Modern Polymer Spect..

76 2 Segnrental Mobilitv of Liquid Crystals cmd Liquid-Crystalline Poljvners
The absorption bands observed in this region can be assigned to overtones and
combinations of characteristic fundamental frequencies. Apart from analytical
applications, NIR spectroscopy has been demonstrated to be of value also to rheooptical
applications [17, 114, 11 51. Thus, although NIR spectra are generally less
informative than their mid-infrared analogs due to overlap of absorption bands, the
details of the changes in crystallinity, conformation, and orientation are also contained
in the overtone and combination vibrations [I 7, 11 31.
2.7.2.4 Experimental
The rheo-optical investigations were performed in the stretching machine shown in
Figure 2-44b. For this purpose, film samples of the LSCE and the reference polymer
(sample thickness about 300-350 pni, original length 8 mm, width 5 mm) were
subjected to an elongation-recovery cycle up to about 45% strain, with a strain
rate of ll'l/o/min and 20-scan interferograms with a spectral resolution of 5 c1n-l
were collected siinultaneously with radiation alternatingly polarized parallel and
perpendicular to the drawing direction. After completion of the experiment the
interferograms were transformed to the corresponding spectra and evaluated in
terms of the order parameter (orientation function) [14, 17-20, 1121 of selected
absorption bands. The experiments discussed here were performed at 27 "C. In
order to study the reorientation mechanism of the preoriented LSCE [ 1 16- 12 I]
and the isotropic reference elastomer during the mechanical treatment of the rheooptical
experiment, the samples were mounted in the clamps of the stretching
machine after a rotation of 90" from the deformation direction in the second
crosslinking step (Figure 2-45). Thus, in the LSCE the mesogens are originally oriented
perpendicular to the subsequent drawing direction. This is reflected in the
direction of deforni;ition
during crosslinking
Q
0%) deformation
direction of mechanical
force during rheo-optical
experiment
maximum strain 45%
n
change in director
orientation during deformation
8
polarization direciian
of NIR-radiation
transparent opaque transparent
Figure 2-45. Experimental sequence for the observation of the director reorientation during the
rheo-optical elongation-recovery cycle.