was to demonstrate the potential of FTIR/FTNIR
polarization spectroscopy at variable temperatures for the characterization of segmental
mobility in liquid crystals and liquid-crystalline polymers under the influence
of external fields. Selected examples have been discussed, where either timeresolved
measurements or static experiments have been utilized to monitor the
effects of electric, electroniagnetic, or mechanical perturbations on the liquidcrystalline
system under investigation. For the extremely fast, reversible switching
processes of low-molecular weight liquid crystals and side-chain liquid-crystalline
polymers in electric fields, the novel step-scan FTIR technique has been applied.
The selective character of vibrational spectroscopy generally allowed us to separate
the orientational effects in different functionalities of the liquid-crystalline compound.
Thus, side-chain liquid-crystalline polymers, for example, have been characterized
under the influence of different external fields in terms of the relative
alignment of their main chain, spacer, and mesogenic groups, respectively. Although
the presented data are necessarily limited to selected examples, the variety of
information and its detailed quantitative Character demonstrate, that FTIR/FTNIR
polarization spectroscopy has certainly become an extremely valuable tool lo elucidate
the dynamics of segmental motion in liquid-crystalline systems.